Blackwell Science, LtdOxford, UK
PCEPlant, Cell and Environment0016-8025Blackwell Science Ltd 2001
25
798
ABA, ethylene and root and shoot growth
R. E. Sharp
Review ArticleBEES SGML
Plant, Cell and Environment (2002) 25, 211–222
Interaction with ethylene: changing views on the role
of abscisic acid in root and shoot growth responses to
water stress
R. E. SHARP
Department of Agronomy, Plant Sciences Unit, 1–87 Agriculture Building, University of Missouri, Columbia, MO 65211, USA
ABSTRACT
Shoot and root growth are differentially sensitive to water
stress. Interest in the involvement of hormones in regulating these responses has focused on abscisic acid (ABA)
because it accumulates in shoot and root tissues under
water-limited conditions, and because it usually inhibits
growth when applied to well-watered plants. However, the
effects of ABA can differ in stressed and non-stressed
plants, and it is therefore advantageous to manipulate
endogenous ABA levels under water-stressed conditions.
Studies utilizing ABA-deficient mutants and inhibitors of
ABA synthesis to decrease endogenous ABA levels, and
experimental strategies to circumvent variation in plant
water status with ABA deficiency, are changing the view of
the role of ABA from the traditional idea that the hormone
is generally involved in growth inhibition. In particular,
studies of several species indicate that an important role of
endogenous ABA is to limit ethylene production, and that
as a result of this interaction ABA may often function to
maintain rather than inhibit shoot and root growth. Despite
early speculation that interaction between these hormones
may influence many of the effects of water deficit, this topic
has received little attention until recently.
Key-words: Abscisic acid; ethylene; root growth; shoot
growth; water stress.
INTRODUCTION
Shoot growth is very sensitive to water-limited conditions.
Much evidence over the past 20 years indicates that the
inhibition of growth is metabolically regulated, rather than
being a direct consequence of altered plant water status
arising from soil drying. For example, studies have shown
that stem and leaf growth can be severely inhibited at low
water potentials despite complete maintenance of turgor in
the growing regions as a result of osmotic adjustment (e.g.
Michelena & Boyer 1982). Furthermore, shoot growth can
Correspondence: Robert E. Sharp. Fax: + 1 573 882 1469;
e-mail: [email protected]
© 2002 Blackwell Science Ltd
be so sensitive to soil drying that substantial inhibition can
occur before the development of decreased water potentials in the aerial plant parts (Saab & Sharp 1989; Gowing,
Davies & Jones 1990), and such observations have led to
much interest in the involvement of non-hydraulic regulatory signals from the roots (Davies & Zhang 1991; Davies et
al. 2000).
Root growth is usually less inhibited than shoot growth,
or even promoted, in plants growing in drying soil, which is
of obvious benefit to maintain an adequate plant water supply (Sharp & Davies 1989). An important feature of the
root system response is the ability of some roots to continue
elongation at water potentials that are low enough to completely inhibit shoot growth. For example, this occurs in
nodal (adventitious) roots of maize, which must penetrate
through the often dry surface soil (Westgate & Boyer 1985),
and in primary roots of a range of species, which helps seedling establishment under dry conditions by ensuring a supply of water before shoot emergence (Sharp, Silk & Hsiao
1988; Spollen et al. 1993; van der Weele et al. 2000). Figure 1
shows for several important agronomic species that the primary root maintains substantial elongation rates at water
potentials as low as −1·6 MPa, whereas shoot growth is
inhibited completely at about −0·8 MPa.
The mechanisms that determine the different sensitivities of root and shoot growth to water stress are not well
understood. Although hormones are likely to play important regulatory roles, and despite considerable early attention to this topic (e.g. Itai & Ben-Zioni 1976; Bradford &
Hsiao 1982), the involvement of these compounds has not
been elucidated. Most interest in this question has concerned the role of abscisic acid (ABA). This review evaluates the evidence for the involvement of ABA in root and
shoot growth responses to water stress, with a focus on
advances in understanding gained from studying effects of
endogenous ABA deficiency. In particular, recent studies
have revealed that an important role of endogenous ABA is
to limit ethylene production, and that this interaction is
involved in the effects of ABA status on root and shoot
growth. Despite early speculation that interaction between
these hormones may influence many of the effects of water
deficit (Wright 1980; Bradford & Hsiao 1982), this topic has
received little attention until recently.
211
212 R. E. Sharp
Maize
Primary Root
Shoot
100
80
Soybean
60
Elongation rate (% control)
40
20
0
0.0
–0.5
–1.0
–1.5
0.0
–0.5
Cotton
100
–1.0
–1.5
Squash
80
60
40
20
0
0.0
–0.5
–1.0
–1.5
0.0
–0.5
–1.0
–1.5
Vermiculite water potential (MPa)
Figure 1. Elongation rates of the primary root () and shoot
() of seedlings of four species at various water potentials. Seeds
were germinated for 36 h at high water potential. The seedlings
were then transplanted to vermiculite at different water potentials
(obtained by thorough mixing with different amounts of water)
and grown at 29 °C in darkness and near saturation humidity to
minimize transpiration. For roots, data were evaluated at root
lengths of 5 cm, when elongation rates were approximately steady.
For shoots, data represent maximum elongation rates obtained
after transplanting. Data are means from 10 to 40 seedlings, and
are plotted as a percentage of the mean rate at high water
potential. (From Spollen et al. 1993. Copyrighted by BIOS
Scientific Publishers and reprinted with permission).
APPROACHES TO STUDY THE ROLE
OF ABA IN GROWTH RESPONSES TO
WATER STRESS
Interest in the involvement of hormones in regulating
growth responses to water stress has focused on ABA
because (a) it accumulates to high concentrations in shoot
and root tissues under water-limited conditions, often correlating with growth inhibition, and (b) it usually inhibits
growth when applied to well-watered plants. Based on
these findings, a commonly proposed function of increased
ABA concentrations in water-stressed plants is growth inhibition (reviewed in Trewavas & Jones 1991; Munns & Sharp
1993; Munns & Cramer 1996). Two of the most compelling
examples of this type of study are those by Creelman et al.
(1990) in soybean and by Zhang & Davies (1990) in maize.
In the former study, relationships between root and shoot
elongation and the ABA contents of the respective growth
zones were compared in seedlings whose growth was inhibited by transfer to vermiculite of low water potential
(−0·3 MPa) or to vermiculite saturated with various concentrations of ABA. Similar to the responses for soybean
shown in Fig. 1, shoot growth was more inhibited than root
growth in the water-stressed seedlings. In the well-watered
seedlings treated with ABA, growth was also more inhibited in the shoot than in the root at all ABA concentrations
tested. The relationships of growth inhibition to ABA content suggested that variation in endogenous ABA accumulation, together with differing sensitivity to ABA, could
largely explain the differential inhibition of shoot and root
growth at low water potential. In the study of maize by
Zhang & Davies (1990), the relationship between inhibition of leaf elongation rate and xylem sap ABA concentration was compared in plants grown in drying soil and in
well-watered plants to which a range of ABA concentrations was supplied to part of the root system. The similarity
between the two data sets suggested that the limitation of
leaf growth resulting from soil drying could be explained
entirely by the increase in endogenous ABA in the xylem
sap.
Similarly to the results of these studies, primary root
growth of well-watered maize seedlings is inhibited progressively when increasing concentrations of ABA are
applied, and comparison of the relationship of growth inhibition with growth zone ABA content to the values
obtained in seedlings grown at low water potential would
suggest that endogenous ABA accumulation could explain
the inhibition of root growth during water stress (Fig. 2).
However, interpretation of these and other similar studies relies on the assumption that when ABA is applied to
well-watered plants and increases to internal levels similar
to those of water-stressed plants, the effects on growth are
the same as those resulting from endogenous ABA accumulation during water stress. Increasing evidence indicates
that this is not necessarily the case. Decreases in plant water
status have been shown to change the compartmentation
(Hartung, Radin & Hendrix 1988; Bacon, Wilkinson &
Davies 1998), apparent sensitivity (Tardieu & Davies 1992;
Dodd & Davies 1996) and response (Sharp et al. 1994) to
ABA in various organs.
To avoid these concerns, it is of obvious advantage to
manipulate endogenous ABA levels under water-stressed
conditions. However, despite the availability of ABA-deficient mutants (and recently, transgenics) in several species,
and of several inhibitors of ABA biosynthesis, very few
studies have used this approach to study the role of ABA in
growth responses to water stress. A particular difficulty for
the study of ABA in growth regulation is its effect on stomatal behaviour and, thereby, on plant water balance.
Hence, ABA-deficient mutants are typically wilty due to
high stomatal conductance even under well-watered conditions (Quarrie 1987), and this problem is exacerbated
under soil drying conditions. The decrease in water status of
ABA-deficient relative to control plants could inhibit
growth independently of direct growth-regulatory properties of ABA. Jones, Sharp & Higgs (1987) subjected several
ABA-deficient tomato mutants to soil drying, and reported
greater decreases in leaf and root biomass than in the wild
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
ABA, ethylene and root and shoot growth 213
Hybrid
Vp5
vp5
Vp14
vp14
Root elongation rate (% control)
100
A
A
80
ψw = −0·03 MPa
A
A
60
40
F
F
20
ψw = −1·6 MPa
F
0
0
20
40
60
80
100
120
140
Root tip ABA content (ng g−1 H2O)
Figure 2. Primary root elongation rate as a function of root tip
(apical 10 mm, encompassing the elongation zone) ABA content
for various maize genotypes growing in vermiculite at high water
potential (ψw) (−0·03 MPa, open symbols) or low water potential
(−1·6 MPa, closed symbols). Seedlings were grown as described in
Fig. 1. At high water potential, the root ABA content of hybrid (cv.
FR27 × FRMo17) seedlings was raised above the normal level by
adding various concentrations of ABA (A) to the vermiculite. At
low water potential, the root ABA content was decreased below
the normal level by treatment with fluridone (F) or by using the
vp5 or vp14 mutants. Data are plotted as a percentage of the rate
for the same genotype at high water potential. Elongation rates of
the mutants at high water potential were similar to their respective
wild types. (Modified from Saab et al. 1990; Sharp et al. 1994 and
unpublished results of I.-J. Cho, B.-C. Tan, D.R. McCarty & R.E.
Sharp.)
type. However, leaf water potentials also decreased more in
the mutants, so interpretation of the role of ABA deficiency
in the greater growth inhibition of the mutants was not possible. Reduced root biomass compared to the wild type of
ABA-deficient and ABA-insensitive mutants of Arabidopsis in drying soil has also been reported (Vartanian, Marcotte & Giraudat 1994). However, plant water relations
were not measured nor were comparative data for wellwatered controls included, so the cause of the smaller root
system development in the mutants could not be assessed.
Accordingly, studies of the effects on growth of endogenous ABA deficiency require experimental strategies that
avoid variation in water status between plants with different levels of ABA. A few such studies have now been published, and the results are changing the view of the role of
ABA from the traditional idea that the hormone is generally involved in growth inhibition.
ABA ACCUMULATION MAINTAINS MAIZE
PRIMARY ROOT GROWTH AT LOW WATER
POTENTIALS
When maize seedlings are grown in vermiculite at a water
potential of −1·6 MPa, at which primary root but not shoot
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
elongation continues (Fig. 1), the ABA content of the root
growth zone increases about five-fold (Fig. 2). Three
approaches have been used to study the effect on root
growth of reducing the accumulation of ABA: (i) the inhibitor fluridone, which blocks carotenoid synthesis (at the
conversion of phytoene to phytofluene) and, thereby, inhibits ABA synthesis although at an early step of the pathway;
(ii) the vp5 mutant, which has a defect at the same step as
that blocked by fluridone; (iii) the vp14 mutant, which has
a defect in the synthesis of xanthoxin (Tan et al. 1997).
Xanthoxin is converted to ABA via ABA-aldehyde, and its
synthesis is considered a key regulatory step in water stressinduced ABA production (Qin & Zeevaart 1999). Initial
studies used the fluridone and vp5 mutant approaches
(Saab et al. 1990; Saab, Sharp & Pritchard 1992; Sharp et al.
1994). Recently, similar studies of vp14 were undertaken to
strengthen the conclusion that the results obtained with fluridone and vp5 were due to ABA deficiency and not to
other effects of their action (I.-J. Cho, B.-C. Tan, D.R.
McCarty & R.E. Sharp, unpublished results). To circumvent variation in plant water status with ABA deficiency,
seedlings were grown under conditions of near-zero transpiration (minimal shoot development, darkness and nearsaturation humidity).
The results obtained with the three approaches were
very similar. At high water potential, root elongation rates
(and ABA contents) were minimally affected. At low water
potential, in contrast, reduced ABA accumulation was
associated with more severe inhibition of root elongation
than in wild-type or untreated seedlings (Fig. 2). The three
methods produced varying degrees of root tip ABA deficiency, but yielded a common relationship of inhibition of
root elongation rate to root tip ABA content. In all cases,
root elongation rate fully recovered when the ABA content
of the elongation zone was restored to normal levels with
exogenous ABA. Similar results were obtained using fluridone-treated and vp5 mutant seedlings grown at a water
potential of −0·3 MPa (Sharp et al. 1994). These studies
demonstrated that accumulation of ABA is required for the
maintenance of maize primary root elongation at low water
potentials.
Additional experiments at a water potential of −0·3 MPa
showed that when the root tip ABA content of fluridonetreated seedlings was increased to twice (or more) the level
in untreated seedlings, recovery of root elongation was
decreased. Also, promotion of root elongation was not
observed with any applied ABA concentration in untreated
seedlings; again, slight inhibition of elongation resulted
when the root tip ABA content was increased to twice the
naturally occurring level (Sharp et al. 1994). Thus, the normal increase in ABA content in roots at low water potential
is optimal for growth maintenance (also see Fig. 7).
It is important to note that the conclusion that the accumulation of ABA in water-stressed roots helps to maintain
growth cannot be inferred by applying ABA to wellwatered seedlings to simulate the increase in content under
water stress (Sharp et al. 1994). Figure 2 shows that in wellwatered seedlings treated with ABA, root growth was sub-
214 R. E. Sharp
stantially inhibited at the root tip ABA content that occurs
in water-stressed roots. These results illustrate that the
maintenance of root elongation at low water potential by
ABA is not solely a function of the increase in ABA content, but also requires the change in environmental conditions that modifies the growth response to ABA.
ABA maintains root growth at low water
potentials by restricting ethylene production
a
30
c
20
a
(b)
b
10
30
20
10
0
b
b
b
0
Cntl +AOA +FLU +FLU
+AOA
Cntl +AOA +FLU +FLU
+AOA
Figure 4. Primary root length increase (a) and ethylene
evolution rate (b) of control (Cntl) and fluridone (FLU)-treated
maize seedlings with or without treatment with aminooxyacetic
acid (AOA) to inhibit ethylene synthesis. Measurements were
made at 42 h (a) and 20 h (b) after transplanting to vermiculite at
a water potential of −1·6 MPa; seedlings were grown as described
in Fig. 1. Within each panel, bars with different letters are
significantly different at the 0·05 level. (Modified from Spollen et
al. 2000.)
all cases the root tip ABA content of the ABA-deficient
seedlings at low water potential remained higher than in
well-watered seedlings (Fig. 2), indicating that an increased
concentration of ABA is necessary to prevent excess ethylene production under water stress. Third, it was shown that
root elongation could be largely restored by each of three
inhibitors of ethylene synthesis or action (Spollen et al.
2000). Significantly, when ethylene evolution was restored
Root elongation rate (% inhibition)
0
Hybrid + FLU
vp 5
vp14
1000
800
20
40
600
Root elongation
400
60
Ethylene evolution
200
80
100
100
Ethylene evolution (% promotion)
Extending the study of ABA-deficient maize seedlings,
recent work has shown that an important function of
endogenous ABA in the maintenance of primary root
growth at low water potential is to prevent excess ethylene
production. First, it was observed that the roots of ABAdeficient seedlings at low water potential not only are
shorter but also exhibit pronounced radial swelling primarily beyond the apical 2 mm (Fig. 3). Exogenous ethylene
inhibits elongation and causes a similar pattern of swelling
in maize primary roots at high water potential (Moss, Hall
& Jackson 1988; Whalen & Feldman 1988). Second, it was
shown that under water stress, ethylene production of
ABA-deficient seedlings was substantially greater than in
control plants (Figs 4 & 5), and this effect was completely
prevented when the root ABA content was restored with
exogenous ABA, together with restoration of root elongation (Spollen et al. 2000). Among the three methods used to
reduce ABA accumulation (fluridone, vp5, vp14), the magnitude of the increase in ethylene evolution correlated both
with the degree of ABA deficiency and the inhibition of
root elongation rate (Fig. 5). It is important to note that in
(a)
b
Ethylene evolution rate
per seedling (pmol h−1)
Root length increase (mm)
40
0
80
60
40
20
0
Root tip ABA content (% inhibition)
Figure 5. Primary root elongation and seedling ethylene
Figure 3. The apical 11 mm of primary roots of (a) untreated
and fluridone (FLU)-treated hybrid, and (b) wild-type and vp5
mutant maize seedlings 48 h after transplanting to vermiculite at a
water potential of −1·6 MPa. Seedlings were grown as described in
Fig. 1. Bar = 1 mm. (From Sharp et al. 1993. Copyrighted by the
American Society of Plant Biologists and reprinted with
permission.)
evolution rate as a function of root tip (apical 10 mm) ABA
content for fluridone (FLU)-treated hybrid, vp5 mutant and vp14
mutant maize seedlings growing in vermiculite at a water potential
of −1·6 MPa. Data are plotted as a percentage of values for
untreated or wild-type seedlings growing at the same water
potential. (Modified from Spollen et al. 2000 and unpublished
results of I.-J. Cho, B.-C. Tan, D.R. McCarty & R.E. Sharp.)
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
ABA, ethylene and root and shoot growth 215
ABA ACCUMULATION CAN INHIBIT
AND PROMOTE MAIZE SEEDLING SHOOT
GROWTH AT LOW WATER POTENTIALS
As discussed above, ABA is generally regarded as an inhibitor of shoot growth (Trewavas & Jones 1991; Davies 1995;
Munns & Cramer 1996). Initial studies of the effect of
decreasing endogenous ABA levels in maize seedlings
grown at low water potential (under conditions of near-zero
transpiration) were consistent with this expectation (Saab
et al. 1990, 1992). Seedlings were grown in vermiculite at a
water potential of −0·3 MPa, at which slow rates of shoot
growth continue (Fig. 1), and fluridone and the vp5 mutant
were again used to decrease endogenous ABA accumulation. Studies were confined to the first 50 h after transplant© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
ing to low water potential, and showed that ABA-deficiency
resulted in substantial promotion of shoot elongation. Similar results of experiments with fluridone are shown in
Fig. 6a and Table 1 (first 40 h after transplanting). These
experiments indicated an involvement of ABA in the inhibition of shoot growth of water-stressed maize seedlings, in
contrast to its role in root growth maintenance.
However, experiments of a longer duration revealed a
greater complexity in the relationship of shoot growth to
ABA status at low water potential (Feng 1996). After 50 h
Control
+ABA
+FLU
+FLU,+ABA
80 (a)
ψw =
−0·03 MPa
60
40
Shoot length increase (mm)
to normal levels [using the inhibitor of ethylene synthesis
aminooxyacetic acid (AOA)], root growth was completely
restored (Fig. 4), suggesting that ABA deficiency did not
also increase the sensitivity of growth to ethylene.
Importantly, since none of the inhibitors of ethylene synthesis or action substantially increased root elongation
when ABA-deficiency was not imposed (results with AOA
are shown in Fig. 4), ethylene does not appear to be an
important cause of the inhibition of elongation in waterstressed roots that accumulate normal levels of ABA
(Figs 1 & 2). In other words, the accumulation of ABA in
water-stressed roots is sufficient to prevent excess ethylene
production, in agreement with the above-mentioned finding that roots at low water potential exhibit an optimum
ABA content for growth maintenance. The possible
involvement of ethylene in the inhibition of growth during
water stress is a long-standing question (El-Beltagy & Hall
1974), but there is no previous information in relation to
root growth. This question is considered further below in
relation to the inhibition of shoot growth in water-stressed
plants.
The discovery that increased levels of endogenous ABA
in roots at low water potential are required to limit ethylene
production confirms ideas first suggested by Wright (1980)
and developed further by Bradford & Hsiao (1982) but
which had not been tested. These suggestions were based
on the finding that pre-treatment with exogenous ABA prevented the increase in ethylene production caused by wilting of excised wheat leaves. It should be noted that
although several other studies reported that ABA treatments of well-watered plants inhibited ethylene production
(e.g. Gertman & Fuchs 1972; Yoshi & Imaseki 1981; Tan &
Thimann 1989), there are also many reports of ABA-stimulated ethylene production (Riov et al. 1990 and references
therein). Interpretation of these results is complicated by
uncertainty that effects of applied ABA in well-watered
plants are predictive of the role of endogenous ABA accumulation in water-stressed plants, as discussed above. The
approach of using chemical and genetic means to manipulate endogenous ABA levels in water-stressed plants
avoided these concerns.
ψw = −0·3 MPa
20
0
0
80 (b)
50
100
150
Control
+STS
60
40
ψw = −0·3 MPa
20
0
0
50
100
Hours after transplanting
150
Figure 6. Shoot length increase of maize seedlings (cv.
FR27 × FRMo17) after transplanting to vermiculite at high
(−0·03 MPa) or low (−0·3 MPa) water potential (ψw). Seeds were
germinated for 55 h at high water potential, then seedlings were
transplanted to high or low water potential and grown at 29 °C in
darkness and near saturation humidity. (a) Fluridone (FLU, 10 µM)
and ABA were supplied during germination and after
transplanting. The ABA concentration supplied during
germination (100 µM) partially restored (by approximately 45%)
the ABA content of fluridone-treated shoots early after
transplanting; higher concentrations were not used because they
severely delayed germination. The ABA concentration supplied
after transplanting (20 µM) was optimal for growth promotion
during the second phase of the experiment. (b) Silver thiosulphate
(STS, 6 mM) was supplied during germination to inhibit ethylene
action; the STS concentration was optimal for growth promotion
during the second phase of the experiment. (STS was not supplied
after transplanting because this resulted in less growth promotion,
presumably due to toxicity from long-term exposure.) Data are
means ±SE, n = 24. (Modified from Feng 1996.)
216 R. E. Sharp
Table 1. Effects of treatments with ABA, fluridone (FLU) and
silver thiosulphate (STS) on shoot elongation rate of maize
seedlings during early and later periods after transplanting to
vermiculite at a water potential of −0·3 MPa. Elongation rates
were calculated from the shoot length increases shown in Fig. 6.
(Modified from Feng 1996)
Treatment
Shoot elongation rate (mm h−1)
0–41 h
72–135 h
Control
+ABA
+FLU
+FLU, +ABA
0.34
0.22
0.54
0.27
0–42 h
0.28
0.64
0.08
0.52
67–138 h
Control
+STS
0.36
0.25
0.18
0.74
from transplanting to a water potential of −0·3 MPa, the
effect of ABA-deficiency reversed so that shoot growth
became inhibited relative to the control (Fig. 6a). Table 1
shows that shoot elongation rate between 72 and 135 h was
inhibited by over 70% in fluridone-treated compared to
control seedlings. Both the fluridone-induced promotion of
shoot growth early after transplanting and the inhibition
during the later period could be prevented with exogenous
ABA. Moreover, when the same ABA treatment was
applied without treatment with fluridone, shoot growth was
further inhibited in the early phase and further promoted
during the later phase (Fig. 6a), such that the shoot elongation rate was more than two-fold greater than in the control
seedlings after 70 h from transplanting (Table 1). These
results indicate that the normal accumulation of ABA in
water-stressed maize seedlings functions to inhibit shoot
growth only at an early stage of development, and subsequently helps to maintain growth, as in roots. In contrast to
the roots, however, in which ABA levels were optimal for
growth maintenance, endogenous ABA accumulation was
apparently insufficient for maximal shoot growth during the
second phase of the experiment.
In the experiment shown in Fig. 6a, supplemental ABA
was supplied both during germination and after transplanting, and the resulting growth promotion after 70 h was
largely attributable to growth of the mesocotyl. Since it is
also the mesocotyl (and coleoptile) that is promoted by
ABA deficiency during the first 40 h (Saab et al. 1990, 1992),
the change in response of shoot growth to ABA from inhibition to promotion was not due to differential sensitivity
among organs but occurred within the same organ. The shift
to growth promotion was not restricted to the mesocotyl,
however. In other experiments in which ABA was supplied
only via the low-water-potential vermiculite into which the
seedlings were transplanted, overall shoot elongation was
similarly promoted after 50 h but the response was specific
to leaf growth (Fig. 7). This difference probably reflects the
different developmental stage of the shoots relative to the
timing of ABA uptake. Importantly, there was no effect of
the supplemental ABA treatment on root growth (Fig. 7),
demonstrating that the promotion of shoot growth was not
attributable indirectly to root growth inhibition (see shoot
and root dry weight data in Fig. 7, legend), and in agreement with the above-mentioned conclusion that the endogenous ABA content of water-stressed roots is optimal for
growth maintenance.
As was the case for root growth (Fig. 2), shoot growth of
seedlings grown at high water potential was inhibited progressively with increasing concentrations of applied ABA
(Feng 1996). Thus, the promotive effect of supplemental
ABA on shoot growth at low water potential appeared to
be specific to the water-stressed condition.
Effects of ABA on shoot growth are mimicked by
inhibiting ethylene action
The change with time after transplanting to low water
potential in the response of shoot growth to supplemental
ABA, from inhibition to promotion, was closely mimicked
by treatment with silver thiosulphate (STS) to inhibit ethylene action (Fig. 6b, Table 1). Thus, treatment with STS
inhibited shoot growth early after transplanting and then
markedly promoted shoot growth relative to the control.
Similar results were obtained using AOA to inhibit ethylene synthesis (Feng 1996). Consistently, preliminary exper-
Figure 7. Representative maize seedlings 209 h after
transplanting to vermiculite at a water potential of −0·3 MPa that
contained either 0 µM (control) or 10 µM ABA. Seedlings were
grown as described in Fig. 6. Dry weights (mg, means ±SE, n = 24)
were: control shoot, 24 ± 4; control root, 69 ± 6; +ABA shoot,
33 ± 3; +ABA root, 70 ± 7. (From Feng 1996.)
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
ABA, ethylene and root and shoot growth 217
iments showed that shoot growth could be increased during
the early phase, and inhibited during the later phase, by
applying either ethylene or the ethylene precursor 1aminocyclopropane-1-carboxylate (ACC), simulating the
effects of ABA deficiency.
Taken together, the results in Fig. 6 and Table 1 suggest
that, as in roots, an important role of ABA in shoot growth
of water-stressed maize seedlings is to restrict ethylene production and/or sensitivity. ABA accumulation thereby
played either a growth-inhibitory (early phase) or growthmaintaining (later phase) role in the response of shoot
growth to water stress, due to a change in the effect of ethylene from promotive to inhibitory.
The substantial promotion of shoot growth at later
stages of development by treatment with STS indicates that
the normal accumulation of ABA in the shoot was suboptimal for growth because it was insufficient to fully prevent
ethylene-induced growth inhibition. This contrasts with the
apparent sufficiency of ABA accumulation in the roots,
and this difference accounted at least partly for the differential ability of the roots and shoots to grow at low water
potential.
It should be noted that the extent to which ethylene
caused the inhibition of shoot growth in water-stressed
seedlings at later stages of development, and therefore the
extent to which growth could be promoted by supplemental
ABA, was influenced by nutritional status. It was subsequently discovered that the seedlings used for the experiments illustrated in Figs 6 and 7 and Table 1 were grown
with suboptimal Ca2+ availability. When seedlings were
grown with a different culture protocol involving supplemental Ca2+ (because the properties of the vermiculite had
changed; Spollen et al. 2000), responses to STS and ABA
after 50 h from transplanting were less pronounced (M.A.
Else & R.E. Sharp, unpublished results). Such an interaction with nutrient status is not unexpected, because nutrient availability can markedly alter plant ethylene relations
(e.g. He, Morgan & Drew 1992).
GENERALITY OF THE PROMOTIVE EFFECT
OF ABA ON SHOOT GROWTH
As discussed above, most of the research underlying the
view that ABA is an inhibitor of shoot growth during water
stress has involved applications of ABA to non-stressed
plants or correlations of growth inhibition to increased
endogenous ABA levels during stress. Indeed, the early
phase of the maize seedling studies described above (Fig. 6;
Saab et al. 1990, 1992) remains the only demonstration of
enhanced shoot growth in response to endogenous ABA
deficiency at low water potentials. The apparent change in
response to ethylene during shoot development in those
experiments, from promotive to inhibitory, is consistent
with reports that ethylene stimulates mesocotyl growth in
some species (Suge 1971; Cornforth & Stevens 1973),
whereas it is usually inhibitory to shoot growth of terrestrial
plants at later stages of development (Abeles, Morgan &
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
Saltveit 1992; Lee & Reid 1997; Hussain et al. 1999). The
following sections summarize recent studies which suggest
that restriction of ethylene production may be a common
function of ABA and therefore that endogenous ABA may
often act to maintain rather than inhibit shoot growth.
Reassessment of the role of ABA in shoot
growth of tomato and Arabidopsis under wellwatered conditions
Paradoxically to the long-standing view that ABA is generally inhibitory to shoot growth, it has been observed for
over 30 years that ABA-deficient mutants are often shorter
and have smaller leaves than the corresponding wild types,
and that leaf and stem growth can be substantially restored
by applying ABA (Imber & Tal 1970; Bradford 1983; Quarrie 1987). As already mentioned, in addition to reduced
growth, ABA-deficient mutants are typically wilty even
when the soil is well supplied with water. This results from
high stomatal conductance and has also been shown to
involve decreased root hydraulic conductance. These
effects can also be prevented by applying ABA (Imber &
Tal 1970; Tal & Nevo 1973; Koornneef et al. 1982; Bradford
1983). Accordingly, the inhibited shoot growth of ABAdeficient mutants of tomato and Arabidopsis has been
attributed to shoot water deficits (Bradford 1983; Neill,
McGaw & Horgan 1986; Nagel, Konings & Lambers 1994;
Léon-Kloosterziel et al. 1996). The growth-promotive effect
of applied ABA in these cases has therefore been assumed
to result from improvement of plant water balance. Consequently, these findings have generally not been regarded as
evidence against a direct inhibitory role of ABA in leaf and
stem growth, although the observations have led some
authors to question this view (Jones et al. 1987; Quarrie
1987; Taylor 1987; Zeevaart & Creelman 1988).
Alternatively, endogenous ABA may be required to
maintain shoot growth of well-watered plants independently of its effect on plant water balance. Consistent with
the finding that ABA restricts ethylene production in waterstressed maize seedlings, it was reported that under wellwatered conditions, ethylene production was greater in
shoots of the flacca (flc) mutant of tomato (Tal et al. 1979)
and in whole plants of the aba1 mutant of Arabidopsis
(Rakitina et al. 1994) than in the corresponding wild types.
In flc, it was also shown that ethylene production could be
reduced to normal levels with exogenous ABA. In addition,
the ABA-deficient mutants of tomato exhibit morphological symptoms characteristic of excess ethylene such as leaf
epinasty and adventitious rooting (Tal 1966; Nagel et al.
1994). Despite these observations, the possibility that ethylene is a cause of reduced shoot growth in ABA-deficient
mutants was not considered until recently.
To distinguish between these possibilities, it is necessary
to assess the growth of ABA-deficient mutants at the same
plant water status as the wild type. If the inhibition of
growth normally observed is caused by adverse water relations, then under such conditions growth should be restored
(a)
a
b
3000
2000
c
c
1000
0
WT flc
flc flc
(sc) +ABA
(b)
a
0·6 a
0·4
b
0·2
0·0
WT flc
b
flc flc
(sc) +ABA
Ethylene evolution (pmol g−1 FW h−1)
Leaf area (cm2)
4000
Leaf ABA content (nmol g−1 FW)
218 R. E. Sharp
200
a
150
a
100
50
0
(c)
b
b
WT flc
flc flc
(sc) +ABA
Figure 8. Total leaf area (a, 35 d after emergence), leaf ABA content (b, 21 d) and leaf ethylene evolution rate (c, 21 d) of wild-type (WT)
tomato (cv. Rheinlands Ruhm), flacca (flc), flacca spray control (sc), and flacca sprayed with 10 µM ABA daily from day 9. Spray control plants
were sprayed with deionized water containing the same concentrations of ethanol and Tween 20 as in the ABA solution. All plants were well
watered, and were grown under controlled-humidity conditions so that throughout development, leaf water potentials of untreated flacca
were equal to or higher than those of the wild type, and leaf water potentials of flacca treated with ABA were equal to or lower than those
of the flacca spray control. Within each panel, bars with different letters are significantly different at the 0·05 (leaf area, ABA) or 0·1 (ethylene)
level. (Modified from Sharp et al. 2000.)
or even enhanced relative to wild-type plants. In contrast, if
endogenous ABA is required to maintain shoot growth
independently of effects on plant water balance, then the
mutants should remain smaller. Jones et al. (1987) grew flc
and two other ABA-deficient mutants of tomato, notabilis
(not) and sitiens (sit), under mist in a greenhouse, and
observed that stem height became greater than in the wild
type, consistent with ABA playing an inhibitory role in
stem elongation. In contrast, later-formed leaves remained
smaller and total leaf biomass remained substantially
decreased. However, leaf water potentials also remained
considerably lower than in the wild type, so interpretation
of the role of ABA in leaf growth was not possible. Taylor
(1987) reported that the inhibition of shoot growth in double mutants of flc, not and sit was partially alleviated when
the plants were grown at high humidity, but information on
plant water relations was not included so, again, full interpretation was not possible. In other studies in which flc was
grown at high humidity, effects on growth were not
reported (Puri & Tal 1977; Tal et al. 1979).
To assess whether the reduced leaf and stem growth of
well-watered flc and not are attributable to water deficits, in
a recent study plants were grown under controlled-humidity conditions in a growth chamber, such that the leaf water
potentials of the mutants were equal to or higher than
those of wild-type plants throughout development (Sharp
et al. 2000). Most parameters of shoot growth remained
markedly impaired; for example, total leaf area in flc was
48% less than in the wild type (Fig. 8a). Root growth was
also greatly reduced. Consistent with the observation of
Jones et al. (1987), the stems of the mutants initially elongated more rapidly than those of the wild types; however,
this effect was not sustained, and the mutants became
shorter at later stages of development. Shoot growth substantially recovered when wild-type levels of ABA were
restored by treatment with exogenous ABA (Fig. 8a,b),
even though the experimental strategy prevented improve-
ment in leaf water potential. The ability of applied ABA to
increase growth was greatest for leaf expansion, which was
restored by 75%. The ethylene evolution rate of growing
leaves was doubled in flc compared with the wild type
(Fig. 8c), and treatment with STS to inhibit ethylene action
partially restored leaf, stem and root growth (Sharp et al.
2000).
Similar results have been obtained in preliminary studies
of Arabidopsis. When grown at the same leaf water potentials as well-watered wild-type plants throughout development, total leaf area of the aba2 mutant (ABA-deficient)
remained greatly inhibited, and was restored by about 50%
in a double mutant of aba2 and etr1 (ethylene-resistant)
(M.E. LeNoble, W.G. Spollen & R.E. Sharp, unpublished
results). The leaf ethylene evolution rate of aba2 was
approximately twice that of the wild type under these conditions.
These results demonstrate that (a) normal levels of
endogenous ABA are required to maintain shoot development, particularly leaf expansion, in well-watered tomato
and Arabidopsis plants independently of effects on plant
water balance, and (b) the impairment of leaf and shoot
growth caused by ABA deficiency is at least partly attributable to increased ethylene production.
It should be noted that these findings do not negate the
necessity of avoiding differences in plant water status
among genotypes in future studies of the effects of ABA
manipulation on growth. Differences in water relations,
when they do occur, might also contribute directly or indirectly to growth responses. It will be particularly critical
(and challenging) to extend the strategy of circumventing
variation in plant and also soil water status between control
and ABA-deficient (or ABA-overproducing) genotypes to
address the function of ABA accumulation in the responses
of plant growth to soil drying.
The conclusion with well-watered tomato and Arabidopsis that endogenous ABA is required to maintain shoot
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
ABA, ethylene and root and shoot growth 219
growth contrasts with the many examples in the literature
where ABA has caused shoot growth inhibition when
applied to well-watered plants. The explanation for this difference might be simply explained by suggesting that the
effects of supplemental ABA in well-watered plants are
non-physiological. However, Bacon et al. (1998) published
compelling evidence that the normal levels of ABA in wellwatered barley can cause leaf growth inhibition. They
showed that the leaf elongation rate of wild-type plants
declined as the pH of artificial xylem sap fed to the leaves
was increased, but that this response did not occur in the
ABA-deficient mutant Az34 unless a normal well-watered
concentration of ABA was supplied in the sap. (The pHinduced reduction in leaf elongation is suggested to result
from increased accumulation of ABA in the apoplast.) This
result is difficult to reconcile with the inhibited growth of
the ABA-deficient mutants of tomato and Arabidopsis. It is
possible that whole plant ABA-deficiency leads to an overriding inhibitory effect on growth of increased ethylene
production. However, the above-described experiments
with water-stressed maize seedlings indicate that, for root
growth at least, this is not the case. In ABA-deficient seedlings in which ethylene evolution was restored to the normal water-stressed rate by treatment with AOA, root
growth recovered but only to the rate of the control seedlings (Fig. 4). In other words, preventing the effect of ABAdeficiency on ethylene production did not uncover a
growth-promotive effect of ABA deficiency.
Resolution of the differing conclusions on the role of
ABA in leaf growth of well-watered plants from the studies
of barley (Bacon et al. 1998) and of tomato and Arabidopsis
will require further investigation.
(1996a, b). Leaf growth of plants growing in compacted soil
was more inhibited in Az34 than in the wild type, and the
evidence suggested that changes in leaf water relations
were not the cause. Recent studies from the same group
demonstrated that ethylene was a major cause of the inhibition of shoot growth in tomato plants grown with their
root system divided between pots of uncompacted and
compacted soil (Hussain et al. 1999, 2000). As shown in
Fig. 9c, ethylene production increased greatly in wild-type
plants under compaction. This response was almost fully
prevented in the ethylene-deficient transgenic ACO1AS
(ACC oxidase antisense; Hamilton, Lycett & Grierson
1990), as was the inhibition of leaf growth (Fig. 9a). The
increase in ethylene production in the wild type occurred
despite an increase in xylem sap ABA concentration
(Fig. 9b). Interestingly, when supplemental ABA was supplied to the wild type, leaf ethylene production was partly
reduced and leaf growth was partially restored. The same
ABA treatment had a minimal effect on growth or ethylene
evolution in ACO1AS, indicating that the effect of ABA on
growth in the wild type was likely an indirect effect via inhibition of ethylene production. These results indicate that
the normal increase in ABA in compaction-stressed plants
was insufficient to fully prevent excess ethylene production
in the leaves.
In addition, Hussain et al. (2000) attempted to use the
ABA-deficient not mutant to demonstrate that the increase
in endogenous ABA in the wild type helped to maintain
leaf growth during compaction by limiting the production
of ethylene. However, ethylene production was similar and
inhibition of shoot growth was actually less in the mutant
than in the wild type in compacted relative to noncompacted plants. Interpretation of these findings was confounded, however, because growth of the mutant was
already substantially inhibited in uncompacted control
plants. The inhibition in the control plants was associated
with, and probably caused by, an already-increased rate of
leaf ethylene evolution, consistent with the results for flc
shown in Fig. 8. The problem of impaired growth of ABAdeficient control plants is discussed further below.
Role of ABA in shoot growth response to
compaction
150
100
ed
pa
c
te
d+
pa
ct
ed
ct
om
C
om
C
om
U
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
AB
A
0
nc
ct
pa
om
50
pa
Ethylene evolution (pmol g−1 FW h−1)
+A
B
200
ed
pa
ct
ed
ed
A
0
(c)
250
C
om
C
50
om
ed
pa
c
te
d+
pa
ct
ed
ct
om
C
pa
nc
om
U
AB
A
0
100
C
40
150
pa
ct
80
200
om
120
(b)
nc
Leaf area (cm2)
160
(a)
U
Wild type
ACO1AS
Xylem ABA concentration ( µ mol m−3)
Plants growing in compacted soil often exhibit reduced
shoot growth. Evidence that endogenous ABA plays a role
in shoot growth maintenance rather than inhibition in compaction-stressed barley was reported by Mulholland et al.
Figure 9. Total leaf area (a), xylem sap
ABA concentration (b), and leaf ethylene
evolution (c) of wild-type (cv. Ailsa Craig) and
ACO1AS transgenic tomato plants at 21 d after
emergence. Plants were well watered, and
were grown in a split-pot system in which
either both compartments contained
uncompacted soil or one compartment
contained uncompacted soil and the other
contained compacted soil. The compartment
containing compacted soil was supplied either
with water or with 100 nM ABA (compacted
+ABA) twice daily from day 5. (Modified
from Hussain et al. 2000.)
220 R. E. Sharp
DOES ABA MAINTAIN SHOOT GROWTH IN
WATER-STRESSED PLANTS AT LATER
STAGES OF DEVELOPMENT?
As described above, in well-watered tomato and Arabidopsis plants, the normal low levels of ABA are sufficient to
prevent excess ethylene production. In contrast, in roots of
water-stressed maize seedlings, ABA deficiency resulted in
increased ethylene production although ABA levels were
still substantially higher than in well-watered plants (Figs 2,
4 & 5). Similarly, in tomato plants exposed to soil compaction, leaf ethylene production increased to growth-inhibitory levels despite an increase in xylem sap ABA
concentration (Fig. 9). Taken together, these results indicate that, for reasons unknown, stressed relative to nonstressed plants require an increased concentration of ABA
to prevent excess ethylene production. Furthermore, in
both water-stressed maize seedlings and compactionstressed tomato plants, normal increases in endogenous
ABA were insufficient to fully prevent ethylene-induced
shoot growth inhibition. In contrast, ethylene was not an
important cause of the inhibition of elongation in waterstressed maize roots that accumulated normal levels of
ABA. Based on these findings, it seems reasonable to
hypothesize that increased levels of endogenous ABA function to maintain rather than inhibit shoot growth of waterstressed plants at later stages of development, but that the
accumulation of ABA may be insufficient to fully prevent
ethylene-induced growth inhibition, hence contributing to
the greater sensitivity of shoot relative to root growth.
Consistent with this hypothesis, there are many reports
that ethylene production can be increased by plant water
deficits (e.g. El-Beltagy & Hall 1974). However, Morgan et
al. (1990) and Narayana, Lalonde & Saini (1991) reported
that in intact cotton, bean and wheat plants subjected to
slow soil drying, ethylene production rates decreased. They
cautioned that many of the earlier observations might have
been erroneous due to the use of excised plant parts and
rapid drying. Restriction of ethylene production resulting
from ABA accumulation provides a possible explanation
for decreased ethylene production under water deficits.
Definitive experiments to test whether increased ethylene
production in drying soil, when it occurs, is a cause of the
inhibition of leaf growth are lacking. In particular, despite
the availability of ethylene deficient or insensitive mutants
and transgenic plants of several species, there are no
reports of the growth responses of these genotypes to soil
drying.
To assess the role of ABA in the responses of growth to
soil drying, a system in which ABA levels are adequate for
normal growth under control conditions is essential. Otherwise, it may not be possible to distinguish the role of
stress-induced increases in ABA concentration from the
function of normal ABA levels. As described above, shoot
growth is already substantially inhibited in ABA-deficient
tomato and Arabidopsis mutants under non-stressed conditions independently of effects on plant water balance, at
least partly due to excess ethylene production. This would
confound interpretation of the effect of decreasing ABA
accumulation on growth under soil drying conditions. It is
also essential that root as well as shoot development is normal under non-stressed conditions. Otherwise, after withholding water, the patterns of soil drying and root stress
exposure would differ between genotypes. Such an effect
would probably influence the shoot growth response
because of the likely involvement of non-hydraulic signals
from roots in drying soil. To avoid the problem of impaired
growth in control plants, a partially complemented line of
the not mutant of tomato has recently been identified that
exhibits normal ABA levels and shoot and root growth
when well watered yet is deficient in water stress-induced
ABA accumulation (E. T. Thorne, A. C. Jackson, A. Burbidge, A. J. Thompson, I. B. Taylor & R. E. Sharp, unpublished results). This genotype is being utilized to examine
specifically the role of increased ABA concentrations in the
response of shoot growth to soil drying.
CONCLUSIONS
The role of ABA in determining plant growth responses to
water stress is a long-standing question. The studies of
ABA-deficient genotypes described in this review have
provided compelling insight into this issue. The commonality of the findings in maize, tomato and Arabidopsis suggests that restriction of ethylene production may be a
widespread function of ABA, and that, as a result, endogenous ABA may often function to maintain rather than
inhibit plant growth. In addition to root and shoot growth
regulation, this hormonal interaction is likely to be relevant
to other stress responses that are thought to involve ethylene, for example early leaf senescence and leaf, flower and
fruit abscission (Morgan & Drew 1997). Moreover, because
increases in ABA concentration and ethylene production
occur in response to a range of environmental stress conditions, the findings may be of wide applicability.
ACKNOWLEDGMENTS
I thank Dr Mary LeNoble for useful discussions and helpful
comments on the manuscript. The work was supported by
the National Science Foundation, the National Research
Initiative Competitive Grants Program of the U.S.
Department of Agriculture, and the University of Missouri
Food for the 21st Century Program. Contribution no.
13,171 from the Missouri Agricultural Experiment Station
Journal Series.
REFERENCES
Abeles F.B., Morgan P.W. & Saltveit M.E. Jr (1992) Ethylene in
Plant Biology, 2nd edn. Academic Press, Inc., San Diego, CA.
Bacon M.A., Wilkinson S. & Davies W.J. (1998) pH-regulated
leaf cell expansion in droughted plants is abscisic acid dependent. Plant Physiology 118, 1507–1515.
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
ABA, ethylene and root and shoot growth 221
Bradford K.J. (1983) Water relations and growth of the flacca
tomato mutant in relation to abscisic acid. Plant Physiology 72,
251–255.
Bradford K.J. & Hsiao T.C. (1982) Physiological responses to
moderate water stress. In Encyclopedia of Plant Physiology
(New Series), Vol. 12: Physiological Plant Ecology II (eds O.L.
Lange, P.S. Nobel, C.B. Osmond & H. Zeigler), pp. 263–324.
Springer-Verlag, Berlin.
Cornforth I.S. & Stevens R.J. (1973) Ethylene and the germination and early growth of barley. Plant and Soil 38, 581–587.
Creelman R.A., Mason H.S., Bensen R.J., Boyer J.S. & Mullet
J.E. (1990) Water deficit and abscisic acid cause differential
inhibition of shoot versus root growth in soybean seedlings.
Analysis of growth, sugar accumulation, and gene expression.
Plant Physiology 92, 205–214.
Davies P.J. (1995) Plant Hormones. Physiology, Biochemistry and
Molecular Biology 2nd edn. Kluwer, Dortrecht, The Netherlands.
Davies W.J. & Zhang J. (1991) Root signals and the regulation of
growth and development of plants in drying soil. Annual Review
of Plant Physiology and Plant Molecular Biology 42, 55–76.
Davies W.J., Bacon M.A., Thompson D.S., Sobeih W. & González Rodríguez L. (2000) Regulation of leaf and fruit growth in
plants growing in drying soil: exploitation of the plants’ chemical
signaling system and hydraulic architecture to increase the efficiency of water use in agriculture. Journal of Experimental Botany 51, 1617–1626.
Dodd I.C. & Davies W.J. (1996) The relationship between leaf
growth and ABA accumulation in the grass leaf elongation zone.
Plant, Cell and Environment 19, 1047–1056.
El-Beltagy A.S. & Hall M.A. (1974) Effect of water stress upon
endogenous ethylene levels in Vicia faba. New Phytologist 73,
47–60.
Feng X. (1996) Is ABA an inhibitor or promoter of shoot growth in
water-stressed plants? MSc Thesis, University of Missouri, Columbia, MO, USA.
Gertman E. & Fuchs Y. (1972) Effect of abscisic acid and its
interaction with other plant hormones on ethylene production in
two plant systems. Plant Physiology 50, 194–195.
Gowing D.J.G., Davies W.J. & Jones H.G. (1990) A positive
root-sourced signal as an indicator of soil drying in apple, Malus
x domestica Borkh. Journal of Experimental Botany 41, 1535–
1540.
Hamilton A.J., Lycett G.W. & Grierson D. (1990) Antisense
gene that inhibits synthesis of the hormone ethylene in transgenic plants. Nature 346, 284–287.
Hartung W., Radin J.W. & Hendrix D.L. (1988) Abscisic acid
movement into the apoplastic solution of water-stressed cotton leaves. Role of apoplastic pH. Plant Physiology 86, 908–
913.
He C.J., Morgan P.W. & Drew M.C. (1992) Enhanced sensitivity to ethylene in nitrogen- or phosphate-starved roots of Zea
mays L. during aerenchyma formation. Plant Physiology 98,
137–142.
Hussain A., Black C.R., Taylor I.B. & Roberts J.A. (1999) Soil
compaction. A role for ethylene in regulating leaf expansion and
shoot growth in tomato? Plant Physiology 121, 1227–1237.
Hussain A., Black C.R., Taylor I.B. & Roberts J.A. (2000)
Does an antagonistic relationship between ABA and ethylene
mediate shoot growth when tomato (Lycopersicon esculentum
Mill.) plants encounter compacted soil? Plant, Cell and Environment 23, 1217–1226.
Imber D. & Tal M. (1970) Phenotypic reversion of flacca, a wilty
mutant of tomato, by abscisic acid. Science 169, 592–593.
Itai C. & Ben-Zioni A. (1976) Water stress and hormonal
response. In Water and Plant Life, Ecological Studies Vol. 19
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222
(eds O.L. Lange, L. Kappen & E.-D. Schulze), pp. 225–242.
Springer-Verlag, Berlin.
Jones H.G., Sharp C.S. & Higgs K.H. (1987) Growth and water
relations of wilty mutants of tomato (Lycopersicon esculentum
Mill.). Journal of Experimental Botany 38, 1848–1856.
Koornneef M., Jorna M.L., Brinkhorst-van der Swan D.L.C. &
Karssen C.M. (1982) The isolation of abscisic acid (ABA) deficient mutants by selection of induced revertants in non-germinating gibberellin sensitive lines of Arabidopsis thaliana (L.)
Heynh. Theoretical and Applied Genetics 61, 385–393.
Lee S.H. & Reid D.M. (1997) The role of endogenous ethylene in
the expansion of Helianthus annuus leaves. Canadian Journal of
Botany 75, 501–508.
Léon-Kloosterziel K.M., Alvaraz Gil M., Ruijs G.J., Jacobsen
S.E., Olszewski N.E., Schwartz S.H., Zeevaart J.A.D. &
Koornneef M. (1996) Isolation and characterization of abscisic
acid-deficient Arabidopsis mutants at two new loci. Plant Journal 10, 655–661.
Michelena V.A. & Boyer J.S. (1982) Complete turgor maintenance at low water potentials in the elongation region of maize
leaves. Plant Physiology 69, 1145–1149.
Morgan P.W. & Drew M.C. (1997) Ethylene and plant responses
to stress. Physiologia Plantarum 100, 620–630.
Morgan P.W., He C.-J., De Greef J.A. & De Proft M.P. (1990)
Does water deficit stress promote ethylene synthesis by intact
plants? Plant Physiology 94, 1616–1624.
Moss G.I., Hall K.C. & Jackson M.B. (1988) Ethylene and the
responses of roots of maize (Zea mays L.) to physical impedance. New Phytologist 109, 303–311.
Mulholland B.J., Black C.R., Taylor I.B., Roberts J.A. & Lenton J.R. (1996a) Effect of soil compaction on barley (Hordeum vulgare L.) growth. I. Possible role for ABA as a rootsourced chemical signal. Journal of Experimental Botany 47,
539–549.
Mulholland B.J., Taylor I.B., Black C.R. & Roberts J.A.
(1996b) Effect of soil compaction on barley (Hordeum vulgare
L.) growth. II. Are increased xylem sap ABA concentrations
involved in maintaining leaf expansion in compacted soils? Journal of Experimental Botany 47, 551–556.
Munns R. & Cramer G.R. (1996) Is coordination of leaf and root
growth mediated by abscisic acid? Opinion. Plant and Soil 185,
33–49.
Munns R. & Sharp R.E. (1993) Involvement of abscisic acid in
controlling plant growth in soils of low water potential. Australian Journal of Plant Physiology 20, 425–437.
Nagel O.W., Konings H. & Lambers H. (1994) Growth rate,
plant development and water relations of the ABA-deficient
tomato mutant sitiens. Physiologia Plantarum 92, 102–108.
Narayana I., Lalonde S. & Saini H.S. (1991) Water-stressinduced ethylene production in wheat. A fact or artifact? Plant
Physiology 96, 406–410.
Neill S.J., McGaw B.A. & Horgan R. (1986) Ethylene and 1aminocyclopropane-1-carboxylic acid production in flacca, a
wilty mutant of tomato, subjected to water deficiency and pretreatment with abscisic acid. Journal of Experimental Botany 37,
535–541.
Puri J. & Tal M. (1977) Abnormal stomatal behavior and hormonal imbalance in flacca, a wilty mutant of tomato. IV. Effect
of abscisic acid and water content on RNase activity and RNA.
Plant Physiology 59, 173–177.
Qin X. & Zeevaart J.A.D. (1999) The 9-cis-epoxycarotenoid
cleavage reaction is the key regulatory step of abscisic acid biosynthesis in water-stressed bean. Proceedings of the National
Academy of Sciences USA 96, 15354–15361.
Quarrie S.A. (1987) Use of genotypes differing in endogenous
abscisic acid levels in studies of physiology and development. In
222 R. E. Sharp
Hormone Action in Plant Development – a Critical Appraisal
(eds G.V. Hoad, J.R. Lenton, M.B. Jackson & R.K. Atkin),
pp. 89–105. Butterworths, London.
Rakitina T., Ya Vlasov P.V., Jalilova F.Kh. & Kefeli V.I. (1994)
Abscisic acid and ethylene in mutants of Arabidopsis thaliana
differing in their resistance to ultraviolet (UV-B) radiation
stress. Russian Journal of Plant Physiology 41, 599–603.
Riov J., Dagan E., Goren R. & Yang S.F. (1990) Characterization of abscisic acid-induced ethylene production in citrus leaf
and tomato fruit tissues. Plant Physiology 92, 48–53.
Saab I.N. & Sharp R.E. (1989) Non-hydraulic signals from maize
roots in drying soil: Inhibition of leaf elongation but not stomatal conductance. Planta 179, 466–474.
Saab I.N., Sharp R.E. & Pritchard J. (1992) Effect of inhibition
of ABA accumulation on the spatial distribution of elongation in
the primary root and mesocotyl of maize at low water potentials.
Plant Physiology 99, 26–33.
Saab I.N., Sharp R.E., Pritchard J. & Voetberg G.S. (1990)
Increased endogenous abscisic acid maintains primary root
growth and inhibits shoot growth of maize seedlings at low water
potentials. Plant Physiology 93, 1329–1336.
Sharp R.E. & Davies W.J. (1989) Regulation of growth and
development of plants growing with a restricted supply of water.
In Plants Under Stress (eds H.G. Jones, T.L. Flowers & M.B.
Jones), pp. 71–93. Cambridge University Press, Cambridge, UK.
Sharp R.E., LeNoble M.E., Else M.A., Thorne E.T. & Gherardi F. (2000) Endogenous ABA maintains shoot growth in
tomato independently of effects on plant water balance: evidence for an interaction with ethylene. Journal of Experimental
Botany 51, 1575–1584.
Sharp R.E., Silk W.K. & Hsiao T.C. (1988) Growth of the maize
primary root at low water potentials. I. Spatial distribution of
expansive growth. Plant Physiology 87, 50–57.
Sharp R.E., Voetberg G.S., Saab I.N. & Bernstein N. (1993)
Role of abscisic acid in the regulation of cell expansion in roots
at low water potentials. In Plant Responses to Cellular Dehydration During Environmental Stress (eds T.J. Close & E.A.
Bray), pp. 57–66. American Society of Plant Physiologists,
Rockville, MD.
Sharp R.E., Wu Y., Voetberg G.S., Saab I.N. & LeNoble M.E.
(1994) Confirmation that abscisic acid accumulation is required
for maize primary root elongation at low water potentials. Journal of Experimental Botany 45, 1743–1751.
Spollen W.G., LeNoble M.E., Samuels T.D., Bernstein N. &
Sharp R.E. (2000) Abscisic acid accumulation maintains maize
primary root elongation at low water potentials by restricting
ethylene production. Plant Physiology 122, 967–976.
Spollen W.G., Sharp R.E., Saab I.N. & Wu Y. (1993) Regulation of cell expansion in roots and shoots at low water potentials.
In Water Deficits. Plant Responses from Cell to Community (eds
J.A.C. Smith & H. Griffiths), pp. 37–52. BIOS Scientific Publishers, Oxford, UK.
Suge H. (1971) Stimulation of oat and rice mesocotyl growth by
ethylene. Plant and Cell Physiology 12, 831–837.
Tal M. (1966) Abnormal stomatal behavior in wilty mutants of
tomato. Plant Physiology 41, 1387–1391.
Tal M., Imber D., Erez A. & Epstein E. (1979) Abnormal stomatal behavior and hormonal imbalance in flacca, a wilty
mutant of tomato. V. Effect of abscisic acid on indoleacetic acid
metabolism and ethylene evolution. Plant Physiology 63, 1044–
1048.
Tal M. & Nevo Y. (1973) Abnormal stomatal behavior and root
resistance, and hormonal imbalance in three wilty mutants of
tomato. Biochemical Genetics 8, 291–300.
Tan B.C., Schwartz S.H., Zeevaart J.A.D. & McCarty D.R.
(1997) Genetic control of abscisic acid biosynthesis in maize.
Proceedings of the National Academy of Sciences USA 94,
12235–12240.
Tan Z.-Y. & Thimann K.V. (1989) The roles of carbon dioxide
and abscisic acid in the production of ethylene. Physiologia
Plantarum 75, 13–19.
Tardieu F. & Davies W.J. (1992) Stomatal response to abscisic
acid is a function of current plant water status. Plant Physiology
98, 540–545.
Taylor I.B. (1987) ABA-deficient tomato mutants. In Developmental Mutants in Plants (eds H. Thomas & D. Grierson), pp.
197–217. SEB Seminar Series 32. Cambridge University Press,
Cambridge, UK.
Trewavas A.J. & Jones H.G. (1991) An assessment of the role of
ABA in plant development. In Abscisic Acid: Physiology and
Biochemistry (eds W.J. Davies & H.G. Jones), pp. 169–188.
BIOS, Oxford, UK.
van der Weele C.M., Spollen W.G., Sharp R.E. & Baskin T.I.
(2000) Growth of Arabidopsis thaliana seedlings under water
deficit studied by control of water potential in nutrient-agar
media. Journal of Experimental Botany 51, 1555–1562.
Vartanian N., Marcotte L. & Giraudat J. (1994) Drought rhizogenesis in Arabidopsis thaliana. Differential responses of hormonal mutants. Plant Physiology 104, 761–767.
Westgate M.E. & Boyer J.S. (1985) Osmotic adjustment and the
inhibition of leaf, root, stem and silk growth at low water potentials in maize. Planta 164, 540–549.
Whalen M.C. & Feldman L.J. (1988) The effect of ethylene on
root growth of Zea mays seedlings. Canadian Journal of Botany
66, 719–723.
Wright S.T.C. (1980) The effect of plant growth regulator treatments on the levels of ethylene emanating from excised turgid
and wilted wheat leaves. Planta 148, 381–388.
Yoshii H. & Imaseki H. (1981) Biosynthesis of auxin-induced
ethylene. Effects of indole-3-acetic acid, benzyladenine and
abscisic acid on endogenous levels of 1-aminocyclopropane-1carboxylic acid (ACC) and ACC synthase. Plant and Cell Physiology 22, 369–379.
Zeevaart J.A.D. & Creelman R.A. (1988) Metabolism and physiology of abscisic acid. Annual Review of Plant Physiology and
Plant Molecular Biology 39, 439–473.
Zhang J. & Davies W.J. (1990) Does ABA in the xylem control
the rate of leaf growth in soil-dried maize and sunflower plants?
Journal of Experimental Botany 41, 1125–1132.
Received 12 June 2001; received in revised form 15 August 2001;
accepted for publication 15 August 2001
© 2002 Blackwell Science Ltd, Plant, Cell and Environment, 25, 211–222