Plant Physiol. (1967) 42, 891-893
Short Communication
Lateral Auxin Transport in Stems and Roots
Stanley P. Burg and Ellen A. Burg
Program in Cellular and Molecular Biology, University of Miami School of Medicine, Miami, Florida 33136
Received February 13, 1967.
influence of gravity; otherwise the Avena and
would not work. Consequiently
when coleoptiles are positioned horizontally, IAA
should only migrate at the flanks from the upper
to the lower half of the coleoptile (see fig 2). In
addition a net movement of IAA from U1 to U.
and L1 to L, wotul(d be expected, giving rise to the
finial distribultioIn illuistrated in figure 2. If this
interpretationi is correct the lower half of a split
coleoptile muist contain a mass of flank cells in
which there is Ino gradient as well as a zone (L1)
partially depleted of auxin, whereas the tipper half
shouil(d containi an atuxin rich zone (U,). Therefore, we believe that atlxin gradients U, :U9 ain(d
L1 :L., are greater 'than the differential meastured
between the tipper aind lower halves of split coleop-
The lateral movement of auixin has been
stuidied with bioassays and also by application of
'4C-IAA. Both techniqutes have demonstrated that
the tupper hal,f of a horizontally positioned stem or
root contains at least 30 to 45 % of the total auxin
present in the tissuie (2). However, such an auxin
gradient is difficult to reconcile with the
Cholodny-WVent theory of root geotropism becatuse
it is too snmall to accouniit for the growth inhibition
which occulrs in the uindlerside of a root experiencing a geotropic curvatuire (2). \Ve shall present
evi(lence that the auixini (lifferential actuially' is
conisiderably larger than these experiments indicate,
an(I dliscliss several problems which 'have obsculred
this fact and hiinldered the measuiremenlt of lateral
aulxin transport.
Previotusly we reported (3) that in 8 experiments, each tising 20 sections and 1 ,LUM 14C-IAA,
the specific activity of the upper half of horizontally positioned etiolated pea stem segments was
37.5 ± 2.7 % that of the lower half. In an equlal
nuimber oif experiments the specific activities of
the uipper quarter, middle half, and lower quarter
relative to that of the intact section were 0.58 40.07, 0.99 + 0.07, and 1.46 ± 0.10 respectively (3).
From these values the distribtution of 14C-IAA
across the diameter of the stem has been calculated
(fig 1). The data show the specific activity to be
fairly uiniform throughotut the middle of the stem,
only rising or falling markedly at the ouiter stirfaces. Apparently each cell passes the atixin it
receives to the next lower cell so that IAA is (lisplace(d from the upper to the lower suirface with
little change in concenitration in the center of the
sectioni where most of the tisstue mass resides.
ConiseqLuentlv when sectioins are halve(d the large
(lifferential between the tipper and(I lower suirfaces
is obscuired by the lack of a gradienit in the central
bulk of tissue included in each sample. The magnitude of the gradient between the sturfaces can be
estiimated by extrapolating the curves ('dotted lines,
fig 1) and may be as large as 10:1. The problem
o-f the 'central tissue mass can be circtumvented by
utilizing hollow cylinders stuch as Avena andl corn
coleoptiles, btut this approach introduces new difficulties. It is well established that auxin does not
spread laterally in these tissues except under the
corn curvature tests
I
1
a
.
axa40 1.5
V.
47I-
E
0
IE
- U)
O Q 10
LA-
0
LE
J_ 0
4'.
4
w
0.
05
U,
4
LOWER
SURFACE
MIDDLE OF
SECTION
UPPER
SURFACE
FIG. 1. Distribution of radioactivity across the diameter of a horizontally positioned pea stem section.
The abscissa represents the relative distance from the
middle of the section.
891
Downloaded from on July 31, 2017 - Published by www.plantphysiol.org
Copyright © 1967 American Society of Plant Biologists. All rights reserved.
892
PLANT PHYSIOLOGY
tiles, and suiggest that in general there may well
be greater differences in auixin content between
top and bottom halves of tropistically stimuilated
tisstues than analysis of the 2 halves wotuld indicate.
In corn and Azena the magnitude of the lateral
gradient is independent of the concentration of
14C-IAA in the donor block except possibly at
extremely high or low auixin levels (6). For example, with corn by the methods of Gillespie and
Thimanin (6) we obtained values of 32.2 ± 1.5
aind 30.4 -+ 4.9 % for the specific activity of the
tpper half relative to that of the lower half using
1 jM and 10 .LAM 14C-IAA respectivelv. In contrast
to the situiation in these modified leaf structures,
suln,flower and pea stems transport less atuxin laterally when the IAA concentration is increased (3, 6).
This must be due to the fact that moderately high
auixin levels stimulate the formation of ethylene
(1, 3), and in these tissues the gas specifically
prevents lateral auixin movement without affecting
auixin uiptake, destruction or polar transport (3).
Thuls whein the IAA concentration in the donor
block applied to pea stem sections is increased from
1 to 10 ,uNi, IAA uptake and polar transport increase 10-foldlbuit ethylene formation is induice(d
and the gas almost completely abolishes lateral
aulxiII movement and geotropic curving (3). Auixii
also sti-mulates corn and 4venci to produce ethylene,
btut the gas does not inhibit lateral transport or
geotropism in these tissues (3), which explains why
lateral transport in coleoptiles is independeint of
IAA concentration. II light grown mulng bean
hypocotyls, etiolate(d sunlflower stem sections (3),
etiolated black bean stem sections, ancd etiolatedl
kidney bean hypocotyls (1), ethylene formation is
ind(utced when the IAA conceintration surpasses 0.1
,,M., and( in most roots when it exceeds 10 to 100
m,um (4). In such instances by currently available
methods it is probably impossible to measture lateral
auxin transport with 14C-IAA, for tissue radioactivity is difficult to detect tinless the donor block
contains at least 1 JUM IAA (even of the highest
available specific activity
10 pc/,M), and this
amount of IAA ought to prevent lateral transport
in these tissues by inducing ethylene formation.
This effect may account for the failure of Ching
and Fang (5) to detect lateral redistributioin of
radioactivity in the shoots of pea seedlings and also
the roots of nuimerous plants treated with 10 "al
14C-JAA. Direct proof of an effect of ethylene
*on lateral transport in roots is lacking blut it must
occur since lima bean, pea, corn, and Avenal roots
becomie ageotropic when they are treate(d with the
gas. T'herefore, we conclude that except in tlle
case of coleoptiles, lateral transport experimenits
with relatively high auxin contents are uinreliable
because they are complicated by the production of
ethylene and consequent suppression of lateral
movement of the auixin.
It has been sugggested that ethylene, produiced
when auxin moves to the lower side oif a horizon-
Ul
FIG. 2. Theoretical lateral miiovemiienit of IAA (indicated by arrows) in a horizontallv positioned coleoptile.
Areas where auixiin accumulates are indicated by stippling.
tally positioned root, causes cellular swelling an(d
the resul-tant geocutrvature (4). However, suich a
mechanisTn must also take into account the fact
that ethylene specifically impedes lateral auixiin
transport, and hence accuimulation of the very
hormone which stimulates its produiction. Possibly a dual feedback mechanism controls root
geotropism, and ethylene not only modifies the
action of auxin, but in addition limits the amounlt
moved laterally.
Acknowledgments
This investigation was supported by research grant
EF-00782 from the United States Public Health Service,
Division of Environmental Engineering and Food Protection, and was carried out while S. P. Burg was the
recipient of Career Research Developrnent Award 1-K3GM-6871 from USPHS.
Literature Cited
1. ABELES, F. B. AND B. RUBINSTEIN. 1964. Regulation of ethylenie evolutioni a;nd leaf albscission by
auxin. Plant Physiol. 39: 963-69.
2. AUDUs, L. J. AND M. E. BROWNBRIDGE. 1957.
Studies on the geotropism of roots. I. Growth rate
distribution during response and the effects of
applied auxin. J. Exptl. Botany 8: 105-24.
3. BURG, S. P. AND E. A. BURG. 1966. The interaction
between auxin and ethylene and its role in plant
growthl. Proc. Natl. Acad. Sci. 55: 262-69.
Downloaded from on July 31, 2017 - Published by www.plantphysiol.org
Copyright © 1967 American Society of Plant Biologists. All rights reserved.
BURG AND BURG-LATERAL TRANSPORT IN STEMS AND ROOTS
4. CHADWICK, A. V. AND S. P. BURG. 1967. An explanation of the inhibition of root growth caused by
indole-3-acetic acid. Plant Physiol. 42: 415-20.
D. CHING, TE-MAY AND S. C. FANG. 1958. The redistribution of radioactivity in geotropically stim-
893
ulated plants pretreated with radioactive indoleacetic acid. Physiol. Plantarum 11: 722-27.
6. GILLESPIE, B. AND K. V. THIMANN. 1963. Transport and distribution of auxin during tropistic reI. The lateral migration of auxin in
sponse.
geotropism. Plant Physiol. 38: 214-25.
Downloaded from on July 31, 2017 - Published by www.plantphysiol.org
Copyright © 1967 American Society of Plant Biologists. All rights reserved.