Pleiotropy in Triazine-Resistant Brassica napus: Leaf and
Environmental Influences on Photosynthetic Regulation
Ja c k D ekker
Weed Biology L aboratory, A gronom y D epartm ent, Iowa State University,
Ames, Iowa, 50011, U .S .A .
Z. Naturforsch. 48c, 2 8 3 - 2 8 7 (1993); received November 9, 1992
Carbon Assimilation, psbA Gene, Chloroplast
Since the first discovery o f s-triazine-resistance (R ) in higher plants, the altered D -l protein
product of the psbA gene has been regarded as less photosynthetically efficient in those R bio­
types of a species. Decreases in electron transport function in the chloroplast have been be­
lieved to be the cause o f decreased carbon assimilation rates and plant productivity in many
reports. W hat is less clear in the literature is whether this change in D -l structure and electron
transport function directly modifies whole-leaf photosynthesis and plant productivity or only
indirectly influences these functions. The dynamic nature o f these responses have led several to
conclude that the primary effect o f R is com plex, involves m ore than one aspect of photosyn­
thesis, and can be mitigated by other processes in the system. Electron transport limitations
are only one possible regulatory point in the photosynthetic pathway leading from light-har­
vesting and the photolysis o f water, through ribulose bisphosphate carboxylase/oxygenase, to
sucrose biosynthesis and utilization. Herein we discuss this complex issue, arguing that D -l
function can ’t be evaluated in isolation from the leaf, the organism, and possibly from the
community response. Carbon assimilation in R and the susceptible wild type (S) is a function
o f several interacting factors. These include the pleiotropic effects resulting from the psbA mu­
tation (the dynamic reorganization o f the R chloroplast) interacting with other regulatory
components o f photosynthesis, microenvironmental conditions, and time (including ontogeny
and time o f day). Previous work in our laboratory indicated a consistant, differential, pattern
o f Chi a fluorescence, carbon assimilation, leaf temperature, and stomatal function between S
and R Brassica napus over the course o f a diurnal light period and with ontogeny, i.e. R is a
chronom utant. Dekker and Sharkey have shown that the primary limitation to photosynthesis
changes with changes in leaf tem perature, and that electron transport limitations in R may be
significant only at higher temperatures. The recognized R plants interact with the environment
in a different way than does S. U nder environmental conditions highly favorable to plant
growth, S often has an advantage over R. Under certain less favorable conditions to plant
growth, stressful conditions, R can be at an advantage over S. These conditions may have been
cool (or hot), low light conditions interacting with other biochemical and diurnal plant factors
early and late in the photoperiod, as well as m ore complex physiological conditions late in the
plant’s development. It can be envisioned that there were environmental conditions in the ab­
sence of s-triazine-herbicides in which R had an adaptive advantage over the more numerous S
individuals in a population o f a species. U nder certain conditions R might have exploited a
photosynthetic niche under-utilized by S.
Introduction
s-T riazin e-resistance (R ) in higher plants was
first discovered in Senecio vulgaris in 1969 [1], S ub­
sequent research has shown R is due to a single
base p air m utation to the psbA ch lo ro p last gene
[2]. The cod on 2 6 4 change in the psbA gene causes
a ch ange in its p rod u ct in Amaranthus hybridus,
the D -l protein, a key functional elem ent in PS II
electron tran sp o rt [2]. s-T riazin e-resistant plants
have been shown to have a d ecreased quan tu m effi-
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ciency o f C 0 2 assim ilation co m p ared to s-triazinesusceptible plants (S) [3]; and have been generally
regarded as less fit than S plants [4]. This decreased
efficiency is credited to an altered red ox state o f
PS II quinone accep to rs and a shift in the equilib­
rium con stan t between Q A~ and Q B in fav o r o f Q A~
[5]W h a t is less clear in the literature is w hether this
change in D -l structure and electron tran sp o rt
function directly modifies w h ole-leaf p h o to sy n ­
thesis and plant p roductivity o r only indirectly in­
fluences these functions [7], The dynam ic nature
o f these responses have led several to con clu d e that
the p rim ary effect o f R is co m p lex, involves m ore
than one aspect o f photosynthesis, and can be mit-
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284
igated by o th er processes in the system [ 7 - 9 ] , F o r
exam ple, it has been pointed out that decreased
Q a ~ to Q b electron tran sp o rt in R is m ore rapid
than the norm ally rate-lim iting oxid ation o f plastoquinol [10, 11], while others studies indicate this
step m ay be rate limiting [12]. Herein we discuss
this com p lex issue, arguing th at D -l function ca n 't
be evaluated in isolation from the leaf, the o rg an ­
ism, and possibly from the com m unity response.
C arb o n assim ilation in R and the susceptible wild
type (S) is a function o f several interacting factors.
T hese include the pleiotropic effects resulting from
the psbA m u tatio n (the dynam ic reorganization o f
the R ch lorop last) interactin g with oth er regu lato­
ry co m p on en ts o f photosynthesis, m icroen viron ­
m ental and m icro h ab itat conditions, and time (in­
cluding on togen y and time o f d ay). It is also
argued herein th at electron tran sp o rt sometimes
indirectly regulates carb o n assim ilation in R , in
oth er instances it has no ap parent effect on regula­
tion.
Discussion
Carbon assimilation in R and S
Several studies have shown lower photosynthet­
ic carb o n assim ilation rates (A ) in R Amaranthus
hybridus [13] and Senecio vulgaris [3, 11] relative to
S. B eversd orf et al. [14] found lower R whole plant
yields in field evaluations o f Brassica napus. A d d i­
tionally, van O o rsch o t and van Leeuwen [15]
found A in S Amaranthus retroflexus was greater
than th at in R ; while R and S carb o n assim ilation
were com p arab le in Chenopodium album, Polygo­
num lapathifolium, Poa annua, Solanum nigrum
and Stellaria media. R biotypes o f Phalaris paradoxa have been found to be photosynthetically su­
perior to their S co u n terp arts [16]. Jansen et al. [17]
observed th at R Chenopodium album chloroplasts
had lower electron tran sp o rt rates between w ater
and plastoquinone co m p ared to S; yet no differ­
ences were found in the rate o f quantum yield o f
w hole-chain electron tran sp o rt, o r in A , between R
and S. These inconsistent responses by R and S
biotypes have led several to conclude that the
change con ferrin g R is not necessarily directly
linked to inferior photosynthetic function [6, 16,
17].
A n assessm ent o f these and o th er studies reveals
several possible reasons why different responses
J. Dekker • Pleiotropy in Triazine-Resistance
m ay have been observed. T hey include pleiotropic
reorganization o f the R ch lo ro p last and the dy­
nam ic interrelationship between com ponents o f
photosynthesis; the role o f environm ent in altering
responses; genetic facto rs, such as differences be­
tween biotypes and genom e interactions within a
biotype; and the possibility o f an unnam ed facto r
controlling photosynthesis [8]. C om p arison o f dif­
ferent p h otosyn th etic responses by R is further
com plicated by the m any different environm ental
and biological con dition s under which they were
conducted. T hese include changes in plant species,
plant age, p lan t u niform ity, plant tissue, tem pera­
ture, P P F D , the degree o f environm ental con trol
(field, glass-house o r con trolled environm ent
ch am b ers) and the diurnal light period length and
variation o f con dition s. A n o th e r m itigating facto r
in com p arin g R and S photosynthetic responses
has been the use o f m odel system s in which oth er
genes, besides the m u tatio n to psbA, have differed
(e.g. [3, 18]). Inferences from these studies have
been confounded because they relied on a non-isogenic m odel system . M cC losk ey and H olt [9] have
suggested n u clear genom e differences m ay co m ­
pensate fo r differences in p roductivity between
non-isogenic R and S selections, and th at d etri­
m ental effects m ay be atten u ated by interactions
o f plastid and n uclear genom es [19]. M an y o f these
lim itations m ay have been o vercom e in studies
with the nearly isonuclear biotypes o f Brassica na­
pus [7, 1 9 - 2 4 ] .
Pleiotropic reorganization o f the R chloroplast
The genetic change in R plants leads to a p ro ­
found reorgan ization o f functional units in the
ch lorop last. T his adaptive reorganization o f p h o ­
tosynthetic co m p o n en ts in the ch lorop last m ay be
a co m p en sato ry m echanism to m aintain a func­
tional in teractio n o f the PS II com plex lipids and
proteins [25]. This pleiotrop ic cascad e includes
both stru ctu ral [26, 27] and functional changes [5],
Several ch anges in thylakoid lipid chem ical c o m ­
position have been observed. R phospholipids had
higher linolenic acid co n cen tratio n s and low er lev­
els o f oleic and linoleic fatty acids [28], R plants
overall were richer in unsatu rated fatty acids, had
higher p ro p o rtio n s (and quantitatively g reater
am ounts of) m o n o g alacto sy l diglyceride and phos­
phatidyl choline, and had low er p rop ortions o f di-
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J. Dekker • Pleiotropy in Triazine-Resistance
galactosyl diglyceride and phosphatidyl choline,
than in S [27]. As a consequence o f a higher p ro ­
p ortion o f appressed thylakoids, R plants had a
greater p rop ortion o f A 3 -/ra«s-h exad ecen o ic acid
in phosphatidylglycerol [28], A lth ough the leaf
an atom y o f R and S is sim ilar, m an y u ltrastru ctu ral ch aracters are different. R has decreased plastid
starch con ten t and increased g ran a stacking (and
the associated ch aracters o f low er Chi a/b ra tio , in­
creased Chi alb light-harvesting com p lex, and
lower P 7 0 0 Chi a and ch lo ro p last coupling fa cto r
am ounts) [24]. M any o f these changes in R are
sim ilar to those found in shade-adapted leaves
[29]. Seeds from the susceptible biotype ap parently
germ inate earlier and m ore quickly than th ose o f
R [30]. The dynam ic n ature o f the ch lo ro p last to
reach a m arkedly different, new, structural and
functional equilibrium in response to the m u tation
o f a key plastidic gene has been observed p reviou s­
ly [ 3 1 - 3 3 ] . M a tto o [25] has suggested th at the ra p ­
id an abolism -catab olism rate o f the D -l protein
could serve as a signal resulting in the reo rg an iza­
tion o f m em branes arou n d the PS II com p lex. This
dynam ic reorganization has consequences for
evaluating and understanding regu latory effects o f
electron tran sp ort in carb o n assim ilation.
Pleiotropy in R: Time, environment and regulation
The equivocal n ature o f ou r understanding o f
how photosynthesis differs between R and S, and
how it is regulated, has led us to focus exp erim en ­
tal efforts on ch ron obiological, environm ental and
regulatory understandings o f R under m o re dy­
nam ic, but closely con trolled , grow th conditions.
M any plant species exhibit an endogenous
rhythm o f carb on assim ilation and sto m atal func­
tion once entrained in a p hotoperiod . This rhythm
is regulated to som e exten t independently o f the
p lan t’s d irect response to P P F D [34], Previous
w ork in ou r lab oratory indicated a con sistan t, dif­
ferential, p attern o f Chi a fluorescence ( F t) [23],
carb on assim ilation, leaf tem peratu re, to tal c o n ­
d uctan ce to w ater vap or (g), and leaf intercellular
C 0 2 p artial pressure (C ;) [21, 22] between S and R
Brassica napus L . over the co u rse o f a diurnal light
period, i.e. R is a ch ro n o m u tan t. R plants varied in
their relative ad vantage (o r disad vantage) o v er S
in term s o f carb o n assim ilation as they aged [22], It
is hypothesized that these on togen etic and diurnal
285
p atterns o f differential photosynthesis m ay be a
consequence o f correlative diurnal fluctuations in
fatty acid biosynthesis and the dynam ic changes in
m em brane lipids over the cou rse o f the light-dark
daily cycle [35], changes in leaf m em brane lipids
with age [27], or m icroenvironm ental tem perature
influences [7, 20, 36],
This research revealed an apparent discrepancy
in the response o f R to tem perature. C arb o n as­
sim ilation in R was m uch lower than th at in S at
high leaf tem peratures (e.g. 35 °C ) when the leaf
tem perature was closely controlled [7], These re­
sults are consistent with those o f others [ 3 7 - 3 9 ] .
W hen leaf tem perature was not directly con trolled ,
but air tem perature was, R carb o n assim ilation ex ­
ceeded th at o f S at relatively high tem peratures
(e.g. 35 °C air tem perature) [20, 21]. In both expe­
rim ental conditions R leaf co n d u ctan ces were
usually greater than in S. A t relatively co o l tem ­
peratures it has been hypothesized th at the change
in lipid satu ration o f ch lorop last m em branes could
con fer cold tolerance to R plants, resulting in
g reater carb on assim ilation rates in R under those
conditions [26, 36, 40], The g reater sto m atal ap er­
ture, and greater leaf co n du ctances apparently
negate this effect [7], A s a consequence, a t all im ­
p o rtan t physiological tem peratures ( 1 0 - 3 5 C ) R
leaves are co o ler than S leaves. S tom atal function
differentially
regulates
carb o n
assim ilation
ointhese two biotypes.
R adaptation to the environment and regulation of
carbon assimilation
R egulation o f photosynthesis in R and S are
controlled by m any different facto rs in those
plants. L im itations in electron tran sp o rt in R are
n ot the only critical fa cto r in yield losses a t the
whole plant level. The pleiotropic effects observed
in R result in a new equilibrium between fu nction­
al and structural com p on ents. It is this new dy­
n am ic pleiotropic reorganization th at regulates
carb o n assim ilation in R . E lectro n tran sp o rt limi­
tation s are only one possible regulatory point in
the p hotosynthetic pathw ay leading from lightharvesting and the photolysis o f w ater, through
ribulose bisphosphate carb o xy lase/o xy g en ase, to
starch /su crose biosynthesis, tran slo catio n , and
utilization. C arb on flux through the leaf is regu­
lated at m any points. E lectro n tran sp o rt, even in
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286
R , is not the only critical regulatory step. In fact,
D ekker and Sharkey [7] have shown that the pri­
m ary lim itation to photosynthesis changes with
changes in leaf tem peratu re, and th at electron
tran sp o rt lim itations in R m ay be significant only
at higher tem peratures.
T he reorganized R plants in teract with the envi­
ron m en t in a different way than does S. It is this
th at causes the functional result observed: under
environm ental conditions highly favorable to
plant grow th, S often has an ad vantage over R .
U n d er certain less favorable conditions to plant
grow th , stressful conditions, R can be at an ad van ­
tage over S. It can be envisioned th at there were
environm ental conditions in the absence o f
s-triazine-herbicides in which R had an adaptive
ad vantage over the m ore num erous S individuals
in a population o f a species. U n d er certain con di­
tions R m ight have exploited a photosynthetic
niche under-utilized by S. These conditions m ay
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J. Dekker • Pleiotropy in Triazine-Resistance
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Acknowledgements
The au th o r w ould like to acknow ledge the sup­
p ort for this research by the Iow a A griculture and
H om e E co n o m ics E xperim en t Station, Iow a State
U niversity; and by the U .S . D ept, o f A griculture
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