Classification of plant growthregulating agentsaccording
to their modes
of action’
2J. Mottley
trates on most of those agents where the precise mode of
action is reasonably clear.
ABSTRACT
Plant growth regulating agents can modify plant
growth and developmentin several indirect ways. They
may affect the metabolism or transport of plant
hormonesor they mayalter the response of plants to
these hormones.A rational classification system based
on the modes of action of some of these chemicals is
presented. It shows which types of agent are presently
available and which types need further research. Suggestions are also given on howthe system maybe used in
classes to predict potential uses of such agents.
THE CLASSIFICATION
Additional index words: Plant growth regulators,
Plant hormones, Classification,
Metabolism, Transport, Response.
ZI~tE
use of plant growth regulators is a short-term
actic for influencing
crop growth and development. It is, however, likely to become a longer-term
strategy when combined with plant breeding for altered
levels of endogenous hormones (12). In addition,
the
emphasis placed on the application
of substances with
direct hormonal activity is likely to gradually give way
to the use of chemical agents which modify some aspect
of the metabolism or transport of plant hormones or influence the response of plants to these hormones. This
approach has the advantage that very fine tuning of
crop development is possible and the substances used
are often more metabolically-stable
and less expensive
than those which have direct hormonal activity.
These
agents may also find use in combination with other
metabolically-labile
plant growth regulators to prevent
their breakdown and/or transport from the site of application.
Although only a limited number of these chemical
agents are in current use, a rational classification
of
their modes of action is desirable. This article presents
such a classification
to stimulate student interest in the
agents currently available and those types requiring development for future use. The term plant growth regulating agent is used loosely here to differentiate
these
substances
from compounds that have direct hormonal
activity,
e.g., plant hormones, their chemical analogues
and hormone mimics, such as 2,4-dichlorophenoxyacetic acid. The following is by no means a comprehensive list, in terms of references to the literature,
of
the agents available or their effects. Instead, it concen’ Contribution of the Dep. of Crop Science, Univ. of Zimbabwe,
P.O. BoxMP167, MountPleasant, Harare, Zimbabwe.
2 Lecturerin cropscience.
64
SYSTEM
Figure l illustrates the six major sites of action of plant
growth regulating agents. The response of plants to different
agents will vary according to: 1) the particular hormoneaffected (auxins, gibberellins, cytokinins, ethylene, abscisic acid,
etc.); 2) the site of action of the agent; and 3) whether inhibition or stimulation occurs at the site of action.
A number of presently available agents are classified in
Table 1. Chlormequat (2-chloroethyl-trimethylammonium
chloride), chlorphonium chloride (2,4-dichlorobenzyl tributyl
phosphonium chloride), AMO-1618[(2-isopropyl-4-trimethylammoniumchloride)-5-methyl-phenyl piperidine carboxylate]
and ancymidol [c~-cyclopropyl-c~-(p-methoxyphenyl)-5-pyrimidinemethanol| inhibit gibberellin biosynthesis (6). This effect
results in reduced levels of gibberellins in plants and a consequential decrease in stem growth. Chlormequat is useful for
producing short, lodging-resistant cereals (17), whereas the
other three agents have been used mainly for the dwarfing of
ornamentals (2, 5). DIHB(3,5-diiodo-4-hydroxybenzoic acid),
÷, AVG (a minoethoxyCo2÷, DNP(2,4-dinitrophenol),
vinylglycine), AOA(aminooxyacetic acid) and cycloheximide
{ 3- [2-(3,5-dimethyl-2-oxocyclohexyl)-2-hydroxyethyl]glutarimide} inhibit ethylene biosynthesis 03, 15, 18, 19). Because
of this effect, DIHBshows potential for the stimulation of
root growth in compacted soils, probably due to the release
of growth inhibition caused by stress-induced ethylene production in the roots (16). Someof the other inhibitors delay
the ripening and abscission of fruits and the senescence of cut
flowers (3, 18).
With regard to the stimulation of the biosynthesis of plant
hormones by some agents, there are probably two modes of
action. Morphactins (e.g. methyl-2-chloro-9-hydroxyfluorene9-carboxylate) and O2 probably stimulate endogenous ethylene
biosynthesis directly (1, 8). Defoliants such as paraquat (l,l’dimethyl-4,4’-bipyridinium ion) and endothall [7-oxabicyclo
(2,2,1) heptane-2,3-dicarboxylic acid] almost certainly increase
ehtylene production indirectly by wounding(10). In addition,
Ca2÷-cytokinins and auxin-type regulators stimulate ethylene
biosynthesis (18) and several of their regulating properties are
likely to be due to increased ethylene production.
Adventitious root initiation is stimulated by auxins and
many phenolic substances are known to reduce auxin breakdown by inhibiting
the catabolic
enzyme IAA oxidase.
Phenolic substances and their derivatives, such as phloridzin
(4,4’,6’-trihydroxy-2’-glucosidodihydrochalcone)
and catechol (1,2-dihydroxybenzene) have thus found use as rooting
stimulants for fruit tree rootstocks (10).
Endogenous auxins and gibberellins are transported down
plant stems and maintain apical dominance by stimulating
stem growth at the expense of axillary bud growth. The agents
NC9634 {[(3-phenyl-l,2,4-thiadiazol-5-yl)thio]
acetic acid},
DPX1840 [3,3a-dihydro-2-[4-methoxyphenyl)-8H-pyrazolo
(5,1a)-isoindol-8-one],
TIBA(2,3,5-triiodobenzoic acid)
MOTTLEY: CLASSIFICATION OF PLANT GROWTHREGULATORS
morphactinsinhibit the transport of endogenousauxins and,
perhaps, gibberellins (4, 7, 10). Theyhavebeenused in situations wherereduced apical dominanceand increased branching, flowering,and lodgingresistance is desirable, such as with
cereals, legumes,and youngfruit trees (10, 17).
Finally, agents such as Ag÷, COs, and PCIB(4-chlorophenoxyisobutyric acid) inhibit the responseof plants to their
owngrowthhormones.At least in somecases this is done by
blocking endogenousreceptor sites in the tissues (10). PCIB
antagonizes auxin action and promotesroot growth(10). The
÷ delays the senesother twoagents inhibit ethyleneaction. Ag
cence of cut flowers, whereas high atmospheric COslevels
have been used to delay fruit ripening during storage and
transport (14).
DISCUSSION
There has been a great deal of research into the discovery, development, and application of plant growth
regulating agents in recent years. A few important general conclusions emergefrom an analysis of the achieveTable 1. Classification
Process
affected1.
of plant growth regulating agents
Effect on
hormone
activity
at site of
interest
Type of
action
Anabolism inhibition
decrease
Hormone
affected/;
gibberellins
ethylene
Catabolism
Transport
Response
stimulation
increase
ethylene
inhibition
increase
auxins
stimulation
decrease
inhibition
decrease
stimulation
increase
inhibition
decrease
stimulation
increase
Agents/;
chlormequat
chlorphonium chloride
AMO-1618
ancymidol
DIHB, Co~’, DNP
Ag÷, AVG, AOA
cycloheximide
morphactins, O2
paraquat, endothall
phenolics, eg.
phloridzin
catechol
-auxins
auxins and
gibberellins
--
NC 9634, DPX-1840
TIBA, morphactins
ethylene
auxins
--
Ag’, CO2
PCIB
Effects on activation and inactivation processes are not included but should
produce similar effects on hormoneactivity as anabolism and catabolism,
respectively.
Dashesindicate that no knownagent is currently available.
inactive
precursors
65
merits so far. All of the agents presently available influence the activity of auxins, gibberellins, and ethylene.
No suitable ones are yet available that influence the activity of other growth hormones, such as abscisic acid
and cytokinins. This is possibly because, until quite recently, there were relatively few commercial applications for altered endogenous activity of these two
types of hormones in crop plants. Abscisic acid is
now known to control stomatal movements and hence
may be involved in the drought resistance of crops (9).
Consequently, research on growth agents that influence
the activity of endogenousabscisic acid is likely to be
muchmore intensive, particularly since it is metabolically labile and suitably active analogues are not yet
available (11). No agents are knownto the author that
are involved with the stimulation of catabolism, transport, or response and further research effort is required
in these areas.
Exercises for students of introductory crop science
courses could involve the prediction of the potential
application of plant growth regulating agents with different modes of action. The answers obtained will obviously depend on: 1) the growth hormoneaffected, 2)
the modeof action of the agent, 3) whether the agents
are used alone or in combination, 4) the type of crop, 5)
the site of application on the plant, 6) the ultimate objective, ie. whether stimulation or inhibition of growth
and development is required, and 7) how muchthe student knownabout the role of plant growth hormones.
The mode of action of hypothetical or actual agents
maybe explained and then their potential application in
different situations assessed. For example, suppose an
agent inhibits both the anabolism of ethylene and the
catabolism of auxins and, at the same time, stimulates
gibberellin
anabolism. This substance should then
reduce ethylene production, maintain or increase auxin
levels and increase the gibberellin concentration in the
organs to which it is applied. Hence, it wouldhave possible applications for reducing the rate of fruit ripening
and organ senescence generally (due to effects on ethylene, auxin and gibberellin levels) and for stimulating
seed germination (effects on gibberellins). It wouldalso
be useful for increasing fruit size and enhancing
parthenocarpic fruit development (both due to effects
on auxins and gibberellins).
It would, however, be
unsuitable for reducing lodging in crops, such as
cereals, because it mayincrease stalk length at flowering
I catabolismI
I anab°lismI
~ plant hormone
inactive
products
or
[I activation
I inactivati°n I
[transport I
locus of response
Fig. 1. The major sites
regulating agents.
of action of plant growth regulating
agents.
Processes placed in boxes indicate
~[ responseJ
the main sites
of action for plant growth
66
JOURNAL OF AGRONOMIC EDUCATION, VOL. 12, 1983
(ethylene and gibberellin effects) and decrease tillering
(auxin and gibberellin effects).
Questions of this type help students to bridge the gap
between the biochemical modes of action of plant
growth regulating agents and their agricultural applications. Using examples of hypothetical agents with various modes of action on plant hormones, a multitude of
effects and practical applications can be predicted. In
the author's experience, this more intellectually creative
approach to questions on plant growth regulation increases the feed-back response from students.