Download GROWTH REGULATOR Plant growth regulators (also called plant

GROWTH REGULATOR
Plant growth regulators (also called plant hormones) are numerous chemical substances that profoundly
influence the growth and differentiation of plant cells, tissues and organs. Plant growth regulators
function as chemical messengers for intercellular communication.
There are currently five recognized groups of plant hormones: auxins, gibberellins, cytokinins, abscisic
acid (ABA) and ethylene. They work together coordinating the growth and development of cells.
Auxins: Auxins stimulate cell elongation and influence a host of other developmental responses, such as
root initiation, vascular differentiation, tropic responses, apical dominance and the development of
auxiliary buds, flowers and fruits. Auxins are synthesized in the stem and root apices and transported
through the plant axis. The principal auxin in plants is indole-3-acetic acid (IAA).
Cytokinins: Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where
cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and
stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are
under the influence of cytokinins, either in conjunction with auxin or another hormone.Cytokinins most
used in tissue culture include zeatin, adenine, 6-(g,g-dimethylallylamino)purine (2 iP) and kinetin.
Cytoknins often inhibit embryogenesis and root induction.
Gibberellins: The main effect of gibberellins in plants is to cause stem elongation and flowering. They
are also prominently involved in mobilization of endosperm reserves during early embryo growth and
seed germination. Gibberellins are an extensive chemical family based on the ent-gibberellane structure.
There exit over 80 different gibberellin compounds in plants but only giberrellic acid (GA3) and GA4+7
are often used in plant tissue culture. In tissue culture, gibberellins are used to induce organogenesis,
particularly adventitious root formation.
Abscisic Acid: Abscisic acid (ABA) in plants is a terpenoid involved primarily in regulating seed
germination, inducing storage protein synthesis and modulating water stress. In plant tissue culture, it is
used to help somatic embryogenesis, particularly during maturation and germination. ABA-mediated
signaling also plays an important part in plant responses to environmental stress and plant pathogens.
Ethylene: Ethylene is a simple gaseous hydrocarbon with the chemical structure H2C=CH2. Ethylene is
apparently not required for normal vegetative growth. However, it can have a significant impact on
development of root and shoots. Usually, ethylene is not used in plant tissue culture.
MODE OF ACTION
1. Mode of action of Auxins
There are two mechanism for mode of actions for Auxin :
i.
Enlargement Mechanism
 Application of IIA in the cell.
 Fixes to the binding sites (PM,ER,Dictyosome)
 Enhance Proton(H+) pump from cytosol to wall
ii.
 Decreasing the pH of cell wall
 Increase the cell elongation
 H2O enters into the wall
 Ultimately decrease the wall pressure
 Activation of some enzymes and loosen the wall pressure
Wall rigidity mechanism
 Fixing on binding sites auxin sends a messenger to nucleus
 Nature of this messenger is unknown to the physiologist
 So that a structural change of DNA happens; mRNA forms from DNA
 These mRNA carries some new information – comes outside the nucleus and
generate new enzyme
 This enzyme produces new cell wall materials which gives wall plasticity and
rigidity. So enlargement get permanency.
 In other cases, new DNA , mRNA formation is the basic principle of mode of
action.
2. Mode of action of cytokinins
The ratio of auxin to cytokinin plays an important role in the effect of cytokinin on plant growth.
Cytokinin alone has no effect on parenchyma cells. When cultured with auxin but no cytokinin,
they grow large but do not divide. When cytokinin is added, the cells expand and differentiate.
When cytokinin and auxin are present in equal levels, the parenchyma cells form an
undifferentiated callus. More cytokinin induces growth of shoot buds, while more auxin induces
root formation.
Cytokinins are involved in many plant processes, including cell division and shoot and root
morphogenesis. They are known to regulate axillary bud growth and apical dominance.
Cytokinins have been shown to slow aging of plant organs by preventing protein breakdown,
activating protein synthesis, and assembling nutrients from nearby tissues.
3. Mode of action of Gibberellins
Gibberellins are involved in the natural process of breaking dormancy and other aspects of
germination.
Gibberellins are produced in greater mass when the plant is exposed to cold temperatures. They
stimulate cell elongation, breaking and budding, seedless fruits, and seed germination. They do
the last by breaking the seed’s dormancy and acting as a chemical messenger. Its hormone binds
to a receptor, and Ca2+ activates the protein calmodulin, and the complex binds to DNA,
producing an enzyme to stimulate growth in the embryo.
4. Mode of action of Abscisic Acid
Abscisic acid owes its names to its role in the abscission of plant leaves. In preparation for
winter, ABA is produced in terminal buds. This slows plant growth and directs leaf primordia to
develop scales to protect the dormant buds during the cold season. ABA also inhibits the
division of cells in the vascular cambium, adjusting to cold conditions in the winter by
suspending primary and secondary growth. Abscisic acid is also produced in the roots in
response to decreased soil water potential and other situations in which the plant may be under
stress. ABA then translocate to the leaves, where it rapidly alters the osmotic potential of
stomatal guard cells, causing them to shrink and stomata to close. The ABA-induced stomatal
closure reduces transpiration, thus preventing further water loss from the leaves in times of low
water availability.
5. Mode of action of Ethylene
(i) Ethylene Receptor Involved in Signaling and Site of Action:
Ethylene has been shown to induce specific changes in genetic expression.By analogy to other
plant hormones, molecular biological phenomena like the synthesis of new mRNA and protein
can be controlled by ethylene.
(ii) Ethylene and Regulation of Gene Expression:
It has been observed that the expression of various target genes is altered by ethylene. Ethylene
has been shown to increase the levels of mRNA transcripts of several genes corresponding to
the enzyme proteins like cellulase, chitinase, β-1, 3-glucanase, peroxidase and chalcone
synthase.
(iii) Biochemical Action:
It is well known that applied ethylene induces its own synthesis which is termed as
‘autocatalytic’ ethylene synthesis. In this model, the mode of action of ethylene at the molecular
level involves the regulation of ACC synthase.