Chapter 30.130.5Control of Plant Growth & Development
30.1 Plant Hormones
hormone is a signaling molecule that regulates or helps coordinate some aspect of the plant’s
growth, metabolism or development
Plant hormones act in response to two general types of cues:
Internal chemical conditions related to growth & development
Conditions in the external environment that affect plant growth such as light or water
Biologists recognize at least seven major classes of plant hormones:
30.1a Auxins Promote Growth
Auxins are synthesized primarily in the shoot apical meristem and young stems and leaves
Their main effect is to stimulate plant growth by promoting cell elongation in stems and coleoptiles
and by governing growth
responses to light and gravity. Experiments Leading to the Discovery of Auxins:
Was first studied by Darwin and his brother Francis.
Apical meristem and plumule of grass seedlings are sheathed by protective coleoptilea structure
that is extremely sensitive to light.
He proposed that when seedlings are illuminated from the side, some influence is transmitted from
the upper to the lower part causing them to bend.
Conclusion: when seedlings are illuminated from one side, an unknown factor transmitted from a
seedling’s top to the tissue below causes it to bend toward the light.
Effects of Auxins:
Auxin stimulates aspects of plant growth and development.
When zygote first divides, forming an embryo that consists of a basal cell and an apical cell, auxin
exported by the basal cell to the apical cell helps guide the development of the various features of
the embryonic shoot.
IAA maintains apical dominance, which inhibits growth of lateral meristems on shoots and restricts
the formation of branches.
Evidence shows that that the shaded side of a shoot tip contains more IAA than the illuminated
Light causes IAA to move laterally from the illuminated to the shaded side of a shoot top
Also the IAA then moves downward in a shoot by way of a toptobottom mechanism called polar
IAA in a coleoptile or shoot tip travels from the apex of the tissue to its base, such as from the tip of
a developing leaf to the stem.
Possible Mechanisms of IAA action:
Plant cell walls grow much faster in an acid environmentthat is when the pH is less then 7.
The acidgrowth hypothesis suggests that auxin causes cells to secrete acid into the cell wall by
stimulating the plasma membrane H+ pumps to move hydrogen ions from the cell interior into the
cell wall; the increased acidity activates proteins called expansins.
Expansins penetrate the cell wall and disrupt bonds between cellulose microfibrils in the wall.
30.1b Gibberellins Also Stimulate Growth, Including the Elongation of Stems
gibberellins stimulate several aspects of plant growth. Like auxin, gibberellin modifies the properties of plant cell walls in ways that promote expansion.
Gibberellins are active in eudicots and in in a few monocots.
Gibberellins can have on internode growth is bolting, growth of a floral stalk in plants that form
vegetative rosettes, such as cabbages and iceberg lettuce.
Gibberellins switch on internode lengthening when environmental conditions favour a shift from
vegetative growth to reproductive growth.
Gibberellin seem to encourage proportionately more male flowers to develop.
This is beneficial because it allows for more pollen to be available for more fruits.
30.1c Cytokinins Enhance Growth and Retard Aging.
Cytokinins play a major role in stimulating cell division.
Researchers found that in addition to a carbon source such as sucrose or glucose, cells in culture
required two other substances.
One was auxin, which promoted the elongation of plant cells but did not stimulate the cells to divide
Other would be coconut milk, which liquid endosperm.
The cultured cells would begin dividing when the two substances were mixed.
The most abundant natural cytokinin is zeatin.
Cytokinins are synthesized largely in root tips and are apparently transported through the plant in
Ex. When two concentrations of hormones were equal, the growing tissue did not differentiate but
instead remained as a loose mass of callus.
When the concentration of cytokinin was increases, chloroplasts in the callus cells matured, the
callus became green and more compact.
Read. Ex. Pg.736
30.1d Ethylene Regulates a Range of Response, Including Senescence.
Ethylene is an unusual hormone because it structurally simple and in part because it is a gas at
normal temperature and pressure. Ethylene helps the process of senescence, which is a closely controlled process of deterioration
that leads to the death of plant cells(ex. Fall leaves).
Senescence is associated with abscission the dropping of flowers fruits and leaves in response
to environmental signals.
30.1e Brassinosteroids Regulate Plant Growth Responses
these stimulate cell division and elongation in a wide rang od plant cell types.
It is been found to promote cell elongation, differentiation of vascular tissue, and elongation of
pollen tube after a flower is pollinated.
It also inhibits the elongation of roots.
Brassinosteroids seem to regulate the expression of genes associated with a plant’s growth
responses to light.
30.1f Abscisic Acid Suppresses Growth and Influences Response to Environmental Stress
plays a major role in abscission.
Functions of ABA:
Opposite of gibberellins
ABA inhibits growth in response to environmental cues, such as seasonal changes in temp and
Plants that produce fleshy fruits, ABA is associated with dormancy of seeds as well.
As seed develops, ABA accumulates in the seed coat, and the embryo does not germinate even if
it becomes hydrated.
Dormant plants suffer less shipping damage, and the effects of the inhibitors can be reversed by
applying a gibberellin.
ABA also tells the stomata to close to prevent water loss
The ABA binds to the guard cells produced by mesophyll and closes the stomata to close.
30.1g Jasmonates and Oligosaccharins Regulate Growth and have roles in defence Jasmonates, a family of 20 compounds from fatty acids
Help late root growth and seed germination
Also helps to manage stresses due to deficiencies of certain nutrients such as K+.
Oligosaccharides could serve as signaling molecules.
Role is to defend the plant against pathogens
Also as growth regulators that adjust the growth and differentiation of plant cells, possibly by
modulating the in