Download Plant Hormones FAQ

Plant Growth Hormones
This FAQ will help you understand what is proposed in one of the grant proposals that
you will evaluate. This refers to the proposal in which the epidermis or cuticle is to be
modified by treatment with plant hormones.
What is a plant growth
hormone?
Plant growth hormones are chemical substances, produced
by plants, which influence plant growth and development at
low concentrations. (The concentrations of naturally
occurring hormones are very low as they regulate normal
development. If we treat a plant with more of a hormone in
the lab or field, it takes very little to have a large effect.)
Some plant growth hormones speed the growth of particular
plant parts and others slow it. There are hormonal effects
on many developmental phenomena, including seed
germination, flowering, the shedding of leaves, branching,
root formation, etc. Major classes of plant growth hormones
include gibberellins, cytokinins, auxins, abscisic acid,
ethylene, and brassinosteroids.
What do gibberellins do?
The gibberellins are a large group of chemically related
plant growth hormones. They are associated with:
 the promotion of stem growth
 seed germination
 the formation of hairs on the epidermis
 many other functions
What do cytokinins do?
The cytokinins are plant growth hormones with many
developmental effects, including:
 aging phenomena in leaves
 the transfer of nutrients from one leaf to another
 control of buds that form branches
 the development of flowers
 seed germination
What do auxins do?
Auxins:
 control stem growth, including the promotion of cell
elongation in isolated pieces of stem
 promote cell division in plant tissue growing in
culture
 promote the formation of lateral roots where stems
have been cut
 in some plants they can promote the formation of
fruits without the plant’s flowers having been
pollinated
What does abscisic acid
do?
 induce the formation of another hormone, ethylene.
Abscisic acid is a plant growth hormone that:
 inhibits the opening of stomata, thus helping to
retard water loss when water is scarce
 inhibits growth
 regulates the maturation and dormancy of seeds.
What does ethylene do?
Ethylene is a plant growth hormone that:
 affects the elongation of stems and roots
 enhances fruit development
 increases the abscission (dropping) of flowers and
fruits
 suppresses the flowering of most species
 is a gas and is the smallest plant hormone: it consists
of two carbon atoms joined to each other, with each
bound also to two hydrogen atoms. Its structure is
H2C=CH2.
What do brassinosteroids
do?
Many of the hormones in our bodies are steroids. The only
steroid hormones in plants are ones called brassinosteroids,
which:
 affect the elongation of pollen tubes
 affect the development of the systems for conducting
water and the products of photosynthesis
 stimulate cell elongation in stems
 inhibit root elongation
Are their other kinds of
Right now plant scientists are studying a newly isolated
plant hormones in addition hormone called florigen. This is the largest of the
to the growth hormones?
hormones—actually a protein. It is the signal causing some
plants to flower. Another plant hormone is systemin. Its
formation is triggered by fungal pathogens and by insect
attacks. Systemin initiates the formation of another
signaling compound (hormone?) called jasmonic acid; that,
in turn, initiates various defensive reactions of the plant.
Do plant hormones
present complications
when we spray them on
crops?
They do. They can be very useful tools for modifying a
plant, but a hormone treatment that does some desirable
thing may also cause other results that we didn’t want. We
need to be very careful about what we’re doing with them.
The normal development of a plant requires precise
interactions among all the naturally occurring hormones.