plant hormones 1

what are plant hormones?

naturally occurring, organic substances that at low concentration exert a profound influence on the physiological processes of a plant (plant growth substances)

do plants have hormones?

yes

• they are chemical messengers that influence growth and development

• they posses a well-developed system of chemical messengers that induce growth and developmental responses

plant growth regulators

• PGRs

• human made forms of plant hormones

what are the five major groups of plant hormones

• IAA- auxin

• Gibberellin A₃

• zeatin- cytoinin

• abscisic aicd

• ethylene

zeatin

• produced abundantly in roots & transported toward the shoot apex

• stimulate cell division

• release apical dominance (counteract auxin)

• prevent senescence (counteract ethylene)

abscisic acid

• produced everywhere, especially under stress

• causes and deepens dormancy (seeds & buds)

• causes stomatal closure

ethlyene

• gas produced in all plant parts, especially under stress

• stimulates senescence process, fruit ripening, and leaf abscission

mechanism of hormone action

hormone → target tissue/ cell → receptor → signal amplification → response

in order for a response to occur from a hormone

• the hormone must be present in sufficient quantity (micro molar)

• the target tissue must be sensitive (sensitized) to the hormone

• there must be a receptor on the target tissue for the hormone to bind to

• the binding of the hormone/ receptor should initiate a change in the receptor (amplification)

• the activated receptor initiates a physiological response

techniques for studying hormones

• bioassays

• immunological studies

• instrumental methods (chemical testing)

bioassay

an experimental technique using sensitive plants or plant tissues to detect, quantify, or assess the biological effects of chemicals, such as herbicides or hormones, within a specific environment

immunological studies

• antibodies are made against the plant hormones and then used as specific probes to localize and quantify

• this technique is very sensitive and specific

methods for regulating endogenous levels

• controlling the rate of hormone synthesis

• forming conjugates (inactive storage forms where the hormone is covalently bonded to a sugar, amino acid, or other molecule)

• enzyme degradation

• transporting the hormone away/toward the site

• compartmentalizing the substance in an organelle such as the chloroplast

auxin

• involved with growth responses (elongate cells, stimulate cell division)

• “to increase or grow”

• first group of plant hormones discovered

• indole-3-acetic acid (IAA) is the most abundant form

auxin structure

all molecules in this family has an aeromatic ring & carboxylic acid

what is an indole?

an aromatic heterocyclic organic compound

• has a strong fecal odor at high concentrations, yet emits a flowery smell in low concentrations, making it useful in perfumes.

where is auxin made?

• in actively growing tissue (apical/ root meristems)

• made in cytosol of cells

what are the two major routes to the production of IAA?

• tryptophan-dependent pathways

• tryptophan-independent pathways

ways of conversion of tryptophan to IAA in tryptophan-dependent pathways

1. a transamination followed by a decarboxylation

2. decarboxylation followed by a transamination

3. or formation of IAA via an oxime and nitrile

basipetal (or polar) transport of auxin

auxin is transported in a basipetal direction. in other words, auxin moves from the shoot tip towards the roots and from the root tip towards the shoot

IAA is involved in the following responses:

• cell elongation & wall relaxation

• cell differentiation

• ethylene production

• inhibition of root growth

• stimulating root initiation

• flowering

• parthenocarpic fruit development (fruit development without fertilization)

• apical dominance

• abscission (a layer of cells on leaves that dies so leave can fall from plants)

what is the function of gibberellin?

it is in young seeds and helps with germination & breakdown of nutrients between the plant

site of gibberellin

• young leaves

• roots

• developing seeds (developing endosperm)

• fruits

what is an endosperm?

a nutrient-rich tissue formed within the seeds of most flowering plants (angiosperms) during double fertilization

• aka aleurone

transport of gibberellin

• made in the tissue in which it is used

• transport occurs through xylem, phloem, cell-to-cell, or diffusion

• phloem seems to be the most important transport route

physiological responses of gibberellin

control of shoot elongation

• gibberellins promote shoot elongation of intact tissue

antigibberellin

• block gibberellin biosynthesis and suppress internode elongation

• can be used to reduce the need for pruning and enhance fruit color and texture

commercial uses of gibberellin

• lessening the bunching of grapes

• increasing seed production of pine nuts

• increasing malting rate in brewing

• applied to oranges to prevent rind disorders and delay ripening

• increase sugar cane production in Hawaii