Plant Hormones - an Overview of There Functions

PLANT HORMONES: AN OVERVIEW OF THERE FUNCTIONS

Submitted By:

Kanika Calvin

Submitted to:

M. Manoharan, Ph.D

Submitted:

Fall 2005

INTRODUCTION

Plant hormones (or plant growth regulators, or PGRs) are internally secreted chemicals in plants that are used for regulating their growth. According to a standard definition, they are signal molecules produced at specific locations, occur in low concentrations, and cause altered processes in target cells at other locations.

It is accepted that there are five major classes of plant hormones:

1. auxins

2. cytokinins(CK’s)

3. ethylene

4. gibberellins (GA’s)

5. abscisic acid (ABA)

Additional suggested hormone classes:

• brassinosteroids (BA’s)

• jasmonates (JA’s)

• salicylates (SA’s)

• polyamines are major classes.

As plants grow their genotype is expressed in the phenotype which is modified by the environmental conditions that they experience. Somehow the rates of growth and differentiation of cells in different parts of the plant are coordinated in response to these inputs.

When growing plants commercially we can ask these questions:

what environmental input will produce the kind of growth that we want?

or can we modify the growth by applying a chemical regulator?

can change the genotype to achieve the kind of growth we want (by traditional breeding or by genetic manipulation)?

The answers to each of these questions depends on an understanding of how plant growth is regulated. Hormones in animals cooordinate body functions by being produced in one place and acting in another. Plants do not have a circulatory system and "action at a distance" may not be a feature of plant hormones. They are molecules that are not directly involved in metabolic or developmental processes but they act at low concentrations to modify those processes.

CLASSES OF HORMONES

Following is the breakdown of the five classes of plant hormones. They are all organic compounds, they may resemble molecules which turn up elsewhere in plant structure or function, but they are not directly involved as nutrients or metabolites.

Auxins

There are many synthetic auxins - aromatic compounds with carboxylic sidechains often affect plant growth in the same way that IAA does. These are used commercially rather than IAA because they are cheaper and more stable. For example naphthalene acetic acid (NAA) is used to control fruit set and sucker growth on trees after pruning. Indole butyric acid is used to promote rooting in cuttings. Far and away the biggest use of auxin-like compounds is as herbicides (2,4-D and MCPA). Applied at high concentration they promote uncoordinated growth and finally death, particularly in broad-leaved weeds.

Cytokinins

There are a number of naturally occurring

cytokinins all related to the nucleotide adenine. They can occur as the free base or as a riboside. Synthetic cytokinins include benzyladenine and kinetin. Cytokinins