PLANT HORMONES

What is a hormone?
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.

There has to be communication between these levels: how does the plant receive and respond to environmental inputs or"signals"? What comunication is there inside the plant to adjust growth and development to the environment?

When growing plants commercially we can ask similar 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.

There are five generally recognized classes of plant hormone, some of the classes are represented by only one compound, others by several different compounds. 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 is only one naturally occurring auxin: indole-3-acetic acid (IAA) and this is chemically related to the amino acid tryptophan.

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 occuring 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 are used in tissue culture media, and for growth control in fruit.

Ethylene

Ethylene is the only gaseous hormone in the plant world; it is a simple hydrocarbon gas that is derived from the amino acid, methionine, via an unusual cyclic compound which is also an amino acid, ACC (1-aminocyclopropane-1-carboxylic acid).

The gas is used commercially for ripening fruit, particularly bananas. There are also synthetic compounds, such as ethephon (chloro-ethanephosphonic acid) that can be sprayed onto plants in solution; once inside the tissues ethephon breaks down to liberate ethylene. Ethephon is used to promote ripening on the tree, leaf abscision in ornamentals, growth control in seedlings and flowering in pineapples.

Abscisic acid

Abscisic acid (ABA) is one of two related compounds (the other is xanthoxin) that are in the isoprenoid group and related to carotenoids ABA is a very expensive material and so far there are no synthetic analogs or practical uses

Gibberellins

The gibberellins (GAs) are the largest group with over 70 compounds although not all are biologically active. Like ABA they are derived from the isoprenoid pathway. Gibberellins are used commercially to break dormancy of"difficult" seeds, and to promote set of grapes and other fruits.

Many growth retardants used on flowering pot plants, woody plants and turf are"anti- gibberellins". Compounds such as ancymidol and uniconazole block GA synthesis and produce dwarf plants. Genetic dwarfs are often deficient in gibberellin.

Hormone action
At the cell level hormones attach to a protein receptor which sends a signal down a transduction pathway to switch on particular genes. Through transcription and translation this leads to production of an enzyme protein which actually causes the change in plant growth. A good example from the early stages of plant development is the role of GA in cereal seed germination. As the seed imbibes water the embryo produces GA. This induces synthesis of amylase in the aleurone layer which secretes the enzyme to the endosperm. Amylase breaks down starch to glucose which diffuses to the embryo and is used for the early stages of plant growth.

Copyright © Michael Knee,
The Ohio State University
All rights reserved.