Phytohormones are substances synthesized by plants which, even in small amounts, have a controlling effect on plant development and differentiation processes, e.g. B. on germination, growth, seed maturity, leaf shedding, flower formation, differentiation and branching. There are many parallels to the effects of hormones in animals, but there are also differences.
In contrast to animals, there is often no separation between the place of education and the place of activity in plants. There are also no clearly defined hormonal glands in plants. Phytohormones can cause opposite reactions in different tissues and organs. Indole acetic acid promotes z. B. the elongation growth in sprouts, but inhibits the growth of the roots in the same concentration.
In phytohormone research, mutants that are not sensitive to certain hormones play a major role. In this way phytohormone receptors could be identified.
The hormonal action is based on the activation of certain genes. The biochemical reaction chains on which this activation is based are in many cases not yet fully understood.
A distinction is made between auxins, cytokinins, gibberellins, ethylene, brassinosteroids, oxylipins (fatty acid derivatives) and peptides (systemin, phytosulfokines).
After the discovery of the growth-promoting effect of auxin (indole-3-acetic acid: IES, indole acetic acid: IAA) in plants, the term was adopted from animal hormones. In contrast to animals, however, plants do not have real, clearly demarcated hormone glands. Rather, phytohormones can be formed in many areas of the vegetation.
Often a certain tissue is only stimulated to synthesize these substances by environmental influences. Phytohormones can also act in the same cells or tissues in which they are produced. Plant hormones control and regulate growth and differentiation processes more strongly than animal hormones.
Another difference is that plant hormones often only have to cover very short transport routes to their place of action. Often this is done simply by diffusion. In addition, plant hormones often cause very different effects, depending on the tissues and organs in which they occur.