The earliest discovered hormone is indoleacetic acid (IAA), which is an auxin and the most studied hormone. Indoleacetic acid is ubiquitous in plants and is the auxin with the strongest physiological activity.
Gibberellin belongs to diterpenoids. Among them, GA3 was discovered the earliest and studied extensively.
Cytokinin (CTK) is an adenine derivative. Zeatin is the first natural cytokinin isolated from higher plants.
The above three hormones mainly promote plant growth, while abscisic acid and ethylene mainly inhibit plant growth.
Abscisic acid is a sesquiterpene derivative.
Ethylene is an unsaturated hydrocarbon with very simple chemical structure.
In addition to these five hormones, brassinolide is considered as the sixth hormone. This is a plant endogenous steroid physiological active substance with sterol as the skeleton, also known as brassinolide.
The mechanism of action of plant hormones is as follows. Hormones in plants show the function of regulating metabolism after binding with a protein called hormone receptor in cells. Hormone receptors have strong specificity and affinity for hormones. There are some receptors on the plasma membrane, which change the proton pump activity on the plasma membrane and affect the membrane permeability after binding with indoleacetic acid. Some receptors exist in cytoplasm and nucleus, which, when combined with hormones, affect the synthesis of DNA, RNAH and protein and regulate the synthesis of special enzymes.
There are various interactions between hormones. The first is synergy. For example, the combination of GA3 and IAA*** can strongly promote the cell division of cambium. For some apple varieties, seedless fruit can only be induced by simultaneous use.
The second is promotion. Exogenous GA3 can promote the synthesis of endogenous auxin, because the applied GA3 can inhibit the activities of IAA oxidase and peroxidase in tissues, thus delaying the decomposition of IAA. High concentration of exogenous auxin promotes the production of ethylene.
The third is coordination. For example, auxin can promote the formation of root primordium, and cytokinin can induce the formation of buds. When cultivating plant cells and tissues, auxin and cytokinin must be combined in a proper proportion in the culture medium to show the totipotency of cells, that is, to grow roots and buds and become complete plants.
The fourth is antagonism. For example, the auxin produced at the top of plants can be transported downwards, which can control the germination and growth of lateral buds and show the top advantage. For example, applying cytokinin to lateral buds can overcome the control of auxin and promote the germination and growth of lateral buds. For another example, ABA can inhibit the production of aleurone layer α -amylase in barley induced by GA3. On the contrary, the inhibitory effect of ABA on potato bud germination can be offset by GA3. Exogenous ethylene promotes the production of IAA oxidase in tissues, thus accelerating the decomposition of IAA and reducing the IAA level in plants.
Synthetic compounds with physiological activity and similar to plant hormones are called plant growth regulators or plant exogenous hormones. A small amount can effectively control the growth and development of plants and increase crop yield, so it is widely used in agriculture and horticulture. These plant growth regulators have the following categories.
1. growth promoter. It is a synthetic substance, similar to auxin, gibberellin and cytokinin. It can promote cell division, elongation, differentiation and the formation of new organs, and prevent fruit from falling off. Include 2,4-D, indoleacetic acid, indolebutyric acid, naphthylacetic acid, 2,4,5-T, 2,4,5-TP, carbaryl, Zengchanling, GA3 gibberellin, kinetin, 6-BA, PBA, zeatin, etc.
2. Growth inhibitors. A compound that slows down the growth rate to inhibit cell division and elongation growth in the lower part of the top of the stem. It leads to the shortening of plant internodes, inducing dwarfing and promoting flowering, but has no effect on leaf size, number of leaves, number of nodes and apical dominance. Growth retardants mainly play a role in preventing gibberellin biosynthesis. These substances include: chlormequat chloride (CCC), B9 (Bijiu), Amo-16 18, phosphine chloride -D (Fosfon -D), growth hormone (Tianan) and so on.
3. Growth inhibitors. Different from growth retardants, it mainly inhibits cell division in apical meristem, resulting in the loss of apical dominance, the increase of lateral branches and the atrophy of leaves. It cannot be reversed by gibberellin. Such substances include: MH (ethylene diamine), decanoic acid, TIBA (triiodobenzoic acid), chloramine (plastic) and glyphosate.
4. Ethylene releasing agent. Synthetic compounds that release ethylene can promote fruit ripening. Ethephon is the most widely used one. Ethephon is stable at pH below 4. When the pH value in plants reaches 5~6, it will slowly degrade and release ethylene gas.
5. defoliant. The defoliant will lead to the release of ethylene and make the leaves age and fall off. Its main substances are tributyl thiobutyrate, calcium cyanamide, adriamycin and aminotriazole. Defoliants are usually herbicides.
6. Desiccant. Desiccant quickly loses water through the damaged cell wall, leading to cell death. It is essentially a contact herbicide. There are paraquat, herbicide Dan, adriamycin and pentachlorophenol.
Although the use of plant growth regulators can regulate the growth of plants, the abuse of hormones often leads to irreparable loss of yield, so the concentration must be appropriate and the number of uses must not be too many.