Phytohormones: Plant hormones

Phytohormones: Plant hormones

Efficient communication between cells, tissues and organs maintains the form and functions of an organism such as regulation and coordination of metabolism, growth and morphogenesis. These activities often depend on signals from one part of higher plants to another. However, it is hormones that mediate communication between cells. At very low concentrations, these molecules have important effects on plant development. The basic plant hormones that regulate plant metabolism, although there are many more, are: auxins, gibberellins, cytokinins and abscisic acid.

An auxin is a compound that can be synthesised inside plant cells, in the cytosol or in chloroplasts. In plants, they grow mainly in shoots, but also in rapidly growing tissues. In particular, they are usually synthesised in the apical bud of the plant and then transported in a polar fashion to the root.

Its functions are varied:

1. They promote the formation of adventitious roots and cell elongation in the stem, thanks to the extensibility of the cell wall. In this case, a dormant state occurs for 10 minutes after the arrival of auxin. After that, the process is initiated. This directed growth mediated by auxins also occurs in response to directional stimuli (tropisms), concentrating on the light side, in phototropism, and on the roots in gravitropism.
2. The regulation of plant metabolism may depend in part on free auxin present in cells, tissues and chloroplasts from the cytosol or chloroplasts.
3. They are involved in apical dominance, lateral root emergence, leaf absorption, vascular differentiation, flower bud formation and fruit development.

GLIBERELLINS

Glyberellins were discovered 20 years after auxins. These hormones are made up of isoprene units, form a large group of terpenoid compounds and more than 125 are known at present. These molecules are synthesised under the control of environmental factors such as temperature, photoperiod or the presence of auxins. Also, the expression of genes involved in their synthesis is controlled at the transcriptional level.

Its main functions are:

1. Induce a drastic elongation in the internode of certain types of plants. Have a stimulating effect on stem growth by elongation and cell division.
2. Produce changes in the youth and sexuality of flowers.
3. Produce changes in fruit promotion and fruit set, fruit growth and seed germination.

CYTOKININS

Cytokinins are hormones that regulate many cellular processes in plants. Zeatin is the main cytokinin in higher plants. These molecules are synthesised in roots, developing embryos, young leaves and fruits.

They are mainly concentrated in young cells, in roots and in cells of poorly differentiated embryonic tissues (meristems) in the apical zone of stems. They are not actively transported, but passively transported from the root to the stem via the xylem together with the processed sap.

Its functions are:

1. Participate in the regulation of various processes such as stem and root morphogenesis, chloroplast maturation or cell senescence.
2. Regulates cell cycle control in growth and development, in collaboration with auxins. Both hormones act synergistically to regulate cell division. It is important to know that a high auxin-cytokinin ratio promotes root differentiation while a low auxin-cytokinin ratio promotes stem differentiation.

ABSCISIC ACID

Abcisic acid, commonly known as ABA, is a hormone that is synthesised in almost all cells containing plastids (organelles found in plant cells). This molecule can be transported through the xylem and phloem. Its concentration in the plant can vary drastically due to developmental and environmental changes. During seed maturation, the concentration of ABA has two maximum peaks of concentration, in the middle of embryogenesis and at the end of embryogenesis.

Its main functions:

1. Inhibit growth and stomatal opening, usually when the plant is under stressful conditions.
2. Regulating seed maturation and dormancy.

Germination is regulated by the ratio of ABA to gibberillae, as high ABA concentration prevents seed germination until the seed is developed, at which point the concentration of gibberillae increases and the seed germinates. During stress events, the ABA concentration can be up to 50 times higher than the basal level. This induces stomatal closure, increases the hydraulic conductivity of the root and increases the root-to-stem ratio at low water potentials.