Plant Physiology - Musiyenko M.M. 2001

General principles of regulation of plant growth and morphogenesis
Molecular mechanisms of phytohormone action

Phytohormone activity is primarily ensured by their interaction with specific receptors in the target Cell. Receptors are protein molecules that recognize the phytohormone and specifically bind to it, forming a hormone-receptor complex.

The receptor is generally viewed as an allosteric protein that lacks physiological activity in the absence of the phytohormone, but acquires a physiologically active conformation when the phytohormone attaches to it. It is precisely this complex that transmits the signal required to trigger the corresponding physiological response of The Cell. Such receptors are located both on membranes and in the Cytosol. Moreover, the same hormone can bind to different receptors, thereby causing different physiological responses. This factor explains the polyfunctional nature of phytohormone action. Phytohormones affect only so-called "competent" Cells, and their competence is determined by the presence of specific receptors.

Membranes are the most sensitive structures in response to phytohormones. Phytohormones primarily regulate membrane permeability. For instance, Auxins and Cytokinins enhance the influx of ions through membranes, whereas ABA and Ethylene induce their efflux from the cell. Gibberellins increase membrane permeability to sugars.

Phytohormones affect the energy function of membranes, in particular, enhancing Oxidative Phosphorylation (auxins) and Photophosphorylation (gibberellins). Auxins increase the operational efficiency of the Respiratory Chain, resulting in membrane hyperpolarization and an increase in the Membrane Potential ΔμН⁺. This mechanism underlies the action of auxins on growth via elongation.

The Effect of phytohormones on cell vitality is closely related to The Biosynthesis of enzyme Proteins. For example, auxins stimulate the biosynthesis of cellulase, Cellulose synthase, pectin methylesterase, ATPase, and others. Gibberellins activate The formation of numerous Hydrolases and Lipid Biosynthesis Enzymes.

It is known that growth-inhibiting phytohormones, particularly ABA, also inhibit Protein Synthesis. Although ethylene, for example, activates the synthesis of enzyme proteins that stimulate fruit ripening processes. The effect of phytohormones on the synthesis of enzyme proteins is mediated through Gene Expression. At the METABOLISM/31.html">Transcription level, it has been found that mRNA synthesis inhibitors suppress the effect of phytohormones. Phytohormones increase Chromatin activity and the de novo formation of messenger RNAs.

Under The Influence of phytohormones, the contact Structure of chromatin changes, resulting in the derepression of a specific region of The Genome. There is evidence that these changes occur with the participation of cyclic AMP (cAMP). In particular, after the interaction of phytohormones with membranes, synthesized ATP can serve as a source for cAMP formation with the participation of the membrane-bound enzyme adenylate cyclase. In turn, cAMP activates protein kinase enzymes, which catalyze the phosphorylation of chromatin proteins, thereby altering their conformation. As a result, they lose their ability to inhibit the transcription process.

Characteristically, regulation is also associated with the participation of phytohormones in the derepression of specific Regions of the genome. Moreover, the same phytohormone can act as a derepressor for different regions of the genome and cause the de novo formation of numerous enzyme proteins. At the same time, different phytohormones can induce the formation of identical enzyme proteins. Auxins, gibberellins, and cytokinins participate in the derepression of certain genomic regions, whereas ABA represses them.

The effect of phytohormones on transcription processes can be exerted not only through The regulation of differential genome activity, but also via changes in The activity of the RNA polymerase enzyme itself. According to O.M. Kulaeva, this is typical, for example, for cytokinins. The effect of Hormones on the biosynthesis of enzyme Proteins can also be implemented at the post-transcriptional level. In particular, it is known that RNA polymerase transcribes information from the entire transcripton (promoter, acceptor zone, and structural zone — the Operon). It should also be taken into account that, according to modern data, coding DNA regions in genes are interrupted by non-coding regions, i.e., those that carry no information. As a result of RNA polymerase functioning, giant molecules of the mRNA precursor (pre-mRNA) are formed. The latter can be stored even in The Nucleus without participating in protein synthesis, for example, in dry seeds, since the initiation of synthesis requires mRNA maturation, or Processing. Processing includes the Cleavage of the acceptor zone, the excision of non-coding codons, as well as the splicing of coding codons (those carrying specific information). Afterwards, the newly formed mRNA joins and emerges from the nucleus into the Cytoplasm in the form of informosomes.

Finally, the influence of phytohormones is also possible at various stages of Translation. It is likely that phytohormones control The process of mRNA entry into the cytoplasm, as well as its lifespan. According to O.M. Kulaeva, cytokinins increase the functional activity of Ribosomes and their number, which in turn leads to an increase in the synthesis of enzyme proteins at the translational level.

Thus, the phytohormone-receptor complex always switches on or switches off a specific metabolic step, thereby determining the corresponding physiological response of the plant Organism.