Hormonal Control and Development

Plant hormones are a group of naturally occurring organic substances that, at low concentrations, influence physiological processes such as growth, differentiation, and development. Many plant hormones are transported from one place in the plant to another, thus coordinating growth throughout the plant, while others act in the tissues in which they are produced.

For a hormone to have an effect it must be synthesized, reach the site of action, be detected, and have that detection transferred into a final biochemical action. The steps following detection are called signal transduction, while the components of the signal transduction chain are referred to as second messengers. Because the concentration of hormone molecules affects the intensity of the response, the level of the hormone is also significant. The level is determined by the biosynthesis of the active hormone molecule and its removal by metabolism to inactive byproducts, or its binding to molecules like sugars, which also has an inactivating effect. Plant scientists have investigated these phenomena by analyzing the levels and biochemical forms of the hormones present in relation to differences in development. Recently the use of mutants and bioengineered plants in which growth or development is abnormal has enabled us to start understanding how hormones work. This research has been coupled with the isolation of the genes and proteins that are needed for the normal functioning of the hormone.

Biosynthesis.

Auxin (indoleacetic acid, IAA) is synthesized from indoleglycerophosphate, the precursor to the amino acid tryptophan, and, in some plants, from tryptophan itself. GA₁, the principal active gibberellin in most plants, is synthesized via the isoprenoid pathway, followed by a series of many other gibberellin intermediates. The level of GA₁ is very tightly regulated. The genes for the enzymatic conversions have been isolated, and the transcription of these genes have been shown to be under both feedback and environmental control. Gregor Mendel's tallness gene encodes a step in the gibberellin biosynthesis pathway just before GA₁. Cytokinins are synthesized by the attachment of an isopentenyl side chain to adenosine phosphate. The enzyme for this process, isopentenyl transferase, is the main regulating step in cytokinin biosynthesis, and its gene has been used in the genetic transformation of plants to enhance cytokinin levels. Abscisic acid is synthesized via carotenoid molecules. Ethylene is derived from methionine via the nonprotein amino acid ACC (1-amino-cyclopropane-1-carboxylic acid). The transcription of the genes for the enzymes making ACC and its conversion into ethylene is under precise developmental control, notably during fruit ripening.

Transport.

Most hormones simply travel along with the contents of the xylem or phloem by a combination of diffusion and bulk transport. Auxin is special in that it is transported primarily in the cells of the vascular cambium or its initials and is moved away from the tip of the stem or root where it is synthesized (termed polar transport ). Auxin enters the cell from the cell wall above as an un-ionized molecule (because the wall has an acidic pH) that can cross the cell membrane. At the neutral pH inside the cell it becomes ionized, preventing its outward diffusion through the cell membrane. Outward transport is on special carrier proteins located only at the base of the cell, so movement is downward. (In roots, the situation is reversed.) When a stem is placed on its side the carriers most likely migrate to the side of the lower cell so that the auxin is transported to the lower side of the stem, causing increased growth on that side and a bending upwards. This is thought to account at least in part for gravitropism, or growth away from the ground. The genes for the transport proteins have been isolated.

Detection.

For a hormone molecule to have an effect it must bind to a receptor protein. Arabidopsis mutants that do not respond to ethylene have been used to study the ethylene receptor. It is located in the cell membrane with parts that react with the next signaling compound exposed on the inside of the cell. Copper has been shown to coordinate the binding of ethylene to the receptor site. The auxin binding protein is located in the endoplasmic reticulum , from which it also migrates to the cell membrane. Its gene has also been isolated.

Signal Transduction.

Following detection, the signal from the presence of the hormone molecule has to be translated into action. There are usually many steps in this process, although a general pattern can be seen. Often the hormone triggers the phosphorylation of an activator protein, which then binds to the regulatory region of a gene, thus turning on gene transcription. This gene may produce the final product, or may itself produce a gene regulator (or transcription factor ). Steps prior to the phosphorylation of the regulatory protein may include an interaction with a membrane G protein that in turn releases other factors and the opening of calcium channels in the membrane permitting an increase in the cytoplasmic level of calcium. Some aspects of action appear, however, to be more direct, not needing gene transcription per se, although some signal transduction is always involved. The mode of action varies from hormone to hormone, and even between different hormone actions, as described below.

Auxin in Cell Elongation.

Cell elongation is a vital part of growth. Auxin causes cell elongation within ten minutes by making the cell walls more extensible. This occurs through a series of steps: Auxin stimulates the pumping of hydrogen ions out of the protoplast via proton pumps driven by adenosine triphosphate (ATP ), so acidifying the wall; this activates an enzyme called expansin, which is activated by acid conditions (about pH 4.5); expansin breaks the hydrogen bonds between the cellulose microfibrils of the wall and the other sugar-chain molecules that cross-link the microfibrils; the cell walls are made more extensible; and the cell then elongates because of the turgor pressure inside the cell.

Auxin also has a rapid action on promoting the transcription of a number of auxin-specific genes, whose exact function is currently unknown. It is uncertain whether auxin activates preexisting proton pumps in the cell membrane or whether it induces synthesis of new proton pumps. Auxin also stimulates the transcription of genes for other enzymes that act on other cell wall polymers.

Gibberellin in Alpha-Amylase Production in Cereal Grains.

Germinating seeds need to mobilize their stored carbohydrates to grow. In germinating cereal grains, gibberellin promotes the synthesis of the enzyme alphaamylase in cells of the aleurone surrounding the endosperm . The alpha-amylase breaks down the starch of the endosperm into sugars for transport to the growing seedling. Gibberellin acts through second messengers to promote transcription of the gene for alpha-amylase. Gibberellin first binds at the surface of the aleurone cell. The initial steps in the transduction chain are unknown, but gibberellin rapidly promotes the biosynthesis of a transcription-promoting factor named GA-myb. GA-myb binds to specific regulatory regions of the alpha-amylase gene, so turning on the transcription of the alpha-amylase mRNA, which is translated to produce alpha-amylase.

Gibberellin in Stem Elongation.

The presence of gibberellin is normally needed for stems to elongate, and gibberellin-deficient mutants are usually dwarf. This has been explained by the idea that a protein factor in the signal transduction chain has the effect of preventing growth, but in the presence of gibberellin this factor is negated, allowing growth to proceed. However, a further mutation of a dwarf Arabidopsis has produced a tall plant, even though the level of gibberellin is still deficient. In the double mutant the inhibitory protein factor is negated because of a mutation in its structure, allowing growth to proceed. There is also genetic evidence of a second negative regulator in the signal transduction chain. At the present time we do not know the end product that actually promotes or inhibits the elongation of the cell.

Abscisic Acid (ABA) and Stomatal Closure.

Stomata are leaf surface pores surrounded by guard cells. ABA promotes stomatal closure by causing the exit of potassium ions from the guard cells. K⁺ is the main solute causing turgor in the guard cells and opening the stoma. ABA binds to a cell-surface receptor on the surface of the guard cells. This causes a calcium ion influx and an increase in the level of inositol triphosphate, a signaling molecule, which causes a release of calcium from internal stores. The Ca⁺⁺ brings about a membrane depolarization, triggering the outward K⁺ ion channels to open. Calcium also has a direct effect on the potassium ion channels via the phosphorylation of a specific protein in guard cell protoplasts.

Ethylene and Seedling Stem Growth.

Exposure of Arabidopsis seedlings to ethylene usually causes stunted growth. However, some mutants are insensitive to ethylene. Other mutants grow stunted, as if they were exposed to ethylene, even when they are not. These mutants have helped the investigation of ethylene signal transduction. Ethylene's receptor interacts with a protein that blocks an ion channel. In the presence of ethylene, the receptor causes the protein to unblock the channel. The entry of (unknown) ions then activates other second messengers. Activation results in the synthesis of a transcription factor, finally triggering the synthesis of specific enzymes that can cause stunted growth. In ripening fruit, ethylene promotes the transcription of the mRNAs that encode for many enzymes that produce the chemical changes, including color, taste, and softening, which we know as ripening. This presumably occurs via a similar transduction chain, but the paucity of mutants makes it more difficult to investigate than in Arabidopsis.

see also Genetic Mechanisms and Development; Hormones.

Peter J. Davies