Molecular Biotechnology: Principles and Applications - Glick B., Pasternak J. 2002

Molecular Biotechnology of Microbial Systems
Plant Growth-Promoting Bacteria
Plant Growth Promotion by Free-Living Bacteria

Plant Growth-Promoting Bacteria exert their effects in several ways: 1) they fix atmospheric nitrogen, which is then utilized by the plant; 2) they synthesize siderophores, which solubilize and sequester iron from the soil and provide it to plant Cells; 3) they synthesize phytohormones that accelerate various growth stages; 4) they solubilize minerals (such as phosphorus), which are then utilized by the plant; 5) they synthesize Enzymes capable of regulating plant hormone levels. Each plant growth-promoting bacterium may employ one or more of these mechanisms.

Nitrogen Fixation contributes only slightly to the positive effect exerted by plant growth-promoting bacteria. Not all such bacteria are diazotrophs, and many of them fix only a limited amount of nitrogen.

To take up iron from the soil, some plants utilize bacterial iron–siderophore complexes; without this, their growth would in most cases be severely stunted. However, while bacterial siderophores undoubtedly contribute to PLANT Nutrition AND, consequently, to their growth, this effect is generally not very pronounced.

Exactly how plant growth-promoting bacteria facilitate the uptake of minerals such as phosphorus by plants is not fully understood. It has been suggested that plants treated with growth-promoting bacteria develop a better ROOT system and therefore take up necessary nutrients from the soil more efficiently, meaning the bacterial effect is indirect. However, experiments with Azospirillum have shown that this Organism specifically increases mineral uptake, possibly by synthesizing and secreting organic acids that solubilize and chelate some of these nutrients.

Very often, the various effects of plant growth-promoting bacteria are attributed to their ability to synthesize phytohormones. Most research in this area has focused on elucidating The Role of one Class of phytohormones, the Auxins. The most common and best-characterized auxin is indole-3-acetic acid (IAA). It stimulates both rapid responses (such as plant Cell elongation) and long-term responses (acceleration of division and differentiation). Plants can also synthesize auxin. This often makes it difficult to determine whether the bacterial or the plant-derived auxin is producing the observed effect. Nevertheless, it can be argued that plant growth-promoting bacteria exert their effects precisely by altering the hormonal balance within the plants. Recently, it was discovered that many plant growth-promoting bacteria synthesize an enzyme capable of regulating levels of the plant hormone Ethylene. This enzyme, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, hydrolyzes ACC, which is the immediate precursor of ethylene in plant Biosynthesis. One explanation for the role of this enzyme is as follows. The bacterium binds to the seed coat or plant roots, then takes up and hydrolyzes ACC, thereby lowering the ethylene concentration in plant Tissues. In many plants, ethylene stimulates seed germination and breaks dormancy; however, if ethylene levels remain too high after germination, root elongation is inhibited. Thus, bacterial ACC deaminase prevents the reduction of root growth rate, allowing the plant to develop faster. Furthermore, many plant growth-promoting bacteria synthesize IAA, and any excess IAA not used to stimulate plant cell elongation or accelerate division activates ACC synthase, leading to an increased ethylene concentration. The presence of active ACC deaminase prevents ACC accumulation even at high IAA concentrations, so the ethylene concentration does not rise to levels that inhibit plant growth (Fig. 14.10). Once the mechanisms through which plant growth-promoting bacteria exert their effects are studied in detail, it will be possible to engineer recombinant microorganisms capable of promoting the growth of A wide variety of plants under diverse conditions.

Fig. 14.10. Schematic representation of the mechanism by which plant growth-promoting bacteria lower ethylene concentration in plant tissues, thereby preventing root growth inhibition. A bacterial cell attached to The surface of a seed or root of a developing plant synthesizes and secretes indole-3-acetic acid (IAA), which stimulates plant growth. Upon entering the plant, bacterial IAA (along with IAA synthesized by the plant itself) stimulates either plant Cell Division and elongation, or the enzyme ACC synthase, which catalyzes The conversion of S-adenosylmethionine (AdoMet) to ACC. A significant portion of the ACC, along with other small molecules typically found in seed or root exudates, is exuded by the plant roots or seeds, taken up by the bacterium, and hydrolyzed by ACC deaminase to ammonia and alpha-ketobutyrate. As a result, The amount of ACC outside the plant decreases. To maintain the equilibrium between internal and external ACC, the plant secretes more of it. Consequently, its concentration, and therefore the concentration of ethylene in plant tissues, decreases. (From Glick et al., J. Theor. Biol., in press.)