What is plant growth-promoting rhizobacteria and endophytes and how plant growth enhancing rhizobacteria act to enhance plant growth?
@ Rk Naresh, plant growth promoting rhizobacteria (PGPR) as well as endophytes act concomitantly on plant growth through direct and indirect mechanisms. they can solubilize soil nutrients (C, N, P, K and Fe) and make them easily available to plants and they also produce hormones like auxin which can modulate plant growth and development. besides this, they have potency to secrete antibiotics which inhibits the growth of several phyto-pathogens.
Bacteria that colonize in the roots of plants or along the rhizopheric axis and promote plant growth are known as PGPR. They promote plant growth directly by regulating nutritional status (phosphate solubilization, N fixation, iron sequestration) and hormone synthesis (IAA, GB, CK, etc.).Plant growth promoting rhizobacteria and endophytes accelerates phytoremediation of metalliferous soils though modulation of (a) plant growth promoting parameters, (b) by providing plants with nutrients, and (c) controlling disease through the production of antifungal metabolites. Plant growth promoting rhizobacteria (PGPR) is a group of bacteria that can be found in the rhizosphere. Plant growth promoting bacteria refers to bacteria that colonize the roots of plants (rhizosphere) that enhance plant growth. Endophytes possess vital ability to mobilize insoluble phosphate and provide nitrogen to their host plants. Microbial endophytes colonize plant tissues without symptomatic behavior and consequently they compete with other microbial pathogens on the same ecological niches. The mechanisms used by endophytic bacteria in plant growth promotion include nitrogen fixation, plant growth stimulation via phytohormone synthesis and modulation, siderophore production, induction of systemic resistance, and synthesis of bioactive compounds against phytopathogens. Endophytes are well known for their ability to produce bioactive compounds. These compounds could be used in the defence mechanism against the microbial pathogens viz. bacteria, fungi, viruses and protozoans by plants. Its involvement in gene activation and signaling can trigger the synthesis of many other phytohormones, such as IAA, ET, and CK. These phytohormones improve hormonal regulatory patterns in plants, photosynthetic rate, plant growth and development under drought stress. Among these free-living bacteria are PGPR that exert beneficial effects on plants through direct and indirect mechanisms. Beneficial rhizobacteria have been utilized to improve water and nutrient uptake, abiotic and biotic stress tolerance. PGPR applied to vegetable crops can act as a biocontrol agent by protecting the plant from pathogens and pests. They achieve this directly by suppressing a broad spectrum of viral, bacterial, fungal, and nematode diseases and indirectly by altering the rhizosphere to favor beneficial microorganisms.Some rhizobacteria are able to produce phytohormones, including cytokinins, auxins, gibberellins, ethylene, and abscisic acid (ABA), which play a role in different growth processes in plants, including cell multiplication, which results in increased cell and root expansion