Plant growth-promoting rhizobacteria: an alternative for NPK fertilizers

In today’s world, fertilizers play an extremely important role in the enrichment of soil fertility which in turn promotes plant growth. The most commonly used fertilizers presently are the NPK fertilizers which are multinutrient fertilizers. These are deemed to be very essential for plant growth because they supply the three most important elements: nitrogen, phosphorus, and potassium, required for leaf growth, development of roots and shoots, and mobility of water throughout the plant, respectively. However, these fertilizers pose a major health risk to humans and are not environment-friendly. This led to the search for a more biologically sound alternative. Plant growth-promoting rhizobacteria (PGPR) are a group that inhabit the rhizosphere and rhizoplane of plants. These actively colonize the roots of the plant, where they carry out a number of functions that ultimately lead to plant growth and a better plant yield. The role of PGPR in improving plant growth has been well characterized. PGPR induces plant growth by a variety of mechanisms like nutrient solubilization (potassium, phosphate, and zinc-solubilizing bacteria); production of siderophores; which are low-molecular-weight proteins that chelates ferric ions from the environment, thus providing plants with iron when it is limiting in the environment, they increase nutrient availability by fixing them which reduces their leaching out. The PGPRs produce biofilm which helps it to better colonize the plant roots and contributes to overall plant health. They also produce important plant hormones like IAA (both in the presence and absence of Tryptophan), GA, etc. that help in the growth of the plant. There are examples of these beneficial microbes which can degrade the residual chemical fertilizers, like urea by the production of agriculturally important enzyme Urease. Apart from that many agriculturally significant enzymes like catalase, phosphatase, sulfatases etc. have been reported to be synthesized by the rhizospheric microbes. PGPR has been established as effective biocontrol agents in the recent years. They exert this role by either producing various antagonistic substances including volatiles or by inducing systemic resistance against the pathogen. Thus taking into consideration the current global scenario of environmental pollution, PGPR would be a better alternative for enhancing plant growth and development.