Plant Growth-Promoting Rhizobacteria (PGPR): Emergence and Future Facets in Medicinal Plants

About three-quarters of the world population rely mainly on plants and plant extracts for health care. India is rich in its biological resources and known for its valuable heritage of herbal medicinal knowledge. It possesses an extremely rich plant biodiversity which is gradually decreasing. Plants that possess therapeutic properties on the animal or plant body are generally designated as medicinal plants; with the development of microorganisms resistant to chemicals applied indiscriminately to crops, research has been done with the goal to search for alternative and safe forms of agrochemical pest control without causing any damage to environment and to humans, maintaining the crop qualitatively and quantitatively. PGPRs exhibit direct/indirect mechanisms as plant growth promoters and biological controlling agents. Direct mechanisms by PGPR include the provision of bioavailable phosphorus for plant uptake; nitrogen fixation for plant use; sequestration of iron for plant by siderophores; production of plant hormones like auxins, cytokinins, and gibberellins; and lowering plant ethylene levels using ACC deaminase that accumulates during biotic and abiotic stresses. Indirect mechanisms of PGPR include the production of antibiotics, viz., 2, 4-diacetyl phloroglucinol (DAPG), phenazine, pyoluteorin, and pyrrolnitrin against pathogenic fungi and bacteria, reduction of iron available to phytopathogens in the rhizosphere, synthesis of fungal cell wall, and insect-gut membrane lysing enzymes. The potentiality of PGPR in medicinal plants is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides, and other supplements. Growth-promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to medicinal plant improvement and their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this chapter.