Westernghat is one of the unique Biodiversity niches, with varied flora, fauna and Landscapes. The increasing demand for crop production with a significant reduction of synthetic chemical fertilizers and pesticides use is a big challenge nowadays. Plant growth promoting bacteria has become of great interest to promote the crop growth and provide protection from phytopathogens. The use of PGPR has been proven to be an environmentally sound ways of increasing crop yields by facilitating plant growth through either a direct or indirect mechanism. This review presents some data on microbial diversity of westernghat, emphasizing the prospects of exploiting them for potential applications that could be imperative for socio-economic development of India.
Introduction
Western Ghats of India, though covering an area of 180,000 km2, or just under 6% of the land area of India, contain more than 30% of all plant, fish, herpeto-fauna, bird, and mammal species found in India. Starting from southern tip of Gujarat and extending from Satpura Range in the north traversing through the States of Maharashtra, Goa, Karnataka and Kerala, hills of Western Ghats ends at southern tip of India, standing tall at an altitude of 2695 m (Anamudi) in Kerala. Unique biodiversity of Western Ghats are protected and conserved through the establishment of biosphere reserves, national parks and several wildlife sanctuaries [1].
A large number of different microorganisms, mostly bacteria, are commonly found in soil. Soil bacteria interact specifically with plant roots in the Rhizosphere, where bacterial density is generally higher. The bacteria that provide benefits to the plant either form symbiotic relationships with the plant or are free-living in the soil, but found near or even within the roots [5].
Plant Growth Promoting Rhizobacteria (PGPR)
Bacteria that colonize plant roots and promote plant growth are referred to as plant growth-promoting rhizobacteria (PGPR). Their effects can occur via local antagonism to soil-borne pathogens or by induction of systemic resistance against pathogens throughout the entire plant. PGPR affect plant growth in two different ways, indirectly or directly. The direct promotion of plant growth by PGPR entails either providing the plant with a compound that is synthesized by the bacterium, for example phytohormone, or facilitating the uptake of certain nutrients from the environment. The indirect promotion of plant growth occurs when PGPR lessen or prevent the deleterious effects of one or more phytopathogenic organisms. This can happen by producing antagonistic substances or by inducing resistance to pathogens. A particular PGPR may affect plant growth and development by using any one, or more, of these mechanisms [2].
Plant growth promoting rhizobacteria (PGPR) shows an important role in the sustainable agriculture industry. The increasing demand for crop production with a significant reduction of synthetic chemical fertilizers and pesticides use is a big challenge nowadays. The use of PGPR has been proven to be an environmentally sound way of increasing crop yields by facilitating plant growth through either a direct or indirect mechanism [4].
Rhizosphere is the soil environment where the plant root is available and is a zone of maximum microbial activity resulting in a confined nutrient pool in which essential macro- and micronutrients are extracted. The microbial population present in the rhizosphere is relatively different from that of its surroundings due to the presence of root exudates that function as a source of nutrients for microbial growth[4].
It is well known that a considerable number of bacterial species, mostly those associated with the plant Rhizosphere, are able to exert a beneficial effect upon plant growth. Therefore, their use as biofertilizers or control agents for agriculture improvement has been a focus of numerous researchers for a number of years. This group of bacteria has been termed ‘plant growth promoting rhizobacteria’ (PGPR) , and among them are strains from genera such as Pseudomonas, Azospirillum, Bacillus, Enterobacter, Rhizobium, Serratia, Alcaligenes, Arthrobacter, Acinetobacter and Flavobacterium [3].
Strains of Pseudomonas putida and Pseudomonas fluorescence have increased root and shoot elongation in canola, lettuce, and tomato as well as crop yields in potato, radishes, rice, sugar beet, tomato, lettuce, apple, citrus, beans, ornamental plants, and wheat. Wheat yield increased up to 30% with Azotobacter inoculation and up to 43% with Bacillus inoculants, and a 10–20% yield increase in the same crop was reported in field trials using a combination of Bacillus megaterium and Azotobacter chroococcum. Azospirillum spp. have increased yield in maize, sorghum, and wheat, and Bacillus spp. has increased yield in peanut, potato, sorghum, and wheat [4].
Root elongation in various agronomic crops by the plant growth promoting rhizobacterium pseudomonas putida gr12-2 Plant growth promoting rhizobacteria (PGPR) such as Pseudomonas putida GR12-2 use a number of different mechanisms to promote the growth of plants, including production Of Siderophore which can sequester iron from the soil and provide it to the plant, nitrogen fixation, out competing phytopathogens, synthesis of phytohormone, and regulation of plant Hormone levels [6].
Screening plant growth-promoting rhizobacteria for improving growth and yield of wheat: In this study bacteria were isolated from the Rhizospheric soil of Wheat plants and they are screened on the basis of their potential for Auxin production and plant growth promoting activity under gnotobiotic conditions.It is concluded that the strain which produced highest amount of Auxins will increase in growth and yield of wheat plants [7].
Phosphate solubilizing bacteria
Several reports have examined the ability of different bacterial species to solubilize insoluble inorganic phosphate compounds, such as tricalcium phosphate, dicalcium phosphate, hydroxyapatite, and rock phosphate. Among the bacterial genera with this capacity are Pseudomonas, Bacillus, Rhizobium, Burkholderia, Achromobacter, Agrobacterium, Micrococcus, Aereobacter, Flavobacterium and Erwinia.[3]
Although several phosphate solubilizing bacteria occur in soil, usually their numbers are not high enough to compete with other bacteria commonly established in the Rhizosphere. Thus, the amount of P liberated by them is generally not sufficient for a substantial increase in situ plant growth. Therefore, inoculation of plants by a target microorganism at a much higher concentration than that normally found in soil is necessary to take advantage of the property of phosphate solubilization for plant yield enhancement. [3]
Soil, the natural dwelling habitat and sweet home for millions and billions of microorganisms has been studied extensively for the presence of many unexplored species of commercially Important strains ().
References
Nampoothiri, K.M., Ramkumar, B. and Pandey, A., 2013. Western Ghats of India: rich source of microbial biodiversity.
Beneduzi, A., Ambrosini, A. and Passaglia, L.M., 2012. Plant growth-promoting rhizobacteria (PGPR): their potential as antagonists and biocontrol agents. Genetics and molecular biology, 35(4), pp.1044-1051.
Rodrı́guez, H. and Fraga, R., 1999. Phosphate solubilizing bacteria and their role in plant growth promotion. Biotechnology advances, 17(4-5), pp.319-339.
Vejan, P., Abdullah, R., Khadiran, T., Ismail, S. and Nasrulhaq Boyce, A., 2016. Role of plant growth promoting rhizobacteria in agricultural sustainability—a review. Molecules, 21(5), p.573.
Gutiérrez‐Mañero, F.J., Ramos‐Solano, B., Probanza, A.N., Mehouachi, J., R. Tadeo, F. and Talon, M., 2001. The plant‐growth‐promoting rhizobacteria Bacillus pumilus and Bacillus licheniformis produce high amounts of physiologically active gibberellins. Physiologia Plantarum, 111(2), pp.206-211.
Hall, J.A., Peirson, D., Ghosh, S. and Glick, B., 1996. Root elongation in various agronomic crops by the plant growth promoting rhizobacterium Pseudomonas putida GR12–2. Israel Journal of Plant Sciences, 44(1), pp.37-42.
Khalid, A., Arshad, M. and Zahir, Z.A., 2004. Screening plant growth‐promoting rhizobacteria for improving growth and yield of wheat. Journal of Applied Microbiology, 96(3), pp.473-480.