4.1 insoluble forms of phosphorus exist as aluminum
4.1 Indole-3-acetic acid is a phytohormone that acts as an important signalling molecule which participates in the regulation of plant development including organogenesis, tropic responses like phototropism, geotropism, cellular responses such as cell division, differentiation, enlargement, gene regulation, apical dominance, increases the rate of xylem and root development, controls the processes of vegetative growth, initiates formation of lateral and adventitious roots, affects photosynthesis, pigment formation, biosynthesis of various metabolites, and provides resistance to stressful conditions.18, 22 IAA production also increases root growth and root length resulting in greater root surface area giving the plant greater access to soil nutrients and water uptake from the soil.24 In our study, five of the 13 studied isolates could produce IAA and KV-5 showed maximum production of indole-3-acetic acid (89.28 µg/ml). Malik et al (2011)25 have reported a maximum of 40.6 µg/ml of IAA production by their Pseudomonas isolate MPS77, upon 4 days of incubation.
4.2Phosphate solubilisation: The bioavailability of phosphorus to the plants is very limited because the majority of phosphorus present in the soil is found in insoluble, immobilized, and precipitated form. Most of the insoluble forms of phosphorus exist as aluminum and iron phosphates in acidic soils 20and calcium phosphates in alkaline soils.21 The insoluble phosphorus exist either as an inorganic mineral such as apatite or as an organic mineral like inositol phosphate (soil phytate), phosphomonesters, and phosphotriesters.26. 27The solubilization of inorganic phosphorus is due to the lowering of pH by the action of low molecular weight organic acids such as gluconate, citrate, lactate, succinate etc. release of protons during the assimilation of ammonia20and chelation of cations like calcium ions that release organic phosphorus and make it available for plant use.28Mineralization of organic phosphorus takes place through the synthesis of different phosphatases that catalyzes the hydrolysis of phosphoric esters. Therefore, phosphate solubilization and mineralization can coexist in the same strain of bacteria.29 Phosphate solubilising bacteria are a group of beneficial bacteria that can solubilize insoluble phosphate into soluble form that can be absorbed as a nutrient by the plants for their overall growth and development. In our study, 5 isolates (KV-5, KV-7, KV-11, KV-12 and KV-13) showed organic acid production and three (KV-5, KV-11 and KV-13) of the 13 studied isolates could solubilize phosphate.
4.3Siderophore production:Iron is involved in cellular growth and metabolism, ATP synthesis, as a cofactor of various enzymes etc. Siderophores are low molecular weight iron chelating compounds by which bacteria take up iron under iron limiting conditions.30 They also form stable complexes with different heavy metals like Al, Cd, Cu etc. and radioisotopes such as neptunium and uranium that pose serious environmental threat thereby increasing soluble metal concentration. Therefore bacterial siderophores help to lower the stress imposed on plants that grow on soil contaminated with heavy metals.31 These iron chelators improve plant growth by bringing ferric ion (Fe3+) to the root surface where it is reduced to ferrous ion (Fe2+) and immediately absorbed by the plant roots that prefer to absorb iron in the more reduced ferrous state thus promoting plant growth.24Siderophore producers have been reported to inhibit growth of various phytopathogenic fungi and bacteria.32In our study, of the 13 bacterial endophytes, 7 isolates viz. KV-2, KV-5, KV- 6, KV-10, KV-11, KV-12 and KV-13 showed siderophore production.
4.4 Nitrogen fixation:Most of the plants cannot take up atmospheric nitrogen. This atmospheric nitrogen is converted into plant-usable forms by Biological Nitrogen Fixation (BNF) which changes nitrogen into ammonia making it available to the plants by various nitrogen fixing endophytic bacteria.31 In our study, a total of 11 isolates KV-1, KV-2, KV-3, KV-5, KV-6, KV-7, KV-9, KV-10, KV-11, KV-12 and KV-13 showed growth on Jensen’s nitrogen free medium and ammonia production.
4.5 HCN production:Indirect mechanism of plant growth promotion includes production of compounds which help host plants combat infections such as hydrocyanic acid production which is a secondary metabolite formed by the decarboxylation of glycine.33It serves as an effective biological control agent against plant pathogens. HCN mainly inhibits electron transport chain and prevents energy supply to the cell, leading to death of the pathogen. 30Of the 13 bacterial endophytes tested, none tested positive for Hydrocyanic acid (HCN) production.
4.6 Potential as bioinoculant:Kumar et al., 201634 isolated fourteen endophytic bacteria from the rhizomes of Curcuma longa L. (Turmeric) viz. Bacillus cereus (ECL1), Bacillus thuringiensis(ECL2), Bacillus sp.(ECL3), Bacillus pumilus(ECL4), Pseudomonasputida (ECL5) and Clavibactermichiganensis (ECL6). Except for B. thuringiensis ECL2, all the strains were capable of solubilizing tricalcium phosphate, all the strains produced IAA; siderophore production was seen only in Bacillus sp. (ECL3) and P. putida (ECL5). These isolates could be used as bioinoculants for increasing yield and productivity of medicinal crops.Shakeelaet al., 201735 isolated forty phosphate solubilizing rhizobacteria and endorhizobacteria from the rhizosphere soil and rhizome/roots of the Picrorhizakurroa. Among them, isolate PkR(7a) showed maximum phosphate solubilization whereas maximum IAA production was showed by PkR(34) and PkR(7b). Maximum siderophore production was observed in the isolate Pk12(b) whereas high HCN production was seen in thee isolates viz., Pk14(a), Pk14(c) and PC7 in which the colour of entire filter paper got changed from yellow to brown. They claimed that these PGPR could be used as bio-fertilizers or biocontrol agents to increase the survival and growth of medicinal plants.