Role of halotolerant phosphate‐solubilising bacteria on growth promotion of peanut (Arachis hypogaea) under saline soil

Soil salinity is a major abiotic stress that limits plant growth, and inoculating plant growth‐promoting rhizobacteria is a well‐known strategy to reduce stressors under adverse soil conditions. This study was conducted to assess the effect of halotolerant phosphate‐solubilising bacteria (PSB) on protecting peanut against salt stress. Four candidate strains: Bacillus megaterium (YM13), Enterobacter sp. (YM14), Providencia rettgeri (TPM23) and Ensifer adhaeren (TPMX5) showed strong tolerance to NaCl and high phosphate‐solubilising ability even at a NaCl concentration of 1.4 M. In addition, all four strains demonstrated variable levels of phosphate solubilisation activity in the presence of various carbon and nitrogen sources, indicating high phosphate‐solubilising efficacy. Germination and radicle length of peanut seedlings increased with inoculation of PSB under both control and saline conditions. Statistically significant increase in the root length (range: 25.71–49%), stem length (19–48%), number of leaves (12.5–37%) and root/shoot biomass were observed. This could be attributed to plant hormones (i.e., indole acetic acid [IAA], abscisic acid [ABA] and gibberellic acid [GA3]) and successful root colonisation by bacterial inoculants. Root colonisation was positively correlated to plant growth and shown to be influenced by soil conditions. In addition, the PSB also improved the levels of available P in soil. The most pronounced beneficial effect on the growth of peanut plants and soil available P content was observed in the inoculation of the PSB isolates with Ca₃(PO₄)₂ addition. This is the first report that describes Providencia rettgeri as a plant growth‐promoting bacterium that may be utilised to alleviate the negative effects of salt stress on peanut plants. This bacterial species may thus be potentially used as a biofertilizer for sustaining the growth of peanut in salt‐stressed soil and in mitigating soil stress conditions.     

The inter‐kingdom volatile signal indole promotes root development by interfering with auxin signalling

Recently, emission of volatile organic compounds (VOCs) has emerged as a mode of communication between bacteria and plants. Although some bacterial VOCs that promote plant growth have been identified, their underlying mechanism of action is unknown. Here we demonstrate that indole, which was identified using a screen for Arabidopsis growth promotion by VOCs from soil‐borne bacteria, is a potent plant‐growth modulator. Its prominent role in increasing the plant secondary root network is mediated by interfering with the auxin‐signalling machinery. Using auxin reporter lines and classic auxin physiological and transport assays we show that the indole signal invades the plant body, reaches zones of auxin activity and acts in a polar auxin transport‐dependent bimodal mechanism to trigger differential cellular auxin responses. Our results suggest that indole, beyond its importance as a bacterial signal molecule, can serve as a remote messenger to manipulate plant growth and development.     

Biological control potential of plant growth‐promoting rhizobacteria suppression of Meloidogyne incognita on cotton and Heterodera glycines on soybean: A review

Over the past decade, we have seen an increasing market for biopesticides and an increase in number of microbial control studies directed towards plant‐parasitic nematodes. This literature survey provides an overview of research on biological control of two economically important plant‐parasitic nematodes, Meloidogyne incognita (Kofoid & White) Chitwood (southern root‐knot nematode) and Heterodera glycines Ichinohe (soybean cyst nematode) using spore‐forming plant growth‐promoting rhizobacteria (PGPR). In this review, the current biological control strategies for the management of those cotton and soybean nematodes, the mechanism of using BacillusPGPR for biological control of plant‐parasitic nematode including induced systemic resistance and antagonism and the future of biological control agents on management of plant‐parasitic nematodes are covered.