Indigenous rhizobacteria possessing abiotic stress tolerant traits promote vigorous growth of coconut seedlings via increased nutrient uptake and positive plant–microbe feedback

A total of 20 bacteria including seven fluorescent Pseudomonas spp. and 13 Bacillus spp., out of 483 previously isolated bacteria from rhizosphere and roots of healthy and high yielding coconut palms from major coconut growing States in India, were tested for their growth promotion efficacy on West Coast Tall variety coconut seedlings. Firstly, they were characterized for (i) ability to grow under different environmental stress parameters, (ii) carbon use pattern determined with BIOLOG and (iii) their genetic diversity using RFLP analysis. Seedling response to PGPR inoculation was measured by recording the leaf numbers, girth, root and shoot fresh and dry biomass weights in relation to nutrient uptake efficiency and plant–microbe feedback. Tolerance studies for abiotic parameters showed that Pseudomonas spp. were efficient in growing in 5.2 to 8.2 pH, 15–30 °C temperatures and 0.5 to 10% NaCl concentration, whereas, Bacillus spp. grew well in pH 4.2–8.2, temperatures 20–40 °C and 0.5 to 10% NaCl concentration. The BIOLOG analysis highlighted Bacillus spp. to be more suited to coconut rhizosphere milieu compared to Pseudomonas spp. based on the carbon oxidizing capabilities. Among the four different enzymes used in RFLP analysis, HaeIII and HinfI were observed to be most discriminatory for Bacillus and Pseudomonas PGPR genera, respectively. Inoculation of coconut seedlings resulted in significant increase in seedling length (1–16% increase), girth of the seedlings (7–27%) and dry weight (7–38% increase) compared with control. Between the two genera, the gram-positive Bacillus spp. were seen to outperform the gram-negative Pseudomonas spp. in the growth promotion studies. The endophytic B. megaterium TEB2 (T2) resulted in highest dry weight biomass increase (38%) in coconut seedlings followed by another endophytic B. cereus HEB10 (T3) and a rhizospheric isolate B. licheniformis RSB14 (T17) indicating their potential for development of bioinoculant formulation. Our studies, thus, offer a bouquet of indigenous PGPR, possessing abiotic stress tolerant traits, to choose upon to develop either an effective singular bioinoculant or a minimum effective microbiome set (MEMS) for raising vigorous and robust coconut seedlings for supply to coconut farmers.