Rhizosphere engineering through exogenous growth-regulating small molecules improves the colonizing efficiency of a plant growth-promoting rhizobacterium in rice.

Enhancing the rhizosphere colonization and persistence of plant growth-promoting rhizobacteria (PGPR) is necessary for maximizing PGPR-mediated benefits for crop growth and fitness in environmentally friendly agriculture. In the present investigation, we attempted manipulation of the rice rhizosphere by spraying of low molecular weight plant-regulating metabolites on the foliage of rice plants to in turn enhance the colonizing efficiency of soil-inoculated PGPR strain. The green fluorescent protein gene-tagged rhizobacterial strain, ZSB15-M2, was inoculated in sterile plant growth medium (vermiculite coco peat mixture) and non-autoclaved agricultural soil. We sprayed different plant growth-regulating small molecules on the foliage of rice seedlings and monitored the colonizing efficiency of ZSB15-M2 in the rice rhizosphere. Among the chemicals assessed, salicylic acid (SA) at 1 mM or cell extract (CGCE, 0.2% w/v) or cell extract (SCCE, 0.2% w/v) showed a tenfold increase in rhizosphere colony-forming units of ZSB15-M2 compared to control with a significant decline in non-rhizosphere bulk soil population. Foliar spray of CGCE enhanced soil organic carbon, microbial biomass carbon and soil protein by 21.86%, 9.68% and 11.57% respectively in the rice rhizosphere as compared to mock control. Additionally, CGCE spray enhanced the key soil enzymes, viz., dehydrogenase and acid- and alkaline phosphatase in the rhizosphere ranging 15-36%. The cumulative effect of this engineered rhizosphere resulted in the elevation of nitrogen, phosphorus, potassium and zinc availability by 21.83%, 28.83%, 23.95% and 61.94%, respectively, in rice rhizosphere as compared to control. On the other hand, SCCE and SA spray had an equal influence on the rhizosphere's biological attributes, which is lower than that of GCGE and higher than that of mock control. From the study, we propose that the aboveground management of rice with microbial-based small molecules will modulate the rice rhizosphere to attract more beneficial PGPR-based inoculants, thus improving the crop and soil health.