ABSTRACT
Plant-growth-promoting rhizobacteria (PGPR) should effectively colonize along the plant root to enhance the plant and soil health. The present investigation aims to improve the PGPR-mediated plant health benefits through above-ground foliar management. A green fluorescent protein-tagged PGPR strain, Pseudomonas chlororaphis (ZSB15-M2) was inoculated in a nonautoclaved agricultural soil before rice culturing. Salicylic acid and cell extracts of Corynebacterium glutamicum and Saccharomyces cerevisiae as a supply of hormonal and inducer compounds were applied on the foliage of the 10-days-old rice plants and subsequently observed the colonizing ability of ZSB15-M2. The cell extracts of Corynebacteria and yeast showed a 100-fold increase in the ZSB15-M2 population in the rhizosphere of rice, whereas salicylic acid had a 10-fold increase in relation to mock control. The rice root exudates collected after the spraying of salicylic acid and microbial extracts showed significantly enhanced release of total carbon, total protein, total sugar, total amino nitrogen, total nitrogen, and phenol content. In vitro assays revealed that these root exudates collected after exogenous spray of these chemicals enhanced the chemotactic motility and biofilm formation of ZSB15-M2 compared to the control plant's root exudate. Metabolomic analysis of root exudates collected from these rice plants by gas chromatography-mass spectrometry revealed that the Corynebacteria and yeast cell extracts enhanced the divergence of metabolites of rice root exudate. Further, due to these cumulative effects in the rice rhizosphere, the total chlorophyll, total protein, total nitrogen, and total phosphorus of rice were significantly improved. These observations provide insights into the rhizosphere functioning of rice plants as modulated by above-ground treatments with improved colonization of inoculant strains as well as the plant growth.
Subject(s)
Agricultural Inoculants/drug effects , Oryza/growth & development , Plant Growth Regulators/pharmacology , Salicylic Acid/pharmacology , Agricultural Inoculants/physiology , Biofilms/drug effects , Chemotaxis/drug effects , Corynebacterium glutamicum/chemistry , Metabolome/drug effects , Oryza/drug effects , Oryza/metabolism , Oryza/microbiology , Plant Exudates/metabolism , Plant Roots/drug effects , Plant Roots/metabolism , Pseudomonas chlororaphis/drug effects , Pseudomonas chlororaphis/physiology , Rhizosphere , Saccharomyces cerevisiae/chemistry , Soil MicrobiologyABSTRACT
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, Pseudomonas chlororaphis 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 Corynebacterium glutamicum cell extract (CGCE, 0.2% w/v) or Saccharomyces cerevisiae 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.