Pseudomonas koreensis strain 69RS promotes rice growth and P bioavailability: results from in vivo experiments

Rice (Oryza sativa, L.) is one of the most important cereal and staple food crop and its production needs to further enhance due to the increase of world’s population. The demand for production is impaired by nowadays lower water availability under climate change scenarios [1]. Concomitantly, cultivation under more aerobic management affect mineral fertilizers bioavailability, phosphate especially. For these reasons, it is important to acquire innovative and eco-friendly agrotechnologies to preserve the productivity and sustainability of rice production [2]. Plant growth-promoting rhizobacteria (PGPR) can be the key position improving soil quality and various nutrients availability, in particular of different phosphate forms present in the soil but not available for plant uptake. The present work aims at determine the efficacy of PGPR inoculation of rice in the solubilization of inorganic phosphate, as determined in pot experiments. More than 200 bacterial strains were isolated from the rice rhizosphere and characterized for different PGP activities including solubilization of three inorganic phosphate forms, mineralization of phytate, nitrogen fixation, indole 3-acetic acid production, aminocyclopropane-1-carboxylate deaminase activity, siderophore production and extra-cellular polymers production. In in-vitro experiments, the best performing bacterial strain belonged to Pseudomonas koreensis. Strain 69RS was able to solubilize different mineral phosphates (tricalcium, Fe and Al) and to mineralize the organic one (phytate), these abilities being confirmed by the presence of microbial genes (gcd, phoD and phnX). The phosphate solubilizing activity as quantified by the molybdenum blue method [3] was major for AlPO4 followed by Ca(CO3)2PO4 and FePO4. Green Fluorescent Protein (GFP) transformants of strain were used to follow rice seedling colonization in growth pouches experiments and successively in pot experiments. The strain was able to colonize rice plants and growth substrates, as evidenced by confocal microscopic analyses. Particularly, in the growth pouches experiment the bacterized seeds showed a significant increase in the rice root growth. Furthermore, pot experiment confirmed that the presence of strain 69RS increase rice growth promotion and phosphate availability with Ca(CO3)2PO4 as only P source. This study highlighted that Pseudomonas koreensis strain 69RS has a successful colonization and persistence into the rice rhizosphere system, possess all principal PGP activities and is able to increment phosphate bioavailability thus encouraging radical growth. Future real soil pot experiments will evaluate the ability of the strain to colonize rice plants and increase growth promotion in the presence of biological competition.