Protist-enhanced survival of a plant growth promoting rhizobacteria, Azospirillum sp. B510, and the growth of rice (Oryza sativa L.) plants

Abstract

Application of plant growth promoting rhizo-bacteria (PGPR) is a promising method for sustainably increasing the plant growth; however, survival of PGPR is the most important factor limiting its efficiency. We studied the effects of heterotrophic protists on the survival of introduced PGPR, Azospirillum sp. B510, on indigenous bacterial community structure, and on the growth of the rice (Oryza sativa L.) plant in the early stages. Sterile rice seedlings grown in sterilized soil were inoculated with Azospirillum sp. B510, four protist isolates, and an indigenous protist-free bacterial community. Impact of protists on bacterial community structure was investigated by high throughput sequencing. Heterotrophic protists significantly increased survival of Azospirillum sp. B510. Heterotrophic protists and Azospirillum sp. B510 significantly increased early plant growth and nitrogen uptake, and their simultaneous inoculation had the largest impact on plant biomass (155.1% increase) and nitrogen uptake (226.0% increase). Protists altered the community composition of bacteria. In particular, Sphingobacteriia, Azospirillum, Rodospirillales, Massilia, Caloramator, and Agrobacterium benefited from the presence of protists, while Sphingomonadales, Ralstonia, Burkholderia, and Rhodoferax decreased in the protist-inoculated microcosms. Impact of different protist isolates was differed on early plant growth, nitrogen uptake and bacterial community structure. Our results suggest that protists could be as beneficial as PGPR for early growth in rice plants, and simultaneous inoculation with protist and PGPR may be a key solution for chemical-free sustainable agriculture.