Rice rhizospheric microbes confer limited Arsenic protection under high Arsenic conditions.

Abstract

Rice (Oryza sativa) is a staple food crop worldwide and plays a critical role in ensuring food security as the global population continues to expand exponentially. Groundwater contamination with Arsenite [As(III)], a naturally occurring inorganic form of arsenic (As), leads to uptake and accumulation within rice plants. As a result, grain yield is lowered, the overall plant health is diminished, and there is a risk of arsenic toxicity from grain consumption. It was previously shown that a novel bacterial strain from the rice rhizosphere may reduce As accumulation in rice plants exposed to low levels of environmental As. We hypothesized that different rice varieties may exhibit varying responses to high As levels, resulting in differences in As uptake and toxicity. Utilizing the natural rice rhizospheric microbes, we initiated a set of hydroponic experiments with two rice varieties, Nipponbare (As tolerant) and IR66 (As susceptible). Rice varieties exposed to high As(III) concentration (50 M) showed changes in both aboveground and belowground traits. As-tolerant Nipponbare varieties show grain production at high As(III) concentrations compared to the As-susceptible IR66 variety. Supplementation of natural rice rhizospheric microbes as single inoculums showed varied responses in both As-tolerant and As-susceptible varieties. Three natural rice rhizospheric microbes Pantoea sps (EA106), Pseudomonas corrugata (EA104), and Arthrobacter oxydans (EA201) were selected based on previously reported high Iron (Fe)-siderophore activity and were used for the hydroponic experiments as well as a non-rice rhizospheric strain, Bacillus subtilis UD1022. Interestingly, treatment with two strains (EA104 and EA201) led to reduction in As(III) uptake in shoots, roots, and grains and the degree of reduction of As(III) was pronounced in As-susceptible IR66 varieties. Non-rice rhizospheric UD1022 showed subtle protection against high As toxicity. High As(III) treatment led to lack or delay of flowering and seed setting in the As-susceptible IR66 variety. The data presented here may further the understanding of how beneficial microbes in the rhizosphere may help rice plants cope with high concentrations of As in the soil or groundwater.