ObjectiveMicrobial arsenic(As) methylation is a vital As resistance mechanism that alters the toxicity and mobility of As, and can be used for the control of As contamination in soils. As-resistant plant growth-promoting bacteria use this mechanism to positively affect the growth of rice in As-contaminated soil. However, the arsenic methylation efficiency of rice rhizosphere bacteria and its effect on rice growth under arsenic stress remain unclear.MethodIn this study, an As-methylating functional Bacillus sp. LH14 was isolated from rice rhizosphere soil contaminated with As. The As methylation efficiency, As resistance and plant growth-promoting traits of the strain were analyzed. Additionally, the effect of strain inoculation on rice growth, rhizosphere As species and microbial interactions in As-contaminated soil were explored.ResultThe results show that strain LH14 promoted As methylation and volatilization and produced dimethylarsenate and trimethylarsenic oxide, which accounted for 54.9% of the initial As(Ⅲ) in the medium. LH14 inoculation significantly increased the copy numbers of arsenic methyltransferase gene(arsM) and methylated As in rhizosphere soil, indicating that LH14 was involved in soil As transformation. LH14 produced IAA under As stress and significantly increased seed germination rate, root and shoot length and biomass in the presence of high As concentration. Also rice growth in the soil was significantly promoted by LH14 inoculation, and this was associated with the enrichment of beneficial microorganisms(e.g., Burkholderiaceae and Gemmatimonadaceae) in the rhizosphere.ConclusionInoculation with As-methylating plant growth-promoting bacteria altered As speciation in rice rhizosphere and directly or indirectly promoted rice growth by producing plant hormones, enriching beneficial bacteria, and alleviating As stress. These findings provide theoretical support for the application of As-methylating bacteria in the remediation of As-contaminated soil and alleviation of As stress in plants.