Rhizobacteria Related to Arsenic Transformation In Paddy Soil – Relevance to Methylated Arsenic In Rice Grains –.

Rice (Oryza sativa) is the staple food for over half the world’s population and a major source of dietary intake of arsenic (As). Paddy rice can absorb the As easier than other crops and vegetables, because anaerobic soil conditions lead to increase of the As mobilization. Among As speciation, inorganic As such as arsenite and arsenate are recognized as analogues by silicate and phosphate transport pathways in rice plant, respectively. As is a variable element, there are many chemical forms in environment including living organisms. Mainly four As species, arsenite, arsenate, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), are found in rice plant. The As toxicity is different in each chemical form, so such an As speciation makes it difficult to assess food standards and guidelines for food safety and human health. Many things have been remained unclear about mechanisms involved in the production of methylated arsenicals. We investigated bacterial effects related to the production of DMAA accumulated in rice plant. In this study, we separated bacteria involved in arsenic methylation from the rice rhizosphere. Although there are some reports showing bacterial arsenic methylation, we think this is the first example in rice and paddy soil environment. A bacterium separated by us was named as strain GSRB54, and the sequencing analysis of 16S rRNA indicated that this strain belongs to Streptomyces spp. Further genetic analysis revealed that strain GSRB54 carries an arsenic methyltransferase, arsM. Here we are going to introduce this bacterium and discuss about the bacterial contribution to As speciation accumulated in the rice grain.