Recent advances in arsenic accumulation and metabolism in rice

Arsenic is commonly present in subsoil and is a carcinogen in humans. Rice takes up arsenic and it accumulates in different plant parts, including grains, at levels several-fold higher than the soil. In high arsenic regions, rice can contribute substantially to arsenic intake by the human population. Arsenic in rice grains is present in the carcinogenic inorganic or the relatively safer organic (methylated) form. A wide variation is noticed in different rice genotypes with respect to the proportion of arsenic in these forms in grains. Mechanisms involved in arsenic uptake, efflux from roots, loading into xylem, transport, partitioning, arsenate reduction, arsenic sequestration in vacuoles, volatilization from leaves, accumulation in grains etc. are poorly understood. Selection of cultivars accumulating low inorganic arsenic is an important trait to be used by breeders to develop rice varieties safer for cultivation in arsenic-contaminated regions. Systematic efforts have not been made to screen rice genotypes for mining the genes involved in arsenic uptake, transport and accumulation in grains. Identification of rice germplasm with varying arsenic uptake and partitioning, and development of mapping populations with contrasting grain arsenic, are required for association studies and QTL mapping for accelerating rice improvement. Efforts on gene expression profiling, deep transcriptome sequencing, high throughput metabolomics and phenotyping of contrasting arsenic accumulating lines need to be increased to develop strategies for design of safer rice varieties. Network research projects need to be developed along these approaches to accelerate the development of crop varieties safer for farming in arsenic-contaminated environments.