Silicon Decreases Dimethylarsinic Acid Concentration in Rice Grain and Mitigates Straighthead Disorder

While root Si transporters play a role in the uptake of arsenite and organic As species dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) in rice (Oryza sativa L.), the impact of Si addition on the accumulation of DMA and MMA in reproductive tissues has not been directly evaluated, particularly in isolation from inorganic As species. Furthermore, DMA and MMA are suspected causal agents of straighthead disorder. We performed a hydroponic study to disentangle the impact of Si on accumulation of DMA and MMA in rice grain. At 5 μM, MMA was toxic to rice, regardless of Si addition, although Si significantly decreased root MMA concentrations. Plants dosed with 5 μM DMA grew well vegetatively but exhibited straighthead disorder at the lowest Si dose, and this DMA-induced yield loss reversed with increasing solution Si. Increasing Si also significantly decreased DMA concentrations in roots, straw, husk, and grain, particularly in mature plants. Si restricted grain DMA through competition for root uptake and downregulation of root Si transporters particularly at later stages of growth when Si uptake was greatest. Our finding that DMA causes straighthead disorder under low Si availability but not under high Si availability suggests Si as a straighthead management strategy.