Defects in ethanol fermentation catalysed by pyruvate decarboxylases reduce arsenic levels in rice.

Reducing arsenic levels in paddy rice is an important agricultural issue. Rice alcohol dehydrogenase 2 (ADH2) deficiency results in reduced arsenic uptake from the soil. ADHs are presumed to be anaerobic metabolism enzymes. However, whether the reduced arsenic uptake is due to changes in anaerobic metabolism remains unclear. Here, we demonstrate the causal relationships between them through analyses of anaerobic metabolism-related mutants. As representative anaerobic metabolism enzymes, two lactate dehydrogenases (LDHs), five pyruvate decarboxylases (PDCs) and three ADHs are found in rice. To eliminate functional redundancy, double LDH mutants, quintuple PDC mutants and triple ADH mutants were generated via genome editing. Arsenic levels in grains were not reduced in the double LDH mutants, suggesting that defects in the lactate fermentation pathway are not responsible for the reduced arsenic uptake. On the other hand, in the quintuple PDC mutant, the arsenic levels in the grains were reduced to the same degree as those in the triple ADH mutant. The PDC and ADH mutants presented reduced silica contents, suggesting reduced uptake of arsenic via silicate transporters. These results indicate that a defect in the ethanol fermentation pathway of anaerobic metabolism triggers reduced arsenic uptake.