Deficiency of alcohol dehydrogenase 2 reduces arsenic in paddy-field rice grains by suppressing silicate transporters during reproductive stage

Paddy-field is anaerobic and facilitates elution of arsenite from the soil. Paddy-field rice accumulates arsenic in the grains because silicate transporters actively uptakes the arsenite during reproductive stage. Reduction of arsenic level in rice grains is an important challenge for agriculture. Using a forward genetic approach, we isolated a rice mutant las3 in which arsenic levels were lower in aerial tissues containing grains. The lower arsenic phenotype was not observed in young plants before heading (emergence of panicle). Genetic analyses revealed that deficiency of alcohol dehydrogenase (ADH) 2 by mutation is responsible for the phenotype. In rice ADH paralogs, ADH2 was most efficiently produced in root tissue under the anaerobic condition. In wildtype, silicon and arsenic contents in aerial tissues increased during heading stage. However, the increase was suppressed in las3. Consistent with this, gene expression of two silicate transporters, Lsi1 and Lsi2, in wildtype was increased around the time of heading, whereas the increase was suppressed in las3. These results indicate that the low arsenic phenotype in las3 is due to the suppression of silicate transporters. We found that gene expression levels of the silicate transporters are reduced by Erythrosine B treatment that induces cytosolic pH decrease. This response suggests that the suppression of the silicate transporters in las3 was due to cytosolic pH decrease, which is known to be facilitated by deficiency of ADH activity under the anaerobic condition. This study would provide a new insight for reducing arsenic level in rice grains.