Exogenous thiocyanate inhibits sulfurtransferase pathway and induces β-cyanoalanine synthase pathway to enhance exogenous cyanide assimilation in rice plants

Cyanide (CN−) assimilation in plants takes place by β-cyanoalanine synthase (β-CAS) and sulfurtransferase (ST), in which the ST pathway converts CN− into thiocyanate (SCN−). Both chemicals (CN− and SCN−) are frequently detected in the effluent of gold mining operations. In this connection, exogenous SCN− was applied to rice plants with CN− and compared with CN− alone to investigate its effects on CN− assimilation and degradation pathways. Interestingly, the CN− and SCN− content in both roots and shoots were increased with the increase in “CN−” treatments, but surprisingly their content under “SCN−+CN−” treatments did not show the similar trend. The increasing trend remained the same for CN− but the SCN− content was constant with increasing CN− concentrations in comparison with the control (SCN− alone). Additionally, the assimilation rates of CN− in rice plants under “SCN−+CN−” treatments were significantly higher than “CN−” treatments. The application of SCN− with CN− mostly alters the expression of both β-CAS and ST-associated genes. On one side, the application of SCN− significantly repressed the expression of genes encoded with ST in rice plants, but on the other side, it significantly up-regulated the expression of the β-CAS gene located in mitochondria. These results reveal that the application of exogenous SCN− increases CN− assimilation rates by inhibiting the ST pathway and stimulating the β-CAS pathway. This study would provide new insight into the positive effects of exogenous SCN− in increasing CN− assimilation by altering the degradation pathways in rice plants.