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.