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

Cyanide (CN-) assimilation in plants takes place by ß-cyanoalanine synthase (ß-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 ß-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 ß-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 ß-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.