Arsenic toxicity in rice (Oryza sativa L.) is influenced by sulfur supply: Impact on the expression of transporters and thiol metabolism

Abstract

Arsenic (As) is a non-essential element whose entry into rice grains is an issue of public concern. The need, therefore, exists to understand the regulatory mechanisms of As accumulation and distribution patterns in plants. This study analyzed the effect of sulfur (S) supply on As accumulation and distribution in rice (Oryza sativa L. var. IR64) plants. Nine day old seedlings were grown in 0.798mM S (Normal S), 0.2mM S (Low S) and 0.003mM S (Zero S) for 7days and then subjected to AsIII (20μM) exposure for 7–15days. The concentration of As varied significantly in different S treatments with a consistent trend of decrease in As concentration in depleted S supply treatments on both 7days and 15days. Arsenic concentration declined from 726μgg−1 DW to 537μgg−1 DW in roots and from 29μgg−1 DW to 8μgg−1 DW in shoots at 7days, and from 1536μgg−1 DW to 1062 in roots and from 58μgg−1 DW to 16μgg−1 DW in shoots at 15days, when S supply declined from normal to zero. The subcellular distribution of As was also found to vary with alteration in S supply and also differentially in shoot and root. In general, an up-regulation of sulfate transporters of groups 1 and 2 was observed while As transporters (Lsi1 and Lsi2) were down-regulated in response to As exposure to maintain sulfate and to regulate As levels. In spite of S depletion, an increase in cysteine, glutathione (GSH) and phytochelatin (PC) levels was observed upon As exposure. However, some negative impact of S depletion and As could be seen on the growth of plants. The study concluded that even in conditions of decreased S availability, plants continue to rely on thiol metabolism to tackle As levels and its toxicity effectively and altered subcellular distribution of As contributes only partially.