The reduction of Cr(VI) in Salvinia minima, possible involvement of an h-type thioredoxin

Hexavalent chromium [Cr(VI)] is extremely toxic to plant cells and has been recognized to possess a high redox potential. Tolerant plant species have shown the ability to reduce Cr(VI), but the operating mechanism involved in this process is not elucidated. Thus, the aim of this study was to investigate the possible involvement of thiolic and phenolic compounds and thioredoxin expression during Cr(VI) reduction in S. minima. In addition, a probable enzymatic reduction of Cr(VI) was investigated. Plants were exposed to 20 mg L−1 Cr(VI) concentration during 7 days under controlled conditions. The amount of metal accumulated in lacinias (root-like submerged leaves) and fronds (floating leaves) indicated that a low percentage of absorbed Cr(VI) was mobilized from lacinias to fronds. X-ray absorption near-edge structure (XANES) analysis revealed that Cr(III) was the only chromium species occurring in S. minima plants. Thiols and phenolics of lacinias and fronds were increased significantly by Cr(VI) treatment, but accumulation patterns were different. The expression of an h-type thioredoxin (Trx h) was demonstrated for the first time in Cr-exposed lacinias. Enzymatic reduction showed a low contribution to the Cr(VI) reduction. Data of this study provide evidences on the involvement of thiols, thioredoxin, and phenolics in the reduction of Cr(VI) to Cr(III) in S. minima tissues.