Differential responses of thiol metabolism and genes involved in arsenic detoxification in tolerant and sensitive genotypes of bioenergy crop Ricinus communis.

Castor, a non-food, dedicated bioenergy crop, has immense potential to be used for phytoremediation/revegetation of heavy metal contaminated sites. In the previous study, we identified arsenate [As(V)]-tolerant (WM) and As(V)-sensitive (GCH 2) genotypes of castor (Ricinus communis L.) with differential accumulation and tolerance of arsenic [As]. The role of thiols in As(V) toxicity and tolerance mechanism in the castor plant is not fully understood. On the one hand, thiol-dependent reduction of As(V) to As(III) by arsenate reductase (AR) makes it capable of reacting with thiol groups of protein leading to disturbed metabolic pathways; on the other hand, reduction of As(V) to arsenite [As(III)] by AR and then complexation of As(III) with phytochelatins (PCs) and compartmentalization of As(III)-PC complex are considered as the major detoxification mechanisms of As(V). In our study, the expression of RcAR increased in leaves and roots of As(V)-tolerant castor genotype WM but decreased in sensitive genotype GCH 2 due to 200 µM As(V) treatment. The activity of glutathione reductase (GR) increased significantly in the tolerant genotype, whereas it remained same in the sensitive genotype. GSH/GSSH ratio declined substantially in the sensitive genotype. The increased expression of phytochelatin synthase 1 isoform 1 (RcPCS1X1) in roots, RcPCS1X2 and metallothionein type 2 (RcMT2) in leaves, and c-type ABC transporter (RcABCC) in roots and leaves of WM was observed, but the expression of these genes declined or remained the same in GCH 2. Overall, our results suggest the essential roles of GR, RcAR, RcPCS1, RcMT2, and RcABCC in the tolerance of WM castor genotype to As(V) toxicity.

Thiamin confers enhanced tolerance to oxidative stress in Arabidopsis.

Thiamin and thiamin pyrophosphate (TPP) are well known for their important roles in human nutrition and enzyme catalysis. In this work, we present new evidence for an additional role of these compounds in the protection of cells against oxidative damage. Arabidopsis (Arabidopsis thaliana) plants subjected to abiotic stress conditions, such as high light, cold, osmotic, salinity, and oxidative treatments, accumulated thiamin and TPP. Moreover, the accumulation of these compounds in plants subjected to oxidative stress was accompanied by enhanced expression of transcripts encoding thiamin biosynthetic enzymes. When supplemented with exogenous thiamin, wild-type plants displayed enhanced tolerance to oxidative stress induced by paraquat. Thiamin application was also found to protect the reactive oxygen species-sensitive ascorbate peroxidase1 mutant from oxidative stress. Thiamin-induced tolerance to oxidative stress was accompanied by decreased production of reactive oxygen species in plants, as evidenced from decreased protein carbonylation and hydrogen peroxide accumulation. Because thiamin could protect the salicylic acid induction-deficient1 mutant against oxidative stress, thiamin-induced oxidative protection is likely independent of salicylic acid signaling or accumulation. Taken together, our studies suggest that thiamin and TPP function as important stress-response molecules that alleviate oxidative stress during different abiotic stress conditions.