A methionine-R-sulfoxide reductase, OsMSRB5, is required for rice defense against copper toxicity

Methionine sulfoxide reductases (MSRs), a family of enzymes catalyzing the conversion of methionine from its oxidized form into its reduced form, has a pivotal role in responding to oxidative stress. In the present study, we found that OsMSRB5 plays a crucial role in rice adaption to Cu stress. OsMSRB5 was mainly expressed in leaves, with low transcriptional levels of OsMSRB5 observed in seeds, stems, and roots; this gene was also induced by the excess copper (Cu) treatment. The functions of OsMSRB5 were analyzed through ectopic expression in Escherichia coli and functional disruption in rice. An in vitro enzymatic activity assay showed that OsMSRB5 had the ability to reduce free methionine-R-sulfoxide (Met-R-SO) and protein-bound-like Met-SO (dabsyl-Met-SO) to Met and dabsyl-Met, respectively. Ectopic expression of OsMSRB5 conferred E. coli cells higher tolerance to excess Cu and methyl viologen (MV). Furthermore, OsMSRB5 mutation decreased Cu and MV tolerance in rice. Our results demonstrate that rice OsMSRB5 is a functional methionine sulfoxide reductase and is involved in defense against Cu toxicity. In addition, rice seedlings responding to MV-induced oxidative stress showed a similar phenotype with excess Cu.