Catalyzes the reduction of methionine sulfoxide (MetSO) to methionine in proteins. Specifically reduces the MetSO R-enantiomer. Plays a protective role against oxidative stress by restoring activity to proteins that have been inactivated by methionine oxidation. May play an essential function in association with MSRB2 in maintaining vegetative growth during environmental constraints, through the preservation of photosynthetic antennae. MSRB1 and MSRB2 account for most of the leaf peptide MSR capacity. [thioredoxin]-disulfide + H2O + L-methionyl-[protein] = [thioredoxin]-dithiol + L-methionyl-(R)-S-oxide-[protein] Binds 1 zinc ion per subunit. kcat is 0.075 sec(-1) with dabsyl-MetSO as substrate. Expressed at low levels in stems, leaves, floral buds, flowers and siliques (at protein level). By photooxidative stress. Lacks the conserved cysteine (here Thr-132) required for the reduction by thioredoxins (TRX) through a dithiol-disulfide exchange involving both redox-active Cys of TRX and MSRB. Reduced by thioredoxin CDSP32 which regenerates MSRB1 through the direct reduction of the sulfenic acid formed on the catalytic Cys, without the help of any other thiol compound. Also reduced by the glutahione/glutaredoxin (GSH/GRX) system via a glutathionylation step of the sulfenic acid and then by GRX reduction of the GSH-MSR adduct. Belongs to the MsrB Met sulfoxide reductase family. Truncated N-terminus. Truncated N-terminus.