Identification and characterization of a novel chloroplast/mitochondria co-localized glutathione reductase 3 involved in salt stress response in rice

Glutathione reductases (GRs) are important components of the antioxidant machinery that plants use to respond against abiotic stresses. In rice, one cytosolic and two chloroplastic GR isoforms have been identified. In this work, we describe the cloning and characterization of the full-length cDNA encoding OsGR3, a chloroplast-localized GR that up to now was considered as a non-functional enzyme because of assumed lack of N-terminal conserved domains. The expression of OsGR3 in E. coli validated that it can be translated as a protein with GR activity. OsGR3 shows 76 and 53 % identity with OsGR1 (chloroplastic) and OsGR2 (cytosolic), respectively. Phylogenetic analysis revealed 2 chloroplastic GRs in Poaceae species, including rice, sorghum and brachypodium, but only one chloroplastic GR in dicots. A plastid transit peptide is located at the N terminus of OsGR3, and genetic transformation of rice with a GR3-GFP fusion construct further confirmed its localization in chloroplasts. Furthermore, OsGR1 and OsGR3 are also targeted to mitochondria, which suggest a combined antioxidant mechanism in both chloroplasts and mitochondria. However, both isoforms showed a distinct response to salinity: the expression of OsGR3 but not OsGR1 was induced by salt stress. In addition, the transcript level of OsGR3 was greatly increased with salicylic acid treatment but was not significantly affected by methyl jasmonate, dehydration or heat shock stress. Our results provide new clues about the possible roles of functional OsGR3 in salt stress and biotic stress tolerance.