Thioredoxin h isoforms from rice are differentially reduced by NADPH/thioredoxin or GSH/glutaredoxin systems.

Rice (Oryza sativa L.) has multiple potential genes encoding thioredoxin (Trx) h and NADP-thioredoxin reductase (NTR). These NTR and Trx h isoforms, known as cytoplasmic NTR/Trx system along with multiple members of glutaredoxin (Grx) family constitute a complex redox control system in rice. In the present study, we investigated the kinetic parameters of two rice NTRs, OsNTRA and OsNTRB, toward three endogenous Trx h isoforms, OsTrx1, OsTrx20, and OsTrx23. The results showed that in contrast with OsTrx1 and OsTrx23, the isoform OsTrx20 was not reduced by OsNTR isoforms. The kcat/Km values of OsNTRB and OsNTRA toward OsTrx1 was six- and 13-fold higher than those values toward OsTrx23, respectively, suggesting that OsNTR isoforms do not reduce different OsTrx h isoforms, equivalently. Furthermore, the possible reduction of OsTrx isoforms by the glutathione (GSH)/Grx system was investigated through the heterologous expression of a gene encoding OsGrx9, a bicysteinic CPYC Grx found in rice. Whereas OsTrx23 was not reduced by GSH, OsTrx20 and with less efficiently OsTrx1 were reduced by GSH or GSH/Grx. Therefore, it seems that OsTrx1 can be reduced either by OsNTR or GSH/Grx. These data for the first time provides an evidence for cross-talking between NTR/Trx and GSH/Grx systems in rice.