Abstract

Kelch-like ECH-associated protein 1 (KEAP1)–nuclear factor E2-related factor 2 (NRF2) is the key antioxidant system in animals. In a previous study, we identified a probable KEAP1 ortholog in rice, OsKEAP1, and demonstrated that the downregulation ofOsKEAP1could alter the redox system and impair plant growth, as well as increase the susceptibility to abscisic acid (ABA) in seed germination. However, no NRF2 orthologs have been identified in plants and the mechanism underlying the phenotype changes of downregulatedoskeap1mutants is yet unknown. An in silico search showed that OsABI5 is the gene that encodes a protein with the highest amino acid identity score (38.78%) to NRF2 in rice. In this study, we demonstrated that, via yeast two-hybrids analysis and bimolecular fluorescence complementation assays, OsKEAP1 interacted with OsABI5 via its Kelch repeat domain in the nucleus. In germinating seeds, the expression ofOsKEAP1was significantly downregulated inoskeap1-1(39.5% that of the wild-type (WT)) andoskeap1-2(64.5% that of WT), while the expression ofOsABI5was significantly increased only inoskeap1-1(247.4% that of WT) but not inoskeap1-2(104.8% that of WT). ABA (0.5 μM) treatment significantly increased the expression ofOsKEAP1andOsABI5in both theoskeap1mutants and WT, and 4 days post treatment, the transcription level ofOsABI5became significantly greater inoskeap1-1(+87.2%) andoskeap1-2(+55.0%) than that in the WT. The ABA-responsive genes (OsRab16Aand threelate embryogenesis abundantgenes), which are known to be activated by OsABI5, became more responsive to ABA in bothoskeap1mutants than in the WT. The transcript abundances of genes that regulate OsABI5, e.g.,OsSnRK2(encodes a kinase that activates OsABI5),OsABI1, andOsABI2(both encode proteins binding to OsSnRK2 and are involved in ABA signaling) were not significantly different between the twooskeap1mutants and the WT. These results demonstrated that OsKEAP1 played a role in the ABA response in rice seed ger