The NAC Transcription Factors OsNAC20 and OsNAC26 Regulate Starch and Storage Protein Synthesis

The NAC transcription factors OsNAC20 and OsNAC26 influence starch and storage protein accumulation by directly regulating the expression of starch and storage protein synthesis-related genes. Starch and storage proteins determine the weight and quality of cereal grains. Synthesis of these two grain components has been comprehensively investigated, but the transcription factors responsible for their regulation remain largely unknown. In this study, we investigated the roles of NAM, ATAF, and CUC (NAC) transcription factors, OsNAC20, and OsNAC26 in starch and storage protein synthesis in rice (Oryza sativa) endosperm. OsNAC20 and OsNAC26 showed high levels of amino acid sequence similarity. Both were localized in the aleurone layer, starchy endosperm, and embryo. Mutation of OsNAC20 or OsNAC26 alone had no effect on the grain, while the osnac20/26 double mutant had significantly decreased starch and storage protein content. OsNAC20 and OsNAC26 alone could directly transactivate the expression of starch synthaseI (SSI), pullulanase (Pul), glutelin A1 (GluA1), glutelin B4/5 (GluB4/5), alpha -globulin, and 16 kD prolamin and indirectly influenced plastidial disproportionating enzyme1 (DPE1) expression to regulate starch and storage protein synthesis. Although they could also bind to the promoters of ADP-Glc pyrophosphorylase small subunit 2b (AGPS2b), ADP-Glc pyrophosphorylase large subunit 2 (AGPL2), and starch branching enzymeI (SBEI), and the expression of the three genes was largely decreased in the osnac20/26 mutant, ADP-Glc pyrophosphorylase and starch branching enzyme activities were unchanged in this double mutant. In addition, OsNAC20 and OsNAC26 are main regulators of Pul, GluB4, alpha -globulin, and 16 kD prolamin. In conclusion, OsNAC20 and OsNAC26 play an essential and redundant role in the regulation of starch and storage protein synthesis.