Multilayered Regulation of Membrane-Bound ONAC054 Is Essential for Abscisic Acid-Induced Leaf Senescence in Rice

In most plants, abscisic acid (ABA) induces premature leaf senescence; however, the mechanisms of ABA signaling during leaf senescence remain largely unknown. Here, we show that the rice (Oryza sativa) NAM/ATAF1/2/CUC2 (NAC) transcription factor ONAC054 plays an important role in ABA-induced leaf senescence. The onac054 knockout mutants maintained green leaves, while ONAC054-overexpressing lines showed early leaf yellowing under dark- and ABA-induced senescence conditions. Genome-wide microarray analysis showed that ABA signaling-associated genes, including ABA INSENSITIVE5 (OsABI5) and senescence-associated genes, including STAY-GREEN and NON-YELLOW COLORING1 (NYC1), were significantly down-regulated in onac054 mutants. Chromatin immunoprecipitation and protoplast transient assays showed that ONAC054 directly activates OsABI5 and NYC1 by binding to the mitochondrial dysfunction motif in their promoters. ONAC054 activity is regulated by proteolytic processing of the C-terminal transmembrane domain (TMD). We found that nuclear import of ONAC054 requires cleavage of the putative C-terminal TMD. Furthermore, the ONAC054 transcript (termed ONAC054α) has an alternatively spliced form (ONAC054β), with seven nucleotides inserted between intron 5 and exon 6, truncating ONAC054α protein at a premature stop codon. ONAC054β lacks the TMD and thus localizes to the nucleus. These findings demonstrate that the activity of ONAC054, which is important for ABA-induced leaf senescence in rice, is precisely controlled by multilayered regulatory processes.