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Degreening, caused by chlorophyll degradation, is the most obvious symptom of senescing leaves. Chlorophyll degradation can be triggered by endogenous and environmental cues, and ethylene is one of the major inducers. ETHYLENE INSENSITIVE3 (EIN3) is a key transcription factor in the ethylene signaling pathway. It was previously reported that EIN3,  miR164 , and a NAC (NAM, ATAF, and CUC) transcription factor ORE1/NAC2 constitute a regulatory network mediating leaf senescence. However, how this network regulates chlorophyll degradation at molecular level is not yet elucidated. Here we report a feed-forward regulation of chlorophyll degradation that involves  EIN3 ,  ORE1 , and chlorophyll catabolic genes ( CCGs ). Gene expression analysis showed that the induction of three major  CCGs ,  NYE1 ,  NYC1  and  PAO , by ethylene was largely repressed in  ein3 eil1  double mutant. Dual-luciferase assay revealed that EIN3 significantly enhanced the promoter activity of  NYE1 ,  NYC1  and  PAO  in  Arabidopsis  protoplasts. Furthermore, Electrophoretic mobility shift assay (EMSA) indicated that EIN3 could directly bind to  NYE1 ,  NYC1  and  PAO  promoters. These results reveal that EIN3 functions as a positive regulator of  CCG  expression during ethylene-mediated chlorophyll degradation. Interestingly, ORE1, a senescence regulator which is a downstream target of EIN3, could also activate the expression of  NYE1 ,  NYC1  and  PAO  by directly binding to their promoters in EMSA and chromatin immunoprecipitation (ChIP) assays. In addition, EIN3 and ORE1 promoted  NYE1  and  NYC1  transcriptions in an additive manner. These results suggest that ORE1 is also involved in the direct regulation of  CCG  transcription. Moreover, ORE1 activated the expression of  ACS2 , a major ethylene biosynthesis gene, and subsequently promoted ethylene production. Collectively, our work reveals that EIN3, ORE1 and CCGs constitute a coherent feed-forward loop involving in the robust regulation of ethylene-mediated chlorophyll degradation during leaf senescence in  Arabidopsis .

EIN3 and ORE1 Accelerate Degreening during Ethylene-Mediated Leaf Senescence by Directly Activating Chlorophyll Catabolic Genes in Arabidopsis