Arabidopsis NAC016 promotes chlorophyll breakdown by directly upregulating STAYGREEN1 transcription

The Arabidopsis transcriptional factor NAC016 directly activates chlorophyll degradation during leaf senescence by binding to the promoter of SGR1 and upregulating its transcription. During leaf senescence or abiotic stress in Arabidopsis thaliana, STAYGREEN1 (SGR1) promotes chlorophyll (Chl) degradation, acting with Chl catabolic enzymes, but the mechanism regulating SGR1 transcription remains largely unknown. Here, we show that the Arabidopsis senescence-associated NAC transcription factor NAC016 directly activates SGR1 transcription. Under senescence-promoting conditions, the expression of SGR1 was downregulated in nac016-1 mutants and upregulated in NAC016-overexpressing (NAC016-OX) plants. By yeast one-hybrid and chromatin immunoprecipitation assays, we found that NAC016 directly binds to the SGR1 promoter, which contains the NAC016-specific binding motif (termed the NAC016BM). Furthermore, nac016-1 SGR1-OX plants showed an early leaf yellowing phenotype, similar to SGR1-OX plants, confirming that NAC016 directly activates SGR1 expression in the leaf senescence regulatory cascade. Although we found that NAC016 activates SGR1 expression in senescing leaves, this transcriptional regulation is considerably weaker in maturing seeds; the seeds of sgr1-1 mutants (also known as nonyellowing1-1, nye1-1) stayed green, while the seeds of nac016-1 mutants turned from green to yellow normally. We also found that the abscisic acid (ABA) signaling-related transcription factor genes ABI5 and EEL and the ABA biosynthesis gene AAO3, which activate SGR1 expression directly or indirectly, were significantly downregulated in nac016-1 mutants and upregulated in NAC016-OX plants. However, the NAC016BM does not exist in their promoter regions, indicating that NAC016 may indirectly activate these ABA signaling and biosynthesis genes, probably by directly activating transcriptional cascades regulated by the NAC transcription factor NAP. The NAC016-mediated regulatory cascades of SGR1 and other Chl degradation-related genes are discussed.