Arabidopsis NAC016 promotes chlorophyll breakdown by directly upregulating STAYGREEN1 transcription

Abstract

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. and chromatin immunoprecipitation assays, we found that NAC016 directly binds to the SGR1 21 promoter, which contains the NAC016-specific binding motif (termed the NAC016BM). 22Furthermore, nac016-1 SGR1-OX plants showed an early leaf yellowing phenotype, similar to 23 SGR1-OX plants, confirming that NAC016 directly activates SGR1 expression in the leaf 24 senescence regulatory cascade. Although we found that NAC016 activates SGR1 expression in 25 senescing leaves, this transcriptional regulation is considerably weaker in maturing seeds; the 26 seeds of sgr1-1 mutants (also known as nonyellowing1-1, nye1-1) stayed green, while the seeds of 27 nac016-1 mutants turned from green to yellow normally. We also found that the abscisic acid (ABA) 28 signaling-related transcription factor genes ABI5 and EEL and the ABA biosynthesis gene AAO3, 29 which activate SGR1 expression directly or indirectly, were significantly downregulated in nac016-1 30 mutants and upregulated in NAC016-OX plants. Howeve...

ABF2, ABF3, and ABF4 Promote ABA-Mediated Chlorophyll Degradation and Leaf Senescence by Transcriptional Activation of Chlorophyll Catabolic Genes and Senescence-Associated Genes in Arabidopsis

“…These findings suggest that NYE1 can be regulated by a distinct type of TFs involved in different but related signaling pathways to ensure a temporally and spatially coordinated regulation of Chl degradation. Recent studies demonstrated that seven SNAC-A genes (ANAC055/ 019/072/002/081/102/032) mediate ABA-triggered leaf senescence via a pathway somehow different from the one regulated by the ABFs, although both of the regulatory pathways require SnRK2 kinases (Takasaki et al, 2015), NAC016 can directly activate NYE1 transcription (Sakuraba et al, 2015), and ANAC046 is a positive regulator of leaf senescence by controlling the expression of CCGs (Oda-Yamamizo et al, 2016). In our previous studies, ANAC019/ 055/072, downstream of MYC2/3/4, could directly promote the expression of CCGs in JA-triggered Chl degradation (Zhu et al, 2015), and NAC092, acting as a downstream target of EIN3, could also activate the expression of CCGs in ethylenetriggered Chl degradation (Qiu et al, 2015).…”

Functional and RNA-Sequencing Analysis Revealed Expression of a Novel Stay-Green Gene from Zoysia japonica (ZjSGR) Caused Chlorophyll Degradation and Accelerated Senescence in Arabidopsis

“…The GO analysis results showed that ZjSGR affected the transcripts of a large portion of genes involved in biotic and abiotic responses in a direct or indirect way, supporting the view that plant defense and senescence extensively co-regulate many genes in these responses (Schenk et al, 2005). The genetic and biochemical regulatory mechanisms of SGR in chlorophyll degradation are largely elusive to date (Zhou et al, 2011; Luo et al, 2013; Sakuraba et al, 2015). In this study, the RNA-seq data revealed that only 3.70% of the DEGs are chloroplast-related genes.…”

“…NAC TFs have been reported to function efficiently in chlorophyll degradation and leaf senescence, and actively participant in ABA, MeJA, and ethylene signaling which could act back on leaf senescence (Bu et al, 2008; Yang et al, 2011; Kim et al, 2014; Sakuraba et al, 2015; Takasaki et al, 2015; Li et al, 2016). Take the significantly up-regulated AtNAC92 as an example in our study, it was reported to work together with other NAC domain proteins to regulate the expression of many genes in senescence (Balazadeh et al, 2010; Breeze and Buchanan-Wollaston, 2011).…”

Pheophorbide a, a chlorophyll catabolite may regulate jasmonate signalling during dark-induced senescence in Arabidopsis

“…In a very similar manner, NAC019 and MYC2 interact with each other to synergistically up-regulate NYE1 (Zhu et al, 2015). Other transcription factors involved in ET signalling (EIN3, EEL, ORE1 and ERF17), ABA signalling (NAC016, NAC046, NAP, ABF2/3/4, ABI5) were also reported as direct interacting transcription factors of some cis -elements in CCG promoters (Kim et al, 2014; Sakuraba et al, 2014; Qiu et al, 2015; Sakuraba et al, 2016; Yin et al, 2016). These multiple intertwined hormonal cues eventually lead to chlorosis, by way of degradation of chl, as a visible landmark of dark-induced, aged-induced and also (a)biotic stress-induced senescence.…”