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3. Transcriptional regulatory network of leaf yellowing by NAC transcription factors

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 NAM/ATAF1/2/CUC2 (NAC) proteins are plant specific transcription factors (TFs), and some of NAC TFs have important role in the regulation of leaf yellowing.

 We revealed that NAC016, the previously uncharacterized senescence-associated NAM/ATAF1/2/CUC2 (senNAC) TF in Arabidopsis thaliana, promotes senescence. Leaves of nac016 mutants remained green under senescence-inducing conditions, and leaves of NAC016-overexpressing (NAC016-OX) plants senesced early. nac016 mutants had low ion leakage, and retained the proper balance of photosystem proteins and normal grana thylakoid shape much longer than wild-type plants, suggesting that NAC016 acts as a functional stay-green type senescence mutant (Kim et al. 2013).

 Using genome-wide gene expression microarray analysis and MEME motif searches, we identified the NAC016-specific binding motif (NAC16BM), GATTGGAT[AT]CA, in the promoters of genes downregulated in nac016-1 mutants. The NAC16BM sequence does not contain the core NAC binding motif CACG (or its reverse complement CGTG). NAC016 directly interacts with the NAC16BM in the promoters of its target genes, including NAP encoding other leaf senescence promoting NAC TF (Sakuraba et al. 2015), and SGR1 encoding chlorophyll catabolic enzyme (Sakuraba et al. 2016). Based on these results, NAC016 has key role in the promotion of leaf yellowing in Arabidopsis. 

We also have characterized the senescence associated NAC TFs, ONAC106 (Sakuraba et al. 2015) and ONAC054 (Sakuraba et al. 2020).

<Related publications>

Kim YS, Sakuraba Y, Han SH, Yoo SC, Paek NC. (2013) Mutation of the Arabidopsis NAC016 transcription factor delays leaf senescence. Plant & Cell Physiology 54:1660-1672. 

Sakuraba Y, Kim YS, Han SH, Lee BD, Paek NC. (2015) The Arabidopsis Transcription Factor NAC016 Promotes Drought Stress Responses by Repressin AREB1 Transcription through a Trifucate Feed-Forward Regulatory Loop Involving NAP. The Plant Cell 27:1771-1787.

Sakuraba Y, Piao W, Lim JH, Han SH, Kim YS, An G, Paek NC. (2015) Rice ONAC106 inhibits Leaf Senescence and Increases Salt Tolerance and Tiller Angle. Plant & Cell Physiology 56:2325-2339.

Sakuraba Y, Han SH, Lee SH, Hortensteiner S, Paek NC. (2016) Arabidopsis NAC016 promotes chlorophyll breakdown by directly upregulating STAYGREEN1 transcription. Plant Cell Reports, 35:155-166. 

Sakuraba Y, Kim D, Han SH, Kim SH, Piao W, Yanagisawa S, An G, and Paek NC. (2020) Multilayered regulation of membrane-bound ONAC054 is essential for abscisic acid-induced leaf senescence in rice. The Plant Cell 32: 630-649.

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