The leaf senescence-promoting transcription factor AtNAP activates its direct target gene CYTOKININ OXIDASE 3 to facilitate senescence processes by degrading cytokinins

Cytokinins(CKs)are a class of adenine-derived plant hormones that plays pervasive roles in plant growth and development including cell division,morphogenesis,lateral bud outgrowth,leaf expansion and senescence.CKs as a“fountain of youth”prolongs leaf longevity by inhibiting leaf senescence,and therefore must be catabolized for senescence to occur.AtNAP,a senescence-specific transcription factor has a key role in promoting leaf senescence.The role of AtNAP in regulating CK catabolism is unknown.Here we report the identification and characterization of AtNAP-AtCKX3(cytokinin oxidase 3)module by which CKs are catabolized during leaf senescence in Arabidopsis.Like AtNAP,AtCKX3 is highly upregulated during leaf senescence.When AtNAP is chemically induced AtCKX3 is co-induced;and when AtNAP is knocked out,the expression of AtCKX3 is abolished.AtNAP physically binds to the cis element of the AtCKX3 promoter to direct its expression as revealed by yeast one-hybrid assays and in planta experiments.Leaves of the atckx3 knockout lines have higher CK concentrations and a delayed senescence phenotype compared with those of WT.In contrast,leaves with inducible expression of AtCKX3 have lower CK concentrations and exhibit a precocious senescence phenotype compared with WT.This research reveals that AtNAP transcription factor˗AtCKX3 module regulates leaf senescence by connecting two antagonist plant hormones abscisic acid and CKs.

A positive feedback regulatory loop,SAAtNAP-SAG202/SARD1-ICS1-SA,in SA biosynthesis involved in leaf senescence but not defense response

Salicylic acid(SA)is an important plant hormone that regulates defense responses and leaf senescence.It is imperative to understand upstream factors that regulate genes of SA biosynthesis.SAG202/SARD1 is a key regulator for isochorismate synthase 1(ICS1)induction and SA biosynthesis in defense responses.The regulatory mechanism of SA biosynthesis during leaf senescence is not well understood.Here we show that AtNAP,a senescence-specific NAC family transcription factor,directly regulates a senescence-associated gene named SAG202 as revealed in yeast one-hybrid and in planta assays.Inducible overexpreesion of AtNAP and SAG202 lead to high levels of SA and precocious senescence in leaves.Individual knockout mutants of sag202 and ics1 have markedly reduced SA levels and display a significantly delayed leaf senescence phenotype.Furthermore,SA positively feedback regulates AtNAP and SAG202.Our research has uncovered a unique positive feedback regulatory loop,SA-AtNAP-SAG202-ICS1-SA,that operates to control SA biosynthesis associated with leaf senescence but not defense response.