NAP-dependent crosstalk between ethylene biosynthesis and abscisic acid signaling pathway coordinately modulates leaf senescence in plants

Abscisic acid (ABA) and ethylene signaling activate the NAC transcription factor NAP, accelerating plant leaf senescence. NAP promotes the synthesis of ABA and ethylene in turn. However, the crosstalk between ABA and ethylene in regulating leaf senescence, and whether NAP plays roles in mediating the crosstalk between these two hormones, still remains to be further clarified. This study identified the tobacco NtNAP and verified its conserved roles in promoting leaf senescence. ChIP assay, transactivation analysis, yeast one-hybrid assay, and electrophoretic mobility shift assay demonstrated that NtNAP directly bound to the promoters and activated the expression of NtACS8a, NtACO4-6, NtPYL4b, and NtSnRK2.6b genes in vivo and in vitro, leading to the production of ethylene and enhanced ABA signaling in tobacco. Blocking ethylene synthesis by AVG treatment or knockout of ACS and ACO delayed ABA-induced leaf senescence in both tobacco and Arabidopsis, while loss of function of PYL4 and SnRK2.6 relieved leaf senescence induced by ACC treatment, which together validated the crosstalk between ABA and ethylene in regulating leaf senescence. Moreover, ABA promoted the production of ethylene in a NAP-dependent manner, and blocking NAP-induced ethylene production relieved leaf senescence caused by ABA treatment in both tobacco and Arabidopsis. Meanwhile, NAP also mediated the enhancement of ABA signaling induced by ACC treatment. Our results revealed a new mechanism by which NAP promotes leaf senescence through synergistic ABA and ethylene signaling.