The NAC transcription factor MdNAC4 positively regulates nitrogen deficiency‑induced leaf senescence by enhancing ABA biosynthesis in apple

Although it is well established that nitrogen(N)deficiency induces leaf senescence,the molecular mechanism of N deficiency-induced leaf senescence remains largely unknown.Here,we show that an abscisic acid(ABA)-responsive NAC transcription factor(TF)is involved in N deficiency-induced leaf senescence.The overexpression of MdNAC4 led to increased ABA levels in apple calli by directly activating the transcription of the ABA biosynthesis gene MdNCED2.In addition,MdNAC4 overexpression promoted N deficiency-induced leaf senescence.Further investigation showed that MdNAC4 directly bound the promoter of the senescence-associated gene(SAG)MdSAG39 and upregulated its expression.Interestingly,the function of MdNAC4 in promoting N deficiency-induced leaf senescence was enhanced in the presence of ABA.Furthermore,we identified an interaction between the ABA receptor protein MdPYL4 and the MdNAC4 protein.Moreover,MdPYL4 showed a function similar to that of MdNAC4 in ABA-mediated N deficiencyinduced leaf senescence.These findings suggest that ABA plays a central role in N deficiency-induced leaf senescence and that MdPYL4 interacts with MdNAC4 to enhance the response of the latter to N deficiency,thus promoting N deficiency-induced leaf senescence.In conclusion,our results provide new insight into how MdNAC4 regulates N deficiency-induced leaf senescence.