MxMPK6‐2‐mediated phosphorylation enhances the response of apple rootstocks to Fe deficiency by activating PM H⁺‐ATPase MxHA2

Abstract

SUMMARYIron (Fe) deficiency significantly affects the growth and development, fruit yield and quality of apples. Apple roots respond to Fe deficiency stress by promoting H+ secretion, which acidifies the soil. In this study, the plasma membrane (PM) H+‐ATPase MxHA2 promoted H+ secretion and root acidification of apple rootstocks under Fe deficiency stress. H+‐ATPase MxHA2 is upregulated in Fe‐efficient apple rootstock of Malus xiaojinensis at the transcription level. Fe deficiency also induced kinase MxMPK6‐2, a positive regulator in Fe absorption that can interact with MxHA2. However, the mechanism involving these two factors under Fe deficiency stress is unclear. MxMPK6‐2 overexpression in apple roots positively regulated PM H+‐ATPase activity, thus enhancing root acidification under Fe deficiency stress. Moreover, co‐expression of MxMPK6‐2 and MxHA2 in apple rootstocks further enhanced PM H+‐ATPase activity under Fe deficiency. MxMPK6‐2 phosphorylated MxHA2 at the Ser909 site of C terminus, Thr320 and Thr412 sites of the Central loop region. Phosphorylation at the Ser909 and Thr320 promoted PM H+‐ATPase activity, while phosphorylation at Thr412 inhibited PM H+‐ATPase activity. MxMPK6‐2 also phosphorylated the Fe deficiency‐induced transcription factor MxbHLH104 at the Ser169 site, which then could bind to the promoter of MxHA2, thus enhancing MxHA2 upregulation. In conclusion, the MAP kinase MxMPK6‐2‐mediated phosphorylation directly and indirectly regulates PM H+‐ATPase MxHA2 activity at the protein post‐translation and transcription levels, thus synergistically enhancing root acidification under Fe deficiency stress.