The apple GARP family gene MdHHO3 regulates the nitrate response and leaf senescence

The regulation of plant gene expression by nitrate is a complex regulatory process. Here, we identified 90 GARP family genes in apples by genome-wide analysis. As a member of the GARP gene family, the expression of MdHHO3 (Malus domestica HYPERSENSITIVITY TO LOW PHOSPHATE-ELICITED PRIMARY ROOT SHORTENING1 HOMOLOG 3) is upregulated under N (nitrogen) supply. The results of DNA-binding site analysis and electrophoretic mobility shift assays (EMSA) showed that MdHHO3 binds to the motif-containing GAATC. Furthermore, MdHHO3 binds to its promoter sequence and inhibits its activity. In addition, the overexpression of MdHHO3 in apple calli resulted in less accumulation of nitrate in 35S:MdHHO3-GFP calli and downregulated the expression of the nitrate transport-related genes but upregulated the expression of the nitrate assimilation-related genes. Similarly, the expression of the nitrate transport-related genes was downregulated and the expression of the nitrate assimilation-related genes was upregulated in MdHHO3 overexpression Arabidopsis and tobacco plants. Interaction experiments showed that MdHHO3 could bind to the promoter MdNRT2.1 (NITRATE TRANSPORTER 2.1) and negatively regulate its expression. Moreover, the exposure of MdHHO3-overexpressing Arabidopsis and tobacco to nitrate deficiency resulted in an early senescence phenotype as compared to the WT plants. These results show that MdHHO3 can not only negatively regulate nitrate accumulation in response to nitrate but also promote early leaf senescence under nitrate deficiency. This information may be useful to further reveal the mechanism of the nitrate response and demonstrates that nitrate deficiency induces leaf senescence in apples.