miR827 increases susceptibility to Alternaria alternata by shearing the mRNAs of low-affinity phosphate transporters PHT5-2 and PHT5-3 in apple.

Apple leaf spot disease, caused by Alternaria alternata, significantly impacts apple production. Phosphorus plays a crucial role in maintaining the healthy growth of plants and enhancing their defense against pathogens. Both low-affinity and high-affinity phosphate transporters are important proteins involved in the response to phosphate starvation and increasing phosphate content. The difference is that the former does not easily cause excessive accumulation of phosphorus, leading to phosphorus toxicity in plants. Currently, the defense mechanisms mediated by low-affinity phosphate transporters in apples are not well understood. In this study, we identified two low-affinity phosphate transporters, PHT5-2 and PHT5-3. Compared to the control, although the overexpression of PHT5-2 and PHT5-3 increased phosphorus content in the plants, it did not result in growth defects. Furthermore, the overexpression of PHT5-2 and PHT5-3 led to increased callose deposition, enhancing resistance to A. alternata. We verified that the non-coding sRNA-miR827 binds to the mRNA of PHT5-2 and PHT5-3 via complementary base pairing and suppresses their expression by cleaving the 5' UTR regions using 5' RLM-RACE and N. benthamiana co-transformation assays. Apple plants overexpressing miR827 showed significantly reduced phosphorus content and severe growth defects, accompanied by decreased callose deposition and weakened disease resistance. In summary, our research results reveal the mechanism by which miR827 regulates phosphate transporters involved in the defense of apples against A. alternata.