Apple vacuolar processing enzyme 4 modulates fruit disease resistance and is regulated by cysteine protease inhibitor

Ring rot is a destructive apple disease caused by Botryosphaeria dothidea. The resistance mechanism of apple plants to B. dothidea remains unclear. Here, we show that apple vacuolar processing enzyme 4 (MdVPE4) is involved in fruit resistance to B. dothidea. MdVPE4 silencing reduced fruit resistance, whereas its overexpression improved fruit resistance. Gene expression analysis revealed that MdVPE4 influenced the expression of fruit resistance-related genes, such as apple polygalacturonase 1 (MdPG1), apple polygalacturonase inhibitor protein 1 (MdPGIP1), apple endochitinase 1 (MdCHI1), and apple thaumatin-like protein 1 (MdTHA1). The expression of the four genes responding to B. dothidea infection decreased in MdVPE4-silenced fruits. Further analysis demonstrated that B. dothidea infection induced MdVPE4 expression and enzyme activation in apple fruits. Moreover, MdVPE4 activity was modulated by apple cysteine proteinase inhibitor 1 (MdCPI1), which also contributed to fruit resistance to B. dothidea as revealed by gene overexpression and silencing analysis. MdCPI1 interacted with MdVPE4 and inhibited its activity. However, MdCPI1 expression was decreased by B. dothidea infection. Taken together, our findings indicate that the interaction of MdVPE4 and MdCPI1 plays an important role in modulating fruit resistance to B. dothidea.