MKK4-MPK3-WRKY17-mediated salicylic acid degradation increases susceptibility to Glomerella leaf spot in apple

Abstract

Glomerella leaf spot (GLS), a fungal disease caused by Colletotrichum fructicola (C. fructicola), severely affects apple quality and yield, yet few resistance genes have been identified in apple (Malus domestica Borkh.). Here we found a transcription factor MdWRKY17 significantly induced by C. fructicola infection in the susceptible apple cultivar ‘Gala’. MdWRKY17 overexpressing transgenic ‘Gala’ plants exhibited increased susceptibility to C. fructicola, whereas MdWRKY17 RNA-interference plants showed opposite phenotypes, indicating MdWRKY17 acts as a plant susceptibility factor during C. fructicola infection. Furthermore, MdWRKY17 directly bound to the promoter of the salicylic acid (SA) degradation gene Downy Mildew Resistant 6 (MdDMR6) and promoted its expression, resulting in reduced resistance to C. fructicola. Additionally, Mitogen-activated protein kinase 3 (MdMPK3)directly interacted with and phosphorylated MdWRKY17. Importantly, predicted phosphorylation residues in MdWRKY17 by MAPK kinase 4 (MdMEK4)-MdMPK3 were critical for the activity of MdWRKY17 to regulate MdDMR6 expression. In the six susceptible germplasms, MdWRKY17 levels were significantly higher than the six tolerant germplasms after infection, which corresponded with lower SA content, confirming the critical role of MdWRKY17-mediated SA degradation in GLS tolerance. Our study reveals a rapid regulatory mechanism of MdWRKY17, which is essential for SA degradation and GLS susceptibility, paving the way to generate GLS resistant apple.

Protein Kinase Signaling Pathways in Plant-Colletotrichum Interaction

“…In contrast to the results in Arabidopsis and N. benthamiana , MPK3 in apple (MdMPK3) has been reported to mediate a pathway leading to the susceptibility to C. fructicola (Shan et al, 2021 ). It is shown that MdMPK3 interacts with upstream kinase MdMKK4 on the plasma membrane and in the nucleus, and interacts with downstream transcription factor MdWRKY17 in the nucleus (Shan et al, 2021 ).…”

“…In contrast to the results in Arabidopsis and N. benthamiana , MPK3 in apple (MdMPK3) has been reported to mediate a pathway leading to the susceptibility to C. fructicola (Shan et al, 2021 ). It is shown that MdMPK3 interacts with upstream kinase MdMKK4 on the plasma membrane and in the nucleus, and interacts with downstream transcription factor MdWRKY17 in the nucleus (Shan et al, 2021 ). In vitro kinase assays have shown that the MdMKK4-MdMPK3 cascade phosphorylates MdWRKY17; and the phosphorylation of MdWRKY17 regulates the activation of its target gene Downy Mildew Resistant 6 (MdDMR6) , which contributes to the degradation of SA (Shan et al, 2021 ).…”

“…It is shown that MdMPK3 interacts with upstream kinase MdMKK4 on the plasma membrane and in the nucleus, and interacts with downstream transcription factor MdWRKY17 in the nucleus (Shan et al, 2021 ). In vitro kinase assays have shown that the MdMKK4-MdMPK3 cascade phosphorylates MdWRKY17; and the phosphorylation of MdWRKY17 regulates the activation of its target gene Downy Mildew Resistant 6 (MdDMR6) , which contributes to the degradation of SA (Shan et al, 2021 ). Interestingly, the expression levels of MdWRKY17 are significantly higher in six susceptible germplasms compared to six tolerant germplasms post infection, which correlates well with lower SA accumulation (Shan et al, 2021 ).…”

“…In vitro kinase assays have shown that the MdMKK4-MdMPK3 cascade phosphorylates MdWRKY17; and the phosphorylation of MdWRKY17 regulates the activation of its target gene Downy Mildew Resistant 6 (MdDMR6) , which contributes to the degradation of SA (Shan et al, 2021 ). Interestingly, the expression levels of MdWRKY17 are significantly higher in six susceptible germplasms compared to six tolerant germplasms post infection, which correlates well with lower SA accumulation (Shan et al, 2021 ). Therefore, the MdMKK4-MdMPK3-MdWRKY17-MdDMR6 pathway reduces the SA levels and increases the susceptibility to C. fructicola ( Figure 1B ).…”

SlS5H silencing reveals specific pathogen-triggered salicylic acid metabolism in tomato

“…Whether that regulation is direct or may occur through the participation of different molecular elements remains unclear. This indirect regulation occurs for the transcription factor from apple MdWRKY17 , which bounds to MdDMR6 promoter and induces its expression (Shan et al, 2021). Our results reinforce the role of SlS5H , also known as SlDMR6-1 , in SA metabolism.…”