Mitogen-activated protein (MAP) kinase signalling pathway VmMkh1-VmMkk1-VmSpm1 regulates cell wall integrity in...

Apple Valsa canker disease, caused by Valsa mali Miyabe et Yamada, seriously endangers the healthy growth of apple trees. Mitogen-Activated Protein Kinase (MAPK) signaling pathway is an important pathway to transmit signals stimulated by environmental stress. In this study, we identify and functionally characterize MAPKKK VmMkh1, MAPKK VmMkk1 and MAPK VmSpm1. VmMkh1 and VmMkk1 positively regulate the phosphorylation of VmSpm1. The radial growth rate of the VmSpm1 deletion mutant was reduced by approximately 31 %. There was no significant difference in growth rate between the VmMkh1 and VmMkk1 mutant and the wild-type. VmMkh1 hyphe branches into a curved shape. The VmMkh1, VmMkk1, and VmSpm1 deletion mutant produced fewer conidia than the wild-type strain at 20 days post inoculation. Moreover, the VmMkh1, VmMkk1, and VmSpm1 deletion mutant slows conidial germination. The hyphal growth of VmMkh1, VmMkk1, and VmSpm1 deletion mutants are significantly inhibited on media containing NaCl, KCl, sorbitol (high osmotic stresses). The hyphal growth of VmMkh1, VmMkk1, and VmSpm1 deletion mutants are significantly inhibited on media containing Congo red, CFW, SDS, and Lysing encymes (Cell wall stress agents). A looser distribution of spacers in VmMkh1, VmMkk1, and VmSpm1 deletion mutants compared with the wild-type strain. The size of lesions on apple fruits and branches inoculated with VmSpm1 deletion mutant showed a reduction of approximately 46 % and 43 %, respectively, after 9 dpi. Overall, our findings demonstrate that VmMkh1, VmMkk1, and VmSpm1 are involved in regulating the growth and development, colony surface hydrophobicity, osmotic stress, cell wall integrity maintenance, carbon and nitrogen source utilization, septa formation, and pathogenicity of Valsa mali.