StMPK4/7 Positively Regulate Potato Immunity To Late Blight Disease

Potato（Solanum tuberosum L.）is cultivated worldwide and is one of the most important vegetable crops in our country.The potato late blight disease caused by the oomycete plant pathogen Phytophthora infestans,seriously threatens potato production.The effective controlling of potato late diseases is of great significance to ensure potato yield,development of national economy and food security.In-depth research on the molecular mechanism of potato resistance to the late blight disease can provide resistance related gene resources for potato disease resistance breeding.The mitogen-activated protein kinase（MAPK）cascades are widely involved in plant stress responses.However,the role of potato MAPKs in resistance to Phytophthora pathogens has not been well studied.Our previous work showed that the Phytophthora infestans RXLR effector PITG20303 targets and stabilizes the potato immune negative regulator mitogen-activated protein kinase kinase（MAPKK）StMKK1,thereby facilitating colonization of Phytophthora infestans on plants.Based on the previous research,we identified the downstream signalling MPK protein of StMKK1 and explored the immune mechanism.The specific results are as follows:（1）Because Arabidopsis thaliana At MPK4 is the downstream phosphorylation target of At MKK1,we conducted a phylogenetic analysis of B group MPK proteins from potato and Arabidopsis etc.We found that potato StMPK4/6/7 are closely related proteins,and they are orthologs of At MPK4/5/11/12.（2）Overexpression of StMPK4/7 in N.benthamiana leaves enhances the resistance of plants to Phytophthora infestans and Phytophthora parasitica.At the same time,the silence of corresponding orthologous genes in N.benthamiana promotes the Phytophthora infestans and Phytophthora parasitica host colonization.These results indicate that StMPK4/7positively regulates plant resistance to Phytophthora pathogens.Potato StMPK7 over-expression（OE）lines showed enhanced resistance to Phytophthora infestans,which indicates StMPK7 positively regulates potato resistance to Phytophthora infestans.（3）The interaction between StMPK7 and StMKK1 was verified by yeast two-hybrid technology（Y2H）,co-immunoprecipitation technology（Co-IP）and luciferase complementary imaging technology（LCI）,And the constitutively actived StMKK1（CA-StMKK1）can phosphorylate StMPK7,indicating that StMPK7 is a direct downstream signaling target of StMKK1.（4）The constitutively activated StMPK7(CA-StMPK7;StMPK7^D198G,E202A)enhances plant immunity to Phytophthora pathogens,and its enhancement of plant immunity is significantly higher than StMPK7.The overexpression of CA-StMPK7 in N.benthamiana leaves triggers SGT1/RAR1-dependent plant cell death.（5）Both of the potato StMPK7-OE transgenic lines and N.benthamiana plants that transiently overexpressing StMPK7 genes promote the expression of the plant basal salicylic acid（SA）-related genes;The co-expression of CA-StMPK7 with the Pseudomonas salicylate hydroxylase Nah G completely inhibited CA-StMPK7-triggered plant cell death.These results further proved that StMPK7 regulates plant immune response via affecting plant salicylic acid signaling pathway.