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Alternative splicing (AS) of pre-mRNAs increases transcriptome and proteome diversity,regulates gene expression through multiple mechanisms,and plays important roles in plant development and stress responses.However,the prevalence of genome-wide plant AS changes during infection and the mechanisms by which pathogens modulate AS remain poorly understood.Here,we examined the global AS changes in tomato leaves infected with Phytophthora infestans,the infamous Irish famine pathogen.We show that more than 2000 genes exhibiting significant changes in AS are not differentially expressed,indicating that AS is a distinct layer of transcriptome reprogramming during plant-pathogen interactions.Furthermore,our results show that P.infestans subverts host immunity by repressing the AS of positive regulators of plant immunity and promoting the AS of susceptibility factors.To study the underlying mechanism,we established a luminescence-based AS reporter system in Nicotiana benthamiana to screen pathogen effectors modulating plant AS.We identified nine splicing regulatory effectors (SREs) from 87 P.infestans effectors.Further studies revealed that SRE3 physically binds U1-70K to manipulate the plant AS machinery and subsequently modulates AS-mediated plant immunity.Our study not only unveils genome-wide plant AS reprogramming during infection but also establishes a novel AS screening tool to identify SREs from a wide range of plant pathogens,providing opportunities to understand the splicing regulatory mechanisms through which pathogens subvert plant immunity.