Alternative splicing (AS) of pre-mRNAs is an important gene regulatory mechanism shaping the transcriptome. AtMC1 is an Arabidopsis thaliana type I metacaspase that positively regulates hypersensitive response (HR). Here, we found that AtMC1 is a negative regulator of plant immunity to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 and is physically associated with Sm-like4 (LSM4), which is involved in pre-mRNA splicing. AtMC1 and LSM4 protein levels both increased with their co-expression as compared with their separate expression in vivo. Like AtMC1, LSM4 negatively regulates plant immunity to Pst DC3000 infection. By RNA-seq, AtMC1 was shown to modulate the splicing of many pre-mRNAs including 4CL3, which is a negative regulator of plant immunity. Moreover, atmc1 mutant plants accumulated microRNAs (miRNAs) such as miR398b, miR399a, miR165a and miR159a at a low level than wild-type Arabidopsis Col-0 plants. Furthermore, AtMC1 affected pri-miRNA processing. RNA immunoprecipitation revealed that AtMC1 interacted with these pri-miRNAs in vivo, suggesting that AtMC1 might suppress their processing by directly binding to them. In addition, we found that MC1-mediated plant immunity is conserved in tomato. Thus, AtMC1 plays a regulatory role in both miRNA expression and pre-mRNA splicing, which might contribute to AtMC1-mediated plant immunity.

The oomycete pathogen Phytophthora capsici encodes hundreds of RXLR effectors to enter plant cells and suppress host defense responses. Only few of them are conserved across different strains and species. Such ‘core effectors’ may target hub immunity pathways that are essential during Phytophthora pathogens interacting with their hosts. However, the underlying mechanisms of core RXLRs-mediated host immunity manipulation are largely unknown. Here, we report the functional characterization of a P. capsici RXLR effector, RXLR242. RXLR242 expression is highly induced during the infection process. Its ectopic expression in Nicotiana benthamiana promotes Phytophthora infection. RXLR242 physically interacts with a group of RAB proteins, which belong to the small GTPase family and function in specifying transport pathways in the intracellular membrane trafficking system. RXLR242 impedes the secretion of PATHOGENESIS-RELATED 1 (PR1) protein to the apoplast by interfering the formation of RABE1-7-labeled vesicles. Further analysis indicated that such phenomenon is resulted from competitive binding of RXLR242 to RABE1-7. RXLR242 also interferes trafficking of the membrane-located receptor FLAGELLIN-SENSING 2 (FLS2) through competitively interacting with RABA4-3. Taken together, our work demonstrates that RXLR242 manipulates plant immunity by targeting RAB proteins and disturbing vesicle-mediated protein transporting pathway in plant hosts.