sORF-Encoded Polypeptide SEP1 Is a Novel Virulence Factor of Phytophthora Pathogens

Diseases caused by the notorious Phytophthora spp. result in enormous economic losses to crops and forests. Increasing evidence suggests that small open reading frame-encoded polypeptides (SEPs) participate in environmental responses of animals, plants, and fungi. However, it remains largely unknown whether Phytophthora pathogens produce SEPs. Here, we systematically predicted and identified 96 SEP candidates in P. capsici. Among them, three may induce stable cell death in Nicotiana benthamiana. Phytophthora-specific and conserved SEP1 facilitated P. capsici infection. PcSEP1-induced cell death is BAK1 and SOBIR1 independent and is correlated with its virulence function. Finally, PcSEP1 may be targeted to the apoplast for carrying out its functions, for which the C terminus is indispensable. Together, our results demonstrated that SEP1 is a new virulence factor, and previously unknown SEPs may act as effector proteins in Phytophthora pathogens.