Plant pathogens employ diverse secreted effector proteins to manipulate host physiology and defense in order to foster diseases. The destructive Phytophthora pathogens encode hundreds of cytoplasmic effectors, which are believed to function inside the plant cells. Many of these cytoplasmic effectors contain the conserved N-terminal RXLR motif. Understanding the virulence function of RXLR effectors will provide important knowledge of Phytophthora pathogenesis. Here, we report the characterization of RXLR effector PcAvh1 from the broad–host range pathogen Phytophthora capsici. Only expressed during infection, PcAvh1 is quickly induced at the early infection stages. CRISPR/Cas9-knockout of PcAvh1 in P. capsici severely impairs virulence while overexpression enhances disease development in Nicotiana benthamiana and bell pepper, demonstrating that PcAvh1 is an essential virulence factor. Ectopic expression of PcAvh1 induces cell death in N. benthamiana, tomato, and bell pepper. Using yeast two-hybrid screening, we found that PcAvh1 interacts with the scaffolding subunit of the protein phosphatase 2A (PP2Aa) in plant cells. Virus-induced gene silencing of PP2Aa in N. benthamiana attenuates resistance to P. capsici and results in dwarfism, suggesting that PP2Aa regulates plant immunity and growth. Collectively, these results suggest that PcAvh1 contributes to P. capsici infection, probably through its interaction with host PP2Aa.

Plant viruses cause devastating diseases in plants, yet no effective viricide is available for agricultural application. We screened cultured filtrates derived from various soil microorganisms cultured in vegetable broth that enhanced plant viral resistance. A cultured filtrate, designated F8 culture filtrate, derived from a fungus belonging to the genus Trichosporon, induced strong resistance to various viruses on different plants. Our inoculation assay found the infection rate of Tobacco mosaic virus (TMV)-inoculated Nicotiana benthamiana with F8 culture filtrate pretreatment may decrease to 0%, whereas salicylic acid (SA)-pretreated N. benthamiana attenuated TMV-caused symptoms but remained 100% infected. Tracking Tobacco mosaic virus tagged with green fluorescence protein in plants revealed pretreatment with F8 culture filtrate affected the initial establishment of the virus infection. From F8 culture filtrate, we identified a previously unknown polysaccharide composed of D-mannose, D-galactose, and D-glucose in the ratio 1.0:1.2:10.0 with a α-D-1,4-glucan linkage to be responsible for the induction of plant resistance against viruses through priming of SA-governed immune-responsive genes. Notably, F8 culture filtrate only triggered local defense but was much more effective than conventional SA-mediated systematic acquired resistance. Our finding revealed that microbial cultured metabolites provided a rich source for identification of potent elicitors in plant defense.

Small RNAs are central players of RNA silencing in eukaryotes. These short RNA molecules (20-25 nucleotides in length) repress target gene expression based on sequence complementarity. While small RNAs are well-known for their essential function in regulating growth and development, recent research has revealed that they also influence plant immunity. Extensive changes in small RNA accumulation have been observed during infection. This review focuses on specific small RNA changes that are involved in plant responses to filamentous eukaryotic pathogens including fungi and oomycetes. We describe how changes in small RNA accumulation influence plant immunity and summarize the cellular processes affected by these small RNAs. In particular, we discuss secondary small interfering RNAs that directly modulate the expression of defense-related genes.

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