SCR 96, a small cysteine‐rich secretory protein of P hytophthora cactorum , can trigger cell death in the Solanaceae and is important for pathogenicity and oxidative stress tolerance

Summary Peptides and small molecules produced by both the plant pathogen P hytophthora and host plants in the apoplastic space mediate the relationship between the interplaying organisms. Various P hytophthora apoplastic effectors, including small cysteine‐rich ( SCR ) secretory proteins, have been identified, but their roles during interaction remain to be determined. Here, we identified an SCR effector encoded by scr96 , one of three novel genes encoding SCR proteins in P . cactorum with similarity to the P . cactorum phytotoxic protein PcF . Together with the other two genes, scr96 was transcriptionally induced throughout the developmental and infection stages of the pathogen. These genes triggered plant cell death ( PCD ) in the S olanaceae, including N icotiana benthamiana and tomato. The scr96 gene did not show single nucleotide polymorphisms in a collection of P . cactorum isolates from different countries and host plants, suggesting that its role is essential and non‐redundant during infection. Homologues of SCR96 were identified only in oomycetes, but not in fungi and other organisms. A stable protoplast transformation protocol was adapted for P . cactorum using green fluorescent protein as a marker. The silencing of scr96 in P . cactorum caused gene‐silenced transformants to lose their pathogenicity on host plants and these transformants were significantly more sensitive to oxidative stress. Transient expression of scr96 partially recovered the virulence of gene‐silenced transformants on plants. Overall, our results indicate that the P . cactorum scr96 gene encodes an important virulence factor that not only causes PCD in host plants, but is also important for pathogenicity and oxidative stress tolerance.