Variation in Capsidiol Sensitivity between <i>Phytophthora infestans</i> and <i>Phytophthora capsici</i> Is Consistent with Their Host Range

<div><p>Plants protect themselves against a variety of invading pathogenic organisms via sophisticated defence mechanisms. These responses include deployment of specialized antimicrobial compounds, such as phytoalexins, that rapidly accumulate at pathogen infection sites. However, the extent to which these compounds contribute to species-level resistance and their spectrum of action remain poorly understood. Capsidiol, a defense related phytoalexin, is produced by several solanaceous plants including pepper and tobacco during microbial attack. Interestingly, capsidiol differentially affects growth and germination of the oomycete pathogens <i>Phytophthora infestans</i> and <i>Phytophthora capsici</i>, although the underlying molecular mechanisms remain unknown. In this study we revisited the differential effect of capsidiol on <i>P. infestans</i> and <i>P. capsici,</i> using highly pure capsidiol preparations obtained from yeast engineered to express the capsidiol biosynthetic pathway. Taking advantage of transgenic <i>Phytophthora</i> strains expressing fluorescent markers, we developed a fluorescence-based method to determine the differential effect of capsidiol on <i>Phytophtora</i> growth. Using these assays, we confirm major differences in capsidiol sensitivity between <i>P. infestans</i> and <i>P. capsici</i> and demonstrate that capsidiol alters the growth behaviour of both <i>Phytophthora</i> species. Finally, we report intraspecific variation within <i>P. infestans</i> isolates towards capsidiol tolerance pointing to an arms race between the plant and the pathogens in deployment of defence related phytoalexins.</p>