The heat shock transcription factor PsHSF1 of Phytophthora sojae is required for oxidative stress tolerance and suppression of plant immunity

P039 - In the interaction between plant and microbial pathogens, reactive oxygen species (ROS) rapidly accumulate upon pathogen recognition at the infection site and play a central role in plant defense. However, ROS detoxification depends on transcription factors (TFs) that are highly conserved in fungi but much less conserved in oomycetes. In this study we indentified the PsHSF1 that acts as a modulator of the oxidative stress response in the soybean stem and root rot pathogen Phytophthora sojae. PsHSF1 belongs to a large family of heat shock transcription factors (HSFs) that is conserved in Phytophthora spp. It is highly up-regulated under oxidative stress, heat shock stress and during cyst germination. Heterologous expression of PsHSF1 in a yeast knock-out mutant, that lacks a TF functioning in stress tolerance (¿skn7), rescued the aberrant oxidative stress response. Phenotypic analyses of P. sojae knock-down mutants revealed that PsHSF1 is required for stress tolerance and virulence, and likely functions as a transcriptional regulator of peroxidase and laccase genes. PsHSF1-silenced transformants showed reduced levels of extracellular peroxidase and laccase activities. Moreover, they were compromised in suppressing hydrogen peroxide accumulation and innate immunity in the host, phenotypes that were partially restored by adding the antioxidant glutathione. Silencing PsLAC4, one of the laccase genes regulated by PsHSF1, resulted in reduced virulence thereby illustrating the contribution of PsLAC4 and PsHSF1 to ROS depletion. Our results identify PsHSF1 as a TF that acts as a positive regulator of extracellular peroxidase and laccase genes which are important for ROS resistance of P. sojae.