Phytophthora infestans is one of the most notorious oomycete plant pathogens which causes late blight, a disease that leads to large yield losses in potato and tomato production worldwide. It’s RXLR effector AVR1 suppresses host defence by targeting Sec5, a subunit of the exocyst complex that is important for mediating polarized exocytosis during plant development and defence against pathogens.
To visualize how the exocyst responds to pathogen attack and what happens at the cellular level when effectors target exocyst subunits, requires detailed imaging of the dynamics of the exocyst complex in living cells by high resolution microscopy. However, live-cell imaging of plant-pathogen interactions is often hampered by the tissue complexity and multi-cell layered nature of plants. In a previous study we showed that P. patens is a suitable host for P. infestans and can invade moss protonema cells and colonize the tissue. This is ideal for studying plant-Phytophthora interactions at the cellular level and is complementary to model systems with seed plants as hosts for Phytophthora. In P. patens lines carrying β-estradiol inducible AVR1 or AVR1ΔT transgenes, expression of AVR1 or AVR1ΔT caused defects in the development of caulonemal protonema cells and abnormal morphology of chloronema cells. Similar phenotypes were observed in Sec5- or Sec6- silenced P. patens lines, suggesting that both AVR1 and AVR1ΔT affect exocyst functioning in P. patens. With respect to Sec5 localization we found no differences between β-estradiol treated and untreated transgenic AVR1 lines. Sec5 localizes at the plasma membrane in growing caulonema cells, also during pathogen attack, and its subcellular localization is the same, with or without AVR1 in the vicinity.