Regulation of Oomycete Autophagy, Lipid Droplet Accumulation and Pathogenesis by Three Rab GTPases

Among eukaryotes, Rab GTPases are critical for intracellular membrane trafficking and possess various functions. Oomycetes, responsible for many devastating plant diseases, pose a significant threat to global agriculture. However, the functions of Rab GTPases in oomycetes are largely uncharted. In this study, we functionally characterised two Rab8 homologues, PlRab8A and PlRab8B, from a plant-pathogenic oomycete, Peronophythora litchii. These genes are crucial for mycelial growth and play significant roles in the production and morphology of sporangia, as well as the formation of zoospores. PlRab8A and PlRab8B also contribute to maintaining cell wall integrity, osmotic stress tolerance and oospore formation, which are vital for the organism's survival and virulence. Moreover, PlRab8A and PlRab8B are essential for the full virulence of P. litchii by influencing laccase activity. We also found that PlRab8B co-localised with lipid droplets (LDs) and negatively affected LD accumulation. Autophagy was enhanced and autophagy-related genes (PlATGs) were up-regulated in PlRab8B mutants. Additionally, PlRab8A was found to interact with PlRab6. Similar to PlRab8, PlRab6 is also required for growth, zoospore formation and pathogenicity of P. litchii. Taken together, this study reported for the first time the functions of three Rab proteins regulating autophagy, LD accumulation and pathogenesis in oomycetes.