The Ankyrin Repeat-Containing Protein PlANK1 Mediates Mycelial Growth, Oospore Development, and Virulence in Peronophythora Litchii

Litchi downy blight, caused by Peronophythora litchii, is one of the most serious diseases in major litchi-producing regions worldwide. The ankyrin (ANK) repeat is one of the most common protein-protein interaction motifs found in all kingdoms of life proteins. ANK-containing proteins have been demonstrated to regulate various biological processes in animals, plants, and fungi. However, their functions in phytopathogenic oomycetes remain unknown. Here, we identified 284 non-redundant genes that encode ANK-containing proteins in P. litchii and classified them into 11 subfamilies. Among them, PlANK1 was found to be highly up-regulated in oospores and from zoospores to the infection process. Loss of PlANK1 in P. litchii resulted in impaired mycelial growth and cyst germination, accelerated zoospore encystment, and increased sensitivity to hyperosmotic stresses and Congo red. Furthermore, PlANK1 deletion mutants were defective in oospore formation and development. Inoculation assays showed that the absence of PlANK1 severely diminished the pathogen's virulence on litchi leaf and fruit. Through transcriptome analysis and nitrogen source utilization assays, we demonstrated that PlANK1 modulates the pathogen's nitrogen metabolism. Altogether, our findings indicate that PlANK1 is a key regulator of sexual and asexual development, and virulence in P. litchii.