Infection assays in Arabidopsis reveal candidate effectors from the poplar rust fungus that promote susceptibility to bacteria and oomycete pathogens H Germain, DL Joly, C Mireault, MB Plourde, C Letanneur, D Stewart, … Molecular Plant Pathology

Abstract

Fungi of the Pucciniales order cause rust diseases, which altogether affect thousands of plant species worldwide and pose major threat to several crops. How rust effectors – virulence proteins delivered into infected tissues to modulate host functions – contribute to pathogen virulence remains poorly understood. Melampsora larici-populina is a devastating and widespread rust pathogen of poplars and its genome encodes 1,184 identified small secreted proteins that could potentially act as effectors. Here, following specific criteria we selected 16 candidate effector proteins and characterized their virulence activities and subcellular localizations in the leaf cells of Arabidopsis thaliana. Infection assays using bacterial (Pseudomonas syringae) and oomycete (Hyaloperonospora arabidopsidis) pathogens revealed subsets of candidate effectors that enhanced or decreased pathogen leaf colonization. Confocal imaging of GFP-tagged candidate effectors constitutively expressed in stable transgenic plants revealed that some protein fusions specifically accumulate in nuclei, chloroplasts, plasmodesmata and punctate cytosolic structures. Altogether, our analysis suggests that rust fungal candidate effectors target distinct cellular components in host cells to promote parasitic growth