SUMMARYFungi of the Pucciniales order cause rust diseases which, altogether, affect thousands of plant species worldwide and pose a major threat to several crops. How rust effectors-virulence proteins delivered into infected tissues to modulate host functionscontribute to pathogen virulence remains poorly understood. Melampsora larici-populina is a devastating and widespread rust pathogen of poplar, and its genome encodes 1184 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 green fluorescent protein-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.

ABSTRACT(MAMP)-triggered immunity (MTI) is the first layer of molecular defense encountered by pathogens. Genetic screens have contributed to our knowledge of MTI, but are limited to phenotype-causing mutations. Here we attempt to identify novel factors involved in the early event leading to plant MTI by comparing the nuclear proteomes of two Arabidopsis genotypes treated with chitosan. Our approach revealed that following chitosan treatment, cerk1 plants had many nuclear accumulating proteins in common, but also some unique ones, when compared with Col-0 plants. Analysis of the identified proteins revealed a nuclear accumulation of DNA-modifying enzymes, RNA-binding proteins and ribosomal proteins. Our results demonstrate that nuclear proteomic is a valid, phenotype-independent approach to uncover factor involved in cellular processes.

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