Comparative sub-cellular proteome analyses reveals metabolic differentiation and production of effector-like molecules in the dieback phytopathogen Phytophthora cinnamomi.

Abstract

Phytopathogenic oomycetes pose a significant threat to global biodiversity and food security. The proteomes of these oomycetes likely contain important factors that contribute to their pathogenic success, making their discovery crucial for elucidating pathogenicity. Oomycetes secrete effector proteins that overcome or elicit a defence response in susceptible hosts. Phytophthora cinnamomi is a root pathogen that causes dieback in a wide variety of crops and a range of native vegetation world-wide. Virulence proteins produced by P. cinnamomi are not well defined and a large-scale approach to understand the biochemistry of this pathogen has not been documented. Here, soluble mycelial, zoospore and secreted proteomes were obtained and label-free quantitative proteomics was used to compare the composition of protein content of the three sub-proteomes by matching the MS/MS data to a sequenced P. cinnamomi genome. Mass spectra matched to a total of 4635 proteins, validating 17.7% of the predicted gene set of the P. cinnamomi genome. The mycelia were abundant in transporters for nutrient acquisition, metabolism and cellular proliferation. The zoospores had less metabolic related ontologies than the mycelia but were abundant in energy generating, motility and signalling associated proteins. Virulence-associated proteins were identified in the secretome such as candidate effector and effector-like proteins, which interfere with the host immune system. These include hydrolases, cell wall degrading enzymes, putative necrosis-inducing proteins and elicitins. The secretome elicited a hypersensitive response on the roots of a model host and thus suggests evidence of effector activity. SignificancePhytophthora cinnamomi is a phytopathogenic oomycete that causes dieback disease in native vegetation and several horticultural crops such as avocado, pineapple and macadamia. Whilst this pathogen has significance world-wide, its pathogenicity and virulence have not been described in depth. We carried out comparative label-free proteomics of the mycelia, zoospores and secretome of P. cinnamomi. This study highlights the differential metabolism and cellular processes between the sub-proteomes. Proteins associated with metabolism, nutrient transport and cellular proliferation were over represented in the mycelia. The zoospores have a specialised proteome showing increased energy generation geared towards motility. Candidate effectors and effector-like secreted proteins were also identified, which can be exploited for genetic resistance. This demonstrates a better understanding of the biology and pathogenicity of P. cinnamomi infection that can subsequently be used to develop effective methods of disease management