Proteomic analysis revealed that the oomyceticide phosphite exhibits multi-modal action in an oomycete pathosystem

ABSTRACT BACKGROUND Phytopathogenic oomycetes constitute some of the most devastating plant pathogens that cause significant crop and horticultural loss. Phytophthora cinnamomi is a phytopathogenic oomycete that causes dieback disease in native vegetation and a variety of crops. This pathogen can survive through harsh environmental conditions which gives it an advantage over its susceptible hosts. The only implemented chemical used to control P. cinnamomi is the oomyceticide phosphite. Despite its widespread use, the mode of action of phosphite is not well understood and it is unclear whether it works directly on the pathogen or through the host. Additionally, resistance to phosphite is emerging in P. cinnamomi isolates and other oomycete phytopathogens. RESULTS The mode of action of phosphite on the pathogen and through a model host was investigated using label-free quantitative proteomics. In vitro treatment of P. cinnamomi with phosphite hinders growth by interfering with metabolism, signalling and gene expression, traits that are not observed in the tolerant isolate. When the model host L. angustifolius was treated with phosphite, enrichment of proteins that are associated with photosynthesis, carbon fixation and lipid metabolism in the host was observed. An increase in the production of a range of defence-related proteins was observed. CONCLUSION We hypothesise direct and indirect models of the multi-modal action of phosphite that directly targets the pathogen as well as alters plant metabolism and immune response.