Metabolic Novelties in the Oomycete Phytopathogen, Phytophthora infestans
Phytophthora infestans, the causal agent of potato late blight, is a significant threat to global food security. Little is known about the metabolism of this oomycete, including how it adapts to different growth and nutrient conditions, while such aspects are one key to understanding host-pathogen interactions. To identify nutrient transporter and metabolic genes involved in pathogenicity, genome and RNA-seq data were mined. One of the major findings was the identification of nitrate assimilation genes that were strongly upregulated at mid time-point tomato leaf tissues compared to late time-point. Mutants silenced for these genes were then generated, which showed a reduction in virulence. This confirmed involvement of the nitrate assimilation genes in pathogenicity. Even though P. infestans can not grow using nitrate as the sole nitrogen source, enzyme assays and isotopic labeling studies indicated that its nitrate assimilation pathway is functional. Enzyme assays revealed few biochemical differences in the activity of nitrate reductase from P. infestans and the enzyme from Phytophthora mirabilis and Pythium ultimum, which are two oomycetes that can grow on nitrate. Isotopic labelling studies indicated that P. infestans and P. mirabilis assimilated little nitrate into amino acids in young cultures, but incorporated substantial nitrate in older cultures. In contrast, Py. ultimum utilized nitrate at both early and late growth stages. Studies of other metabolic pathways revealed the presence of mitochondrial glycolytic pay-off phase in addition to the canonical cytosolic one, and a mitochondrial serine biosynthesis pathway, which is cytoplasmic in other species; both pathways are linked through the intermediate, 3-phosphoglycerate. These novel mitochondrial genes are restricted to stramenopiles. Such novel enzymes could be used as targets for the chemical control of oomycetes. I purpose that manipulation of metabolism is a promising approach for control of P. infestans, one of the most devastating phytopathogens.