Oomycetes are eukaryotic microbes that differ from canonical eukaryotes in subcellular organization of central carbon metabolism. Whereas glycolysis is typically an exclusively cytosolic pathway in most eukaryotes, oomycetes express both cytosolic and mitochondrial enzyme sets for the last six steps of glycolysis. Linked to glycolysis via 3-phosphoglycerate is the phosphorylated serine biosynthesis (PSB) pathway, which in most eukaryotes is cytosolic but in oomycetes has an atypical mitochondrial location. Previous work validated the mitochondrial localization of these enzymes despite their nuclear encoding, but their actual contributions to metabolism and organismal fitness remained unresolved.
To assess the functional importance of the mitochondrial glycolytic branch, the authors used Phytophthora infestans as a model. They generated single-gene knockouts targeting the mitochondrial forms of phosphoglycerate kinase and enolase. Each single knockout exhibited little or no measurable effect on growth or pathogenicity, indicating partial redundancy or compensatory activity from other enzyme pools. However, strains deleted for both mitochondrial phosphoglycerate kinase and enolase displayed severely impaired growth and pathogenic traits. These genetic results demonstrate that the combined activity of the mitochondrial forms of these two enzymes is important to organismal fitness in P. infestans.
Metabolomic profiling of the double knockout strains revealed widespread metabolic perturbations. The study reported altered levels of glycolytic intermediates and tricarboxylic acid (TCA) cycle metabolites, indicating disruption of central carbon flow. Adenylate pools were affected, consistent with changes in cellular energy status. Concentrations of vitamins and other small molecules important to cellular function were also perturbed. Together, these metabolomic shifts link loss of the mitochondrial glycolytic branch to broad changes in metabolic homeostasis, supporting the physiological relevance of the mitochondrial enzyme set beyond localization alone.
The authors disrupted the PSB pathway by knocking out phosphoserine aminotransferase, a key enzyme in the pathway. Blocking PSB also compromised growth of P. infestans, albeit with fewer downstream metabolic changes than observed for the glycolytic double knockouts. The metabolomic pattern and experimental findings suggested that, in this organism, the principal function of the mitochondrial PSB pathway may be to generate 3-phosphoglycerate for glycolysis rather than to serve primarily as a serine production route. Thus, PSB appears integrated with mitochondrial glycolytic metabolism and contributes to fitness primarily through provision of glycolytic substrate.
Phylogenetic analysis reported in the study indicates that most of the mitochondrial-targeted enzymes examined appear to have been acquired by lateral gene transfer into the stramenopile lineage. That lineage includes oomycetes as well as diatoms and brown algae. The presence of these genes in the nuclear genome with mitochondrial targeting, together with the functional data from P. infestans, implies an evolutionary reconfiguration of metabolic pathways where enzymes relocated and assumed indispensable roles within mitochondrial metabolism in oomycetes.
This work links the unusual mitochondrial localization of terminal glycolytic enzymes and the PSB pathway in oomycetes to measurable contributions to metabolism and organismal fitness. In Phytophthora infestans, loss of both mitochondrial phosphoglycerate kinase and enolase produces strong growth and pathogenicity defects and broad metabolic dysregulation, while disruption of PSB reduces growth and appears mainly to limit supply of 3-phosphoglycerate for glycolysis. The findings support a model in which metabolism in oomycetes has adapted to depend on these mitochondrial enzyme sets, many of which likely entered the lineage via lateral gene transfer. Details on experimental methods, quantitative metabolite changes, and additional datasets are provided in the original preprint; specific numeric results and experimental parameters were not reproduced here and should be consulted in the source document.