Department of Microbiology and Plant Pathology, University of California, Riverside, United States of America.
PLoS Pathog. 2019 Apr 19;15(4):e1007729. doi: 10.1371/journal.ppat.1007729. eCollection 2019 Apr.
The use of host nutrients to support pathogen growth is central to disease. We addressed the relationship between metabolism and trophic behavior by comparing metabolic gene expression during potato tuber colonization by two oomycetes, the hemibiotroph Phytophthora infestans and the necrotroph Pythium ultimum. Genes for several pathways including amino acid, nucleotide, and cofactor biosynthesis were expressed more by Ph. infestans during its biotrophic stage compared to Py. ultimum. In contrast, Py. ultimum had higher expression of genes for metabolizing compounds that are normally sequestered within plant cells but released to the pathogen upon plant cell lysis, such as starch and triacylglycerides. The transcription pattern of metabolic genes in Ph. infestans during late infection became more like that of Py. ultimum, consistent with the former's transition to necrotrophy. Interspecific variation in metabolic gene content was limited but included the presence of γ-amylase only in Py. ultimum. The pathogens were also found to employ strikingly distinct strategies for using nitrate. Measurements of mRNA, 15N labeling studies, enzyme assays, and immunoblotting indicated that the assimilation pathway in Ph. infestans was nitrate-insensitive but induced during amino acid and ammonium starvation. In contrast, the pathway was nitrate-induced but not amino acid-repressed in Py. ultimum. The lack of amino acid repression in Py. ultimum appears due to the absence of a transcription factor common to fungi and Phytophthora that acts as a nitrogen metabolite repressor. Evidence for functional diversification in nitrate reductase protein was also observed. Its temperature optimum was adapted to each organism's growth range, and its Km was much lower in Py. ultimum. In summary, we observed divergence in patterns of gene expression, gene content, and enzyme function which contribute to the fitness of each species in its niche.
宿主营养物质的利用来支持病原体的生长是疾病的核心。我们通过比较两种卵菌——半活体营养的致病疫霉（Phytophthora infestans）和活体营养的茄腐镰孢（Pythium ultimum）在马铃薯块茎定殖过程中的代谢基因表达，来研究代谢与营养行为之间的关系。与茄腐镰孢相比，致病疫霉在其活体营养阶段表达了更多参与氨基酸、核苷酸和辅助因子生物合成等途径的基因。相比之下，茄腐镰孢具有更高的表达水平，用于代谢那些通常在植物细胞内被隔离但在植物细胞裂解后被释放到病原体中的化合物，如淀粉和三酰基甘油。在晚期感染过程中，致病疫霉代谢基因的转录模式变得更像茄腐镰孢，这与前者向坏死营养的转变是一致的。代谢基因在种间的变化有限，但仅在茄腐镰孢中存在γ-淀粉酶。研究还发现，这些病原体在利用硝酸盐方面采用了截然不同的策略。mRNA 测量、15N 标记研究、酶测定和免疫印迹表明，致病疫霉的同化途径对硝酸盐不敏感，但在氨基酸和铵饥饿时会被诱导。相比之下，该途径在茄腐镰孢中是被硝酸盐诱导的，但不受氨基酸抑制。茄腐镰孢中缺乏氨基酸抑制似乎是由于缺乏一种作为氮代谢抑制剂的转录因子，这种转录因子在真菌和卵菌中普遍存在。还观察到硝酸盐还原酶蛋白功能多样化的证据。它的最适温度适应了每个生物体的生长范围，并且在茄腐镰孢中的 Km 值要低得多。总之，我们观察到基因表达模式、基因含量和酶功能的趋异，这有助于每个物种在其生态位中的适应性。
