Evidence for rapid adaptive evolution of tolerance to chemical treatments in <i>Phytophthora</i> species and its practical implications

Chemical treatments are used widely in agricultural and natural settings to protect plants from diseases; however, they may exert an important selection pressure on plant pathogens, promoting the development of tolerant isolates through adaptive evolution. Phosphite is used to manage diseases caused by Phytophthora species which include a large number of the most economically damaging plant pathogens worldwide. Phosphite controls the growth of Phytophthora species in planta without killing it; as a result, isolates can develop tolerance to phosphite after prolonged exposure. We investigated the inter- and intra-specific variability in phosphite tolerance of eleven Phytophthora species, including P. ramorum, an internationally important, highly regulated pathogen. Phytophthora ramorum is a good model system because it is comprised of multiple genetically homogeneous lineages. Seven species were found to be consistently sensitive to phosphite based on the low Effective Concentration (EC) 50 values of all isolates tested (amount of phosphite required to inhibit mycelial growth by 50% relative to growth in the absence of phosphite). However, P. ramorum, P. lateralis, P. crassamura and P. cambivora showed intraspecific variability in sensitivity to phosphite, with at least one isolate showing significantly higher tolerance than the other isolates. Within the three P. ramorum evolutionarily divergent lineages tested, NA1 was the most susceptible to phosphite, the NA1 and EU1 lineages showed intralineage variability and the NA2 lineage showed a decreased sensitivity to phosphite overall as all isolates were relatively tolerant. This finding is relevant because NA1 is dominant in the wild and can be controlled using phosphite, while the EU1 lineage has recently been identified in the wild and is phosphite-tolerant, making the treatment approach potentially less effective. Phytophthora ramorum, P. lateralis and P. crassamura are either selfing, homothallic species, or are known to reproduce exclusively clonally, indicating tolerance to phosphite can emerge even in the absence of sexual recombination.化学防治手段广泛应用于农业生产与自然生态系统中，用以保护植物免受病害侵害；但此类措施可能对植物病原菌施加显著选择压力，通过适应性进化推动耐受性菌株的形成与演化。亚磷酸酯（phosphite）被用于防控疫霉属（Phytophthora）物种引发的植物病害——该属包含全球范围内诸多经济危害最为严重的植物病原菌类群。亚磷酸酯可在植物体内抑制疫霉属物种的生长，而非直接将其杀灭；因此菌株经长期暴露后，可演化出对亚磷酸酯的耐受性。本研究针对11个疫霉属物种的亚磷酸酯耐受性的种间与种内差异展开探究，其中包括国际上备受关注、受到严格管控的病原菌栎树猝死疫霉（Phytophthora ramorum，简称P. ramorum）。栎树猝死疫霉是理想的模式研究系统，因其包含多个遗传均一的进化谱系。基于所有供试菌株的半有效浓度（Effective Concentration, EC₅₀）值（即相较于无亚磷酸酯的对照组，抑制菌丝生长50%所需的亚磷酸酯剂量）较低，研究发现有7个疫霉属物种对亚磷酸酯始终表现出敏感性。但栎树猝死疫霉、侧生疫霉（P. lateralis）、厚垣孢囊疫霉（P. crassamura）以及坎比伏疫霉（P. cambivora）则表现出种内敏感性差异，至少存在一个菌株的耐受性显著高于其余菌株。在供试的3个栎树猝死疫霉进化分歧谱系中，NA1谱系对亚磷酸酯的敏感性最高；NA1与EU1谱系表现出谱系内敏感性差异，而NA2谱系整体敏感性下降，因其所有供试菌株均表现出较强的耐受性。这一发现具有重要实践价值：NA1谱系在野外种群中占主导地位，可通过亚磷酸酯进行有效防控；而EU1谱系近期才在野外被检出，且对亚磷酸酯具有耐受性，这使得该防治手段的应用效果可能大打折扣。栎树猝死疫霉、侧生疫霉与厚垣孢囊疫霉要么属于自交型同宗配合物种，要么仅能进行无性繁殖，这表明即便不存在有性重组，菌株也可演化出对亚磷酸酯的耐受性。