Abstract

Mitochondrial complex III inhibitors, such as ametoctradin, are crucial for controlling oomycete diseases. However, the increasing severity of fungicide resistance necessitates urgent clarification of its evolutionary mechanisms to optimize disease management and delay resistance development. The DddA-derived cytosine base editors (DdCBEs)–mediated base editing in oomycetes confirmed that the PsCytb S33L mutation is sufficient to confer ametoctradin resistance in Phytophthora sojae . We clarified that low-resistant and medium-resistant mutants serve as transitional populations during the resistance evolution process, with high-resistant mutants eventually becoming the dominant population. We revealed the dynamic changes in mitochondrial heterogeneity under fungicide selection, proving that the gradual increase of mutated mitochondria drives resistance evolution. There is a trade-off between ametoctradin resistance and fitness. While the PsCytb S33L mutation confers this resistance, it impairs mitochondrial function and thereby reduces fitness. Up-regulation of TFAM1 , however, can partially offset this fitness cost. Resistance evolution in plant pathogens involves not only standing variation but also de novo mutations, providing unique insights into the “chicken-and-egg” dilemma.