Gene editing with an oxathiapiprolin resistance selection marker reveals that PuLLP, a loricrin-like protein, is required for oospore development in Pythium ultimum

Abstract Oomycetes, such as Pythium species, contain numerous devastating plant pathogens that inflict substantial economic losses worldwide. Although CRISPR/Cas9-based genome editing is available, the selection markers available for genetic transformation in these species are limited. In this study, a mutated version of the Phytophthora capsici oxysterol-binding protein-related protein 1 (PcMuORP1), known to confer oxathiapiprolin resistance, was introduced into the CRISPR/Cas9 system for in situ complementation in Pythium ultimum. We targeted PuLLP, which encodes a loricrin-like protein, and showed significant downregulation when the Puf RNA-binding protein-encoding gene PuM90 was knocked out. The PuLLP knockout mutants could not produce oospores, indicating a similar biological function as PuM90. The reintroduction of PuLLP into the knockout mutant using PcMuORP1 as a selection marker restored oospore production. Further comparisons with the conventional selection marker NPTII indicated that PcMuORP1 could be applied at a lower concentration and cost, resulting in a higher screening efficiency. Successive subculturing in the absence of selective pressure showed that PcMuORP1 had little long-term effect on the fitness of transformants. Hence, it could be reused as an alternative selection marker. This study demonstrates the successful implementation of the PcMuORP1 gene as a selection marker in the genetic transformation of Py. ultimum and reveals the loricrin-like protein PuLLP as a sexual reproduction-related factor downstream of the Puf RNA-binding protein PuM90. Overall, these results will help accelerate the functional genomic investigation of oomycetes.