A cluster of NIF transcriptional regulators with divergent patterns of spore-specific expression in Phytophthora infestans

Seven genes encoding putative nuclear LIM interactor-interacting factors (NIFs) from Phytophthora infestans were characterized. Four exhibited spore-specific expression and clustered within 20 kb, and three hyphal forms were also identified. Pi-nifS mRNA accumulated during sporulation, and Pi-nifC1, Pi-nifC2, and Pi-nifC3 mRNA during zoosporogenesis. By expressing fusions with the β-glucuronidase reporter gene in transformants, promoter fragments determining patterns of transcription specific to sporangia formation or zoosporogenesis were identified. Such studies also indicated that Pi-nifS was expressed at a late stage of sporulation. Pi-nifC1, Pi-nifC2, and Pi-nifC3 induction involved inositol-trisphosphate signaling based on studies with 2-aminoethoxydiphenylborate and U-73122, was independent of verapamil-sensitive signals, and required the same cool temperatures that trigger zoosporogenesis. In contrast, warmer temperatures favoring direct sporangial germination caused Pi-nifS mRNA to rapidly decay, while verapamil increased mRNA levels. The divergent expression and clustering of the spore-specific genes may help reveal how genomes and developmental pathways evolved within the genus.