The heterotrimeric G-protein pathway regulates cellular responses to a wide range of extracellular signals in virtually all eukaryotes. It also controls various developmental processes in the oomycete plant pathogen Phytophthora infestans, as was concluded from previous studies on the role of the G-protein α-subunit PiGPA1 in this organism. The expression of the P. infestans G-protein β-subunit gene Pigpb1 was induced in nutrient-starved mycelium before the onset of sporangium formation. The gene was hardly expressed in mycelium incubated in rich growth medium. The introduction of additional copies of Pigpb1 into the genome led to silencing of the gene and resulted in transformants deficient in PiGPB1. These Pigpb1-silenced mutants formed very few asexual spores (sporangia) when cultured in rye sucrose medium and produced a denser mat of aerial mycelium than the wild type. Partially Pigpb1-silenced mutants showed intermediate phenotypes with regard to sporulation, and a relatively large number of their sporangia were malformed. The results show that PiGPB1 is important for vegetative growth and sporulation and, therefore, for the pathogenicity of this organism.
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