Phospholipase Cδ regulates germination of Dictyostelium spores

Plc-null spores germinate at alkaline pH, reduced temperature or increased osmolarity, conditions at which the emerging amoebae can not grow. In contrast, food-activated wild-type spores return to dormancy till conditions in the environment allow growth. The analysis of inositol 1,4,5-trisphosphate (IP3) levels and the effect of added IP3 uncover an unexpected mechanism how PLC regulates spore germination: i) deletion of PLC induces the enhanced activity of an IP5 phosphatase leading to high IP3 levels in plc-null cells; ii) in wild-type spores unfavourable conditions inhibit PLC leading to a reduction of IP3 levels; addition of exogenous IP3 to wild-type spores induces germination at unfavourable conditions; iii) in plc-null spores IP3 levels remain high, also at unfavourable environmental conditions.The results imply that environmental conditions regulate PLC activity and that IP3 induces spore germination; the uncontrolled germination of plc-null spores is not due to a lack of PLC activity but to the constitutive activation of an alternative IP3-forming pathway.Many extracellular signals activate inositide-specific phospholipase C (PLC) thereby producing the second messengers Ins(1,4,5)P3 and diacylglycerol [1]. Three types of PLC are known, PLC-β, PLC-γ and PLC-δ which are regulated by G-proteins, receptor tyrosine kinases, and Ca2+, respectively [2]. Animals such as human and rat, but also C. elegans, Artemia, Loligo forbesi and Drosophila possess all three PLC isoforms [3-6]. However, in non-animals exclusively PLC-δ has been identified, e.g. in soybean [7] and catfish [8], and the lower eukaryotes Dictyostelium discoideum[9], Saccharomyces cerevisiae[10-12] and Schizosaccharomyces pombe[13]. This phylogenetic distribution of PLC isozymes is in accordance with the deduced three dimensional structure, suggesting that PLC-δ is the ancient isoform to which specific domains were added in PLC-β and PLC-γ [14,15].Dictyostelium cells live in the soil where they feed