Evidence for an inositol lipid signal pathway in the yeast-mycelium transition of Ophiostoma ulmi, the Dutch elm disease fungus

Abstract

Results presented in this paper indicate the involvement of an inositol lipid signalling system, including protein kinase C, in the yeast-mycelium transition of Ophiostoma ulmi . The exogenous supply of inositol 1,4,5-trisphosphate (Inauthor,4,5P 3 ) induced germ tube formation in O. ulmi , maximum germ tube formation resulting at a concentration of 30 μ m . The addition of Inauthor,4,5P 3 10 min after initiation of Ca 2+ uptake by non-growing yeast cells of O. ulmi resulted in a slight increase in Ca 2+ uptake. The protein kinase C-activating phorbol ester, phorbol 12-myristate 13-acetate (PMA), also induced a significant germination response in O. ulmi and approximately 50% of the population exhibited germ tube formation after 12 h incubation in the presence of 16 n m -PMA. This indicated a possible role for protein kinase C, and therefore diacylglycerol, in the germination response. Ca 2+ uptake by yeast extract-(1% w/v) induced germ tubes of O. ulmi was slower than that which occurred in PMA- or Inauthor,4,5P 3 -induced germ tubes under similar conditions, PMA-induced germ tubes showing the highest levels of Ca 2+ uptake. The anti-calmodulin drug R24571, at 3 μ m , caused almost complete suppression of a yeast extract-induced yeast-mycelium (Y-M) transition which confirmed the involvement of calmodulin in germ tube formation. The addition of 16 n m -PMA in conjunction with yeast extract had no effect on germ tube formation yet, when added with R24571, the inhibitory effect of R24571 was reversed and an almost complete Y-M transition resulted after 16 h incubation. This provided further evidence that activation of protein kinase C was required for germ tube formation in O. ulmi . Ca 2+ uptake by yeast cells or during the growth of yeast extract- and PMA-induced germ tubes of O. ulmi increased markedly over the first 12 h of growth with highest Ca 2+ uptake occurring in yeast extract-induced germ tubes. Ca 2+ uptake by 12 h yeast cells of O. ulmi was inhibited after preincubation for 10 min with PMA suggesting either inhibition of Ca 2+ influx or stimulation of Ca 2+ efflux. Use of Ca 2+ -loaded cells showed that efflux of Ca 2+ from Ca 2+ -loaded yeast cells of O. ulmi was similar in the absence or presence of 70 m m glucose but increased rapidly on addition of PMA to the incubation medium.