Germ tube growth and the microtubule cytoskeleton in Phytophthora infestans: Effects of antagonists of hyphal growth, microtubule inhibitors, and ionophores

We carried out a set of experiments designed to test hypotheses on the relationship between the microtubule (MT) cytoskeleton and tip growth in fungal hyphae. Using a growth assay with 1·5 h-old germ tubes of Phytophthora infestans we tested various chemicals that inhibit growth or disrupt MTs, or that affect both. Germ tubes were measured after 2 h exposure to the test compounds and MTs were revealed by immunofluorescence. The phytoalexin glyceollin had a dual effect as it stimulated germ tube growth at concentrations of 10 −5 m and 10 −6 m, but inhibited it at 10 −4 m. It affected the MT cytoskeleton at neither of these concentrations. Metalaxyl, the active ingredient of the fungicide Ridomil, inhibited germ tube growth slightly at 10 −4 m and 10 −6 m, but it had no effect on MTs. Benomyl, the active ingredient of the fungicide Benlate, had no effect on MTs at a concentration of 10 −5 m, although it partially inhibited germ tube growth. Nocodazole, a benzimidazole inhibitor of MTs, disrupted the subapical MT cytoskeleton in germ tubes of P. infestans at 10 −4 m, at which growth was still 79% of the control. The potassium ionophore nigericin and the calcium ionophore A23187 stimulated germ tube growth at concentrations between 10 −6 m and 10 −9 m, but inhibited it at 10 −4 m. Nigericin, but not A23187, disrupted MTs at this concentration. These results point out that no simple relationship exists between an intact MT cytoskeleton and germ tube growth in P. infestans.