The conidial germling of Metarhizium anisopliae produces an appressorium upon contact with a hard hydrophobic surface. We have conducted an investigation into how this entomopathogen mediates intracellularly the inductive signal to shift from polarized germ-tube growth to non-polarized appressorial growth. During sporulation, conidia accumulated 45Ca2+ but there was no evidence for a gradient of Ca2+ in the spore which could establish the initial polarity. Calmodulin, however, was localized at the poles of the conidia, near the site of germ-tube emergence. Exposing conidia to Ca2+ deprivation or calmodulin antagonists inhibited germination and polar growth. Disruption of Ca2+ gradients by ionophoresis did not prevent germination but caused the multiple emergence of branched germ-tubes from conidia. These findings indicate that Ca2+ plays a fundamental role in establishing the dominance of apical growth. Although an external source of Ca2+ is not required for appressorium formation, germlings producing appressoria took up 45Ca2+ when available. The 45Ca2+ accumulated in the cell wall, plasma membrane and organelles suggesting that these may function as Ca2+ stores for Ca2+-stimulated exocytosis of cell-wall materials. Mitochondria and vacuoles sequestered 45Ca2+ indicating that they play a role in maintaining low cytoplasmic concentrations of 45Ca2+ consistent with the reorganization of the cytoskeleton required for appressorial growth. Several Ca2+-binding proteins in appressoria may provide an energy-independent component of Ca2+ buffering in the cytoplasm. The results indicate that the apical Ca2+ gradient is disrupted during differentiation and subsequent differential Ca2+ redistribution in the cell enlargement zone coincides with germ-tube swelling. cAMP may also be involved by potentiating the effects of small changes in Ca2+ concentration and stimulating exocytosis of mucus components required for adhesion, which is a prerequisite for differentiation.
Second messenger involvement in differentiation of the entomopathogenic fungus, Metarhizium anisopliaeJournal of General Microbiology
Citation InformationSt. Leger, R.J., T.M. Butt, R.C. Staples and D.W. Roberts. 1990. Second messenger involvement in differentiation of the entomopathogenic fungus, Metarhizium anisopliae. Journal of General Microbiology 136: 1779-1789.