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Summary: 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 anisopliae