Summary: The conidial germling of 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 Ca but there was no evidence for a gradient of Ca 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 Ca deprivation or calmodulin antagonists inhibited germination and polar growth. Disruption of Ca gradients by ionophoresis did not prevent germination but caused the multiple emergence of branched germ-tubes from conidia. These findings indicate that Ca plays a fundamental role in establishing the dominance of apical growth. Although an external source of Ca is not required for appressorium formation, germlings producing appressoria took up Ca when available. The Ca accumulated in the cell wall, plasma membrane and organelles suggesting that these may function as Ca stores for Ca-stimulated exocytosis of cell-wall materials. Mitochondria and vacuoles sequestered Ca indicating that they play a role in maintaining low cytoplasmic concentrations of Ca consistent with the reorganization of the cytoskeleton required for appressorial growth. Several Ca-binding proteins in appressoria may provide an energy-independent component of Ca buffering in the cytoplasm. The results indicate that the apical Ca gradient is disrupted during differentiation and subsequent differential Ca redistribution in the cell enlargement zone coincides with germ-tube swelling. cAMP may also be involved by potentiating the effects of small changes in Ca concentration and stimulating exocytosis of mucus components required for adhesion, which is a prerequisite for differentiation.


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