A model to explain differentiation of appressoria by germlings of Metarhizium anisopliae
Journal of Invertebrate Pathology
The roles of the cell wall and cytoskeleton in cell differentiation of Metarhizium anisopliae were examined by treating germlings with antagonists of wall formation or with drugs which bind to microtubules and filamentous actin. Evidence for the greater plasticity of the appressorial wall compared with nongrowing regions comes from differential staining with fluorescent brighteners (Uvitex and Calcofluor) and fluorescently labeled wheat germ agglutinin; agents which detect chitin. Sorbose, which interferes with primary wall formation, was a potent inhibitor of differentiation which indicates that formation of appressoria involves addition of materials to the wall which matches the wall extension. Treatment with the anti-microfilament agent, cytochalasin A, disrupted polar growth and produced “ballooning” of the hyphal tip, indicating that actin stabilizes the tip against turgor pressure and may be involved in the transition to nonpolar growth of appressoria. Flooding differentiated germlings with 0.5% acetic acid produced bursting of appressoria and extrusion of the cytoplasm, indicating that growing appressoria are also turgescent and that internal hydrostatic pressure is a component in their growth. The effects of anti-microtubule agents are consistent with the well-known role for microtubules in the transport of vesicles. Taken together, our studies indicate that the microtubule-microfilament network is involved in the mediation of extracellular signals but not in initial reception. Inhibition of differentiation by vanadate (an inhibitor of the plasma membrane ATPase) and gadilonium (a blocker of stretch-activated ion channels) implies that changes in membrane potential may play a role in signal reception. We propose a model for the sequence of events during differentiation in which an induction signal, perhaps involving a localized change in transmembrane potential produced by deformation of the plastic hyphal tip, disrupts the apical Ca2+ gradient required for maintenance of polar growth and the actin cytoskeleton, the consequent differential redistribution of Ca2+ in the cell enlargement zone initiates germ tube swelling and appressorium formation by turgor pressure against an expanded area of the cell wall produced by disruption of exocytosis at the original apex and randomly dispersed intussusception of new wall material over the entire cell surface. A possible role for the Spitzenkörper (vesicle generating apparatus) is discussed.
St. Leger, R.J., D.W. Roberts and R.C. Staples. 1991. A model to explain differentiation of appressoria by germlings of Metarhizium anisopliae. J. Invertebrate Pathology 57: 299-310.