Second messenger involvement in differentiation of the entomopathogenic fungus Metarhizium anisopliae

Abstract

The conidial germling of Metarhizium anisopliue 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 cellwall 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 germtube 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.

Ultrastructural observations of Beauveria bassiana infection in Xylotrechus rusticus larvae

“…The activation of a number of cAMPase‐dependent kinases eventually leads to the formation of the appressorium from the germ tube (St Leger et al . ; Clarkson & Charnley ). Invasive infection pegs are formed at one end of the appressorium (Fig.…”

“…The signal transduction pathway for appressorium formation involves the germ tube reaching a specific area on the host's body (surface) like insects internode area, activation of a pressure-sensitive ion channel, which damages the Ca 2+ gradient on the tip of the germ tube, and subsequent depolarization of the germ tube; the instantaneous depolarization of the germ tube changes the surface structure of the cell membrane and activates Gproteins on the cell membrane, which, in turn, activate cAMPase. The activation of a number of cAMPase-dependent kinases eventually leads to the formation of the appressorium from the germ tube (St Leger et al 1990;Clarkson & Charnley 1996). Invasive infection pegs are formed at one end of the appressorium (Fig.…”

Interaction between Metarhizium anisopliae and Its Host, the Subterranean Termite Coptotermes curvignathus during the Infection Process

“…At 4 PI, a penetrant hypha (Figure 2a) that had earlier elongated from an appressorium on t termite's epicuticle (as observed at 6 h PI in Figure 1b), continued to press against t epidermal cell of the termite to gain entry (Figure 2a). Besides providing a major site adhesion, the appresorium plays a role in mechanical pressuring to pierce the infect host [35,36]. During the invasion at 96 h PI, blastospores are seen being dispersed throug Metarhizium anisopliae's development within the host started at 48 h PI.…”

Expression of exogenous dsRNA by Lecanicillium attenuatum enhances its virulence to Dialeurodes citri

“…Because the insect body wall is the first barrier against fungal infection, hyphae require the proliferation of appressoria that mechanically stress the body wall of an insect and secrete body wall‐degrading enzymes . In addition, the presence of Ca 2+ , cAMP, and hydrophobins in fungi can affect the formation of appressoria . As fungi penetrate the walls of insect bodies, body wall‐degrading enzymes such as chitinases, proteases, and lipases play an important role .…”

“…16 In addition, the presence of Ca 2+ , cAMP, and hydrophobins in fungi can affect the formation of appressoria. 17,18 As fungi penetrate the walls of insect bodies, body wall-degrading enzymes such as chitinases, proteases, and lipases play an important role. 16 Among these enzymes, the proteases include PR1 (subtilisin-like enzymes) and PR2 (trypsin-like enzymes).…”