A Mitogen-Activated Protein Kinase Kinase Required for Induction of Cytokinesis and Appressorium Formation by Host Signals in the Conidia of Colletotrichum gloeosporioides

Abstract

Differentiation of fungal conidia of phytopathogens into the infection structure, appressorium, requires contact with a hard surface and host signals. The molecular signaling involved in the induction of this differentiation is poorly understood. We report the cloning of a mitogen-activated protein kinase kinase (MEK), CgMEK, from Colletotrichum gloeosporioides and its role in the induction of these developmental processes involved in pathogenesis. Disruption of CgMEK1 resulted in the loss of its ability to form appressoria in response to the host's signals and a loss of virulence. Results of confocal microscopic examination of germinating conidia of the gene-disrupted mutants were similar to those for wild-type conidia treated with an MEK inhibitor, suggesting that CgMEK1 is involved in two developmental processes in the differentiation into appressorium: (1) polarized cell division, with the preferential increase in F-actin in one of the daughter nuclei after nuclear division and the formation of septum; and (2) differentiation of the germ tube into an appressorium. CgMEK1 is required for the differentiation. INTRODUCTIONMany phytopathogenic fungi are known to require hard surface contact of conidia before they can differentiate into infection structures called appressoria (Emmett and Parbery, 1975; Xiao et al., 1997). Physical or chemical signals from the host are also known to be required for this differentiation (Grover, 1971;Lapp and Skoropad, 1978; Parbery and Blakeman, 1978; Staples and Hoch, 1987;. Anthracnose disease, caused by the Colletotrichum (Gloeosporium) or the Glomerella group, is very common and destructive of numerous crop and ornamental plants worldwide. The conidia of Colletotrichum gloeosporioides , a causal agent of anthracnose disease on fruits, need a hard surface contact for 2 hr before they can differentiate into appressoria in response to host signals such as surface wax and the ripening hormone ethylene (Flaishman and Kolattukudy, 1994;Flaishman et al., 1995;. Protein phosphorylation is involved in this signal-induced process of appressorium formation because inhibition of protein phosphorylation by kinase inhibitors severely inhibits the appressorium formation induced by ethylene and host wax. The proteins phosphorylated in response to treatment with ethylene and wax include those similar in size to the mitogen-activated protein kinase, MAP kinase (MAPK;Flaishman et al., 1995).MAPK signaling pathways play a pivotal role in sensing extracellular signals and relaying the signals to control gene expression. (Nishida and Gotoh, 1993;Herskowitz, 1995). They involve three sequentially acting kinases-MAPK, MAPK kinase (MEK), and MEK kinase (MEKK) (Marshall, 1994)-the primary structures of which are remarkably conserved among eukaryotes from Saccharomyces cerevisiae to Homo sapiens . MAPK pathways are involved in a variety of developmental processes in yeast and filamentous fungi (Madhani and Fink, 1998a, 1998b). MAPKs have been cloned from phytopathogenic fungi (Li et al., 1997;Hamer an...

A Mitogen-Activated Protein Kinase Cascade Regulating Infection-Related Morphogenesis in Magnaporthe grisea

“…The PMK1 homologs are essential for appressorium formation in all four appressorium-forming fungal pathogens examined to date: M. grisea, Colletotrichum lagenarium, Cochliobolus heterostrophus, and Pyrenophora teres (Lev et al, 1999;Takano et al, 2000;Ruiz-Roldan et al, 2001). In Colletotrichum gloeosporioides, CgMEK1 encodes a MEK that is required for appressorium formation and virulence (Kim et al, 2000). Similar to pmk1 mutants in M. grisea, gene replacement mutants of PTK1 in P. teres and CMK1 in C. lagenarium are nonpathogenic and fail to colonize healthy or wounded host tissues (Takano et al, 2000;Ruiz-Roldan et al, 2001).…”

Nonpathogenic Strains ofColletotrichum lindemuthianumTrigger Progressive Bean Defense Responses during Appressorium-Mediated Penetration

“…Differentiation of the appressorium seems to depend on perception of several putative inductive extracellular signals that are physical and/or biological, such as hydrophobicity, surface hardness, and specific chemical compounds (22,27,30). It has been demonstrated that cyclic AMP (29), mitogen-activated protein kinases (23,42,(50)(51)(52), and Ca 2ϩ /calmodulin-mediated signaling pathways (24,30,49) are all directly involved in triggering the morphogenesis program that leads to appressorium formation. Once appressorium differentiation has occurred, appressorium maturation takes place.…”

Mitogen-activated protein kinases MpkA and MpkB independently affect micafungin sensitivity in Aspergillus nidulans

“…[16][17][18][19][20]27) In particular, Fus3/ Kss1 orthologs in plant pathogens such as M. grisea and Colletotrichum lagenarium are involved in the formation of infection structures (appresorila) and in infectious processes. 18,27) The Fus3/Kss1 ortholog Chk1 in C. heterostrophus is involved not only in virulence but also in responses against oxidative and heavy-metal stresses.…”

“…13,14) In several phytopathogenic fungi, MAPK genes orthologous to yeast Fus3/Kss1 have been characterized and appear to be well conserved in the regulation of plant infection processes. [16][17][18][19][20] For example, Chk1, an ortholog of yeast Fus3/Kss1 in the southern corn leaf blight fungus Cochliobolus heterostrophus, regulates not only sexual-asexual development and pathogenicity but also adaptation to oxidative and heavy-metal stresses. 20) MAPK genes orthologous to yeast Fus3/Kss1 apparently are well conserved in regard to their involvement in sexual development in filamentous fungi.…”