A Mitogen-Activated Protein Kinase Pathway Modulates the Expression of Two Cellulase Genes in Cochliobolus heterostrophus during Plant Infection

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Abstract
Conserved eukaryotic signaling elements play an important role in the development of fungal pathogens on their hosts. Chk1, a mitogen-activated protein kinase (MAPK), functions in virulence, mating, and sporulation of the maize leaf pathogen Cochliobolus heterostrophus. Suppression subtractive hybridization was used to identify fungal genes whose expression on the host plant is affected in chk1 deletion mutants. Two of the genes isolated in this screen were predicted to encode cellulolytic enzymes: a cellobiohydrolase, CBH7, and an endoglucanase, EG6. Expression of EG6 and CBH7 was followed by the fusion of their upstream regulatory regions to the coding sequence of the green fluorescent protein. Induction of both genes began at the onset of invasive growth and reached its maximal extent during leaf necrosis. Furthermore, EG6 was induced preferentially within necrotic lesions. Disruption of MAPK CHK1 resulted in a delay in the penetration of hyphae into the leaf and a concomitant delay in the induction of expression of both cellulase genes. In saprophytic culture, the absence of Chk1 resulted in a marked delay in the induction of CBH7 expression by crystalline cellulose. EG6 was expressed at a basal level in culture, and this expression was found to depend strictly on Chk1. Thus, the Chk1 MAPK signaling pathway is involved in the regulation of two cellulase-encoding genes and is necessary for their timely induction by environmental signals.