Phytopathogenic fungi are heterotrophic organisms that excrete a complex array of enzymes for digestion of plant host tissues. Regulation and coordination of extracellular enzyme production, according to growth conditions and fungus nutritional needs, may be controlled by conserved eukaryotic signaling elements such as G-protein subunits and mitogen-activated protein kinase (MAPK). These pathways are known to mediate a complex set of responses in fungi involved in development, reproduction and pathogenicity. Here, we used a series of mutants, deficient in G-protein α (cga1) or/and β subunits or in MAPK, to test their contribution to the ability of Cochliobolus heterostrophus to utilize different carbon sources. In saprophytic culture, the G-protein α subunit mutant strains had WT levels of cellulase, pectinase and protease degradation activities, but it grew significantly slower on minimal medium containing maltose. This weakened ability implies an essential role of the CGA1 signaling in some poor nutritional environments. Remarkably, the MAPK null mutant failed to achieve the WT (and cga1) growth rate on cellulose as a sole carbon and did not grow at all for the first seven days of culture. An enzymatic activity test revealed that this strain significantly reduced cellulose extracellular degradation activity when grew on this medium. Deficiency in the MAPK encoding gene also led to reduced ability to grow on pectin, protein sources and maltose as a sole carbon. The evidence presented indicates a significant and nutrient-specific role of the G-protein and MAPK pathways in mediating growth of this fungus in different environments.
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