Role of rumen fungi in fiber degradation.

Anaerobic fungi inhabit the rumen and actively degrade plant cell walls. Rumen fungi produce high levels of cellulases and hemicellulases and are particularly proficient in producing xylanases. These enzymes are regulated by substrate (especially soluble sugars) available to the organisms. Fungi degrade unlignified (i.e., no histochemical reaction for phenolics) plant walls totally, indicating that enzymes are able to hydrolyze or solubilize the entire plant wall. These organisms are better able to colonize and degrade the lignin-containing tissues than are bacteria; phenolics are solubilized but not metabolized from the plant wall by fungi. Anaerobic fungi are unique among rumen microorganisms in that they penetrate the cuticle. Residues after incubation with fungi are physically weaker than those incubated with whole rumen fluid or with rumen bacteria, suggesting that fungi could alter the fibrous residue for easier mastication by the animal. Data indicate that cocultures of anaerobic fungi with methanogenic bacteria stimulate cellulose degradation; other data suggest that fungi are inhibited by certain rumen microorganisms. The interaction of rumen fungi with other organisms in relation to fiber degradation in the rumen requires additional study. Rumen fungi have the potential to degrade the more recalcitrant plant walls in forages, but this potential is not always reached in the rumen.