The present invention relates to an isolated .beta.-mannanase protein having an amino acid sequence which is 90% similar to the amino acid sequence of SEQ ID NO:1, as well as isolated polynucleotides encoding the .beta.-mannanase protein, and isolated expression systems and host cells containing the polynucleotides. The present invention also relates to a method of recombinantly producing .beta.-mannanase protein. Also disclosed is a method of degrading mannans and polysaccharides in plant material, which involves providing plant material and contacting the plant material with the .beta.-mannanase protein of the present invention under conditions effective to degrade mannans and polysaccharides in the plant material.