Hypocrea jecorina (anamorph: Trichoderma reesei) can grow on plant arabinans by the aid of secreted arabinan-degrading enzymes. This growth on arabinan and its degradation product L-arabinose requires the operation of the aldose reductase XYL1 and the L-arabinitol dehydrogenase LAD1. Growth on arabinan and L-arabinose is also severely affected in a strain deficient in the general cellulase and hemicellulase regulator XYR1, but this impairment can be overcome by constitutive expression of the xyl1 encoding the aldose reductase. An inspection of the genome of H. jecorina reveals four genes capable of degrading arabinan, i.e., the ␣-L-arabinofuranosidase encoding genes abf1, abf2, and abf3 and also bxl1, which encodes a ␤-xylosidase with a separate ␣-L-arabinofuranosidase domain and activity but no endo-arabinanase. Transcriptional analysis reveals that in the parent strain QM9414 the expression of all of these genes is induced by L-arabinose and to a lesser extent by L-arabinitol and absent on D-glucose. Induction by L-arabinitol, however, is strongly enhanced in a ⌬lad1 strain lacking L-arabinitol dehydrogenase activity and severely impaired in an aldose reductase (⌬xyl1) strain, suggesting a cross talk between L-arabinitol and the aldose reductase XYL1 in an ␣-L-arabinofuranosidase gene expression. Strains bearing a knockout in the cellulase regulator xyr1 do not show any induction of abf2 and bxl1, and this phenotype cannot be reverted by constitutive expression of xyl1. The loss of function of xyr1 has also a slight effect on the expression of abf1 and abf3. We conclude that the expression of the four ␣-Larabinofuranosidases of H. jecorina for growth on arabinan requires an early pathway intermediate (L-arabinitol or L-arabinose), the first enzyme of the pathway XYL1, and in the case of abf2 and bxl1 also the function of the cellulase regulator XYR1.The recently reinitiated interest in second-generation biofuel production (i.e., from renewable plant material whose use does not compete with use for food and feed production) also led to a renaissance in cellulase and hemicellulase production by the ascomycete Hypocrea jecorina (anamorph Trichoderma reesei), the current best producer of these enzymes (27). The efficient use of these enzymes for plant biomass hydrolysis, however, is still limited by several factors, including incomplete knowledge of regulation of production of these enzymes.The L-arabinose polymer arabinan is another polysaccharide found in plant cell wall heteropolysaccharides as a side chain of pectin (9), and L-arabinose is particularly abundant in the form of glucuronoarabinoxylans in monocotyledon primary cell walls of Commelinoid flowery plants and as arabinoxylans in cereal grains, where it can mount up to 25 to 30% (4, 11). H. jecorina is also able to degrade arabinan to L-arabinose by an arabinanolytic system. To date, two arabinofuranosidases have been characterized: an ␣-L-arabinofuranosidase (ABF1) (26) and a ␤-xylosidase which has a separate ␣ L-arabinofuranosidase domain and activity...