Diferulate cross-links impede the enzymatic degradation of non-lignified maize walls

We assessed the effect of ferulate substitution and diferulate cross-linking of xylans on the degradation of cell walls by two fungal enzyme mixtures, one of which contained feruloyl esterase and high xylanase activities. Non-lignified cell suspensions of maize (Zea mays) were grown with 0 or 40 μM 2-aminoindan-2-phosphonic acid to produce walls with normal (17·2 mg g⁻¹) or reduced (5·1 mg g⁻¹) ferulate concentrations. Walls were incubated with mercaptoethanol to inhibit diferulate formation or with hydrogen peroxide to stimulate diferulate formation by wall bound peroxidases. Varying the ferulate substitution of xylans did not affect cell wall hydrolysis. In contrast, increasing ferulate dimerisation from 18 to 40% reduced carbohydrate release by 94–122 mg g⁻¹ after 3 h and by 0–48 mg g⁻¹ after 54 h of enzymatic hydrolysis. Diferulate cross-links impeded the release of xylans, cellulose and pectins from walls. These results provide compelling evidence that diferulate cross-links reduce the rate and, to a lesser degree, the extent of wall hydrolysis by fungal enzymes. Our results also suggest that enzyme mixtures containing high xylanase activity but not feruloyl esterase activity can partially overcome the inhibitory effects of diferulate cross-linking on wall hydrolysis. © 1998 SCI.