DIFERULATE CROSS-LINKS IMPEDE THE ENZYMATIC DEGRADATION OF NON-LIGNIFIED MAIZE WALLS

We assessed the effect of ferulate substitution and diferulate crossli nking of xylans on the degradation of cell walls by two fungal enzyme mixtures, one of which contained feruloyl esterase and high xylanase a ctivities. Nonlignified cell. suspensions of maize (Zea mays) were gro wn with 0 or 40 mu M 2-aminoindan-2-phosphonic acid to produce walls w ith normal (17.2 mg g(-1)) or reduced (5.1 mg g(-1)) ferulate concentr ations. Walls were incubated with mercaptoethanol to inhibit diferulat e formation or with hydrogen peroxide to stimulate diferulate formatio n by wall bound peroxidases. Varying the ferulate substitution of xyla ns did not affect cell wall hydrolysis. In contrast, increasing ferula te 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 hydrol ysis. Diferulate crosslinks impeded the release of xylans, cellulose a nd pectins from walls. These results provide compelling evidence that diferulate cross-links reduce the rate and, to a lesser degree, the ex tent of wall hydrolysis by fungal enzymes. Our results also suggest th at enzyme mixtures containing high xylanase activity but not feruroyl esterase activity can partially overcome the inhibitory effects of dif erulate cross-linking on wall hydrolysis. (C) 1998 SCI.