Topic 15.8

Glucanases and Other Hydrolytic Enzymes May Modify the Matrix

Several types of experiments implicate endo-(1,4)-β-D-glucanases (EGases) in cell wall loosening, especially during auxin-induced cell elongation (see Chapter 19). Matrix glucans such as xyloglucan show enhanced hydrolysis and turnover in excised segments when growth is stimulated by auxin. EGases cut (1,4)-β-D-glucans randomly along their backbone, potentially cutting the xyloglucans that bind cellulose microfibrils together. Because of its microfibrillar structure, cellulose not readily digested by these plant enzymes. Interference with this hydrolytic activity by antibodies or lectins reduces growth in excised segments.

EGase activity is associated with growing tissues, and promotion of EGase activity may stimulate cell growth (Park et al. 2004). Such results support the idea that glucanases promote wall stress relaxation and expansion. However, plant EGases have not been found to cause extension of isolated cell walls. They may instead act indirectly by weakening the cell wall in a way that promotes expansin-mediated polymer creep.

Plants contain two gene families that encode enzymes with EGase activity. The first family consists of the large XTH family discussed earlier, some members of which have xyloglucan endotransglucosylase activity (see textbook Figure 15.16), while others have hydrolase activity. XTH enzymes may affect wall mechanics via their action on xyloglucan.

The second family of plant EGases is also a large family (Urbanowicz et al. 2007) with diverse presumptive functions, including wall softening during fruit ripening and abscission, cellulose formation (i.e., trimming nascent glucans for microfibril formation), and growth. These enzymes may act on xyloglucan or on the noncrystalline regions of cellulose microfibrils (Park et al. 2003). Some EGases act on xylans as well as (1,4)-β-D-glucans.