Oxidative scission of plant cell wall polysaccharides by ascorbate-induced hydroxyl radicals

Abstract

Scission of plant cell wall polysaccharides in vivo has generally been assumed to be enzymic. However, in the presence of l-ascorbate, such polysaccharides are shown to undergo non-enzymic scission under physiologically relevant conditions. Scission of xyloglucan by 1 mM ascorbate had a pH optimum of 4.5, and the maximum scission rate was reached after a 10–25-min delay. Catalase prevented the scission, whereas added H2O2 (0.1–10 mM) increased the scission rate and shortened the delay. Ascorbate caused detectable xyloglucan scission above approx. 5 µM. Dehydroascorbate was much less effective. Added Cu2+ (> 0.3 µM) also increased the rate of ascorbate-induced scission; EDTA was inhibitory. The rate of scission in the absence of added metals appeared to be attributable to the traces of Cu (2.8 mg·kg-1) present in the xyloglucan. Ascorbate-induced scission of xyloglucan was inhibited by radical scavengers; their effectiveness was proportional to their rate constants for reaction with hydroxyl radicals (•OH). It is proposed that ascorbate non-enzymically reduces O2 to H2O2, and Cu2+ to Cu+, and that H2O2 and Cu+ react to form •OH, which causes oxidative scission of polysaccharide chains. Evidence is reviewed to suggest that, in the wall of a living plant cell, Cu+ and H2O2 are formed by reactions involving ascorbate and its products, dehydroascorbate and oxalate. Systems may thus be in place to produce apoplastic •OH radicals in vivo. Although •OH radicals are often regarded as detrimental, they are so short-lived that they could act as site-specific oxidants targeted to play a useful role in loosening the cell wall, e.g. during cell expansion, fruit ripening and organ abscission.