Reversible accumulation of (1-->3,1-->4)-beta-glucan endohydrolase in wheat leaves under sugar depletion.

A (1-->3,1-->4)-beta-D-glucan endohydrolase [(1-->3,1-->4)-beta-glucanase, EC 3.2.1.73] was detected in wheat (Triticum aestivum L.) leaves by Western analyses and activity measurements. This enzyme is able to degrade the (1-->3,1-->4)-beta-glucans present in the cell walls of cereals and other grass species. In wheat, enzyme levels clearly increased during leaf development, reaching maximum values at full expansion and then decreasing upon leaf ageing. To test whether the abundance of (1-->3,1-->4)-beta-glucanase might be controlled by the carbohydrate status, environmental and nutritional conditions capable of altering the leaf soluble sugar contents were used. Both the activity and enzyme protein levels rapidly and markedly increased when mature leaves were depleted of sugars (e.g. during extended dark periods), whereas elevated carbohydrate contents (e.g. following continuous illumination, glucose supply in the dark or nitrogen deficiency during a light/dark cycle) caused a rapid decrease in (1-->3,1-->4)-beta-glucanase abundance or prevented its accumulation in the leaves. The physiological significance of (1-->3,1-->4)-beta-glucanase accumulation under sugar depletion remains to be elucidated.