Effect of senescence and hormone treatment on the activity of a β-1,3-glucan hydrolase in Nicotiana glutinosa leaves.

A high level of activity of a β-1,3-glucan hydrolase is present in leaves of Nicotiana glutinosa and the enzyme is also present in the roots, midribs, petioles and stems. By comparison, very low levels of β-1,4-glucan hydrolase are found throughout the plant. The activity of the β-1,3-glucan hydrolase in leaves aged on the plant was found to increase 14-fold during the course of leaf senescence and to reach a maximum in yellow-green leaves. Detached leaves and leaf discs floated on water in the dark showed similar patterns of change.The increase in β-1,3-glucan hydrolase activity during senescence is apparently not due to the loss of an inhibitor from young green leaves or to the formation of an enzyme activator in yellow leaves. The enzyme in yellow leaves was electrophoretically indistinguishable from that in green leaves. The hydrolase is not firmly attached to the cell walls and is not present in the particulate fraction sedimenting at 105400xg for 60 min. Within the leaf cell it is therefore likely to be located either in the cytoplasm or in an easily disrupted structure such as a vacuole.The relationship of the hydrolase to leaf senescence was investigated by examining the effect of plant hormones on the changes in level of hydrolase, protein and chlorophyll in leaf discs during senescence. IAA (10 μM) and GA3 (50 μM) did not alter the normal patterns of change, whilst Kin (50 μM) delayed the loss of protein and chlorophyll and also delayed and decreased the rise in hydrolase activity. In contrast, ABA (190 μM) which increased the rate of loss of protein and chlorophyll, also caused a decrease in the rate and extent of the rise in hydrolase.Possible functions of the hydrolase in the leaf are discussed.