Metabolism of Oat Leaves during Senescence : VIII. The Role of L-Serine in Modifying Senescence

The mechanism whereby l-serine specifically promotes the dark senescence of detached oat (Avena) leaves has been examined. The fact that this promotion is strong in darkness but very weak in white light has been explained, at least in part, by the finding that added serine is partly converted to reducing sugars in light. Labeled serine gives rise to (14)C-sugars and (14)CO(2). In the absence of CO(2), serine does cause chlorophyll loss in light and undergoes a decreased conversion to sugar.As to the large promotion of protease activity which accompanies senescence in the dark, reported earlier, careful purification of the proteases shows that the l-[(14)C]serine is not incorporated into these enzymes, although it is incorporated into the total protein. Cycloheximide decreases the overall synthesis both of protease and of total protein, but again [(14)C]serine does not impart radioactivity to the purified acid proteases. Even when serine is simply added to the protease assay the proteolysis is significantly increased. It is concluded that serine promotes the protease activity by synergizing with the enzyme, or by activating an apoenzyme.