EMERGENCE OF PROTEASES IN GERMINATING CUCUMBER COTYLEDONS AND THEIR ROLES IN THE 2-STEP DEGRADATION OF STORAGE PROTEIN

The degradation of storage protein in germinating cucumber seeds was s hown to proceed via two distinct steps. First, several proteases with acidic isoelectric points (pIs) were involved in solubilization and pa rtial degradation of 11S globulin, Treatment of seedlings with cyclohe ximide inhibited this step and the expression of these proteases. Thus , the first step appeared to be governed by these proteases, which wer e synthesized de novo after imbibition, The first step was observed in dark-grown cotyledons, but the complete degradation of 11S globulin d id not occur in the absence of illumination, An additional protease, w ith a pI of 4.5, was induced by illumination, and it was involved in t he further cleavage of the partially degraded products of 11S globulin . Thus, the complete degradation of the storage protein proceeded via a two-step process in illuminated germinating seedlings, Light is need ed to induce the second step in the degradation of 11S globulin that s upplies the nitrogen required for development of the photosynthetic ap paratus in the greening cotyledon.