Emergence of proteases in germinating cucumber cotyledons and their roles in the two-step degradation of storage protein.

The degradation of storage protein in germinating cucumber seeds was shown to proceed via two distinct steps. First, several proteases with acidic isoelectric points (pIs) were involved in solubilization and partial degradation of 11S globulin. Treatment of seedlings with cycloheximide inhibited this step and the expression of these proteases. Thus, the first step appeared to be governed by these proteases, which were synthesized de novo after imbibition. The first step was observed in dark-grown cotyledons, but the complete degradation of 11S globulin did not occur in the absence of illumination. An additional protease, with a pI of 4.5, was induced by illumination, and it was involved in the 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 needed to induce the second step in the degradation of 11S globulin that supplies the nitrogen required for development of the photosynthetic apparatus in the greening cotyledon.