A CLOSE RELATIONSHIP BETWEEN INCREASES IN GALACTOSYLTRANSFERASE ACTIVITY AND THE ACCUMULATION OF GALACTOLIPIDS DURING PLASTID DEVELOPMENT IN CUCUMBER SEEDLINGS

Changes in the activity of UDP-galactose:diacylglycerol galactosyltran sferase (UDGT), a key enzyme in galactolipid biosynthesis, during germ ination were investigated in cucumber (Cucumis sativus L. cv. Aonagaji bai) seedlings. After germination, UDGT activity increased during grow th in darkness for 4 days, reaching 10 times the activity in ungermina ted seeds. Illumination of 4-day-old dark-grown seedlings strongly sti mulated the activity. By contrast, in seedlings grown continuously in darkness, the increase in UDGT activity ceased after 4 days and the ac tivity remained constant thereafter. A similar increase in the specifi c activity of UDGT was observed in the envelope fraction from seedling s, indicating that the increase in the enzymatic activity preceded syn thesis of other proteins in the envelope membrane. Coincident with the change in the enzymatic activity, here was an increase in levels of m onogalactosyl diacylglycerol (MGDG) and digalactosyl diacylglycerol (D GDG), two major constituents of chloroplast membrane lipids, in the ge rminated seedlings. Cycloheximide inhibited the light-mediated increas e in the enzymatic activity by illumination of 4-day-old dark-grown se edlings, and, as a consequence, it inhibited the accumulation of MGDG and DGDG. It was clear, therefore, that protein synthesis was necessar y during this activation. Addition of a cytokinin, benzyladenine (BA), stimulated the increase in the UDGT activity. The increase in the UDG T activity caused by BA was accompanied by the accumulation of galacto lipids, as in the case of the activation by light. These results sugge st that activation of the final reaction in the synthesis of MGDG, whi ch is catalyzed by the galactosyltransferase, contributes to the accum ulation of galactolipids during the development of the chloroplast mem brane.