UDP-Glucose Metabolism during Differentiation and Dedifferentiation of Riella helicophylla

Abstract

Summary A very high activity of UDP-glucose-PPiase found in the cells of the liverwort Riella helicophylla could not be explained only by utilization of UDP-glucose in sucrose synthesis. Therefore, the amount of UDP-glucose and the activities of enzymes involved in production of precursors of hemicelluloses were tested in extracts of tissue fragments in different stages of differentiation and dedifferentiation. During differentiation the amount of UDP-glucose increased from the meristematic cells to the expanding cells and decreased in the mature cells. The levels of UDP-glucose-dehydrogenase activity were correlated directly with UDP-glucose content. That means that in the expanding cells the production of UDPglucuronate via UDP-glucose-dehydrogenase predominates. In contrast, UDP-glucose-epimerase activity rose continuously from the meristem to the mature cells, in which the production of UDP-galactose was highest. UDP-glucuronate-PPiase activity was highest in the expanding and the mature cells. It is assumed that in these cells the synthesis of pentosans and hexosans, respectively, is increased. After 48 h isolation of a fragment of mature tissue, the UDP-glucose amount increased drastically in its adaxial region. As during differentiation, UDP-glucose-dehydrogenase activity correlated directly with UDP-glucose amount. The activities of UDP-glucuronate-PPiase and UDP-glucose-epimerase increased also during dedifferentiation, whereas the activity of UDP-glucose-PPiase remained unchanged. It is concluded that the interruption in the correlations between the different tissues induces an increase in activities of enzymes involved in production of hemicellulose precursors.