Carbohydrate Metabolism and Differentiation in Seedling Roots

SUMMARY Information relating to the changing activities of the Embden—Meyerhof—Parnas pathway, the pentose phosphate pathway and tricarboxylic acid cycle during the differentiation of roots is reviewed. Attention is particularly drawn to the operation in roots of a short series of reactions involving the enzymes phosphoenol pyruvate carboxylase, malate dehydrogenase and malic enzyme. The possible role of this pathway in the provision of NADPH for reductive biosynthesis is discussed. Sucrose is the major carbon source for carbohydrate metabolism in the root. Present information regarding its mode of entry into the root cells is sparse and confused. An attempt is made to pinpoint particular anomalies in the available data and to discuss some of the drawbacks associated with present approaches. Interactions between the different pathways of carbohydrate oxidation are extremely complex. Some attempts to define these have been made, with specific reference to the roles played in metabolism by the individual pathways. The presence of the pentose phosphate pathway dehydrogenases, the malic enzyme and isocitrate dehydrogenase in roots raises the question of the origin of NADPH for biosynthesis. There is some evidence to suggest a degree of association between specific reaction systems, based on ‘supply’ and ‘demand’ for NADPH. Very little information is available on the regulation of carbohydrate metabolism in roots. Some possible approaches to gaining more data on this important topic are outlined.