Invertase in leaves: Conundrum or control point?

Abstract

to catalyse other glycosyl transferase reactions. A number of diVerent genes are involved ( Koch, 1996). Sucrose The distribution and activity of invertase in mature synthase ( EC 2.4.1.13; UDP glucose: d-fructose source leaves is discussed in relation to the anabolic 2-glucosyl transferase) catalyses the cleavage of sucrose and catabolic processes that occur in such tissues. in the presence of UDP to give UDP-glucose ( UDPG) Anomalously high levels of extractable activity are not, and fructose. The reaction is reversible and the K m for at first sight, consistent with the perceived anabolic sucrose high (ap Rees, 1988). In this reaction, a significant function of leaves. The mechanisms of sucrose break- proportion of the free energy of the glycosidic linkage in down in leaves are discussed with particular reference sucrose is retained in the UDPG produced, whereas this to the ways in which in vivo activity could be modulated energy is lost in inv-mediated hydrolysis. by factors such as product inhibition or developmental There are a number of instances in sink tissues where control. The possible roles of invertase in leaves are strong correlations have been established between discussed in relation to (a) the metabolism of sucrose increases in sink activity and increases in enzyme activity in heterotrophic tissues (epidermis and vasculature); and/or mRNA abundance (ap Rees, 1988; Koch, 1996). (b) sensing changes in assimilate abundance and This provides evidence for the central importance of transducing these into altered patterns of gene modulating sucrose catabolism in sink tissues, and a view expression and (c) the metabolism of oligosaccharides of sucrose metabolism which is consistent both with based upon sucrose. Evidence is presented that indi- theories of long-distance transport and with the ‘procates a significant degree of tissue-level compart- tected’ nature of sucrose as a non-reducing neutral sugar mentation in the activity of invertase. (Arnold, 1968; Farrar, 1992). The situation in mature source leaves is much less clear.