Involved In Sucrose Metabolism

Since it had been found in the 1960s that wild forms of sugarcane and high-yield commercial varieties are distinguished by different enzyme activities of sucrose metabolism (3), a correlation between sucrose storage (or sucrose concentration in the internodes) and enzyme activities was sought for. As pointed out before (Fig.3), sucrose undergoes a cycle of synthesis and breakdown. In theory, sucrose concentration can be increased by shifting up sucrose synthesis activity or by switching down the sucrose hydrolysis activity, whereby each of these metabolic reactions is catalysed by different enzymes in parallel, not considering even that each of these enzymes most likely is comprising a family of genes. Zhu et al. (20) analysed the progeny of a defined cross between Saccharum officinarum and Saccharum robustum and followed sucrose concentration and enzyme activities during internode ripening of low and high sucrose clones. The best correlation found, comprising all internodes and all selected clones of the progeny, was between sucrose concentration and the difference of sucrose-phosphate synthase minus soluble acid invertase (SPS-SAI, Fig.6), whereby soluble acid invertase activity alone gave a nearly as good fit. The other enzymes, sucrose synthase, neutral invertase or sucrose-phosphate synthase alone did not correlate with sucrose. The correlation was very robust, because it remained valid even if sucrose concentration and soluble acid invertase activity of the entire stalk as a whole (i.e. not separated into individual internodes), was used for calculation.

The same correlation between SPS-SAI (or SAI alone) and sucrose concentration was obtained, when internodes of various commercial strains were compared during development (21), or when internodes of sugarcane

Fig.6: Relationship between sucrose concentration and enzyme activities in individual internodes of stalks of 9 months old progeny from a cross of S. officinarum and S. robustum. Graph taken from Zhu et al. (20).

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