inbreds has no discernible phenotypic consequence (77).
There is substantial evidence that phosphorylated glucose is directly imported into the amy-loplasts of tuber cells to sustain starch synthesis (78, 79, 80). Competition for these substrates between starch synthesis and other (cytosolic) pathways is likely to exist. Of particular relevance is the sucrose phosphate synthase (SPS) mediated conversion of UDPGlc and Fru6P into sucrose. The function of SPS-mediated sucrose synthesis during tuber filling is unknown. However, there is little doubt that cycles of sucrose degradation and synthesis occur in isolated tuber parenchyma (36). Reimholz et al. (81) showed that potato tuber SPS is regulated by metabolites and protein phosphorylation, similarly to the enzyme in leaves. In particular, Glc6P and inorganic phosphate (Pi) act as activator and inhibitor of the activity, respectively. Moreover, high turgor induces a decrease in SPS activation state (82). Gleixner et al. (83) examined the stable isotope composition of leaf and tuber sucrose in glasshouse-grown plants (Table 1). The relative 13C content in tuber sucrose was significantly higher than that of leaf sucrose and the difference was attributable to the selective ^C enrichment of intermediates originated during de novo sucrose synthesis in the tubers. On the basis of this finding, the authors speculated that sucrose import and mobilization into the tubers exceeded the rate of starch synthesis in intact, naturally grown plants. Thus, it is feasible that sucrose synthesis and compartmentalization to the vacuole may serve the purpose of maintaining a sucrose concentration gradient between the SECC and the cytosol of tuber parenchyma cells under conditions where starch synthesis is sub-optimal. Indeed, SPS-mediated sucrose synthesis in the tuber appears to be stimulated under conditions that inhibit starch synthesis. These include removal of developing tubers from the mother plant (58-60), exposure of tuber tissue to temperatures above 25° C (84), water stress (82) or treatment with sodium fluoride (85). Moreover, antisense repression of AGPase in potato results in tubers almost devoid of starch and containing up to 30% of sucrose on a dry weight basis (23).
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