11.4.2 Sugar storage

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Figure 11.10 Grapes undergo an abrupt change in their physiology midway through development. For about 8 weeks after flowering, berry volume increases steadily but fruit are hard (low deformability) and sugar content low. At 'veraison' invertase activity rises abruptly and reducing sugar content increases rapidly, reaching about 20% of fresh weight when ripe. Berries attain full size by 10-12 weeks, and approach an asymptote in sugar content 2-3 weeks later.

(Based on Davies and Robinson 1996)

In sugar-storing fruits a major shift in metabolism generally takes place when fruit expansion is almost complete, heralding a rapid increase in sugar content (Frommer and Sonnewald 1995). Control points for sugar entry and accumulation by fruits include:

1. rate of sugar production by leaves and delivery to transport pathways;

2. reallocation of sugar from supporting vegetative growth towards fruit growth;

3. enhanced unloading of sugar from transport streams into fruits;

4. enhanced transfer of sugar across plasma membranes into cells or through plasmodesmal connections between cells;

5. onward metabolism of sugar in the cytoplasm, or transfer to storage in vacuoles;

6. increased respiratory utilisation of sugar to provide energy for metabolic processes.

As with any biological system, multiple controls operate concurrently to drive a given pattern of maturation. Such events lead us to more robust indicators of ripeness, and improved ways of manipulating maturation to yield higher sugar content and better handling properties. These options are outlined below.

In melon, where sucrose is the main sugar to increase, there is a corresponding decrease in acid invertase and an increase in sucrose phosphate synthase (SPS) activity (this synthesises sucrose from hexose phosphate and adenylated precursors). In grape (Figure 11.10), hexoses accumulate; SPS, sucrose synthase and hexokinase activities all increase (Manning 1993), but acid invertase does not. In addition, expression of mRNA coding for acid invertase and activity both peak just prior to or at veraison (Davies and Robinson 1996).

In tomato there are different genotypes that accumulate either hexoses or sucrose. Most cultivars are hexose accumu-lators and acid invertase is active during growth and ripening (Klann et al. 1993). SSC % hardly changes in commercial tomatoes subsequent to 20 d postanthesis. Sucrose accumu-lators lack acid invertase.

One transgenic tomato has been reported (Ohyama et al. 1995) where the type of sugar stored by fruit has been manipulated. Acid invertase activity was suppressed by antisense RNA resulting in sucrose accumulation in a normally hexose-accumulating cultivar. Conventional breeding studies using crosses between the two types of tomato also showed that an acid invertase gene is not transcribed during ripening of the sucrose accumulators (Harada et al. 1995).