The Supply of Sucrose
In photosynthetic and gluconeogenic tissues sucrose is predominantly exported from cells, most probably by facilitated diffusion, and subsequently taken up by the phloem complex through a specific sucrose/H+ co-transport mechanism (Riesmeier et al., 1994; Frommer and Sonnewald, 1995). Once in the phloem complex, sucrose is transported to cells of heterotrophic ‘sink’ organs. Sucrose obtained through translocation can enter a cell via the symplasm (Figure 25.1) or the apoplasm (Figure 25.2) and in many species the nature of the predominantly used route is hotly debated. Several studies using assymetrically labelled sucrose suggest that carbon obtained by heterotrophic cells moves primarily through the symplasmic route and is not cleaved to glucose and fructose during transport. It seems likely that cells of many species receive most of their sucrose by such a route (Patrick, 1990; Tegeder et al., 1999; Lalonde et al., 1999).
However, in certain tissues it is clear that sucrose must be supplied through the apoplasm. This is certainly the case in developing seeds in which protoplasmic connections between maternal and embryonic tissue simply do not exist. Thus, studies on the pathway of uptake of sucrose from the apoplast revealed that there is not a single route of uptake. Hydrolysis of sucrose precedes uptake by developing seeds of maize, sorghum and pearl-millet, whereas in wheat, rye and barley sucrose appears to be transferred without cleavage (Thorne, 1985; Weschke et al., 2000). However, even in species where apoplasmic hydrolysis of sucrose occurs this does not seem to be a prerequisite for uptake since the invertase-resistant sucrose analogue 1-fluorosucrose is taken up by maize seeds at similar rates to that of sucrose (Schmalstig and Hitz, 1987). Studies on the minature-1 mutant of maize, deficient in apoplasmic invertase activity, revealed that seeds were only one fifth the normal weight (Miller and Chourey, 1992), suggesting that apoplasmic hydrolysis of sucrose may play an important role in the maintenance of source to sink sucrose gradients.
The pathway of phloem unloading in the tuber has been the subject of much debate. It has been clear for some time that plasmodesmatal connections between the phloem and the surrounding parenchyma cells exist in the tuber (Oparka and Prior, 1987) and that plasmolysis of growing tubers has an inhibitory effect on the flux of sucrose into the tuber (Oparka and Wright, 1988) suggesting that unloading occurs via a symplastic mechanism. However, isolated tuber discs display a substantial capacity to take up sucrose supplied to the surrounding media (Geigenberger et al., 2000; Fernie et al., 2001a). Furthermore transgenic expression of a yeast-derived invertase in the apoplast under the control of the tuber specific B33-patatin promoter significantly altered tuber yield (Sonnewald et al., 1997) indicating the importance of apoplastic sucrose.
Recent studies using a combination of confocal microscopy, autoradiography and biochemical analyses have provided definitive evidence that unloading in the potato tuber is predominantly apoplastic during stolon elongation and becomes primarily symplastic during the initial phases of tuberisation (Viola et al., 2001). This is in direct contrast to the situation observed in the developing tomato fruit in which sucrose unloading is predominantly symplasmic during early, starch accumulating, stages of development (Damon et al., 1988; Ruan and Patrick, 1995) and apoplasmic during later, hexose accumulating, stages (Patrick et al., 1990; Ruan and Patrick, 1995).
Given that sucrose unloading is essentially symplastic in the developing potato tuber the impact on tuber morphology following expression of a heterologous invertase in the apoplast at this developmental stage (Sonnewald et al., 1997; Hajirezaei et al., 2000) is intriguing. However, despite the large morphological changes apoplastic expression of invertase had no effect on the levels of cellular metabolites (Hajirezaei et al., 2000) and the role of sugars in the apoplastic space of the tubers remains unsolved. Interestingly, when the rate of glucose consumption in these transgenic lines was increased by the cytosolic expression of a bacterial glucokinase the total hexose content of the tuber was reduced implying that apoplastic hexose is somehow able to enter cytosolic metabolism (Fernie et al., 2000). Comparison of transgenic plants exhibiting apoplastic with those exhibiting cytosolic expression of the invertase reveals that a completely different phenotype is produced depending upon the compartment to which the enzyme is targeted. This observation together with results from biochemical studies, suggests that the route of entry of hexoses into metabolism differ according to whether they are generated in the cytosol or the apoplast.
For sugars synthesised in the apoplast this could imply either an endocytotic-like mechanism of transport to the vacuole and subsequent release to the cytosol or delivery into the cytosol by a specific hexose transporter in the plasma membrane which have a signalling capacity (Lalonde et al., 1999; Fernie et al., 2000). It is clear that an apoplasmic unloading mechanism needs the presence of one or both types (monosaccharide and sucrose) of transporter at the plasma membrane. Tables 25.1 and 25.2 list the currently sequenced monosaccharide and sucrose transporters of agronomically important plant species and of Arabidopsis. Since these transporters have been the subject of two excellent recent reviews (Lalonde et al., 1999; Lemoine, 2000) and to our knowledge there has been very little success, from a biotechnological standpoint, in the genetic manipulation of these transporters we will not discuss them in detail here. It is however worth pointing out that although the transport mechanism of the much studied potato sucrose proton transporter SUT1 has been characterised by expression in Xenopus oocytes (Boorer et al., 1996) its precise role in planta is yet to be fully elucidated. Since this is one of the best characterised transporters it, therefore, follows that much work is required before the factors controlling the intracellular movement of sugars within the heterotrophic cell can be fully resolved.
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