Sucrose accumulation

J. M. Rohwer and F. C. Botha, Analysis of sucrose accumulation in the sugar cane culm on the basis of in vitro kinetic data. Biochem. J. 358, 437 445 (2001). 

During the day, the rates of starch and sucrose synthesis and the rate of photosynthetic carbon assimilation must be coordinated. There is a clear need to determine how much assimilated carbon can be diverted into sucrose and starch synthesis without decreasing too much the amount that returns to the RPPP. Conversely, when sucrose accumulates in the cytosol because the rate of export diminishes (and/or photosynthesis increases), starch begins to accumulate inside the chloroplast. During the night, the... 

Figure 1. Source leaf minor vein phloem. (A) Autoradiograph of leaf tissues following l C-sucrose accumulation showing radioactivity (white) in veins. (B) Tracing of an electron micrograph of a cross section of minor vein, x, xylem, vp, vascular parenchyma cc, companion cell se, sieve element pp, phloem parenchyma, me, mesophyll cell. Reproduced with permission from Ref. 6. Copyright 1983. Annual Reviews.

In addition to this feed-forward mechanism, Fru-2,6-P2 functions in feedback control of sucrose synthesis. When sucrose accumulates in the leaf, the hexose phosphate concentration increases [51] (the reason for this is still unclear), leading to an activation of Fru-6-P,2K. The resultant increase of Fru-2,6-P2 then restricts the activity of cytosolic FBPase so that less sucrose, and more starch, is synthesized. In this way, when photosynthesis exceeds the rate at which sucrose can be exported, or stored in the leaf, an increasing proportion of the photosynthate is diverted into starch, which provides a store of carbohydrate that is especially important at night (see above). 

Zhu, Y.J., Komor, E., and Morre, P.H., Sucrose accumulation in the sugarcane stem is regulated by the difference between the activities of soluble acid invertase and sucrose phosphate synthase. Plant Physiol., 115, 609, 1997. 

Plants also contain the enzymes necessary for interconverting the hexoses. The reactions shown in Fig. 6 represent some of the known pathways for these interconversions. When barley plants were infiltrated 71) with D-glucose, D-fructose, D-galactose, or D-mannose, sucrose accumulated hence the enzymes were present for interconverting these sugars. Mechanisms for these interconversions are discussed in several reviews 72 73) (see also Section 3B of this chapter). 

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