Parenchyma Cells

Sugarcane storage parenchyma cells can take up hexoses (glucose and fructose) and sucrose at different rates and with different kinetic properties. Suspension cells (developmentally more comparable to fast growing meristematic cells) take up hexoses by an active, proton-symport system (15-16). Sucrose is only taken up after hydrolysis by cell wall-bound invertase. Tissue slices from immature internodes take up hexoses by far faster than sucrose at all concentrations between 5 and 150 mM (Fig.5, internode 0). The concentration dependance reveals a saturable uptake phase with Km of 5-10 mM for hexoses, superimposed by a "linear phase" apparent at high concentrations. Sucrose uptake seems to be governed only by a very slow "linear phase". Tissue slices from a mature internode generally have low sugar uptake rates (Fig.5, internode 13), only one tenth of that of immature tissue. The rates for hexoses and sucrose are rather similar and obviously not saturable.

In the literature apparently conflicting results are presented: Feeding the storage tissue with asymmetrically labeled sucrose via the phloem (from the leaf midrib) label randomization was small. In contrast feeding sucrose over the apoplast of the storage cells a high or even complete randomization was observed. Feeding sucrose under conditions, where extracellular hydrolysis was low, or feeding non-hydrolysable fluoro-sucrose resulted in sucrose uptake with low label randomization (or none in case of fluoro-sucrose) (1718). Although not all data from the literature can be fully reconciled with a uniform model (see the excellent discussion of the data in Moore (4)), it appears that label randomization (i.e. involvement of cell wall invertase and hexose uptake) is small when sucrose enters the storage tissue symplastically, namely via the phloem, and high if sucrose enters from the apoplast. When sucrose cleavage in the apoplast is minimized with antibodies or by extreme extracellular dilution of produced hexoses, the sucrose uptake by the definitely existing sucrose permeation system may be seen to a measurable extent. Further complication in interpretation of label randomization lies in sucrose synthase, which may synthesize sucrose from hexose precursors (derived from sucrose cleavage) without randomization in

Fig.4: Representation of fluxes into and out of sucrose and hexoses between apoplast and cytosol and cytosol and vacuole.The strength of the arrows represent the flux rate, the height of the letters of sucrose and hexoses represent the prevailing concentrations. Immature tissue is internode 0, ripening tissue is internode 3 and mature tissue is internode 13. the arrow to the left indicates the net import of sugar into the storage cell. The data and the graph are taken from Zingsheim (13).

IMMATURE ,/ TISSUE

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