uole of photosynthetic cells may serve to buffer chloroplast from the adverse changes in cytosolic metabo-
lites that occur when phloem transport is limited. In addition, utilization of the vacuole provides a larger
compartment for short-term carbohydrate storage than either the chloroplast or the cytoplasm, while poly-
merization avoids the osmotic problems that would occur if the large amounts of carbon partitioned into
fructan were stored in the form of sucrose [15–17]. The mechanisms that control carbon partitioning into
fructans are not yet established. Also, the question of how fructan chain length is determined has not been
elucidated.
D. Polyols
Polyhydroxy alcohols, or polyols, are probably ubiquitous in all plant species, but only in relatively few
plant families are these compounds found to be synthesized from photosynthetically fixed carbon in
source leaves [23,24]. The most commonly occurring polyols are derivatives of hexose sugars in which
the aldose or ketose group has been reduced to a hydroxyl group. Thus, mannitol, sorbitol, and dulcitol
(Figure 5) are the polyol equivalents of the hexoses glucose, fructose, and galactose, respectively.
Formation of a polyol from a hexose sugar requires reduction of the aldehyde or ketone group. In
higher plants, this reduction takes place through a hexose phosphate intermediate, as indicated in Figure
- In source leaves of celery [25,26] and privet [25], reduction of mannose-6-P to mannitol-1-P is cat-
alyzed by the enzyme mannonse-6-P reductase (M6PR), which utilizes NADPH as reductant. Similarly,
in leaves of apple, peach, pear, apricot [27], and loquat [28], an aldose 6-phosphate reductase catalyzes
the reduction of glucose-6-P to sorbitol-6-P, again using NADPH. A similar NADPH-dependent enzyme
is also present in Euonymusleaves, producing dulcitol [29].
As indicated in Figure 6, synthesis of polyols always occurs in addition to sucrose synthesis, not in
substitution for it. The regulatory mechanisms that control the allocation of carbon between sucrose and
polyols are not yet known. Immunological evidence clearly indicates that polyol synthesis is a cytoplas-
mic event [30], but the regulation of this biosynthetic pathway has not been deciphered. Both sucrose and
polyols are exported in the phloem and/or may be stored in the vacuole for later export, but again, the reg-
ulation of compartmentation between storage and export pools is not fully understood.
Because polyols are not rapidly utilized by source leaf tissues, which lack enzymes to reconvert them
to hexose or hexose phosphate, they are particularly useful as storage and transport forms of carbon in
source leaves. Polyols may also play a role as compatible solutes in source leaves, allowing continuation
of photosynthetic activity and carbon metabolism under adverse environmental conditions such as water
stress. Also, the intriguing hypothesis has been put forward that the utilization of reductant in polyol syn-
thesis allows recycling of NADPH between the chloroplast and cytosol, preventing photoinhibition un-
der stress conditions. This possibility could also explain the unusually high photosynthetic rates com-
monly seen in polyol-synthesizing plants [23,24]. The underlying mechanisms that regulate the synthesis
of polyols in source leaves have not yet been established.
Because of the apparent properties of polyols in promoting stress tolerance in plants, there have been
attempts to transform genetically plants that normally do not make polyols with polyol synthesis genes.
To enhance polyol production and accumulation, a bacterial gene for mannitol synthesis has been suc-
cessfully transformed into tobacco [31,32]. Transgenic plants that synthesized mannitol appeared to grow
better under salt stress, supporting the conclusion that mannitol might be involved in stress tolerance. The
actual mechanism is unknown, but it is believed that mannitol may either act as an osmoticum or have
474 PATTANAGUL ET AL.Figure 5 Structure of commonly occurring plant polyols.