In 1989, Keller  concluded that research on sugar alcohols was in its infancy. While many new findings have completed our knowledge since that date, most of them concern sorbitol in Rosaceous trees or mannitol in celery and olive. Sugar alcohols are photosynthetic products which can be temporarily stored in the leaf mesophyll, then translocated in the phloem towards sinks where they can be immediately utilised or stored transitorily. These features depend on the genotype, developmental stage and environment (i.e. season, stress). Despite many recent results on the physiological role and metabolism of sorbitol and mannitol, further studies are required to determine if all sugar alcohols behave like those which have been the most studied. For instance, very little is known about the occurrence and metabolism of branched-chain sugar alcohols such as hamamelitol , The role of cyclitols. which sometimes function like sugar alcohols in physiological terms , needs to be studied further. Moreover, the role of sugar alcohols in species that do not accumulate them, such as corn [155-157], and soybean , needs to be clarified. It has been suggested that the presence of a sorbitol pathway in the seeds of these species allows the interconversion of glucose and fructose with sorbitol as an intermediate .
However, even for 'model sugar alcohols', some of the aspects cited below have hitherto been neglected. Still very little is known about the regulation of the developmental and tissue expression of the genes coding for the enzymes of sugar alcohol synthesis or degradation. Similarly, little is known about the post-translational regulation of the enzymes. Studies on the regulation of the expression (mRNA and protein level) and activity of the enzymes involved in synthesising and degrading sugar alcohols, as a function of developmental stage and stress level for instance, are rare. In the near future, the use of molecular tools will help to fill the information gaps (when compared to sucrose) on the characterisation of sugar alcohol transporters. Moreover, the characterisation of the gene promoters seems promising for fundamental and practical purposes. Studies on the role of mannitol or sorbitol could also be developed using transgenic plants. So far, to determine the advantages of metabolising sugar alcohols, mannitol or sorbitol accumulation was conferred to plants which usually do not accumulate sugar alcohols . Suppression or co-suppression of one enzyme of sugar alcohol metabolism (synthesis, degradation or membrane transport) in a plant which normally accumulates a sugar alcohol would also improve our understanding of the physiological role of these metabolites.
The outcome of all these studies on the regulation of sugar alcohol metabolism could be an increase in plant productivity through improved stress tolerance. Moreover, further studies will have to be carried out on the non-osmotic roles that sugar alcohols seem to play in regulating morphogenesis and metabolism in plants that do not produce polyols as primary photosynthetic products .
We thank Dr J.P. Gaudillere for numerous scientific discussions and critical reading of the manuscript, Drs A. Fer and R. Lemoine for critical reading of the manuscript, and A.M. Wall for language corrections.
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