Introduction

Sugar alcohols are acyclic polyols which play a key role in the metabolism of some higher plants. They are also often called alditols. poly-alcohols or polyols. Loescher and Everard [1] noted that the term sugar alcohol is more appropriate than alditol. since these compounds can be obtained by reduction of either aldoses or ketoses. We will hereafter use the term sugar alcohol.

As mentioned by Loescher [2], sugar alcohols resemble the sugars from which they are derived in terms of their chemical, physical and biological characteristics. Therefore, most of the physiological functions of sugar alcohols are similar to those of sucrose.

Sugar alcohols are primary photosynthetic products which can be transported in the phloem. According to Drew [3] and Lewis [4], a storage carbohydrate will accumulate to more than 1% of the dry weight of the tissue in which it resides, and is available for reutilisation some time after deposition. This definition applies to sugar alcohols. Following metabolic and physiological studies. Fellman and Loescher [5] also concluded that sugar alcohols act as carbon storage compounds. Sugar alcohol metabolism is also involved in response to abiotic and biotic stresses.

2. OCCURRENCE AND DISTRIBUTION OF SUGAR ALCOHOLS IN HIGHER PLANTS

The distribution of sugar alcohols in higher plants was first described extensively by Plouvier [6] and Lewis and Smith [7], Wallaart [8. 9] studied more specifically the occurence of sorbitol in the Rosaceae family, and sorbitol and mannitol in the Plantaginaceae family. Later, Lewis [4] listed 17 sugar alcohols that occur naturally in vascular plants, with 13 occuring in Angiosperms. At this time, he noted that only those with six carbon atoms (mannitol, sorbitol, galactitol. allitol and hamamelitol) have been studied in detail from physiological and biochemical points of view. This remark remains true.

The distribution of sugar alcohols has been used as a chemotaxonomic marker in some genera [6], It is summarised in Figure 1 [4, 6. 9. 10], Three alditols are widely distributed in Angiosperms: galactitol, mannitol and sorbitol. Sugar alcohols are rare in monocotyledonous Angiosperms [7]. Mannitol is the most widely distributed of the hexitols in dictotyledons, and is present in some economically important species in Rubiaceae (coffee), Oleaceae (olive) and Apiaceae (celery). In the Rosaceae family, different studies have compared the sorbitol content of different species [8, 11] and of different varieties of one species [12], Sorbitol is present in woody Rosaceae such as apple, pear, plum, prune, peach and apricot.

Sugar alcohols are present in some parasite weeds [13-16]. Sugar alcohols are also widely distributed in fungi, algae and lichens [7, 17], but this is not considered in the present review.

3. METABOLISM OF SUGAR ALCOHOLS IN HIGHER PLANTS

The metabolism of sugar alcohols in sink and source organs was recently reviewed by Loescher and Everard [1], The description of sugar alcohol enzymology is made complicated by unclear nomenclature, with synonymous names and an incomplete list of EC numbers. The main features are summarised in Tables 1 and 2 according to Loescher and Everard's review [1] and some more recent references. Sorbitol and mannitol are the sugar alcohols which have been studied the most extensively with respect to synthesis and degradation, and up to purification, immunolocalisation. and gene sequencing for some of the enzymes. Pharr et at. [18] discussed the energetic benefits of mannitol synthesis and catabolism (a photoassimilate which is chemically reduced to a greater extent than sucrose) compared to sucrose.

One characteristic feature of sugar alcohols is that the capacity of synthesis and degradation is compartmented in different tissues [19-21], This tissue compartmentation has direct consequences on the capacity of sugar alcohol accumulation. For sorbitol and mannitol species, the synthesis of sugar alcohol mainly occurs in source tissue (i.e. mature leaf), and its degradation mainly occurs in sink tissue (i.e. fruit, root tips and roots, bark, young leaf). In apple and peach, when developing leaves undergo the transition from sink to source, sorbitol dehydrogenase activity (sorbitol degradation) decreases and aldose-6-phosphate reductase

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Figure 1. Distribution of sugar alcohols in the phylogenetic tree of Takhtajan [24] modified for Umbellalcs and Plantaginales according to Heywood [25]. The distribution of galactitol (1), mannitol (2) and sorbitol (3) is taken from Plouvier (1963) [6], Wallaart (1981) [9] and Lewis (1984) [4], Figure modified from [10] with the permission of the publisher.

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Figure 1. Distribution of sugar alcohols in the phylogenetic tree of Takhtajan [24] modified for Umbellalcs and Plantaginales according to Heywood [25]. The distribution of galactitol (1), mannitol (2) and sorbitol (3) is taken from Plouvier (1963) [6], Wallaart (1981) [9] and Lewis (1984) [4], Figure modified from [10] with the permission of the publisher.

Synthesis of some sugar alcohols in higher plants (modified from Loescher and Everard [1 ] and updated).

Sugar alcohol

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