Stress Response

Several studies suggest that sugar alcohols play a role in abiotic stress tolerance. They mainly concern tolerance to low temperature, drought and salt-stress. However, it is not always clear whether this role is related to osmotic adjustment, accumulation of a compatible solute or the transitory storage of carbon reserves. In their recent review on the metabolic engineering of plants for osmotic stress resistance. Nuccio el al. [123] mentioned 6 studies where mannitol or sorbitol accumulation is conferred to tobacco or Arabidopsis. Such studies may help to discriminate between the different roles of sugar alcohols in stress tolerance. Since Loescher and Everard [1] have discussed the biophysical roles of sugar alcohols as compatible solutes, this aspect will not be developed here.

Some studies also describe a potential role of sugar alcohol metabolism in biotic stress. Stoop et al. [124] reviewed the role of mannitol metabolism on plant response to salt-stress, osmotic-stress and biotic stresses. Pharr et al. [50] discussed the regulation of mannitol dehydrogenase in terms of stress tolerance. The main aspects are summarised below.

6.1. Effect of low temperatures

Sorbitol may play a role in cold hardiness of Rosaceae trees. Williams and Raese [125] reported that the accumulation of sorbitol was regulated by temperature in apple trees. Whetter and Tapper [126] observed that sorbitol accumulates to high concentrations in tissues and tracheal sap during cold periods. Interestingly, the amount and activity of aldose-6-

phosphate reductase (the key enzyme of sorbitol synthesis) increase in loquat leaves at low temperature [30],

In olive, the highest mannitol content in bark and leaf tissues was observed in winter, and was correlated with the low temperatures [95], In Opuntla humifusa, mannitol may also play a role in tolerance to low temperatures [127]. The role of sugar alcohols in cold hardiness could be related to the thermoprotective effect of these compounds on proteins during dehvdration [128],

6.2. Effect of drought stress

Sorbitol has been implicated in the osmotic adjustment of apple [129, 130], and cherry [131] tissues in response to drought. In apple leaves [130], sorbitol, glucose, and fructose concentrations increased while sucrose and starch levels decreased significantly as water stress developed, indicating that sorbitol and monosaccharides were the most important osmotica for adjustment. Sorbitol accounted for more than 50% of total osmotic adjustment. Carbohydrate partitioning and metabolism were determined in apple mature source leaves, young sink leaves, and stems in response to water stress, by applying [l4C]glucose, [,4C]sucrose or [l4C]sorbitol to shoots which had previously experienced water stress, compared to non-stressed plants. It was suggested that sorbitol accumulation in water-stressed apple mature leaves could be due to increased rates of glucose conversion to sorbitol [132].

Conversely, in peach [133], the relationship between osmotic potential at full turgor and water potential showed that neither mild nor severe drought stress induced significant active osmotic adjustment in mature leaves, although sorbitol was the major organic component of osmotic potential. The partitioning of newly-fixed C was affected by drought stress, but the changes appeared to originate from the inhibition of photosynthesis induced by drought stress. At low photosynthetic rates, sorbitol synthesis was favoured over the synthesis of sucrose. Drought stress did not affect the in vitro activity of sucrose phosphate synthase, the key enzyme in sucrose synthesis but the activity of aldose-6-phosphate reductase, the key enzyme in sorbitol synthesis, tended to increase in response to stress. The authors concluded that peaches did not seem to benefit from sorbitol synthesis during short-term drought stress with respect to active osmotic adjustment in mature leaves. However, in phloem sap, increases in sucrose and especially sorbitol concentrations were observed in stressed plants.

The role of mannitol in drought stress tolerance has also been cited for Orobanche, Thesium and Fraxinus. In Orobanche species, which are holoparasites. a high accumulation of mannitol was demonstrated in drought stress experiments ([119] cited by Harloff and Wegman [35]). In Orobanche ramosa and Orobanche crenata, the enzymes of the mannitol synthesising pathway are identical to those in celery leaves. Their activities increased under drought stress [14], In water-stressed Thesium humile, a hemiparasite of wheat, both the host and the parasite responded to water stress by decreasing their osmotic potentials through the accumulation of species-specific solutes. Of the other solutes, mannitol seemed to contribute to osmotic potential in T. humile [120]. In Fraxinus excelsior L, mannitol has been implicated in leaf osmotic adjustment under drought conditions together with malate [134].

6.3. Effect of salt stress

Loescher and Everard [1] summarised the potential role of sugar alcohols in species that do not regulate salt entry. During salt stress in these types of salt tolerant plants, inorganic ions usually accumulate in the vacuole up to a vacuole water potential balancing that of the external milieu. In the cytoplasm, inorganic ions must be maintained at a low concentration, and compatible solutes (i.e. sugar alcohols) can accumulate to a high concentration without disrupting biological processes, maintaining osmotic equilibrium with the vacuole. Although a large proportion of the sugar alcohol is usually stored in the vacuole, salt stress tolerance may imply a reallocation of that compound to the cytoplasm.

Biochemical studies on Plantaginaceae [135] have provided some evidence of an adaptative function of sorbitol in salt resistance in these species. Correlative data exist indicating a role of mannitol in some mangrove species [136], The role of mannitol in salt stress has been studied extensively in celery [18. 137] and olive [138. 139. 140. 141],

In olive, over the course of the diurnal period, and under high irradiance, leaf mannitol increased more in salt-treated plants than in controls, whereas the contents of other nonstructural carbohydrates were not affected by the treatment [139], Salt stress (100 mM NaCl) also caused an increase in the radioactive C partitioned into mannitol. and a decrease in the radioactive C recovered as glucose [140].

The increase in mannitol accumulation, observed in salt-stressed celery, is not passive but involves the specific regulation of mannitol synthesising and catabolising enzymes [18]. Increasing salinity up to 300 mM increased mannitol accumulation and decreased sucrose and starch pools in celery leaf tissues [137], ,4C labelling revealed that these changes were partly due to shifts in photosynthetic carbon partitioning from sucrose to mannitol. The salt treatment also increased the activity of the key enzyme of mannitol biosynthesis, mannose-6-phosphate reductase, in young and mature leaves. Moreover, a decrease in the activity and protein level of mannitol dehydrogenase, a key enzyme of mannitol degradation, was observed in celery plants subjected to an excess of macronutrient salts [142], This induced an accumulation of mannitol, functioning as an osmoprotectant according to Pharr et al. [18] throughout the plant.

6.4. Effect of biotic stress

Salicylic acid, an inducer of pathogenesis-related proteins, induces mannitol dehydrogenase activity and RNA in celery cell cultures [65]. It has been suggested that the increase in mannitol dehydrogenase activity increases the ability to utilise mannitol. thus providing an additional source of carbon and energy for response to pathogen attack.

Many fungi synthesise mannitol [7, 17] which is considered to be a potential quencher of reactive oxygen species [143, 144], Recently, the induction of expression of the catabolic enzyme mannitol dehydrogenase was discovered and reported in tobacco, a plant which does not contain mannitol, in the presence of Alternaria alternata and specific inducers of plant defence responses. Mannitol production and secretion was also induced in A. alternata in the presence of host plant extracts. The authors suggested that mannitol dehydrogenase represents a new class of non-specific pathogen resistance gene which can counteract the fungal suppression of reactive oxygen species-mediated defences, by catabolising mannitol of fungal origin [145], This is in line with the earlier hypothesis of Stoop et al. [124] who suggested that the removal of mannitol, by mannitol dehydrogenase during pathogen attack, might help the hypersensitive response to proceed.

Sorbitol may also be involved in the expression of resistance to damage caused by bacteria. In apple, the soluble carbohydrate content and solute potential of individual leaves could be related to the progress of fire blight symptom development after infection by Erwinia amylovora [146]. The progressive expression of resistance with leafage paralleled an increase in sorbitol concentration.

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