Physiological Roles of Polyols in Horticultural Crops

Review Physiological Roles of Polyols in Horticultural Crops

Yoshinori Kanayama

Graduate School of Agricultural Science, Tohoku University, Sendai 981-8555, Japan

Sucrose is generally considered the primary photosynthate in plants; however, many horticultural crops, including rosaceous fruit trees, synthesize and use polyols (sugar alcohols). This review describes recent progress in physiological research on the metabolism and transport of sorbitol in rosaceous fruit trees, and of mannitol, another common polyol, in horticultural crops. Studies on various polyols other than sorbitol (in rosaceous fruit trees) and mannitol are then described. Polyols play a role not only in the translocation and storage of photosynthates but also in biotic and abiotic responses in many horticultural crops; therefore, this review also provides insights into the effects of polyol metabolism on the biochemical mechanisms of pathogenicity and environmental stress tolerance, including a novel strategy for engineering stress tolerance.

Key Words: mannitol, polyol, sorbitol, sugar alcohol.

single nucleotide polymorphism in the cell wall invertase Introduction directly affects sugar accumulation in tomato fruit. This Sucrose is generally considered the primary photo- finding indicates the horticultural importance of research synthate in plants; therefore, sucrose synthesis, transport, into the metabolism and transport of soluble carbohy- and catabolism have been well studied. Glucose, drates. fructose, and their phosphate esters, which are Soluble carbohydrates also play a role in abiotic stress metabolites in sucrose catabolism, generally play a role responses. Since polyols are involved in osmotic in primary metabolism in plant cells; however, many adjustment, the genes for polyol synthesis have been horticultural crops, including rosaceous fruit trees, used to enhance abiotic stress tolerance using synthesize and use polyols (sugar alcohols). Polyols are transformation techniques. Interestingly, there is evi- carbohydrates that are generally formed through the dence that the role of polyols in abiotic stress tolerance reduction of reducing sugars or their phosphate esters. is not solely osmotic adjustment. Recent interesting work Polyols are non-reducing carbohydrates of high has shown the important role of polyols in pathogenicity. solubility and are thus suitable as translocatable This review describes recent progress in physiological carbohydrates, similar to sucrose, although a recent study research and genetic engineering regarding sorbitol in claimed that reducing sugars are transported in the rosaceous fruit trees and other polyols in horticultural phloem (van Bel and Hess, 2008). crops. It also describes the role of polyols in Soluble carbohydrates are used for respiration and pathogenicity and abiotic stress tolerance. Myo-inositol macromolecule synthesis in plant cells and also generate is not considered here because it is common in plant cell turgor during the cell enlargement stage of cells, where it plays a primary role, and has also been development. Accumulated soluble carbohydrates well studied. increase the sweetness of fruit during maturation and 1. Physiological studies of sorbitol metabolism and ripening. Sucrose phosphate synthase plays a role in the transport in rosaceous fruit trees biosynthesis of sucrose in leaves, while sucrose synthase and acid invertase play roles in the metabolism of sucrose Sorbitol is a primary photosynthate and a major in fruit. Cell wall invertase in fruit hydrolyzes sucrose soluble carbohydrate in the phloem of rosaceous fruit to hexoses to enhance the uptake of sugars into fruit trees (Moing et al., 1997a). Sorbitol 6-phosphate cells. Importantly, Fridman et al. (2004) reported that a dehydrogenase (S6PDH), also called aldose 6-phosphate reductase, is a key enzyme in sorbitol biosynthesis in Received; August 26, 2008. Accepted; December 12, 2008. source leaves. S6PDH converts glucose 6-phosphate to E-mail: [email protected]. sorbitol 6-phosphate, which is then converted to sorbitol

158 J. Japan. Soc. Hort. Sci. 78 (2): 158–168. 2009. 159 by sorbitol 6-phosphate-specific phosphatase (Zhou et which is separate from the bacterial sorbitol 6-phosphate al., 2003). A key enzyme in the sorbitol metabolic metabolizing enzyme (Fig. 1). S6PDH mRNA, protein, pathway in fruit is believed to be NAD-dependent and activity are high in source leaves and cotyledons of sorbitol dehydrogenase (SDH), which catalyzes the apple seedlings (Kanayama et al., 1995). Furthermore, oxidation of sorbitol to fructose. Other important the concentration of S6PDH mRNA is higher in mature components are the sorbitol transporters, which play a than immature leaves, which are sink organs on role in membrane transport of sorbitol. Sorbitol developing shoots of peach and pear trees (Sakanishi et metabolism and transport are important not only in sugar al., 1998; Suzue et al., 2006). S6PDH may therefore be accumulation into fruit but also in retaining high controlled by sink-source conversion in leaves of photosynthetic ability in source leaves (Cheng et al., developing shoots. Suzue et al. (2006) further reported 2008). Sorbitol also facilitates phloem boron transport in ‘La France’ pear that S6PDH activity is more than by the formation of boron-sorbitol complexes in ten-fold higher than sucrose phosphate synthase activity rosaceous fruit trees (Brown and Hu, 1996; Hu et al., in mature leaves, and that sucrose phosphate synthase 1997). A role for sorbitol in boron mobility has been activity remains at an almost constant level throughout reported in tobacco plants transformed with S6PDH developmental stages of leaves. This suggests that (Brown et al., 1999a). S6PDH, rather than sucrose phosphate synthase, is the main source enzyme in rosaceous fruit trees. 1) Sorbitol 6-phosphate dehydrogenase (S6PDH) Using transgenic tobacco plants, Tao et al. (1995) S6PDH has been purified and characterized from apple showed that apple S6PDH plays a role in sorbitol seedlings (Kanayama and Yamaki, 1993). ATP is a biosynthesis. Kanamaru et al. (2004) demonstrated that potent inhibitor of S6PDH activity, that is, there is S6PDH is essential in sorbitol biosynthesis by transgenic competitive inhibition against NADPH in the direction apple plants. They showed that sorbitol content is of sorbitol 6-phosphate synthesis. Zhou et al. (2003) markedly decreased and sucrose content is increased in showed the regulation of S6PDH activity by divalent transgenic apple plants in which the expression of cations and inorganic phosphate. Several effectors of S6PDH is suppressed. This finding indicates that sucrose sucrose-related enzymes regulate carbon flow into production can compensate for a decrease in sorbitol sucrose. Regulation of S6PDH activity might also be production and that the expression of S6PDH regulates important in regulating sorbitol biosynthesis in leaves. the distribution of carbon flow into sorbitol or sucrose. Kanayama et al. (1992) first cloned cDNA encoding Cheng et al. (2005) further showed that starch is also S6PDH from plants. In phylogenetic analysis, S6PDH increased in transgenic apple plants with suppressed is included in the plant aldose phosphate reductase group, expression of S6PDH. Fruit from apples transformed

Fig. 1. Phylogenetic tree of amino acid sequences of polyol-related enzymes. Unrooted neighbor-joining phylogenetic tree was constructed using the ClustalW multiple sequence alignment. Accession numbers are X97606 (Medicago sativa, alfalfa), AF133841 (Xerophyta viscosa), X57526 (Hordeum vulgare, barley), AB016256 (Malus pumila, apple, SDH), AB025969 (Prunus persica, peach, SDH), AB183015 (Lycopersicon esculentum, tomato), U83687 (Apium graveolens, celery), AF055910 (Orobanche ramosa, broomrape), BT001243 (Arabidopsis thaliana), BT025872 (Arabidopsis thaliana), AK104885 (Oryza sativa, rice), EF576940 (Prunus persica, peach, S6PDH), D11080 (Malus pumila, apple, S6PDH), AF415012 (Sorbus aucuparia, European rowan), AJ002527 (Clostridium beijerinckii), AF132127 (Streptococcus mutans), AF396831 (Lactobacillus casei), and Y14603 (Erwinia amylovora). 160 Y. Kanayama with S6PDH cDNA was analyzed by Teo et al. (2006), Lohaus (2008) also suggested a mixture of apoplastic who found that although fruit weight and firmness did and symplastic phloem loading in peach plants by not change, there was an increase in glucose and a measuring subcellular concentrations of sorbitol. decrease in malate content after suppression of S6PDH Apoplastic unloading could also be important in fruit. expression. This increase in Brix value and decrease in Gao et al. (2003) isolated cDNAs encoding sorbitol acidity favor high fruit quality. Kanamaru et al. (2004) transporters from sour cherry and suggested their role and Teo et al. (2006) examined apple vegetative growth in sink activity in fruit and young leaves. Zhang et al. and found some inhibition after suppression of S6PDH (2004) provided evidence of the spatial localization of expression and promotion after overexpression of a functional monosaccharide transporter for apoplastic S6PDH, suggesting that sorbitol is a favorable phloem unloading in developing apple fruit. For future photosynthate for apple vegetative growth. studies, noninvasive analysis using a positron-emitting tracer imaging system will be useful for real-time 2) NAD-dependent sorbitol dehydrogenase (SDH) observations of photoassimilate translocation (Kikuchi Oura et al. (2000) purified SDH from Japanese pear et al., 2008). fruit and used kinetic analysis to show how it favors the 2. Physiological studies of mannitol metabolism conversion of sorbitol to fructose. Yamada et al. (1998) and transport in horticultural crops first isolated cDNA encoding SDH from apple fruit, and expression analyses have been carried out in apples and Mannitol is widely found in plants and microorgan- other rosaceous fruit trees (Bantog et al., 2000; Yamada isms. In some horticultural crops (e.g., celery, olive, et al., 1999, 2001, 2006c). These reports indicate the delphinium), it is a major soluble carbohydrate. importance of SDH in sugar accumulation into fruit. Mannitol is a primary photosynthate and translocat- Kanayama et al. (2005) used divergent peach cultivars able carbohydrate in celery. It is synthesized through to show that SDH activity is likely responsible for mannose 6-phosphate reductase (M6PR) from mannose fructose concentration in fruit. Recently, four to nine 6-phosphate, which is formed from fructose 6-phosphate members of the SDH gene family and the physiological by phosphomannose isomerase. In celery, M6PR, the roles of some of these in sink organs have been reported key enzyme in mannitol biosynthesis, has been purified in apple and Japanese pear trees (Ito et al., 2005; and characterized (Loescher et al., 1992), and M6PR Nosarzewski and Archbold, 2007; Nosarzewski et al., cDNA has been isolated and its role in source organs 2004: Park et al., 2002). Nosarzewski et al. (2004) shown (Everard et al., 1997). The physiological role and demonstrated the importance of SDH in early fruit amino acid sequence of celery M6PR are similar to those development by finding that SDH protein comprises 7– of S6PDH in rosaceous fruit trees. Celery and broomrape 8% of the total extractable protein in young fruit 1–3 (Orobanche, a parasitic herbaceous plant) M6PR are weeks after flowering. phylogenetically separated from S6PDH, although both Interesting studies have reported the regulation of are included in the plant aldose phosphate reductase sorbitol metabolism by soluble carbohydrates and plant group (Fig. 1). Aldose phosphate reductase homologs hormones. Sorbitol increases SDH protein concentration from rice and Arabidopsis are also included in the M6PR and activity in apple fruit tissue while fructose decreases group rather than the S6PDH group. them (Iida et al., 2004). Sorbitol may function as a signal Mannitol is oxidized to mannose by mannitol molecule in the utilization of soluble carbohydrates. This dehydrogenase (MTD), and the resultant mannose is hypothesis is supported by the finding that stem girdling used through subsequent hexokinase and isomerase and suppression of S6PDH expression decrease the reactions. Celery MTD has been purified and its cDNA supply of sorbitol into sink tissues and affect SDH cloned (Stoop et al., 1995; Williamson et al., 1995). activity (Berüter and Feusi, 1997; Zhou et al., 2006). Mannitol transporters have also been isolated and Roles for gibberellins in increasing sink strength and characterized from celery, and play a role in mannitol SDH activity have also been reported in Japanese pear transport in phloem tissues through the proton symport fruit (Zhang et al., 2007). mechanism (Juchaux-Cachau et al., 2007; Noiraud et al., 2001). 3) Sorbitol transporters In olives, mannitol is also a major soluble Isolation and expression analysis of sorbitol transport- carbohydrate, and mannitol transporters and MTD play ers recently provided useful information on loading and a role in mannitol metabolism and transport (Conde et unloading systems, which are important in sorbitol al., 2007). Transcripts for the olive mannitol transporter translocation in rosaceous fruit trees. These transporters increase throughout the maturation of olive fruit, likely act as proton/sorbitol cotransporters for active suggesting that the transporter is involved in mannitol uptake across the plasma membrane with the proton accumulation during fruit maturation. Where boron is pump. Watari et al. (2004) found sorbitol transporters limited, increased mannitol maintains boron remobiliza- expressed in phloem tissues in apple source leaves, tion in olive plants (Liakopoulos et al., 2005). Hu et al. suggesting apoplastic phloem loading. Nadwodnik and (1997) reported that boron is present in the phloem of J. Japan. Soc. Hort. Sci. 78 (2): 158–168. 2009. 161 celery as the boron-mannitol complex. In some reduced by the wilting of sepals and the development ornamental plants, the occurrence of polyols determines of pistils. Kikuchi et al. (2003) suggested that MTD is boron toxicity symptoms (Brown et al., 1999b) related to pistil development and that sucrose-related Delphinium contains a considerable amount of enzymes are important in sepal wilting. mannitol. Tanase et al. (2005) reported that decreases 3. Physiological studies on various polyols in mannitol and other sugars in flowers under low light likely facilitate ethylene evolution and flower abscission SDH-like sequences are widespread in the plant in potted delphinium. Flower abscission can be prevented kingdom, as shown in genome projects on plant species by ethylene inhibition; however, flower quality is still that do not synthesize sorbitol as a major photosynthate.

Fig. 2. Multiple alignment of amino acid sequences of tomato SDH, apple SDH, and SDH-like proteins in EST from species of different plant families. The zinc-containing alcohol dehydrogenase signature is underlined. Asterisks and plus signs indicate conserved amino acid residues at the catalytic zinc binding site and structural zinc binding site, respectively. SDL, tomato SDH. Source: Ohta et al. (2005). 162 Y. Kanayama

In spite of this, little information is available at the dominant soluble carbohydrate in sweetpea (Ichimura et molecular level on SDH-like sequences in non-rosaceous al., 1999). plants. Ohta et al. (2005) first found molecular evidence Volemitol, a seven-carbon polyol, is the major soluble of the presence of SDH in a non-rosaceous plant, tomato. carbohydrate in Primula (Hafliger et al., 1999), They produced recombinant protein from a tomato SDH- accounting for 25% of leaf dry weight and 24% of the like sequence and identified the protein as SDH by phloem sap carbohydrate, suggesting its importance as enzymatic characterization. In addition, the antisense a translocatable and storage carbohydrate. A key enzyme transformation of tomato with the SDH-like sequence in volemitol biosynthesis is a novel ketose reductase, decreased SDH activity, indicating the contribution of sedoheptulose reductase, which forms volemitol by the the sequence to SDH activity in tomato. Cataldi et al. reduction of sedoheptulose, which is generally found in (1998) and Roessner-Tunali et al. (2003) detected a small the form of its phosphate ester in the pentose phosphate amount of sorbitol in tomato. These findings suggest the cycle in plants. presence of sorbitol metabolism in tomato. Furthermore, Although the physiological roles of these various arabitol, a substrate for tomato SDH, accumulates in the polyols are not always clear, they may be major leaves of tomato plants infected with a fungal pathogen carbohydrates in storage and translocation. Alternatively, (Clark et al., 2003). Because arabitol accumulation is they may be related to biotic and abiotic stress tolerance, important in pathogenicity, SDH possibly has antifungal which is a polyol-specific function. It is necessary to activity by catabolizing arabitol. mRNAs for SDH-like identify key enzymes for their biosynthesis, as found for proteins whose amino acid sequences are 80% or more sorbitol, mannitol, and volemitol. The roles of polyols identical to that of tomato SDH can be expressed in non- will be uncovered by suppressing key enzyme rosaceous plants of various families, including mono- expressions or divergences in polyol biosynthesis. cotyledons and gymnosperms, suggesting that functional 4. Polyols in pathogenicity SDH is widespread in the plant kingdom (Fig. 2). SDH expression and activity are also observed in an- High polyol concentrations accumulate in tomato after other fruit, strawberry, which is a sucrose-translocating infection with a fungal pathogen (Clark et al., 2003). plant and contains only a small amount of sorbitol The role of this accumulated arabitol is likely (Duangsrisai et al., 2007, 2008). Strawberry SDH osmoregulation by the pathogen, because water expression is high in the young and mature stages of acquisition is important in pathogenicity. Polyol fruit and is controlled by soluble carbohydrates and accumulation during infection has also been reported in phytohormones, suggesting that SDH may play a role cucumber and sunflower (Abood and Losel, 2003; Jobic in development and response to the environment. et al., 2007). Plantain (Plantago major) is a non-rosaceous, sucrose- Voegele et al. (2005) reported that mannitol and sorbitol-translocating species that has been well accumulation in broad bean after fungal infection is due studied at the molecular level. Polyol transporters have to fungal mannitol dehydrogenase, which could be been cloned from plantain, and their expression is related to carbon storage for the fungal pathogen and reportedly induced by salt stress (Pommerrenig et al., scavenging of reactive oxygen species (ROS), which are 2007; Ramsperger-Gleixner et al., 2004). In contrast, the important in plant defense reactions. Because broad bean expression of SDH, which catabolizes sorbitol, is does not normally synthesize and use mannitol, this decreased by salt stress, resulting in an increase of strategy of the fungal pathogen could be effective for sorbitol in plantain. Plantain is considered salt tolerant pathogenicity. Arabitol is also accumulated after fungal because of the above molecular mechanism. infection of broad bean, possibly as a scavenger of ROS D-mannoheptulose, a seven-carbon sugar, and persei- (Link et al., 2005). Using insertional mutagenesis of the tol, a seven-carbon polyol, are the dominant soluble fungal mannitol 1-phosphate dehydrogenase gene for carbohydrates in avocado (Liu et al., 1999a, 1999b). Liu mannitol synthesis, Solomon et al. (2005) showed that et al. (2002) suggested that the seven-carbon sugar plays it is required for sporulation in planta of a wheat important roles in carbon allocation in avocado trees and pathogen. These findings all indicate the importance of inhibits fruit ripening. polyol accumulation by fungal pathogens in fungus-plant Various polyols are found in ornamental plants of interactions. many species; for example, carnation and redbud contain Analysis of transgenic host plants in relation to plant pinitol (Griffin et al., 2004; Norikoshi et al., 2008). mannitol catabolism and transport confirms the role of Redbud accumulates pinitol under environmental stress mannitol during infection and enhanced disease and pinitol is the methyl ester of chiro-inositol, a cyclitol resistance. The overexpression of celery MTD, which (cyclic polyol). Interestingly, soybean contains pinitol, plays a role in mannitol catabolism, in tobacco plants which reportedly improves health in conditions enhances disease resistance (Jennings et al., 2002). When associated with insulin resistance, such as diabetes the celery mannitol transporter is expressed in tobacco mellitus and obesity (Morinaga et al., 2006). Bornesitol, plants, sensitivity to mannitol-secreting pathogenic fungi a methyl ester of another cyclitol, myo-inositol, is a is decreased, suggesting a role for polyol transporters in J. Japan. Soc. Hort. Sci. 78 (2): 158–168. 2009. 163 defense mechanisms (Juchaux-Cachau et al., 2007). A pathogen is known to use a polyol synthesized by plants. Oh and Beer (2005) suggested that the capability of Erwinia amylovora to use sorbitol is an important factor affecting disease-causing activity in apple shoots. This is likely because E. amylovora, a bacterial plant pathogen, causes fire blight in rosaceous plants such as apple and pear. 5. Genetic engineering of polyol biosynthetic pathways Sorbitol content is increased by drought stress in apple, Fig. 3. Proposed sucrose metabolic pathway in sink tissues and peach, and loquat plants (Cui et al., 2003, 2004; Sircelj engineered sorbitol cycle (Deguchi et al., 2006). Arrows indicate et al., 2005, 2007). Sorbitol is a reliable indicator of the main direction of reactions in the tissues. Susy, sucrose drought stress in apple trees, together with zeaxanthin, synthase; Inv, invertase; UGPase, UDP-glucose pyrophospho- glutathione, and ascorbate (Sircelj et al., 2007). Since rylase; PGM, phosphoglucomutase; HXK, hexokinase; Pase, sorbitol accumulation under environmental stress is phosphatase; PGI, phosphoglucoisomerase; FRK, fructokinase. The sorbitol cycle consists of S6PDH, Pase, SDH, FRK, and related to the induction of S6PDH expression, and PGI in plants transformed with the genes for SDH and S6PDH. S6PDH expression is also induced by abscisic acid, S6PDH is a multifunctional gene for the synthesis of translocatable carbohydrates and response to stress enhancement of stress tolerance (unpublished data); (Deguchi et al., 2002a, 2002b; Kanayama et al., 2006). therefore, the introduction of a gene to convert polyols The activity of celery MTD promoter is inhibited by to other metabolizable carbohydrates, in addition to the salt stress, osmotic stress, and abscisic acid (Zamski et gene for polyol synthesis, is a useful strategy for al., 2001); therefore, mannitol may accumulate under engineering stress tolerance. Alternatively, inducible decreased MTD activity in celery as a response to abiotic expression systems for stress tolerance determinants can stress. In a halophytic plant (Mesembryanthemum be useful for preventing growth inhibition. crystallinum), ononitol and pinitol are produced as a 6. Function of polyol molecules stress response through the action of inositol methyl transferase (Vernon and Bohnert, 1992). The inositol Engineering a polyol cycle represents a strategy that methyl transferase gene can be used for the production avoids polyol-induced growth inhibition by introducing of pinitol and ononitol using transformation techniques the transgene, as described above. In this strategy, plants (Chiera et al., 2006). do not always accumulate excess polyols. On the other Since polyols are involved as compatible solutes, as hand, many reports on polyols and other osmolytes have mentioned above, there have been many efforts to shown that marginal accumulation of these solutes engineer stress tolerance by accumulating polyols increases stress tolerance (Abebe et al., 2003; Chiang et (Abebe et al., 2003; Deguchi et al., 2004; Karakas et al., 2005; Holmström et al., 1996; Karakas et al., 1997; al., 1997; Sheveleva et al., 1997, 1998; Sickler et al., Kavi Kishor et al., 1995; Macaluso et al., 2007; 2007; Tarczynski et al., 1993). Such transformants often Tarczynski et al., 1993; Thomas et al., 1995). In these show growth inhibition (Abebe et al., 2003; Deguchi et cases, the enhanced stress tolerance is not due to osmotic al., 2004; Karakas et al., 1997; Sheveleva et al., 1998), adjustment but involves other mechanisms, such as ROS which could be a serious problem for engineering stress scavenging and stabilization of macromolecular struc- tolerance although it can be useful for the production of tures; therefore, engineering a polyol cycle is a useful dwarf fruit trees (Deguchi et al., 2004). When Sheveleva method to easily obtain plants that accumulate a medium et al. (1998) produced more than 100 tobacco plants amount of polyols and grow normally. transformed with S6PDH that accumulated various Previous studies have shown that osmolytes, including concentrations of sorbitol, many had reduced growth. polyols, have free radical scavenging properties (Shen Abebe et al. (2003) reported that most transgenic wheat et al., 1997; Smirnoff and Cumbes, 1989). Most of these plants with the bacterial mltD gene for biosynthesis of results, however, were from in vitro experiments using mannitol are short and sterile and have twisted leaves relatively high concentrations of compatible solutes. and heads. These transgenic plants show abnormal Cuin and Shabala (2007) found evidence of an in situ growth, possibly because polyols are synthesized as a mitigating effect of low concentrations of compatible dead-end product in plants that do not naturally have solutes, such as mannitol, on ROS by monitoring ROS- polyols; thus, Deguchi et al. (2006) developed a novel induced ion fluxes across the plasma membrane. strategy in which a sorbitol cycle was engineered by Recent studies have shown that polyols protect introducing SDH in addition to S6PDH (Fig. 3). The proteins against denaturation. Jaindl and Popp (2006) engineered sorbitol cycle resulted in normal growth and measured the activity and circular dichroism spectra of 164 Y. Kanayama glutamine synthase and malate dehydrogenase and are also expressed in rosaceous fruit trees (data not showed that cyclitols protect these enzymes from shown). Although SDH is believed to be the main thermally induced denaturation and deactivation. sorbitol-catabolizing enzyme, aldose reductase (NADP- Anderson (2007) determined structure-function relation- dependent sorbitol dehydrogenase), which reduces ships between polyols and the thermal stability of pepper glucose to sorbitol, has been partially purified from apple leaf proteins, finding that the thermal stability of proteins (Yamaki, 1984), and sorbitol can be converted to glucose increased with increasing numbers of OH groups and in apple and pear fruits, as shown in tracer experiments the polarity of polyols. Since mixtures of stabilizing and using 14C-soribitol (Ohkawa et al., 2008); therefore, destabilizing compounds negate each other, the further studies on aldose reductase will be necessary at identification and removal of destabilizing compounds the molecular level in rosaceous fruit trees. As for SDH, should have a similar effect to increasing stabilizers as transgenic studies or other genetic approaches are a strategy for protecting protein conformation. necessary to confirm the physiological roles of the SDH gene family. The relationship between sorbitol catabo- 7. Concluding remarks lism and transport and watercore should also be Polyols play a role not only in the translocation and investigated. Gao et al. (2005) suggested that decreased storage of photosynthates, but also in biotic and abiotic expression of a sorbitol transporter results in late responses in horticultural crops. Polyol metabolism watercore in apple fruit, while Yamada et al. (2006a, might not be essential for plant development, as shown 2006b) found active metabolism and uptake of sorbitol by the suppression of S6PDH expression, although it is in parenchyma cells of early watercored apple fruit. possible that a very small amount of sorbitol is important Literature Cited or that less sorbitol decreases stress tolerance, and this cannot be ruled out until knockout plants are tested. Abebe, T., A. C. Guenzi, B. Martin and J. C. Cushman. 2003. Polyols are likely to be an alternative pathway and Tolerance of mannitol-accumulating transgenic wheat to function specifically in stress responses. water stress and salinity. Plant Physiol. 131: 1748–1755. Abood, J. K. and D. M. Losel. 2003. Changes in carbohydrate The sorbitol/sucrose ratio diverges in the genus Prunus composition of cucumber leaves during the development of according to Moing et al. (1997b). Because sorbitol is powdery mildew infection. Plant Pathol. 52: 256–265. a compatible solute, the ratio to sucrose could be related Anderson, J. A. 2007. Additive effects of alcohols and polyols on to environmental conditions. Some eucalypt species thermostability of pepper leaf extracts. J. Amer. Soc. Hort. accumulate quercitol, a cyclitol derived from inositol, Sci. 132: 67–72. in drought stress conditions (Merchant et al., 2006). The Bantog, N. A., K. Yamada, N. Niwa, K. Shiratake and S. Yamaki. + presence and concentration of quercitol is segregated 2000. Gene expression of NAD -dependent sorbitol dehydrogenase and NADP+-dependent sorbitol-6-phosphate dehydro- among eucalypt species according to taxonomy and genase during development of loquat (Eriobotrya japonica geographic/environmental distribution (Merchant et al., Lindl.) fruit. J. Japan. Soc. Hort. Sci. 69: 231–236. 2007); therefore, the determinant of polyol levels could Berüter, J. and M. E. S. Feusi. 1997. The effect of girdling on be related to evolutionary adaptation to environmental carbohydrate partitioning in the growing apple fruit. J. Plant stress. Since a polyol molecule has multiple functions Physiol. 151: 277–285. in relation to plant stress tolerance, engineering the Brown, P. H., N. Bellaloui, H. Hu and A. Dandekar. 1999a. polyol synthetic pathway will be a powerful tool if the Transgenically enhanced sorbitol synthesis facilitates phloem boron transport and increases tolerance of tobacco to boron side effects of growth inhibition can be avoided. deficiency. Plant Physiol. 119: 17–20. Nucleotide sequences of polyol-related enzymes have Brown, P. H. and H. Hu. 1996. Phloem mobility of boron is species been found in various plant species that do not synthesize dependent: Evidence for phloem mobility in sorbitol-rich polyols as a major photosynthate as a result of recent species. Ann. Bot. 77: 497–505. progress in the genome project. Metabolome analysis Brown, P. H., H. Hu and W. G. Roberts. 1999b. Occurrence of also shows that polyols are synthesized as a minor sugar alcohols determines boron toxicity symptoms of component in various plant species, such as tomato and ornamental species. J. Amer. Soc. Hort. Sci. 124: 347–352. Cataldi, T. R. I., G. Margiotta and C. G. Zambonin. 1998. bean (Hernández et al., 2007; Roessner-Tunali et al., Determination of sugars and alditols in food samples by 2003); their physiological roles can be clarified by HPAEC with integrated pulsed amperometric detection using reverse genetic approaches. It is possible that a novel alkaline eluents containing barium or strontium ions. Food function for polyols will be found, because minor soluble Chem. 62: 109–115. carbohydrates can be important in plant growth, as Cheng, Y., O. Arakawa, M. Kasai and S. Sawada. 2008. Analysis described for trehalose (Schluepmann et al., 2003). of reduced photosynthesis in the apple leaf under sink-limited A cluster of the aldose reductase group in the conditions due to girdling. J. Japan. Soc. Hort. Sci. 77: 115– 121. phylogenetic tree of polyol-related enzymes is shown in Cheng, L., R. Zhou, E. J. Reidel, T. D. Sharkey and A. M. Figure 1. Aldose reductases have been investigated in Dandekar. 2005. Antisense inhibition of sorbitol synthesis various non-rosaceous plants (Gavidia et al., 2002; leads to up-regulation of starch synthesis without altering Mundree et al., 2000), although their substrates are not CO2 assimilation in apple leaves. Planta 220: 767–776. always glucose and sorbitol. Aldose reductase homologs Chiang, Y., C. Stushnoff, A. E. McSay, M. L. Jones and H. J. J. Japan. Soc. Hort. Sci. 78 (2): 158–168. 2009. 165

Bohnert. 2005. Overexpression of mannitol-1-phosphate of sorbitol transporters from developing sour cherry fruit and dehydrogenase increases mannitol accumulation and adds leaf sink tissues. Plant Physiol. 131: 1566–1575. protection against chilling injury in petunia. J. Amer. Soc. Gavidia, I., P. Pérez-Bermúdez and H. U. Seitz. 2002. Cloning Hort. Sci. 130: 605–610. and expression of two novel aldo-keto reductases from Chiera, J. M., J. G. Streeter and J. J. Finer. 2006. Ononitol and Digitalis purpurea leaves. Eur. J. Biochem. 269: 2842–2850. pinitol production in transgenic soybean containing the Griffin, J. J., T. G. Ranney and D. M. Pharr. 2004. Heat and inositol methyl transferase gene from Mesembryanthemum drought influence photosynthesis, water relations, and soluble crystallinum. Plant Sci. 171: 647–654. carbohydrates of two ecotypes of redbud (Cercis canadensis). Clark, A. J., K. J. Blissett and R. P. Oliver. 2003. Investigating J. Amer. Soc. Hort. Sci. 129: 497–502. the role of polyols in Cladosporium fulvum during growth Hafliger, B., E. Kindhauser and F. Keller. 1999. Metabolism of under hyper-osmotic stress and in planta. Planta 216: 614– D-glycero-D-manno-heptitol, volemitol, in polyanthus. Dis- 619. covery of a novel ketose reductase. Plant Physiol. 119: 191– Conde, C., P. Silva, A. Agasse, R. Lemoine, S. Delrot, R. Tavares 197. and H. Geros. 2007. Utilization and transport of mannitol in Hernández, G., M. Ramírez, O. Valdés-López, M. Tesfaye, M. A. Olea europaea and implications for salt stress tolerance. Plant Graham, T. Czechowski, A. Schlereth, M. Wandrey, A. Erban, Cell Physiol. 48: 42–53. F. Cheung, H. C. Wu, M. Lara, C. D. Town, J. Kopka, M. Cui, S., G. L. Chen and N. Nii. 2003. Effects of water stress on K. Udvardi and C. P. Vance. 2007. Phosphorus stress in sorbitol production and anatomical changes in the nuclei of common bean: Root transcript and metabolic responses. Plant leaf and root cells of young loquat trees. J. Japan. Soc. Hort. Physiol. 144: 752–767. Sci. 72: 359–365. Holmström, K-O., E. Mäntylä, B. Welin, A. Mandal and E. T. Cui, S., K. Sadayoshi, Y. Ogawa and N. Nii. 2004. Effects of Palva. 1996. Drought tolerance in tobacco. Nature 379: 683– water stress on sorbitol content in leaves and roots, anatomical 684. changes in cell nuclei, and starch accumulation in leaves of Hu, H., S. G. Penn, C. B. Lebrilla, P. H. Brown and H. Patrick. young peach trees. J. Japan. Soc. Hort. Sci. 73: 25–30. 1997. Isolation and characterization of soluble boron Cuin, T. A. and S. Shabala. 2007. Compatible solutes reduce ROS- complexes in higher plants: The mechanism of phloem induced potassium efflux in Arabidopsis roots. Plant Cell mobility of boron. Plant Physiol. 113: 649–655. Environ. 30: 875–885. Ichimura, K., K. Kohata, Y. Mukasa, Y. Yamaguchi, R. Goto and Deguchi, M., A. B. Bennett, S. Yamaki, K. Yamada, K. Kanahama K. Suto. 1999. Identification of L-bornesitol and changes in and Y. Kanayama. 2006. An engineered sorbitol cycle alters its content during flower bud development in sweet pea sugar composition, not growth, in transformed tobacco. Plant (Lathyrus odoratus L.). Biosci. Biotech. Biochem. 63: 189– Cell Environ. 29: 1980–1988. 191. Deguchi, M., Y. Koshita, M. Gao, R. Tao, T. Tetsumura, S. Yamaki Iida, M., N. A. Bantog, K. Yamada, K. Shiratake and S. Yamaki. and Y. Kanayama. 2004. Engineered sorbitol accumulation 2004. Sorbitol- and other sugar-induced expressions of the induces dwarfism in Japanese persimmon. J. Plant Physiol. NAD+-dependent sorbitol dehydrogenase gene in Japanese 161: 1177–1184. pear fruit. J. Amer. Soc. Hort. Sci. 129: 870–875. Deguchi, M., H. Saeki, W. Ohkawa, K. Kanahama and Y. Ito, A., H. Hayama and Y. Kashimura. 2005. Partial cloning and Kanayama. 2002a. Effects of low temperature on sorbitol expression analysis of genes encoding NAD+-dependent biosythesis in peach leaves. J. Japan. Soc. Hort. Sci. 71: 446– sorbitol dehydrogenase in pear bud during flower bud 448. formation. Sci. Hort. 103: 413–420. Deguchi, M., M. Watanabe and Y. Kanayama. 2002b. Increase in Jaindl, M. and M. Popp. 2006. Cyclitols protect glutamine sorbitol biosynthesis in stressed Japanese pear leaves. Acta synthetase and malate dehydrogenase against heat induced Hort. 587: 511–517. deactivation and thermal denaturation. Biochem. Biophys. Duangsrisai, S., K. Yamada, N. A. Bantog, K. Shiratake, Y. Res. Commun. 345: 761–765. Kanayama and S. Yamaki. 2007. Presence and expression of Jennings, D. B., M. E. Daub, D. M. Pharr and J. D. Williamson. NAD+-dependent sorbitol dehydrogenase and sorbitol-6- 2002. Constitutive expression of a celery mannitol phosphate dehydrogenase genes in strawberry. J. Hort. Sci. dehydrogenase in tobacco enhances resistance to the Biotech. 82: 191–198. mannitol-secreting fungal pathogen Alternaria alternata. Duangsrisai, S., K. Yamada, N. A. Bantog, K. Shiratake, Y. Plant J. 32: 41–49. Kanayama and S. Yamaki. 2008. Properties of sorbitol Jobic, C., A. Boisson, E. Gout, C. Rascle, M. Fevre, P. Cotton dehydrogenase in strawberry fruit and enhancement of the and R. Bligny. 2007. Metabolic processes and carbon nutrient activity by fructose and auxin. J. Japan. Soc. Hort. Sci. 77: exchanges between host and pathogen sustain the disease 318–323. development during sunflower infection by Sclerotinia Everard, J. D., C. Cantini, R. Crumet, J. Plummer and W. H. sclerotiorum. Planta 226: 251–265. Loescher. 1997. Molecular cloning of mannose-6-phosphate Juchaux-Cachau, M., L. Landouar-Arsivaud, J. Pichaut, C. reductase and its developmental expression in celery. Plant Campion, B. Porcheron, J. Jeauffre, N. Noiraud-Romy, P. Physiol. 113: 1427–1435. Simoneau, L. Maurousset and R. Lemoine. 2007. Character- Fridman, E., F. Carrari, Y. L. Liu, A. R. Fernie and D. Zamir. ization of AgMaT2, a plasma membrane mannitol transporter 2004. Zooming in on quantative trait for tomato yield using from celery, expressed in phloem cells, including phloem interspecific introgressions. Science 305: 1785–1789. parenchyma cells. Plant Physiol. 145: 62–74. Gao, Z., S. Jayanty, R. Beaudry and W. Loescher. 2005. Sorbitol Kanamaru, N., Y. Ito, S. Komori, M. Saito, H. Kato, S. Takahashi, transporter expression in apple sink tissues: implications for M. Omura, J. Soejima, K. Shiratake, K. Yamada and S. fruit sugar accumulation and watercore development. J. Amer. Yamaki. 2004. Transgenic apple transformed by sorbitol-6- Soc. Hort. Sci. 130: 261–268. phosphate dehydrogenase cDNA switch between sorbitol and Gao, Z., L. Maurousset, R. Lemoine, S. Yoo, S. van Nocker and sucrose supply due to its gene expression. Plant Sci. 167: W. Loescher. 2003. Cloning, expression, and characterization 55–61. 166 Y. Kanayama

Kanayama, Y., M. Kogawa, M. Yamaguchi and K. Kanahama. temperature and paraquat. J. Hort. Sci. Biotech. 82: 979–985. 2005. Fructose content and the activity of fructose-related Merchant, A., P. Y. Ladiges and M. A. Adams. 2007. Quercitol enzymes in the fruit of eating-quality peach cultivars and links the physiology, taxonomy and evolution of 279 eucalypt native-type peach cultivars. J. Japan. Soc. Hort. Sci. 74: 431– species. Global Ecol. Biogeogr. 16: 810–819. 436. Merchant, A., M. Tausz, S. K. Arndt and M. A. Adams. 2006. Kanayama, Y., H. Mori, H. Imaseki and S. Yamaki. 1992. Cyclitols and carbohydrates in leaves and roots of 13 Nucleotide sequence of a cDNA encoding NADP-sorbitol-6- Eucalyptus species suggest contrasting physiological phosphate dehydrogenase from apple. Plant Physiol. 100: responses to water deficit. Plant Cell Environ. 29: 2017–2029. 1607–1608. Moing, A., F. Carbonne, B. Zipperlin, L. Svanella and J. P. Kanayama, Y., K. Sakanishi, H. Mori and S. Yamaki. 1995. Gaudillere. 1997a. Phloem loading in peach: Symplastic or Expression of the gene encoding NADP-dependent sorbitol- apoplastic? Physiol. Plant. 101: 489–496. 6-phosphate dehydrogenase in apple seedlings. Plant Cell Moing, A., N. Langlois, L. Svanella, A. Zanetto and J. P. Physiol. 36: 1139–1141. Gaudillere. 1997b. Variability in sorbitol: sucrose ratio in Kanayama, Y., M. Watanabe, R. Moriguchi, M. Deguchi, K. mature leaves of different Prunus species. J. Amer. Soc. Hort. Kanahama and S. Yamaki. 2006. Effects of low temperature Sci. 122: 83–90. and abscisic acid on the expression of the sorbitol-6-phosphate Morinaga, T., M. Yamaguchi, Y. Makino, H. Nanamiya, K. dehydrogenase gene in apple leaves. J. Japan. Soc. Hort. Sci. Takahashi, H. Yoshikawa, F. Kawamura, H. Ashida and K. 75: 20–25. Yoshida. 2006. Functional myo-inositol catabolic genes of Kanayama, Y. and S. Yamaki. 1993. Purification and properties Bacillus subtilis natto are involved in depletion of pinitol in of NADP-dependent sorbitol-6-phosphate dehydrogenase natto (fermented soybean). Biosci. Biotech. Biochem. 70: from apple seedlings. Plant Cell Physiol. 34: 819–823. 1913–1920. Karakas, B., P. Ozias-Akins, C. Stushnoff, M. Suefferheld and Mundree, S. G., A. Whittaker, J. A. Thomson and J. M. Farrant. M. Rieger. 1997. Salinity and drought tolerance of mannitol- 2000. An aldose reductase homolog from the resurrection accumulating transgenic tobacco. Plant Cell Environ. 20: plant Xerophyta viscosa Baker. Planta 211: 693–700. 609–616. Nadwodnik, J. and G. Lohaus. 2008. Subcellular concentrations Kavi Kishor, B. P., Z. Hong, G. Miao, C. A. Hu and D. P. S. of sugar alcohols and sugars in relation to phloem Verma. 1995. Overexpression of Δ1-pyrroline-5-carboxylate translocation in Plantago major, Plantago maritima, Prunus synthetase increases proline production and confers osmotol- persica, and Apium graveolens. Planta 227: 1079–1089. erance in transgenic plants. Plant Physiol. 108: 1387–1394. Noiraud, N., L. Maurousset and R. Lemoine. 2001. Identification Kikuchi, K., S. Ishii, S. Fujimaki, N. Suzui, S. Matsuhashi, I. of a mannitol transporter, AgMaT1, in celery phloem. Plant Honda, Y. Shishido and N. Kawachi. 2008. Real-time analysis Cell 13: 695–705. of photoassimilate translocation in intact eggplant fruit using Norikoshi, R., H. Imanishi and K. Ichimura. 2008. A simple and 11 CO2 and a positron-emitting tracer imaging system. J. Japan. rapid extraction method of carbohydrates from petals or sepals Soc. Hort. Sci. 77: 199–205. of four floricultural plants for determination of their content. Kikuchi, K., K. Kanahama and Y. Kanayama. 2003. Changes in J. Japan. Soc. Hort. Sci. 77: 289–295. sugar-related enzymes during wilting of cut delphinium Nosarzewski, M. and D. D. Archbold. 2007. Tissue-specific flowers. J. Japan. Soc. Hort. Sci. 72: 37–42. expression of SORBITOL DEHYDROGENASE in apple fruit Liakopoulos, G., S. Stavrianakou, M. Filippou, C. Fasseas, C. during early development. J. Exp. Bot. 58: 1863–1872. Tsadilas, I. Drossopoulos and G. Karabourniotis. 2005. Boron Nosarzewski, M., A. M. Clements, A. B. Downie and D. D. remobilization at low boron supply in olive (Olea europaea) Archbold. 2004. Sorbitol dehydrogenase expression and in relation to leaf and phloem mannitol concentrations. Tree activity during apple fruit set and early development. Physiol. Physiol. 25: 157–165. Plant. 121: 391–398. Link, T., G. Lohaus, I. Heiser, K. Mendgen, M. Hahn and R. T. Oh, C. and S. V. Beer. 2005. Molecular genetics of Erwinia Voegele. 2005. Characterization of a novel NADP+-dependent amylovora involved in the development of fire blight. FEMS D-arabitol dehydrogenase from the plant pathogen Uromyces Microbiol. Lett. 253: 185–192. fabae. Biochem. J. 389: 289–295. Ohkawa, W., S. Moriya, K. Kanahama and Y. Kanayama. 2008. Liu, X., P. W. Robinson, M. A. Madore, G. W. Witney and Reevaluation of sorbitol metabolism in the fruit of Rosaceae M. L. Arpaia. 1999a. ‘Hass’ avocado carbohydrate fluctua- fruit trees. Acta Hort. 772: 159–166. tions. I. Growth and phenology. J. Amer. Soc. Hort. Sci. 124: Ohta, K., R. Moriguchi, K. Kanahama, S. Yamaki and Y. 671–675. Kanayama. 2005. Molecular evidence of sorbitol dehydroge- Liu, X., P. W. Robinson, M. A. Madore, G. W. Witney and nase in tomato, a non-Rosaceae plant. Phytochemistry 66: M. L. Arpaia. 1999b. ‘Hass’ avocado carbohydrate fluctua- 2822–2828. tions. II. Fruit growth and ripening. J. Amer. Soc. Hort. Sci. Oura, Y., K. Yamada, K. Shiratake and S. Yamaki. 2000. 124: 676–681. Purification and characterization of a NAD-dependent Liu, X., J. Sievert, M. L. Arpaia and M. A. Madore. 2002. sorbitol dehydrogenase from Japanese pear fruit. Phytochem- Postulated physiological roles of the seven-carbon sugars, istry 54: 567–572. mannoheptulose, and perseitol in avocado. J. Amer. Soc. Park, S. W., K. J. Song, M. Y. Kim, J. H. Hwang, Y. U. Shin, Hort. Sci. 127: 108–114. W. C. Kim and W. I. Chung. 2002. Molecular cloning and Loescher, W., R. H. Tyson, J. D. Everard, R. J. Redgwell and characterization of four cDNAs encoding the isoforms of R. L. Bieleski. 1992. Mannitol synthesis in higher plants. NAD-dependent sorbitol dehydrogenase from the Fuji apple. Evidence for the role and characterization of a NADPH- Plant Sci. 162: 513–519. dependent mannose 6-phosphate reductase. Plant Physiol. 98: Pommerrenig, B., F. S. Papini-Terzi and N. Sauer. 2007. 1396–1402. Differential regulation of sorbitol and sucrose loading into Macaluso, L., R. Lo Bianco and M. Rieger. 2007. Mannitol- the phloem of Plantago major in response to salt stress. Plant producing tobacco exposed to varying levels of water, light, Physiol. 144: 1029–1038. J. Japan. Soc. Hort. Sci. 78 (2): 158–168. 2009. 167

Ramsperger-Gleixner, M., D. Geiger, R. Hedrich and N. Sauer. Teo, G., Y. Suzuki, S. L. Uratsu, B. Lampinen, N. Ormonde, 2004. Differential expression of sucrose transporter and W. K. Hu, T. M. DoJong and A. M. Dandekar. 2006. Silencing polyol transporter genes during maturation of common leaf sorbitol synthesis alters long-distance partitioning and plantain companion cells. Plant Physiol. 134: 147–160. apple fruit quality. Proc. Natl. Acad. Sci. USA 103: 18842– Roessner-Tunali, U., H. Bjorn, A. Lytovchenko, F. Carrari, C. 18847. Bruediagam, D. Granot and A. R. Fernie. 2003. Metabolic Thomas, J. C., M. Sepahi, B. Arendall and H. J. Bohnert. 1995. profiling of transgenic tomato plants overexpression Enhancement of seed germination in high salinity by hexokinase reveals that the influence of hexose phosphory- engineering mannitol expression in Arabidopsis thaliana. lation diminishes during fruit development. Plant Physiol. Plant Cell Environ. 18: 801–806. 133: 84–99. van Bel, A. J. E. and P. H. Hess. 2008. Hexoses as phloem transport Sakanishi, K., Y. Kanayama, H. Mori, K. Yamada and S. Yamaki. sugars: the end of a dogma? J. Exp. Bot. 59: 261–272. 1998. Expression of the gene for NADP-dependent sorbtiol- Vernon, D. M. and H. J. Bohnert. 1992. Increased expression of 6-phosphate dehydrogenase in peach leaves of various myo-inositol methyl transferase in Mesembryanthemum developmental stages. Plant Cell Physiol. 39: 1372–1374. crystallinum is part of a stress response distinct from Schluepmannm, H., T. Pellny, A. van Dijken, S. Smeekens and Crassulacean acid metabolism induction. Plant Physiol. 99: M. Paul. 2003. Trehalose 6-phosphate is indispensable for 1695–1698. carbohydrate utilization and growth in Arabidopsis thaliana. Voegele, R. T., M. Hahn, G. Lohaus, T. Link, I. Heiser and K. Proc. Natl. Acad. Sci. USA 100: 6849–6854. Mendgen. 2005. Possible roles for mannitol and mannitol Shen, B., R. G. Jensen and H. J. Bohnert. 1997. Mannitol protects dehydrogenase in the biotrophic plant pathogen Uromyces against oxidation by hydroxyl radicals. Plant Physiol. 115: fabae. Plant Physiol. 137: 190–198. 527–532. Watari, J., Y. Kobae, S. Yamaki, K. Yamada, K. Toyofuku, T. Sheveleva, E., W. Chmara, H. J. Bohnert and R. G. Jensen. 1997. Tabuchi and K. Shiratake. 2004. Identification of sorbitol Increase in salt and drought tolerance by D-ononitol transporters expressed in the phloem of apple source leaves. production in transgenic Nicotiana tabacum L. Plant Physiol. Plant Cell Physiol. 45: 1032–1041. 115: 1211–1219. Williamson, J. D., J. M. H. Stoop, M. O. Massel, M. A. Conkling Sheveleva, E., S. Marquez, W. Chmara, A. Zegeer, R. G. Jensen and D. M. Pharr. 1995. Sequence analysis of a mannitol and H. J. Bohnert. 1998. Sorbitol-6-phosphate dehydrogenase dehydrogenase cDNA from plants reveals a function for the expression in transgenic tobacco. High amounts of sorbitol pathogenesis-related protein ELI3. Proc. Natl. Acad. Sci. lead to necrotic lesions. Plant Physiol. 117: 831–839. USA 92: 7148–7152. Sickler, C. M., G. E. Edwards, O. Kiirats, Z. Gao and W. Loescher. Yamada, H., Y. Kaga and S. Amano. 2006a. Cellular 2007. Response of mannitol-producing Arabidopsis thaliana compartmentation and membrane permeability to sugars in to abiotic stress. Funct. Plant Biol. 34: 382–391. relation to early or high temperature-induced watercore in Sircelj, H., M. Tausz, D. Grill and F. Batic. 2005. Biochemical apples. Sci. Hort. 108: 29–34. responses in leaves of two apple tree cultivars subjected to Yamada, H., T. Okumura, Y. Miyazaki and S. Amano. 2006b. progressing drought. J. Plant Physiol. 162: 1308–1318. Seasonal changes in cellular compartmentation and mem- Sircelj, H., M. Tausz, D. Grill and F. Batic. 2007. Detecting brane permeability to sugars in relation to early or high different levels of drought stress in apple trees (Malus temperature-induced watercore in ‘Orin’ apples. J. Hort. Sci. domestica Borkh.) with selected biochemical and physiolog- Biotech. 81: 1069–1073. ical parameters. Sci. Hort. 113: 362–369. Yamada, K., H. Mori and S. Yamaki. 1999. Gene expression of Smirnoff, N. and Q. J. Cumbes. 1989. Hydroxyl radical scavenging NAD-dependent sorbitol dehydrogenase during fruit devel- activity of compatible solutes. Phytochemistry 28: 1057– opment of apple (Malus pumila Mill. var. domestica 1060. Schneid.). J. Japan. Soc. Hort. Sci. 68: 1099–1103. Solomon, P. S., K. Tan and R. P. Oliver. 2005. Mannitol 1- Yamada, K., N. Niwa, K. Shiratake and S. Yamaki. 2001. cDNA phosphate metabolism is required for sporulation in planta cloning of NAD-dependent sorbitol dehydrogenase from of the wheat pathogen Stagonospora nodorum. Mol. Plant- peach fruit and its expression during fruit development. J. Microbe Interact. 18: 110–115. Hort. Sci. Biotech. 76: 581–587. Stoop, J. M. H., J. D. Willamson, M. A. Conkling and D. M. Yamada, K., Y. Oura, H. Mori and S. Yamaki. 1998. Cloning of Pharr. 1995. Purification of NAD-dependent mannitol NAD-dependent sorbitol dehydrogenase from apple fruit and dehydrogenase from celery suspension cultures. Plant gene expression. Plant Cell Physiol. 39: 1375–1379. Physiol. 108: 1219–1225. Yamada, K., Y. Suzue, S. Hatano, M. Tsukuda, Y. Kanayama, K. Suzue, Y., M. Tsukuda, S. Hatano, Y. Kanayama, K. Yamada, K. Shiratake and S. Yamaki. 2006c. Changes in the activity and Shiratake and S. Yamaki. 2006. Changes in the activity and gene expression of sorbitol- and sucrose-related enzymes gene expression of sorbtiol- and sucrose-related enzymes with associated with development of ‘La France’ pear fruit. J. leaf development of ‘La France’ pear. J. Japan. Soc. Hort. Japan. Soc. Hort. Sci. 75: 38–44. Sci. 75: 45–50. Yamaki, S. 1984. NADP+-dependent sorbitol dehydrogenase found Tanase, K., Y. Ushio and K. Ichimura. 2005. Effects of light in apple leaves. Plant Cell Physiol. 25: 1323–1327. intensity on flower life of potted Delphinium plants. J. Japan. Zamski, E., W. Guo, Y. T. Yamamoto, D. M. Pharr and J. D. Soc. Hort. Sci. 74: 395–397. Williamson. 2001. Analysis of celery (Apium graveolens) Tarczynski, M. C., R. G. Jensena and H. J. Bohnert. 1993. Stress mannitol dehydrogenase (Mtd) promoter regulation in protection of transgenic tobacco by production of the Arabidopsis suggests roles for MTD in key environmental osmolyte mannitol. Science 259: 508–510. and metabolic responses. Plant Mol. Biol. 47: 621–631. Tao, R., S. L. Uratsu and A. M. Dandekar. 1995. Sorbitol synthesis Zhang, L., Y. Peng, S. Pelleschi-Travier, Y. Fan, Y. Lu, Y. Lu, in transgenic tobacco with apple cDNA encoding NADP- X. Gao, Y. Shen, S. Delrot and D. Zhang. 2004. Evidence dependent sorbitol-6-phosphate dehydrogenase. Plant Cell for apoplasmic phloem unloading in developing apple fruit. Physiol. 36: 525–532. Plant Physiol. 135: 574–586. 168 Y. Kanayama

Zhang, C., K. Tanabe, F. Tamura, A. Itai and M. Yoshida. 2007. synthase in shoot tips of the transgenic apple trees with Role of gibberellins in increasing sink demand in Japanese decreased synthesis. J. Exp. Bot. 57: 3647–3657. pear fruit during rapid fruit growth. Plant Growth Reg. 52: Zhou, R., L. Cheng and R. Wayne. 2003. Purification and 161–172. characterization of sorbitol-6-phosphate phosphatase from Zhou, R., L. Cheng and A. M. Dandekar. 2006. Down-regulation apple leaves. Plant Sci. 165: 227–232. of sorbitol dehydrogenase and up-regulation of sucrose