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Dongfeng Liu, Junbei Ni, Ruiyuan Wu, and Yuanwen Teng

almost equal in concentration to sucrose content at fruit maturity ( Chen et al., 2011 ; Yao et al., 2010 ). Sorbitol metabolism in Rosaceae fruit also changes with fruit development and is related to fruit growth rate and sink strength ( Bianco et al

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Riccardo Lo Bianco, Mark Rieger, and She-Jean S. Sung

Sorbitol is the major photosynthetic product in peach. In sink tissues, sorbitol is converted to fructose via the NAD-dependent enzyme sorbitol dehydrogenase (SDH). A new assay is described that allows rapid, simple quantitation of SDH activity in growing shoot tips, root tips, and fruits. The activity was measured on the crude extract desalted with a Saphadex G-25 column to eliminate small molecules such as sugars and nucleotides. Optimum buffer type and pH for the enzyme as well as degradation by proteolytic enzymes and stability over time were determined in the present study. Inhibition by dithiothreitol (DTT) was detected at an inhibitor concentration as low as 2 mM, proving the similarity with mammalian SDH. Storage of samples at 4°C overnight resulted in significant loss of enzyme activity. Using this assay, we also correlated SDH activity with sink strength in peach.

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Dwight S. Fujii and Abhaya M. Dandekar

Many tree crops belonging to the Rosaceae family translocate and metabolize sorbitol. We have determined that some species of bacteria belonging to the genus Agrobacterium, Pseudomonas, and Erwinia pathogenic to the Rosaceae demonstrate the ability to metabolize sorbitol while those that were isolated from other hosts could not utilize sorbitol. Employing cellulose acetate electrophoresis (CAE) we have been able to demonstrate the presence of isoenzymes of sorbitol dehydrogenase (SDH) that correlate with the ability to metabolize sorbitol in these organisms. In order to study the properties of SDH in these organisms we carried out a detailed enzymatic analysis of the enzyme from A. tumefaciens. We found that the enzyme displayed activity when mannitol or xylitol were used as substrates, in addition to sorbitol. Michaelis constants (Km) were 32.8 mM, 0.19 mM, and 38.2 mM for sorbitol, mannitol, and xylitol respectively. To further distinguish the reactions with the different substrates the enzymatic extracts were further characterized on CAE using different substrates to visualize patterns of isoenzymes for a particular sugar alcohol. These analyses revealed the presence of unique isoenzymes for SDH. In addition we observed the presence of mannitol dehydrogenase (MDH) representing in most species a non-specific polyol dehydrogenase.

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Douglas D. Archbold* and Marta Nosarszewski

Acquiring sufficient carbohydrate is essential for successful apple fruit set. Sorbitol may be the dominant carbohydrate imported by growing fruit, and the rate of sorbitol accumulation may be a function of NAD-dependent sorbitol dehydrogenase (SDH; EC activity. Prior work indicated that SDH activity from whole fruit (seeds plus cortex) increased for 2 or 3 weeks after initiation of fruit growth and then declined through 5 weeks. Using SDH activity assays, an SDH-specific antibody, and SDH-specific probes in Northern analyses, it is evident that SDH is expressed and is active in both apple seed and cortex tissue during the first few weeks of fruit growth. On a per unit protein basis, SDH activity in seeds increased by the pattern described above while that in fruit was generally lower and constant. During this same period of time, the sorbitol content of the expressed sap of apple shoots was analyzed. The sorbitol concentration was 50- to 100-fold higher than the sucrose concentration. The concentrations of both carbohydrates changed in parallel to the change in SDH activity of whole fruit and seeds. The lowest SDH activity and sap sorbitol levels preceded and/or coincided with the beginning of the natural fruit drop (or June drop) period.

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Takaya Moriguchi, Tetsuro Sanada, and Shohei Yamaki

Sugar levels and composition were determined in developing `Hakuto' peach (Prunus persica Batsch var. vulgaris Maxim.) fruit. Glucose and fructose in nearly equal amounts were the predominant sugars detected during the early stage of development. Sucrose subsequently began to accumulate and was the predominant sugar in mature fruit. Sorbitol remained at a low level throughout development. The large increase in the amount of sucrose was accompanied by a rapid increase in sucrose synthase (EC activity. Sucrose phosphate synthase (EC was also detected in flesh extracts, but the activities were low throughout development. Acid invertase (EC activity was highest in young fruit and declined with development. Activity, however, increased again at a later stage of development. Peach fruit contained appreciable sorbitol oxidase activity, while other sorbitol-related enzymes were barely detectable, suggesting that transported sorbitol was predominantly converted to glucose. These results suggest that the supply of glucose and fructose depends on acid invertase and sorbitol oxidase, and that accumulation of sucrose depends on-sucrose synthase.

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Hideaki Yamaguchi, Yoshinori Kanayama, Junichi Soejima, and Shohei Yamaki

Seasonal changes in the amounts of the NAD-dependent sorbitol dehydrogenase (NAD-SDH) (enzyme code, protein in developing apple (Malus pumila Mill var. domestica Schneid) fruit were determined by immunoblotting analysis. The amounts of the enzyme protein were very low in young fruit and rose as fruit matured. The weak correlation between enzyme protein and NAD-SDH activity and also the changes in NAD-SDH specific activity suggested that there could be posttranslational modification to the pre-existing enzyme or isoenzyme(s) of NAD-SDH. The changes in the amounts of NAD-SDH protein did not show the same pattern as those in relative growth rate, which is used to express sink activity, especially in young fruit. The role of NAD-SDH on sink activity in apple fruit, therefore, could not be explained simply by the amount and activity of the enzyme. In young fruit, it seems that enzymes other than NAD-SDH would be more directly related with fruit growth.

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Kenji Kobashi, Hiroshi Gemma, and Shuichi Iwahori

A water stress treatment was imposed on peach trees [Prunus persica (L.) Batsch `Kansuke Hakuto' (Peach Group)] to elucidate the relationship among sugar accumulation, sugar metabolism, and abscisic acid (ABA) in fruit under water stress. Treatment was carried out on peach trees grown in containers from 8 July 1996 [80 days after full bloom (DAFB)] for 16 days, to achieve a predawn water potential of -0.8 to -1.1 MPa compared to that of -0.4 to -0.6 MPa in control trees. Levels of sorbitol, sucrose, and total sugars, as well as the activity of sorbitol oxidase increased in fruit of water stressed trees under moderate water stress (-0.8 MPa), whereas under severe water stress (-1.1 MPa), no difference between the waterstressed trees and the controls was observed. Water stress also induced an increase in ABA in the fruit. These initial results indicated that water-stress-induced ABA accelerated sugar accumulation in peaches by activating sorbitol metabolism.

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Hisashi Yamada, Hirokazu Ohmura, Chizuru Arai, and Makoto Terui

The influence of controlling temperature during apple (Malus domestica Borkh.) maturation on sugars, fruit maturity, and watercore occurrence was investigated in watercore-susceptible `Himekami' and `Fuji' apples. The incidence of watercore at 13/5 and 23/15C was greater than at 33/25C or ambient temperatures in `Himekami' apples in 1991 and was greater at 18/10C than at other temperatures in `Fuji' apples in 1990. In 1992, the extent of watercore increased as fruit temperature decreased from 28 to 14C and 23 to 9C in `Himekami' and `Fuji' apples, respectively. Watercore occurrence affected by fruit temperature was not related to fruit maturity, as judged by ethylene evolution. The effect of fruit temperature on sorbitol was relatively small compared with that on other sugars, and no relationship was found between watercore development and sugars. These results suggest that fruit temperature affects watercore expression independently of fruit maturity or sorbitol metabolism in the fruit in watercore-susceptible apple cultivars.

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Riccardo Lo Bianco, Brunella Morandi, and Mark Rieger

Along with sucrose, sorbitol represents the major photosynthetic product and the main form of translocated carbon in peach. The objective of the present study was to determine whether in peach fruit, sorbitol and sucrose enzyme activities are source-regulated, and more specifically modulated by sorbitol or sucrose availability. In two separate trials, peach fruit relative growth rate (RGR), enzyme activities, and carbohydrates were measured 1) at cell division stage before and after girdling of the shoot subtending the fruit; and 2) on 14 shoots with different leaf to fruit ratio (L:F) at cell division and cell expansion stages. Fruit RGR and sorbitol dehydrogenase (SDH) activity were significantly reduced by girdling, whereas sucrose synthase (SS), acid invertase (AI), and neutral invertase (NI) where equally active in girdled and control fruits on the fourth day after girdling. All major carbohydrates (sorbitol, sucrose, glucose, fructose and starch) were reduced on the fourth day after girdling. SDH activity was the only enzyme activity proportional to L:F in both fruit developmental stages. Peach fruit incubation in sorbitol for 24 hours also resulted in SDH activities higher than those of fruits incubated in buffer and similar to those of freshly extracted samples. Overall, our data provide some evidence for regulation of sorbitol metabolism, but not sucrose metabolism, by photoassimilate availability in peach fruit. In particular, sorbitol translocated to the fruit may function as a signal for modulating SDH activity.

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Valeria Sigal-Escalada and Douglas D. Archbold

Sorbitol dehydrogenase (SDH) is a key enzyme in apple fruit converting sorbitol into fructose. SDH activity in `Fuji' apple was reported to increase close to harvest, perhaps as part of the ripening process. Aminoethoxyvinylglycine (AVG) is used to delay fruit ripening and prevent fruit drop, though its effect on sorbitol metabolism is not known. To determine if the late season increase in SDH activity is common among apple cultivars and if AVG use affects SDH expression and activity, AVG was applied to `Lodi', `Redchief Delicious', and `Red Fuji' trees 4 weeks before harvest. Control and AVG-treated fruit were collected 1 week prior to, at, and 1 week after the normal harvest date for assessment of ethylene production over time after harvest and SDH presence and activity at harvest. Ethylene production in control fruit increased after harvest and AVG reduced it in all cultivars. `Redchief Delicious' fruit had the highest ethylene production of the treated samples. The levels of SDH activity in controls were similar across dates for `Redchief Delicious' and showed no consistent pattern in the other cultivars. `Redchief Delicious' and `Red Fuji' showed the highest and lowest levels of SDH activity, respectively. AVG did not affect SDH activity in `Redchief Delicious', and substantially increased SDH activity in `Red Fuji' on each of the three harvest dates, and, in `Lodi', only 1 week after normal harvest. SDH presence was confirmed through immunoblotting for all cultivars and harvest dates. Overall, fruit with the greatest reduction in ethylene production in response to AVG also showed changes in SDH activity.