Mannitol is the most abundant sugar alcohol in nature, occurring in bacteria, fungi, lichens, and many species of vascular plants. Celery (Apium graveolens L.), a plant that forms mannitol photosynthetically, has high photosynthetic rates thought to result from intrinsic differences in the biosynthesis of hexitols vs. sugars. Celery also exhibits high salt tolerance due to the function of mannitol as an osmoprotectant. A mannitol catabolic enzyme that oxidizes mannitol to mannose (mannitol dehydrogenase, MTD) has been identified. In celery plants, MTD Mannitol is a six-carbon noncyclic sugar alcohol found in diverse organisms ranging from bacteria to higher plants. Mannitol is present in more than 100 species of higher plants, where it can be a significant portion of the soluble carbohydrate (1-3). For instance, celery (Apium graveolens) translocates up to 50% of its photoassimilate as mannitol, with the remainder being sucrose (4). Both translocated carbohydrates are assimilated during growth of nonphotosynthetic heterotrophic (i.e., sink) tissues. Other postulated roles for mannitol include carbon storage, free radical scavenging, and osmoprotection (4-7).The use of mannitol as a photoassimilate and translocated carbohydrate is reported to be advantageous to the plant in several ways. Celery, a C3 plant, has carbon fixation rates equivalent to those of many C4 plants (8). This may result from both increased NADP/NADPH turnover compared to plants that exclusively form sugars and from the additional cytosolic sink for photosynthetically fixed CO2 provided by mannitol synthesis (7,9,10). In addition to the increased carbon fixation that accompanies mannitol biosynthesis, the initial step of mannitol utilization generates NADH, thus giving a higher net ATP yield than the catabolism of an equal amount of sucrose (7). Finally, mannitol-producing plants also exhibit a high degree of salt tolerance due to the function of mannitol as an osmoregulator and compatible solute (6,11,12). Celery plants grown in hydroponic culture with a salinity equivalent to 30% that of sea water show dry weight gains equal to plants grown at normal nutrient levels (12). In addition, tobacco that was genetically engineered to synthesize mannitol through the introduction of the Escherichia coli NAD-dependent mannitol-1-phosphate dehydrogenase acquired significant salt tolerance (6).Metabolite pool sizes in plants are usually determined by relative rates of synthesis and utilization. The isolation and characterization of a plant NAD-dependent mannitol dehydrogenase (MTD), the enzyme responsible for the oxidation of mannitol to mannose in celery, was reported by our laboratory (13). MTD is a monomeric mannitol:mannose 1-oxidoreductase with a molecular mass of "40 kDa (13,14). In celery plants, the expression of MTD is highly regulated. MTD activity is highest in young actively growing root tips, is also high in young rapidly growing (sink) leaves, but is not detected in mature photosynthetic (source) leaves. Extractable MTD activity ...

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Microbial interactions are important for the success and safety of food fermentations. Although much indispensable work has described the microbial succession in various fermentations, little is known about how the microbes present interact. Here, we discuss the various mechanisms of microbial interaction from trophic interactions to cell signaling. We also examine the role these interactions play in the dominant microbiota associated with various food fermentations.

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Mannitol, a six carbon sugar alcohol, is widely distributed in nature and is a major phloem-translocated photoassimilate in celery. II may also function as a compatible osmolyte providing stress tolerance. Until recently, little was known about the route of mannitol catabolism in sink tissues of higher plants. An enzyme. mannitol dehydrogenase. (MDH) that oxidizes mannitol to mannose utilizing NAD as the electron acceptor was discovered (Arch. Biochem. Biophys. 1991. 298:612-619) in “sink” tissues of celery and celeriac plants. The activity of the enzyme is inversely related to tissue mannitol concentration in various parts of celery plants suggesting a role for the enzyme in mannitol catabolism. In osmostressed celery plants, the activity of the enzyme in sink tissues decreases as mannitol accumulates. Celery cells growing heterotrophically in suspension culture utilize either sucrose or mannitol as the sole carbon source and grow equally well on either carbohydrate. Mannitol-grown cells contain more MDI-I activity than sucrose-grown cells, and the activity of the enzyme is correlated with the rate of depletion of mannitol from the culture medium. Cells growing on mannitol contain an internal pool of mannitol but little sugar. Cells growing on sucrose contain internal sugar pools but no mannitol. Mannitol-grown cells are also more salt tolerant than cells grown on sucrose. Our laboratory is involved in studies of the physiological role of the mannitol oxidizing enzyme in regulating mannitol utilization and the role of the enzyme in regulating mannitol pool size during salt and osmostress in both celery plants and celery suspension cultures. Current studies on the molecular control of expression of the enzyme will be discussed.