Sequence analysis of a mannitol dehydrogenase cDNA from plants reveals a function for the pathogenesis-related protein ELI3.

Abstract

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 ...

Expression Pattern of Two Paralogs Encoding Cinnamyl Alcohol Dehydrogenases in Arabidopsis. Isolation and Characterization of the Corresponding Mutants

“…The AtCAD-B paralogs corresponding to ELI-3 proteins have been studied previously and were originally identified as part of the defense response in parsley (Petroselinum crispum) and in Arabidopsis (Kiedrowski et al, 1992). Williamson et al (1995) and Somssich et al (1996) demonstrated that ELI-3 was neither a CAD nor a malate dehydrogenase but rather a benzyl alcohol dehydrogenase, which accepts various benzaldehyde substrates. In our phylogenetic analysis, these proteins (AtCAD-A, AtCAD-B1, and -B2) fall within the same cluster as PtSAD, which has been recently identified and characterized in poplar (Li et al, 2001).…”

Characterization of three cinnamyl alcohol dehydrogenases from Carthamus tinctorius

“…Although CtCAD1 may not be dominant in monolignol biosynthesis, it may have other roles in plant metabolism, such as in stress-induced defenses, similar to the ELI3 genes encoding pathogen defense-related CADs in Apium graveolens, Arabidopsis, or P. crispum (parsley) (Logemann et al 1997;Somssich et al 1996;Williamson et al 1995). It is also possible that CtCAD1 is involved in extremely localized processes, such as lignification during Arabidopsis silique or anther dehiscence (Liljegren et al 2000;Mitsuda et al 2005).…”

“…A phylogenetic tree (Figure 2) constructed using the neighbor-joining method showed that CtCAD1 and CtCAD3 were located in a clade that included P. tremuloides sinapaldehyde-specific SAD (PtreSAD) and plant defense-related CADs from A. thaliana (Somssich et al 1996), Apium graveolens (Williamson et al 1995), Camptotheca acuminata (Valletta et al 2010), Ocimum basilicum (Iijima et al 2006), Petrosilinum crispum (Kiedrowski et al 1992), and the lignin-related CAD from Fragaria×ananassa cv. Chandler (Blanco-Portales et al 2002).…”

Molecular Cloning of Mannose-6-Phosphate Reductase and Its Developmental Expression in Celery

“…Such studies should provide insight into the roles of polyols in primary carbon metabolism. Given the increasing number of studies linking sugar alcohols to stress tolerance (Ahmad et al, 1979;Hirai, 1983;Ranney et al, 1991;Tarczynski et al, 1993;Everard et al, 1994;Stoop and Pharr, 1994;Pharr et al, 1995;Thomas et al, 1995;Tattini et al, 1996), we should also gain further insight into the mechanisms involved in the regulation of sugar alcohol levels in plants subjected to abiotic stress.…”

“…It is a mannito1 producer and approximately 50% of newly assimilated carbon is partitioned into this compound in mature leaves. Both the anabolic (Rumpho et al, 1983;Loescher et al, 1992) and catabolic (Pharr et al, 1995) pathways have been described. The synthetic pathway appears to be present in other mannitol-synthesizing higher plant species Harloff and Wegmann, 1993;Simier et al, 1994) and comprises three unique enzymatic steps: (a) isomerization of Fru-6-P to Man-6-P by Man-6-P isomerase; (b) reduction of Man-6-P to mannitol-1-phosphate by M6PR; and (c) dephosphorylation of mannitol-1-phosphate to mannitol by mannitol-1-phosphate phosphatase.…”