Vitamin C (L-ascorbic acid) has important antioxidant and metabolic functions in both plants and animals, but humans, and a few other animal species, have lost the capacity to synthesize it Plant-derived ascorbate is thus the major source of vitamin C in the human diet Although the biosynthetic pathway of L-ascorbic acid in animals is well understood, the plant pathway has remained unknown-one of the few primary plant metabolic pathways for which this is the case L-ascorbate is abundant in plants (found at concentrations of 1-5 mM in leaves and 25 mM in chloroplasts) and may have roles in photosynthesis and transmembrane electron transport We found that D-mannose and L-galactose are efficient precursors for ascorbate synthesis and are interconverted by GDP-D-mannose-3,5-epimerase We have identified an enzyme in pea and Arabidopsis thaliana, L-galactose dehydrogenase, that catalyses oxidation of L-galactose to L-galactono-1,4-lactone We propose an ascorbate biosynthesis pathway involving GDP-D-mannose, GDP-L-galactose, L-galactose and L-galactono-1,4-lactone, and have synthesized ascorbate from GDP-D-mannose by way of these intermediates in vitro The definition of this biosynthetic pathway should allow engineering of plants for increased ascorbate production, thus increasing their nutritional value and stress tolerance

The biosynthetic pathway of vitamin C in higher plants

Sphingolipids are potential heat stress signals in Saccharomyces.

Sucrose and Other Disaccharides

The cellular pathway of postphloem sugar transport was elucidated in the outer pericarp of tomato (Lycopersicon esculentum Mill cv. Floradade) fruit at 13–14 and 23–25 days after anthesis (DAA). These developmental stages are characterized by phloem-imported sugars being accumulated as starch and hexose, respectively. The symplasmic tracer, 5(6)-carboxyfluorescein, loaded into the storage parenchyma cells of pericarp discs, moved readily in the younger fruit but was immobile in fruit at 23–25 DAA. Symplasmic mobility of [14C]glucose was found to be identical to 5(6)-carboxyfluorescein. For the older fruit, the pericarp apoplasm was shown to be freely permeable to the apoplasmic tracer, trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate. Indeed, the transport capacity of the pericarp apoplasm was such that the steady-state rate of in-vitro glucose uptake by pericarp discs accounted fully for the estimated rate of in-vivo glucose accumulation. For fruit at 23–25 DAA, the inhibitory effects of the sulfhydryl group modifier, p-chloromer-curibenzenesulfonic acid (PCMBS), on [14C]glucose and [14C]fructose uptake by the pericarp discs depended on the osmolality of the external solution. The inhibition was most pronounced for pericarp discs enriched in storage parenchyma. Consistent with the PCMBS study, strong fluorescent signals were exhibited by the storage parenchyma cells of pericarp discs exposed to the membrane-impermeable thiol-binding fluorochrome, mono-bromotrimethylammoniobimane. The fluorescent weak acid, sulphorhodamine G, was accumulated preferentially by the storage parenchyma cells. Accumulation of sulphorhodamine G was halted by the ATPase inhibitor erythrosin B, suggesting the presence of a plasma-membrane-bound H+-ATPase. A linkage between the putative H+-ATPase activity and hexose transport was demonstrated by an erythrosin-B inhibition of [14C]glucose and [14C]fructose uptake. In contrast, comparable evidence for an energy-coupled hexose porter could not be found in the pericarp of younger fruit at 13–14 DAA. Overall, the data are interpreted to indicate that: (i) The postphloem cellular pathway in the outer fruit pericarp shifts from the symplasm during starch accumulation (13–14 DAA) to the apoplasm for rapid hexose accumulation (23–25 DAA). (ii) An energy-coupled plasma-membrane hexose carrier is expressed specifically in storage parenchyma cells at the latter stage of fruit development.

The cellular pathway of postphloem sugar transport in developing tomato fruit

Structural Characterization of the Pseudomonas aeruginosa 1244 Pilin Glycan

Thin-layer chromatography of carbohydrates

Improved thin-layer chromatographic method for sugar separations

Thin-layer chromatography of oligosaccharides with tungstic or molybdic acid as impregnant.

Coupled layers: a new technique for the two-dimensional thin-layer chromatography of carbohydrates.

A two-dimensional thin-layer chromatographic method for screening carbohydrate anomalies.

S. Rufini

Papers

Thin-layer chromatography of carbohydrates

Improved thin-layer chromatographic method for sugar separations

Thin-layer chromatography of oligosaccharides with tungstic or molybdic acid as impregnant.

Coupled layers: a new technique for the two-dimensional thin-layer chromatography of carbohydrates.

A two-dimensional thin-layer chromatographic method for screening carbohydrate anomalies.