Synthesis of L-ascorbic acid in the phloem.
BACKGROUND
Although plants are the main source of vitamin C in the human diet, we still have a limited understanding of how plants synthesise L-ascorbic acid (AsA) and what regulates its concentration in different plant tissues. In particular, the enormous variability in the vitamin C content of storage organs from different plants remains unexplained. Possible sources of AsA in plant storage organs include in situ synthesis and long-distance transport of AsA synthesised in other tissues via the phloem. In this paper we examine a third possibility, that of synthesis within the phloem.


RESULTS
We provide evidence for the presence of AsA in the phloem sap of a wide range of crop species using aphid stylectomy and histochemical approaches. The activity of almost all the enzymes of the primary AsA biosynthetic pathway were detected in phloem-rich vascular exudates from Cucurbita pepo fruits and AsA biosynthesis was demonstrated in isolated phloem strands from Apium graveolens petioles incubated with a range of precursors (D-glucose, D-mannose, L-galactose and L-galactono-1,4-lactone). Phloem uptake of D-[U-14C]mannose and L-[1-14C]galactose (intermediates of the AsA biosynthetic pathway) as well as L-[1-14C]AsA and L-[1-14C]DHA, was observed in Nicotiana benthamiana leaf discs.


CONCLUSIONS
We present the novel finding that active AsA biosynthesis occurs in the phloem. This process must now be considered in the context of mechanisms implicated in whole plant AsA distribution. This work should provoke studies aimed at elucidation of the in vivo substrates for phloem AsA biosynthesis and its contribution to AsA accumulation in plant storage organs.
Hancock
Robert D
RD
Unit of Plant Biochemistry, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom. rhanco@scri.sari.ac.uk
McRae
Diane
D
Haupt
Sophie
S
Viola
Roberto
R
D000077102:Apium / Q000737:chemistry / Q000201:enzymology / Q000378:metabolism; D001205:Ascorbic Acid / Q000096:biosynthesis*; D001345:Autoradiography; D050260:Carbohydrate Metabolism; D002250:Carbon Radioisotopes; D002851:Chromatography, High Pressure Liquid; D019662:Cucurbitaceae / Q000737:chemistry / Q000201:enzymology / Q000378:metabolism; D005690:Galactose / Q000378:metabolism; D005691:Galactose Dehydrogenases / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D005956:Glucose-6-Phosphate Isomerase / Q000378:metabolism; D006593:Hexokinase / Q000378:metabolism; D008358:Mannose / Q000378:metabolism; D008359:Mannose-6-Phosphate Isomerase / Q000378:metabolism; D010936:Plant Extracts / Q000737:chemistry / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism; D018514:Plant Structures / Q000737:chemistry / Q000201:enzymology / Q000378:metabolism*; D011755:Pyrophosphatases / Q000378:metabolism; D014026:Nicotiana / Q000378:metabolism
D002250:Carbon Radioisotopes; D010936:Plant Extracts; D005691:Galactose Dehydrogenases; C047959:galactose dehydrogenase; D006593:Hexokinase; D011755:Pyrophosphatases; D008359:Mannose-6-Phosphate Isomerase; D005956:Glucose-6-Phosphate Isomerase; D005947:Glucose; D008358:Mannose; D001205:Ascorbic Acid; D005690:Galactose