Abstract
BackgroundFollowing on from recent advances in plant AsA biosynthesis there is increasing interest in elucidating the factors contributing to the L-ascorbic acid (AsA) content of edible crops. One main objective is to establish whether in sink organs such as fruits and tubers, AsA is synthesised in situ from imported photoassimilates or synthesised in source tissues and translocated via the phloem. In the current work we test the hypothesis that long-distance transport is involved in AsA accumulation within the potato tuber, the most significant source of AsA in the European diet.ResultsUsing the EDTA exudation technique we confirm the presence of AsA in the phloem of potato plants and demonstrate a correlation between changes in the AsA content of source leaves and that of phloem exudates. Comparison of carboxyflourescein and AgNO3 staining is suggestive of symplastic unloading of AsA in developing tubers. This hypothesis was further supported by the changes in AsA distribution during tuber development which closely resembled those of imported photoassimilates. Manipulation of leaf AsA content by supply of precursors to source leaves resulted in increased AsA content of developing tubers.ConclusionOur data provide strong support to the hypothesis that long-distance transport of AsA occurs in potato. We also show that phloem AsA content and AsA accumulation in sink organs can be directly increased via manipulation of AsA content in the foliage. We are now attempting to establish the quantitative contribution of imported AsA to overall AsA accumulation in developing potato tubers via transgenic approaches.
Highlights
Following on from recent advances in plant AsA biosynthesis there is increasing interest in elucidating the factors contributing to the L-ascorbic acid (AsA) content of edible crops
In experiments to test the stability of authentic AsA in the presence of EDTA or CaCl2 exudation buffers less than 5% oxidation was observed in either case over 90 min
Our findings indicate that changes in source leaf AsA biosynthesis rapidly impact on the phloem AsAt content resulting in transport of de novo synthesised AsA directly to developing sinks
Summary
Following on from recent advances in plant AsA biosynthesis there is increasing interest in elucidating the factors contributing to the L-ascorbic acid (AsA) content of edible crops. In spite of its obvious relevance for humankind, our understanding of how plants synthesise AsA is still rudimentary It was only in 1998 that an evidence-backed AsA biosynthetic pathway in (page number not for citation purposes). Up-regulation of AsA accumulation in plants has been achieved via ectopic over-expression of biosynthetic genes unrelated to the proposed biosynthetic pathway [6,7,8,9]. This has led to proposals of additional steps, branches or alternative routes to the original pathway [7,9,10]
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