Abstract
BackgroundStarch is the predominant storage compound in underground plant tissues like roots and tubers. An exception is sugar beet tap-root (Beta vulgaris ssp altissima) which exclusively stores sucrose. The underlying mechanism behind this divergent storage accumulation in sugar beet is currently not fully known. From the general presence of starch in roots and tubers it could be speculated that the lack in sugar beet tap-roots would originate from deficiency in pathways leading to starch. Therefore with emphasis on starch accumulation, we studied tap-roots of sugar beet using parsnip (Pastinaca sativa) as a comparator.ResultsMetabolic and structural analyses of sugar beet tap-root confirmed sucrose as the exclusive storage component. No starch granules could be detected in tap-roots of sugar beet or the wild ancestor sea beet (Beta vulgaris ssp. maritima). Analyses of parsnip showed that the main storage component was starch but tap-root tissue was also found to contain significant levels of sugars. Surprisingly, activities of four main starch biosynthetic enzymes, phosphoglucomutase, ADP-glucose pyrophosphorylase, starch synthase and starch branching enzyme, were similar in sugar beet and parsnip tap-roots. Transcriptional analysis confirmed expression of corresponding genes. Additionally, expression of genes involved in starch accumulation such as for plastidial hexose transportation and starch tuning functions could be determined in tap-roots of both plant species.ConclusionConsidering underground storage organs, sugar beet tap-root upholds a unique property in exclusively storing sucrose. Lack of starch also in the ancestor sea beet indicates an evolved trait of biological importance.Our findings in this study show that gene expression and enzymatic activity of main starch biosynthetic functions are present in sugar beet tap-root during storage accumulation. In view of this, the complete lack of starch in sugar beet tap-roots is enigmatic.
Highlights
Starch is the predominant storage compound in underground plant tissues like roots and tubers
As parsnip partially stores starch while sugar beet stores sucrose, a comparison between these two species will provide a better understanding of carbon allocation in these underground storage tissues and an understanding of the genetic and enzymatic factors governing the accumulation of the two different carbon storage compounds
In conclusion, gene expression and enzymatic activities could be found for the major participants in starch biosynthesis in sugar beet, despite that structural analyses and chemical analysis failed to indicate any presence of starch
Summary
Starch is the predominant storage compound in underground plant tissues like roots and tubers. The underlying mechanism behind this divergent storage accumulation in sugar beet is currently not fully known. With emphasis on starch accumulation, we studied tap-roots of sugar beet using parsnip (Pastinaca sativa) as a comparator. A major use of these energy reserves is to facilitate growth and propagation of the generation and they are laid down in sink tissues, e.g. seeds and tubers. The plant storage reserves, starch, oil and sugars, are supplying mankind with the majority of calories but have important industrial applications. The biosynthesis of storage compounds, starch, oil and sugars, is known in quite detail but the knowledge of why a certain type of these products accumulates and the underlying mechanisms are largely lacking [1,2]. Plants might be engineered to accumulate additional storage compounds than naturally occurring
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