Starch biosynthetic genes and enzymes are expressed and active in the absence of starch accumulation in sugar beet tap-root

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.

Integrative transcriptomics reveals genotypic impact on sugar beet storability

“…The upregulation of the starch metabolism in well storable varieties during storage seen in our study seems very interesting as it is known that sugar beet stores energy only in form of sucrose instead of starch, despite an expression of starch biosynthesis genes (Turesson et al 2014). However, a gene that appeared highly upregulated in well storable varieties was an alpha-glucan phosphorylase, which is part of the starch metabolic process that is known to endure water stress deficit but does not alter starch content in Arabidopsis (Zeeman et al 2004).…”

“…S4). Under the top 20 upregulated DEGs (sorted by padj-value) in well storable varieties at T4 we found among genes without annotation two genes involved in starch metabolism (starch synthase 1, Turesson et al 2014 ; alpha-glucan phosphorylase, Zeeman et al 2004 ) and genes important for oxidative stress and pathogen response, e.g. probable thimet oligopeptidase (Moreau et al 2013 ), copper methylamine oxidase (Rea et al 2002 ) as well as genes positive regulating cell proliferation ( Arabidopsis homolog At3g07870, Baute et al 2017 ; ELP4, Zhou et al 2009 ) and one gene involved in sterol biosynthesis (probable 3-beta-hydroxysteroid-Delta(8),Delta(7)-isomerase).…”

Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere

“…Most taproots species accumulate C resources as starch, with the exception of sugar beet which produces sucrose during tap‐root development. The reason why beet root accumulates sucrose instead of starch is not well understood but may come from the adaptation of the wild ancestor to saline growth conditions (Turesson et al ). Recent data showed the absence of granule accumulation in both sugar beet and in the sea beet ancestor ( B. vulgaris ssp.…”

“…The authors speculated that starch deficiency in sugar beet may be related to high‐degradation activities but, surprisingly, further analyses revealed the expression level of the enzymes implicated in this process was lower in beetroot than in parsnip ( Pastinaca sativa ), a starch‐storing tuber. Therefore, the lack of starch accumulation in sugar beet remains unexplained (Turesson et al ).…”

Varying water deficit stress (WDS) tolerance in grain amaranths involves multifactorial shifts in WDS-related responses

“…10B). This contrast suggests that additional starch degradation mechanisms contributed to the starch depletion observed in leaves and roots of WDS-tolerant amaranths (Grennan, 2006; Turesson et al , 2014).…”