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

Abstract

Background: Starch 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.

Results: Metabolic 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.

Conclusion: Considering 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.

Figure 1

Light micrographs of parsnip and sugar beet tap-root storage tissue. Sections of parsnip 48 days after planting (a), parsnip 61 days after planting (b), sugar beet 41 days after planting (c) and sugar beet 54 days after planting (d) were stained with MAS (Triple staining methylene blue-azur A-safranin O). Scale bar 10 μm.

Figure 2

Average fresh weight of parsnip and sugar beet tap-roots. Parsnip tap-roots were harvested 48 and 61 days after planting (DAP) and sugar beet tap-roots harvested 41 and 54 DAP (n = 40). Vertical bars correspond to the standard deviation of the average.

Figure 3

Starch and sugars content in parsnip and sugar beet tap-roots. Parsnip tap-roots harvested 48 and 61 days after planting (DAP) and sugar beet tap-roots harvested 41 and 54 DAP. Results are reported as % of dry weight (n = 2). Each sample consists of 3 pooled roots. Vertical bars correspond to the standard deviation of the average.

Figure 4

Expression levels of parsnip and sugar beet genes encoding functions in starch accumulation. Number of tap-root Illumina HiSeq 2000 reads per million reads (RPM) mapped on reference assemblies of P. sativa (Psa) and B. vulgaris (Bvu) corresponding to different cDNAs of functions involved in starch accumulation. The closest homologous Arabidopsis thaliana loci by BLASTx are given in the figure. GPT – glucose phosphate transporter, PGM1 – plastidic phosphoglucomutase, APS – ADP-glucose pyrophosphorylase small subunit, APL – ADP-glucose pyrophosphorylase large subunit, SS – soluble starch synthase, GBSS – granule bound starch synthase, SBE – starch branching enzyme, ISA – isoamylase, PHS – starch phosphorylase, NTT – ATP/ADP translocator and PPa6 – plastidic pyrophosphorylase.