Turesson Helle, Andersson Mariette, Marttila Salla, Thulin Ingela, Hofvander Per
Department of Plant Breeding, Swedish University of Agricultural Sciences, P,O, Box 101, SE-23053 Alnarp, Sweden.
BMC Plant Biol. 2014 Apr 23;14:104. doi: 10.1186/1471-2229-14-104.
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.
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.
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.
淀粉是根和块茎等地下植物组织中主要的储存化合物。糖用甜菜主根（Beta vulgaris ssp altissima）是个例外，它只储存蔗糖。目前，糖用甜菜这种不同的储存积累背后的潜在机制尚不完全清楚。从根和块茎中普遍存在淀粉可以推测，糖用甜菜主根中缺乏淀粉可能源于淀粉合成途径的缺陷。因此，我们以欧洲防风（Pastinaca sativa）作为对照，重点研究了糖用甜菜的主根，以探讨淀粉积累情况。
对糖用甜菜主根的代谢和结构分析证实蔗糖是唯一的储存成分。在糖用甜菜或其野生祖先海甜菜（Beta vulgaris ssp. maritima）的主根中未检测到淀粉颗粒。对欧洲防风的分析表明，其主要储存成分是淀粉，但主根组织中也含有大量的糖类。令人惊讶的是，糖用甜菜和欧洲防风主根中四种主要淀粉生物合成酶，即磷酸葡萄糖变位酶、ADP - 葡萄糖焦磷酸化酶、淀粉合酶和淀粉分支酶的活性相似。转录分析证实了相应基因的表达。此外，在这两种植物的主根中都可以检测到参与淀粉积累的基因表达，如质体己糖转运和淀粉调控功能相关基因。
考虑到地下储存器官，糖用甜菜主根具有仅储存蔗糖的独特特性。其祖先海甜菜中也缺乏淀粉，这表明这是一种具有生物学重要性的进化特征。我们在本研究中的发现表明，在储存积累过程中，糖用甜菜主根中存在主要淀粉生物合成功能的基因表达和酶活性。鉴于此，糖用甜菜主根中完全缺乏淀粉令人费解。
