Abstract
Anthocyanins are natural pigments that have important functions in plant growth and development. Radish taproots are rich in anthocyanins which confer different taproot colors and are potentially beneficial to human health. The crop differentially accumulates anthocyanin during various stages of growth, yet molecular mechanisms underlying this differential anthocyanin accumulation remains unknown. In the present study, transcriptome analysis was used to concisely identify putative genes involved in anthocyanin biosynthesis in radish. Spatial-temporal transcript expressions were then profiled in four color variant radish cultivars. From the total transcript sequences obtained through illumina sequencing, 102 assembled unigenes, and 20 candidate genes were identified to be involved in anthocyanin biosynthesis. Fifteen genomic sequences were isolated and sequenced from radish taproot. The length of these sequences was between 900 and 1,579 bp, and the unigene coverage to all of the corresponding cloned sequences was more than 93%. Gene structure analysis revealed that RsF3′H is intronless and anthocyanin biosynthesis genes (ABGs) bear asymmetrical exons, except RsSAM. Anthocyanin accumulation showed a gradual increase in the leaf of the red radish and the taproot of colored cultivars during development, with a rapid increase at 30 days after sowing (DAS), and the highest content at maturity. Spatial-temporal transcriptional analysis of 14 genes revealed detectable expressions of 12 ABGs in various tissues at different growth levels. The investigation of anthocyanin accumulation and gene expression in four color variant radish cultivars, at different stages of development, indicated that total anthocyanin correlated with transcript levels of ABGs, particularly RsUFGT, RsF3H, RsANS, RsCHS3 and RsF3′H1. Our results suggest that these candidate genes play key roles in phenotypic and spatial-temporal anthocyanin accumulation in radish through coordinated regulation and the major control point in anthocyanin biosynthesis in radish is RsUFGT. The present findings lend invaluable insights into anthocyanin biosynthesis and may facilitate genetic manipulation for enhanced anthocyanin content in radish.
Highlights
Anthocyanins are a big group of naturally occurring watersoluble pigments that belong to a larger group of ubiquitous secondary metabolites referred to as flavonoids (RodriguezSaona et al, 1999)
The anthocyanin concentrations of radish under study increased with increase in days after sowing, resulting in higher concentrations in mature taproots than sprouts of colored tissues
The increase in anthocyanin accumulation as the crop advances in growth could be due to increased accumulation of anthocyanin content in tissues, resulting from increase in size and the capacity of the plant’s biosynthetic machinery
Summary
Anthocyanins are a big group of naturally occurring watersoluble pigments that belong to a larger group of ubiquitous secondary metabolites referred to as flavonoids (RodriguezSaona et al, 1999). They play integral biological functions in plants by protecting plant tissues or senescing leaves against extreme temperatures, photo-oxidative injury and irradiation (Nhukarume et al, 2010). The anthocyanin pathway branches from the general phenylpropanoid pathway when enzymes chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H) and flavonoid 3′-hydroxylase (F3′H), catalyze the synthesis of tetrahydroxychalcone (THC) from the combination of a 4-coumaroyl CoA molecule and three of malonyl-CoA (Tanaka et al, 2008). The flavonoid genes were found to exhibit upregulation with advance toward ripening stage, resulting in color development in strawberry (F. ananassa), which establishes
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