Abstract
The genus Allium is a rich source of steroidal saponins, and its medicinal properties have been attributed to these bioactive compounds. The saponin compounds with diverse structures play a pivotal role in Allium’s defense mechanism. Despite numerous studies on the occurrence and chemical structure of steroidal saponins, their biosynthetic pathway in Allium species is poorly understood. The monosomic addition lines (MALs) of the Japanese bunching onion (A. fistulosum, FF) with an extra chromosome from the shallot (A. cepa Aggregatum group, AA) are powerful genetic resources that enable us to understand many physiological traits of Allium. In the present study, we were able to isolate and identify Alliospiroside A saponin compound in A. fistulosum with extra chromosome 2A from shallot (FF2A) and its role in the defense mechanism against Fusarium pathogens. Furthermore, to gain molecular insight into the Allium saponin biosynthesis pathway, high-throughput RNA-Seq of the root, bulb, and leaf of AA, MALs, and FF was carried out using Illumina's HiSeq 2500 platform. An open access Allium Transcript Database (Allium TDB, http://alliumtdb.kazusa.or.jp) was generated based on RNA-Seq data. The resulting assembled transcripts were functionally annotated, revealing 50 unigenes involved in saponin biosynthesis. Differential gene expression (DGE) analyses of AA and MALs as compared with FF (as a control) revealed a strong up-regulation of the saponin downstream pathway, including cytochrome P450, glycosyltransferase, and beta-glucosidase in chromosome 2A. An understanding of the saponin compounds and biosynthesis-related genes would facilitate the development of plants with unique saponin content and, subsequently, improved disease resistance.
Highlights
Allium is an enormous genus (850 species) that stretches broadly across the northern hemisphere from the boreal zone to the dry subtropics [1,2,3]
The utilization of monosomic addition lines (MALs) as valuable genetic resources for understanding physiological traits has been reported in several plant species, including the Beta vulgaris L. genome with the addition of chromosome 9 from B. corolliflora to improve salt stress [10, 11] and the Brassica napus genome mediated by one alien chromosome from Orychophragmus violaceus for understanding the metabolism pathways regulating brassinosteroid (BR) biosynthesis and the role of auxin signaling in gynoecium development [12]
Total saponin contents were highly abundant in the root followed by bulb and leaf tissues (Fig 2), and the highest saponin accumulation was detected in Aggregatum group (AA), FF1A and FF2A root relative to FF and other MALs root (Fig 2A)
Summary
Allium is an enormous genus (850 species) that stretches broadly across the northern hemisphere from the boreal zone to the dry subtropics [1,2,3]. A region with diverse ecological niches led to the development of an astonishing number of Allium species with different morphological and physiological traits [1] Due to their culinary and medicinal properties, many plants of this genus [A. cepa (onion), A. cepa Aggregatum group (shallot), A. fistulosum (Japanese bunching onion), A. sativum (garlic), A. ampeloprasum (leek), and A. tuberosum (Chinese chives)] have significant economic importance worldwide as vegetables or medicinal plants [4,5,6]. The increased saponin content in FF2A line was positively correlated with increased Fusarium disease resistance index [18] These findings give insight into the significant role of shallot saponin in the disease resistance improvement of A. fistulosum despite the fact that the casual genes regulating this biosynthesis process in Allium are still unknown
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