Selenium Application During Radish (Raphanus sativus) Plant Development Alters Glucosinolate Metabolic Gene Expression and Results in the Production of 4-(methylseleno)but-3-enyl glucosinolate

Abstract

Selenium (Se) is an essential micronutrient for human health, entering the diet mainly through the consumption of plant material. Members of the Brassicaceae are Se-accumulators that can accumulate up to 1g Se kg−1 dry weight (DW) from the environment without apparent ill effect. The Brassicaceae also produce glucosinolates (GSLs), sulfur (S)-rich compounds that benefit human health. Radish (Raphanus sativus) has a unique GSL profile and is a Se-accumulating species that is part of the human diet as sprouts, greens and roots. In this report we describe the effects of Se-fertilisation on GSL production in radish during five stages of early development (from seed to mature salad greens) and on the transcript abundance of eight genes encoding enzymes involved in GSL metabolism. We tentatively identified (by tandem mass spectrometry) the selenium-containing glucosinolate, 4-(methylseleno)but-3-enyl glucosinolate, with the double bond geometry not resolved. Two related isothiocyanates were tentatively identified by Gas Chromatography-Mass Spectrometry as (E/Z?) isomers of 4-(methylseleno)but-3-enyl isothiocyanate. Se fertilisation of mature radish led to the presence of selenoglucosinolates in the seed. While GSL concentration generally reduced during radish development, GSL content was generally not affected by Se fertilisation, aside from the indole GSL, indol-3-ylmethyl glucosinolate, which increased on Se treatment, and the Se-GSLs, which significantly increased during development. The transcript abundance of genes involved in aliphatic GSL biosynthesis declined with Se treatment while that of genes involved in indole GSL biosynthesis tended to increase. APS kinase transcript abundance increased significantly in three of the four developmental stages following Se treatment. The remaining genes investigated were not significantly changed following Se treatment. We hypothesise that increased APS kinase expression in response to Se treatment is part of a general protection mechanism controlling the uptake of S and the production of S-containing compounds such as GSLs. The upregulation of genes encoding enzymes involved in indole GSL biosynthesis and a decrease in those involved in aliphatic GSL biosynthesis may be part of a similar mechanism protecting the plant’s GSL complement whilst limiting the amount of Se-GSLs produced.

Selenium biofortification in the 21st century: status and challenges for healthy human nutrition

“…This is likely because Se and S compete for the uptake and metabolic processes in plants (McKenzie et al 2019;Schiavon and Pilon-Smits 2017a;Tian et al 2016Tian et al 2018. Negative effects of Se/S competition on the synthesis of GSH and GLS precursor amino acids, and on the expression of genes within the GLS pathway have been largely described (Dall'Acqua et al 2019;McKenzie et al 2019;Schiavon et al 2016;Tian et al 2016). Methionine (Met) is the precursor amino acid for the synthesis of aliphatic GLS, and the decrease of its content in response to Se fertilization has been associated with lower accumulation of Metderived-GLS in salad rocket (Eruca sativa Mill.…”

“…Beside S-glucosinolates, Brassicaceae spp. supplem e n t e d w i t h S e c a n p r o d u c e (methylseleno)glucosinolates and their Se-containing aglycons (Matich et al 2012(Matich et al 2015McKenzie et al 2019), whose bioactivity as anticancer and antimicrobial agents is considered higher compared to GLS (Emmert et al 2010). The majority of Se-GSL identified so far are aliphatic, with Se in the position of the S donated by Met and not in the sulphate group or on the bond with glucose (Matich et al 2012(Matich et al 2015.…”

“…The majority of Se-GSL identified so far are aliphatic, with Se in the position of the S donated by Met and not in the sulphate group or on the bond with glucose (Matich et al 2012(Matich et al 2015. The Seglucosinolates 3-(methylseleno)propyl glucosinolate, 4 -( m e t h y l s e l e n o ) b u t y l g l u c o s i n o l a t e , 5 -( m e t h y l s e l e n o ) p e n t y l g l u c o s i n o l a t e , 4-(methylseleninyl)butyl glucosinolate and seleno-2phenylethyl glucosinolate have been described in broccoli, cauliflower and forage rape (Matich et al 2012(Matich et al 2015, while 4-(methylseleno)but-3-enyl glucosinolate a n d r e l a t e d i s o t h i o c y a n a t e s ( i s o m e r s o f 4-(methylseleno)but-3-enyl isothiocyanate) have been tentatively identified in radish plants (McKenzie et al 2019). In Se-enriched radish and broccoli, the upregulation of the gene coding for APS kinase (APSK), the key branch point enzyme of GSL biosynthesis, was observed and explained as a protective mechanism activated by plants to control S uptake and limit Se-GLS production (McKenzie et al 2017(McKenzie et al 2019.…”

“…The Seglucosinolates 3-(methylseleno)propyl glucosinolate, 4 -( m e t h y l s e l e n o ) b u t y l g l u c o s i n o l a t e , 5 -( m e t h y l s e l e n o ) p e n t y l g l u c o s i n o l a t e , 4-(methylseleninyl)butyl glucosinolate and seleno-2phenylethyl glucosinolate have been described in broccoli, cauliflower and forage rape (Matich et al 2012(Matich et al 2015, while 4-(methylseleno)but-3-enyl glucosinolate a n d r e l a t e d i s o t h i o c y a n a t e s ( i s o m e r s o f 4-(methylseleno)but-3-enyl isothiocyanate) have been tentatively identified in radish plants (McKenzie et al 2019). In Se-enriched radish and broccoli, the upregulation of the gene coding for APS kinase (APSK), the key branch point enzyme of GSL biosynthesis, was observed and explained as a protective mechanism activated by plants to control S uptake and limit Se-GLS production (McKenzie et al 2017(McKenzie et al 2019. The repression of genes encoding enzymes promoting aliphatic GLS biosynthesis and the concomitant up-regulation of genes coding for enzymes involved in indole GLS generation were also hypothesized as part of the same protective mechanism that aims to generate indole GLSs over aliphatic GLS to restrict the synthesis of Se-GLS (McKenzie et al 2019).…”

The Role of Brassica Bioactives on Human Health: Are We Studying It the Right Way?

“…Similarly, efforts on health-oriented breeding (e.g., manipulation of the activity of nitrile specifier protein in broccoli by Roman et al [60]) as well as the very promising bioactivities of exotic GSLs like glucomoringin and its ITC moringin from a tropical tree not from the Brassicaceae family but within the order Brassicales [57]. This area of work together with additional efforts on the discovery of brassicaceous phytochemicals is another perspective: the widespread occurrence of epithionitriles in cabbage and new GSL products in these plants [10]; the poorly known chemistry of some vegetables, e.g., commercial chemotype of watercress in the USA [61]; and other derivatives including seleno-glucosinolates [62,63].…”