Analysis of sulfur and selenium assimilation in Astragalus plants with varying capacities to accumulate selenium

Abstract

SummarySeveral Astragalus species have the ability to hyperaccumulate selenium (Se) when growing in their native habitat. Given that the biochemical properties of Se parallel those of sulfur (S), we examined the activity of key S assimilatory enzymes ATP sulfurylase (ATPS), APS reductase (APR), and serine acetyltransferase (SAT), as well as selenocysteine methyltransferase (SMT), in eight Astragalus species with varying abilities to accumulate Se. Se hyperaccumulation was found to positively correlate with shoot accumulation of S-methylcysteine (MeCys) and Se-methylselenocysteine (MeSeCys), in addition to the level of SMT enzymatic activity. However, no correlation was observed between Se hyperaccumulation and ATPS, APR, and SAT activities in shoot tissue. Transgenic Arabidopsis thaliana overexpressing both ATPS and APR had a significant enhancement of selenate reduction as a proportion of total Se, whereas SAT overexpression resulted in only a slight increase in selenate reduction to organic forms. In general, total Se accumulation in shoots was lower in the transgenic plants overexpressing ATPS, PaAPR, and SAT. Root growth was adversely affected by selenate treatment in both ATPS and SAT overexpressors and less so in the PaAPR transgenic plants. Such observations support our conclusions that ATPS and APR are major contributors of selenate reduction in planta. However, Se hyperaccumulation in Astragalus is not driven by an overall increase in the capacity of these enzymes, but rather by either an increased Se flux through the S assimilatory pathway, generated by the biosynthesis of the sink metabolites MeCys or MeSeCys, or through an as yet unidentified Se assimilation pathway.

Molecular Mechanisms of Selenium Tolerance and Hyperaccumulation in Stanleya pinnata

“…Total Se accumulation in the SMT transgenic plants was much lower than commonly found in hyperaccumulators, which suggests that SMT is an important enzyme for hyperaccumulation and tolerance, but additional processes are likely involved that have a synergistic effect on Se hyperaccumulation. Sors et al (2005a) found no evidence that enzymatic differences in the capacity to reduce or assimilate S is important for Se hyperaccumulation in Astragalus, while SMT enzyme activity correlated with Se hyperaccumulation. Additionally, a nonaccumulator SMT homolog lacked the SMT activity in vitro, explaining why little or no detectable MeSeCys accumulation was observed in the nonaccumulator species (Sors et al, 2009).…”

“…Like A. bisulcatus, the Brassicaceae hyperaccumulator S. pinnata accumulates mainly MeSeCys Freeman et al, 2006b). In addition to the specific methylation of SeCys by SMT, it is not clear at this point which mechanisms may contribute to Se hyperaccumulation and tolerance in A. bisulcatus or other hyperaccumulators (Sors et al, 2005a(Sors et al, , 2005b(Sors et al, , 2009. Se hyperaccumulators do possess some unexplained physiological traits associated with growing on Se-enriched soils.…”

“…When growing plants to measure Se accumulation, physiology of Se toxicity, SMT immunoblot, and Se protein incorporation, macroarray and metabolic analyses experiments followed an arid western plant growth protocol previously used for Se hyperaccumulators and described by Sors et al (2005a). Twenty plants per treatment from each species were grown via a controlled drip system in a sterilized, pathogen-free laboratory growth room on presoaked and water-rinsed gravel (Turface) in round 25-cm-diameter pots, for 16 weeks, with and without 20 mM selenate.…”

Selenium induced selenocysteine methyltransferase gene expression and antioxidant enzyme activities in Astragalus chrysochlorus

“…Selenium hyperaccumulation is involved with SMT enzymatic activity and accumulation of S-methylcysteine (MeCys) and MeSeCys (Sors et al 2005). In this study, our main object was to investigate the expression profi le of the SMT gene (AchSMT, Accession number:GQ844862) involved in selenium metabolism in A. chrysochlorus callus under increasing concentrations of selenium.…”

Selen v vodnih in kopenskih rastlinah / Selenium in water and in terrestrial plants

“…V Slovenji je selena v tleh malo, vsebnosti so navadno nižje od 0,1 mg/kg, ponekod pa vsebnost selena v tleh doseže tudi vrednosti od 0,3 do 0,7 mg/kg (Golob 2017). Količina selena, ki je prisotna v tleh, je odvisna od vrste tal, organske snovi v tleh in padavin (SORS et al 2005). Gorate države kot so Finska, Švedska in Škot-ska, imajo navadno pomanjkanje selena v tleh, med tem ko imajo sušna območja običajno večje količine selena v tleh.…”