Effects of metal amendment and metalloid supplementation on foliar defences are plant accession-specific in the hyperaccumulator Arabidopsis halleri

Abstract

Soil pollution by metals and metalloids as a consequence of anthropogenic industrialisation exerts a seriously damaging impact on ecosystems. However, certain plant species, termed hyperaccumulators, are able to accumulate extraordinarily high concentrations of these metal(loid)s in their aboveground tissues. Such hyperaccumulation of metal(loid)s is known to act as a defence against various antagonists, such as herbivores and pathogens. We investigated the influences of metal(loid)s on potential defence traits, such as foliar elemental, organic and mechanical defences, in the hyperaccumulator plant species Arabidopsis halleri (Brassicaceae) by artificially amending the soil with common metallic pollutants, namely cadmium (Cd) and zinc (Zn). Additionally, unamended and metal-amended soils were supplemented with the metalloid silicon (Si) to study whether Si could alleviate metal excess. Individuals originating from one non-/low- and two moderately to highly metal-contaminated sites with different metal concentrations (hereafter called accessions) were grown for eight weeks in a full-factorial design under standardised conditions. There were significant interactive effects of metal amendment and Si supplementation on foliar concentrations of certain elements (Zn, Si, aluminium (Al), iron (Fe), potassium (K) and sulfur (S), but these were accession-specific. Profiles of glucosinolates, characteristic organic defences of Brassicaceae, were distinct among accessions, and the composition was affected by soil metal amendment. Moreover, plants grown on metal-amended soil contained lower concentrations of total glucosinolates in one of the accessions, which suggests a potential trade-off between inorganic defence acquisition and biosynthesis of organic defence. The density of foliar trichomes, as a proxy for the first layer of mechanical defence, was also influenced by metal amendment and/or Si supplementation in an accession-dependent manner. Our study highlights the importance of examining the effects of co-occurring metal(loid)s in soil on various foliar defence traits in different accessions of a hyperaccumulating species.

Keywords: Elemental defence; Glucosinolate; Intraspecific variation; Metal hyperaccumulation; Silicon; Trade-off hypothesis.

Fig. 1

Score plot of principal component analysis (PCA) of 14 elements found in leaves of three accessions of Arabidopsis halleri (Wall: Wallenfels, Best: Bestwig and Lan: Langelsheim) grown in soil without or with metal amendment and metalloid (Si) supplementation. Scores (coloured symbols) and loadings (arrows) are presented

Fig. 2

Foliar concentrations (mean ± SE in µg/g d.w.) of Cd, Zn and Si in plants of three accessions of Arabidopsis halleri (Wall: Wallenfels, Best: Bestwig and Lan: Langelsheim) grown on soil without or with metal amendment and metalloid (Si) supplementation. Solid circles indicate data points: n = 4–10 per treatment combination, except n = 2 for the Lan accession with metal amendment and +Si supplementation. Statistical outcomes are indicated as: ∗∗∗P < 0.001, ∗P < 0.05 and n.s. P > 0.1 (non-significant). Different letters above the bars indicate significant differences based on the Tukey’s HSD post-hoc test

Fig. 3

Foliar concentrations (mean ± SE in μg/g d.w.) of Al, Fe, K, Mn and S in plants of the three accessions of Arabidopsis halleri (Wall: Wallenfels, Best: Bestwig and Lan: Langelsheim) grown on soil without or with metal amendment and metalloid (Si) supplementation. Solid circles indicate data points:  n = 4–10 per treatment combination, except n = 2 for the Lan accession with metal amendment and +Si supplementation. Statistical outcomes are indicated as: ∗∗∗P < 0.001, ∗∗P < 0.01, ∗P < 0.05, •P < 0.1 (marginally significant) and n.s. P > 0.1 (non-significant). Different letters above the bars indicate significant differences based on the Tukey’s HSD post-hoc test

Fig. 3

Foliar concentrations (mean ± SE in μg/g d.w.) of Al, Fe, K, Mn and S in plants of the three accessions of Arabidopsis halleri (Wall: Wallenfels, Best: Bestwig and Lan: Langelsheim) grown on soil without or with metal amendment and metalloid (Si) supplementation. Solid circles indicate data points:  n = 4–10 per treatment combination, except n = 2 for the Lan accession with metal amendment and +Si supplementation. Statistical outcomes are indicated as: ∗∗∗P < 0.001, ∗∗P < 0.01, ∗P < 0.05, •P < 0.1 (marginally significant) and n.s. P > 0.1 (non-significant). Different letters above the bars indicate significant differences based on the Tukey’s HSD post-hoc test

Fig. 4

Score plot of principal component analysis (PCA) of 10 glucosinolates found in leaves of three accessions of Arabidopsis halleri (Wall: Wallenfels, Best: Bestwig and Lan: Langelsheim) grown on soil without and with metal amendment and metalloid (Si) supplementation. Scores (coloured symbols) and loadings (arrows) are presented. Glucosinolates are abbreviated as: 5MSOP: 5-methylsulfinylpentyl glucosinolate; 6MSOH: 6-methylsulfinylhexyl glucosinolate; 7MSOH: 7-methylsulfinylheptyl glucosinolate; 8MSOO: 8-methylsulfinyloctyl glucosinolate; 6MTH: 6-methylthiohexyl glucosinolate; 7MTH: 7-methylthioheptyl glucosinolate; 8MTO: 8-methylthiooctyl glucosinolate; I3M: indol-3-ylmethyl glucosinolate; 4MOI3M: 4-methoxyindol-3-ylmethyl glucosinolate and 4OHI3M: 4-hydroxyindol-3-ylmethyl glucosinolate. The core structure of glucosinolates is shown in the upper right

Fig. 5

Composition of glucosinolates according to mean concentrations (µmol/g d.w.) found in leaves of three accessions of Arabidopsis halleri (Wall: Wallenfels, Best: Bestwig and Lan: Langelsheim) grown on soil without and with metal amendment and metalloid (Si) supplementation. n = 3–10 per treatment combination, except n = 2 for the Best accession with metal amendment and −Si supplementation and n = 1 for the Lan accession with metal amendment and +Si supplementation. Glucosinolates are abbreviated as: 5MSOP: 5-methylsulfinylpentyl glucosinolate; 6MSOH: 6-methylsulfinylhexyl glucosinolate; 7MSOH: 7-methylsulfinylheptyl glucosinolate; 8MSOO: 8-methylsulfinyloctyl glucosinolate; 6MTH: 6-methylthiohexyl glucosinolate; 7MTH: 7-methylthioheptyl glucosinolate; 8MTO: 8-methylthiooctyl glucosinolate; I3M: indol-3-ylmethyl glucosinolate; 4MOI3M: 4-methoxyindol-3-ylmethyl glucosinolate and 4OHI3M: 4-hydroxyindol-3-ylmethyl glucosinolate

Fig. 6

Foliar trichome density (mean ± SE of count per 12.6 mm²) and shoot dry biomass (mean ± SE in mg) in plants of the three accessions of Arabidopsis halleri (Wall: Wallenfels, Best: Bestwig and Lan: Langelsheim) grown on soil without and with metal amendment and metalloid (Si) supplementation. Solid circles indicate data points: n = 3–10 per treatment combination. Statistical outcomes are indicated as: ∗∗∗P < 0.001, ∗∗P < 0.01, ∗P < 0.05 and n.s. P > 0.1 (non-significant). Different letters above the bars indicate significant differences based on the Tukey’s HSD post-hoc test