Isolation and identification of 4-α-rhamnosyloxy benzyl glucosinolate in Noccaea caerulescens showing intraspecific variation

Glucosinolates are secondary plant compounds typically found in members of the Brassicaceae and a few other plant families. Usually each plant species contains a specific subset of the ∼ 130 different glucosinolates identified to date. However, intraspecific variation in glucosinolate profiles is commonly found. Sinalbin (4-hydroxybenzyl glucosinolate) so far has been identified as the main glucosinolate of the heavy metal accumulating plant species Noccaea caerulescens (Brassicaceae). However, a screening of 13 N. caerulescens populations revealed that in 10 populations a structurally related glucosinolate was found as the major component. Based on nuclear magnetic resonance (NMR) and mass spectrometry analyses of the intact glucosinolate as well as of the products formed after enzymatic conversion by sulfatase or myrosinase, this compound was identified as 4-α-rhamnosyloxy benzyl glucosinolate (glucomoringin). So far, glucomoringin had only been reported as the main glucosinolate of Moringa spp. (Moringaceae) which are tropical tree species. There was no apparent relation between the level of soil pollution at the location of origin, and the presence of glucomoringin. The isothiocyanate that is formed after conversion of glucomoringin is a potent antimicrobial and antitumor agent. It has yet to be established whether glucomoringin or its breakdown product have an added benefit to the plant in its natural habitat.

The current status of the elemental defense hypothesis in relation to pathogens

Metal hyperaccumulating plants are able to accumulate exceptionally high concentrations of metals, such as zinc, nickel, or cadmium, in their aerial tissues. These metals reach concentrations that would be toxic to most other plant species. This trait has evolved multiple times independently in the plant kingdom. Recent studies have provided new insight into the ecological and evolutionary significance of this trait, by showing that some metal hyperaccumulating plants can use high concentrations of accumulated metals to defend themselves against attack by pathogenic microorganisms and herbivores. Here, we review the evidence that metal hyperaccumulation acts as a defensive trait in plants, with particular emphasis on plant-pathogen interactions. We discuss the mechanisms by which defense against pathogens might have driven the evolution of metal hyperaccumulation, including the interaction of this trait with other forms of defense. In particular, we consider how physiological adaptations and fitness costs associated with metal hyperaccumulation could have resulted in trade-offs between metal hyperaccumulation and other defenses. Drawing on current understanding of the population ecology of metal hyperaccumulator plants, we consider the conditions that might have been necessary for metal hyperaccumulation to be selected as a defensive trait, and discuss the likelihood that these were fulfilled. Based on these conditions, we propose a possible scenario for the evolution of metal hyperaccumulation, in which selective pressure for resistance to pathogens or herbivores, combined with gene flow from non-metallicolous populations, increases the likelihood that the metal hyperaccumulating trait becomes established in plant populations.

Identifying model metal hyperaccumulating plants: germplasm analysis of 20 Brassicaceae accessions from a wide geographical area

•  Here we report on the first phase of a funded programme to select a wild relative of Arabidopsis thaliana for use in large-scale genomic strategies, including forward and reverse genetic screens for the identification of genes involved in metal hyperaccumulation. •  Twenty accessions of metal accumulating species of the Brassicaceae collected from Austria, France, Turkey and the USA during spring-summer 2001 were evaluated. •  The criteria established for selection were: hyperaccumulation of metal (Ni, Zn); compact growth habit; reasonable time to flowering; production of ≥ 1000 seeds per plant; self-fertility; a compact diploid genome; high sequence identity with A. thaliana; and ≥ 0.1% transformation efficiency with easy selection. As part of this selection process we also report, for the first time, the stable genetic transformation of various hyperaccumulator species with both the green fluorescence protein (GFP) and the bar selectable marker. •  We conclude that metal hyperaccumulation ability, self-fertility, seed set, transformation efficiency and a diploid genome were the most important selection criteria. Based on an overall assessment of the performance of all 20 accessions, Thlaspi caerulescens Félix de Pallières showed the most promise as a model hyperaccumulator.