Thlaspi caerulescens, an attractive model species to study heavy metal hyperaccumulation in plants.

Studying heavy metal hyperaccumulation is becoming more and more interesting for ecological, evolutionary, nutritional, and environmental reasons. One model species, especially in the era of high throughput genomics, transcriptomics, proteomics and metabolomics technologies, would be very advantageous. Although there are several hyperaccumulator species known, there is no single model species yet. The Zn, Cd and Ni hyperaccumulator species Thlaspi caerulescens has been studied to a great extent, especially for Zn and Cd hyperaccumulation and tolerance. Its physiological, morphological and genetic characteristics, and its close relationship to Arabidopsis thaliana, the general plant reference species, make it an excellent candidate to be the plant heavy metal hyperaccumulation model species.

Dynamics of Ni-based defence and organic defences in the Ni hyperaccumulator, Streptanthus polygaloides (Brassicaceae).

Plants use chemical defences to reduce damage from herbivores and the effectiveness of these defences can be altered by biotic and abiotic factors, such as herbivory and soil resource availability. Streptanthus polygaloides, a nickel (Ni) hyperaccumulator, possesses both Ni-based defences and organic defences (glucosinolates), but the extent to which these defences interact and respond to environmental conditions is unknown. S. polygaloides plants were grown on high-Ni and low-Ni soil and concentrations of Ni and glucosinolates were compared with those of the congeneric non-hyperaccumulator, S. insignus spp. insignus, grown under the same conditions. Ni contents were highest (4000 µg g-1 dry tissue) in S. polygaloides plants grown on high-Ni soil. Glucosinolate content was significantly higher in S. insignus than in S. polygaloides suggesting that plants defended by Ni produce a lower concentration of organic defences. In a separate experiment, high-Ni S. polygaloides plants were exposed to simulated herbivory or live folivores to determine the inducibility of Ni-based and organic defences. Contents of Ni were not affected by either herbivory treatment, whereas glucosinolate concentrations were >30% higher in damaged plants. We concluded that the Ni-based defence of S. polygaloides is not induced by herbivory.

The defensive function of Ni in plants: response of the polyphagous herbivore Spodoptera exigua (Lepidoptera: Noctuidae) to hyperaccumulator and accumulator species of Streptanthus (Brassicaceae).

Metals sequestered by plants may defend them against herbivores and/or pathogens. We explored the effect of plant metal content on a polyphagous herbivore, Spodoptera exigua. Plant experiments used a Ni hyperaccumulator (Streptanthus polygaloides) and two Ni accumulator species (S. breweri and S. tortuosus). High- and low-Ni plants of each species were produced by growing plants on either Ni-amended or unamended soil. Mean leaf Ni contents for plants grown on Ni-amended soil and control soil, respectively, were: 1500 and 20 mg Ni kg-1 for S. polygaloides, 40 and 9 mg kg-1 for S. breweri, and 93 and 0.5 mg kg-1 for S. tortuosus. Neonate or second-instar Sp. exigua larvae were fed high- or low-metal leaves of each plant species, and survival and other parameters were monitored. High-Ni leaves of S. polygaloides were acutely toxic, resulting in 96% mortality within 10 days, whereas only 48% of larvae fed low-Ni leaves died. Low- and high-Ni leaves of S. breweri did not differ in their effects on larval survival, larval weight, adult weight, and duration of pupation. Leaves of S. tortuosus from high-Ni soil did not significantly affect larval survival relative to low-Ni leaves. However, larvae eating high-Ni leaves weighed significantly less and pupation was significantly delayed. Larval feeding experiments using artificial diet amended with Ni demonstrated a toxic threshold at 963 mg Ni kg-1 and a sublethal threshold at 535 mg Ni kg-1. Because plant material containing less Ni had detectable sublethal effects, we suggest that Ni interacts with other plant qualities (including secondary defensive compounds) to produce those effects. We conclude that hyperaccumulated Ni is a potent defense against polyphagous folivorous insects, but suggest that the sublethal impacts of the lesser Ni levels found in accumulator plant species may play only a minor defensive role against herbivores.

A Ni hyperaccumulator and a congeneric non-accumulator reveal equally effective defenses against herbivory.

The defense hypothesis is commonly used to explain the adaptive role of metal hyperaccumulation. We tested this hypothesis using two Brassicaceae congeneric species: Alyssum pintodasilvae, a Ni hyperaccumulator, and the non-accumulator Alyssum simplex both growing on serpentine soils in Portugal. Artificial diet disks amended with powdered leaves from each plant species were used to compare the performance (mortality, biomass change) and feeding behavior of Tribolium castaneum in no-choice and choice tests. The performance of T. castaneum was not affected at several concentrations of A. pintodasilvae or A. simplex in no-choice tests. However, the consumption of plant-amended disks was significantly lower than that of control disks, irrespectively of the species fed. Accordingly, when insects were given an alternative food choice, disks of both plant species were significantly less consumed than control disks. Moreover, insects did not discriminate between disks in the combination "A. pintodasilvae+A. simplex". Contrary to our expectations, these results suggest that both plant species have equally effective defenses against herbivory. While Ni is believed to be part of the deterrence mechanism in the hyperaccumulator A. pintodasilvae, it seems likely that organic compounds, possibly glucosinolates, play an important role in the defense of A. simplex or in both species.