Zinc hyperaccumulation :: science

Zinc hyperaccumulation Zinc hyperaccumulation in Thlaspi caerulescens as a chemical defence against herbivory ABSTRACT Thlaspi caerulescens is one of several plant species known to accumulate heavy metals in excess of 2% of their above ground plant biomass. The reasons for hyperaccumulation are unknown, but several studies conclude that it may be a plant chemical defence. This has been of interest to biologists because these metals are usually toxic. The accumulation of these metals may serve as a model for coevolution. We examined the effects of zinc hyperaccumulation in Thlaspi on Xanthomonas campestris and found that the plants containing zinc thrived when inoculated with this bacteria, while plants not containing zinc showed signs of deterioration. INTRODUCTION There are several wild plant species that have the ability to accumulate high quantities of heavy metals in their above ground biomass, up to three percent or more. Many of these plants are found in the Brassicaceae family throughout Europe and the British Isles. These plants thrive on mineral outcrops with calamine and serpentine soils rich with high levels of zinc, cadmium, and nickel (Baker et al, 1994). Several theories have been advanced on the reasons for this hyperaccumulation. Boyd and Martens propose that it could be a form of drought resistance, inadvertent uptake, interference, tolerance or disposal of metal from the plant, or a chemical defence against herbivory or pathogens. Several studies have supported the chemical defence hypothesis. Martens and Boyd (1994 and Boyd and Martens, 1994) showed that nickel hyperaccumulation is an effective defence against insect herbivores in two different feeding experiments. Boyd et at (1994) also demonstrated that nickel hyperaccumulating plants resisted pathogens including Xanthomonas campestris. Thlaspi caerulescens J. and C. Presl (Brassicaceae) is a hyperaccumulating plant found in the British Isles. It has been shown to accumulate 10,000 ppm (>1%) of its biomass in zinc (Bakeret at, 1994), and Pollard and Baker (1997) suggest that this is an effective defence against herbivory for this species. This paper explores the effects of zinc hyperaccumulation in Thlaspi as a defence against Xanthomonas campestris. MATERIALS AND METHODS Thlaspi caerulescens seeds were collected in Cloughwood, U. K. These seeds germinated on polyester beads supported in expanded polystyrene rafts floating on one-tenth strength Rorison's solution (Hewitt, 1966). These containers were placed in a Conviron E-15 environmental growth chamber at the following settings: 20 C, 90% RH, 16 hr day, and 8 hr night. After three weeks, twenty seedlings were transferred to 4 rafts composed of expanded styrene on polyethylene, each supporting five plants individually.