Plants Essay, Research Paper

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 (Baker

et 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. Ten

plants floated on one-tenth strength Rorison’s, and ten plants

floated on a solution containing Rorison’s and 10ppm zinc, as ZnSO

The solutions were freshened every four days to inhibit any

possible algal growth.

After twenty days, each plant was transferred to an individual

beaker containing 25ml of solution. The ten Rorison’s plants

retained the same solution as did the zinc plants. Parafilm held

the plants in place. The plants were then inoculated with three

different strains of Xanthomonas campestris, a bacteria known to

harm plants. Each plant had each strain inoculated on three

different leaves. The plants grew in the growth chamber for one

week and then were examined.

The plants were analyzed with a ranking scale based on

appearance by three people who did not know which plants contained

zinc.

RESULTS

Scale:

1= Healthy, green leaf with no brown, small puncture hole

2= Longer puncture hole with some white spots

3= Some leaf discoloration, expanded hole, some shriveling leaves

4= Shriveling of several leaves, whole plant not thriving

5= Many leaves dead, small shriveled plants

Plant Solution Average

Rank 1 Non-zinc 5.00 2 Zinc 1.67 3 Zinc 2.00 4 Non-zinc 4.67

5 Non-zinc 2.67 6 Non-zinc 3.67 7 Zinc 2.67 8 Non-zinc 2.00 9

Non-zinc 4.00 10 Zinc 1.33 11 Non-zinc 1.67 12 Non-zinc 5.00 13

Zinc 2.33 14 Zinc 2.00 15 Zinc 1.00 16 Zinc 1.33 17 Zinc

2.00 18 Non-zinc 1.67 19 Zinc 1.67 20 Non-zinc 1.00

The rankings showed that the zinc plants were on average

healthier than were the non-zinc plants after being inoculated.

However, the non-zinc plants did show a variety of rankings, but

they were statistically unhealthier than the zinc plants. The

Mann-Whitney U Test showed a one-tailed probability of 0.031. We

are 97% confident that the results were significant. The zinc

plants showed a healthier response to the bacteria than did the

non-zinc plants.

DISCUSSION

These results demonstrate that zinc hyperaccumulation in

Thlaspi caerulescens is an effective defence against the pathogen

Xanthonomas campestris. Since the experiment was a double blind

investigation, the results were not biased.

Our results were consistent with other studies in this area.

Boyd et al. found that nickel accumulation in S. polygaloides was

an effective defence against pathogens such as Xanthomonas. Of the

plants inoculated with this bacteria, the nickel accumulating

plants inhibited the growth of a powdery mildew. Growth of the

fungus Alternaria brasssicola was also inhibited by nickel

concentration in the plants.

Martens and Boyd (1993) showed in feeding experiments that

nickel accumulation is an effective defence against insect

herbivory. The insects fed non nickel bearing leaves survived or

showed weight gain while the insects fed nickel bearing leaves did

not. The nickel accumulation is effective because of broad

toxicity, low cost, and high lethality.

Pollard and Baker (1997) conducted studies showing preferences

of locusts (Schistocerca gregaria), slugs (Deroceras caruanae), and

caterpillars (Pieris brassicea) to Thlaspi caerulescens grown in

low zinc and zinc amended solutions. They all showed preferential

feeding on plants with low zinc concentrations.

This is an important finding because zinc in these quantities

is normally lethal to a plant. Boyd and Martens (1993) suggest

that it is reasonable to assume that the hyperaccumulated metals,

especially nickel, might also be toxic to pathogens and herbivores

since they are widely used in fungicides and bactericides. Studies

show that hyperaccumulators are more susceptible to fungi when

grown on non-serpentine soil. Studies by Martens and Boyd (1993)

suggest that metal accumulation may provide a useful example of the

coevolution of defence mechanisms because of the increased fitness

it allows for these plants. Herbivores were given a choice between

accumulating and non-accumulating S. polygaloides, and the fitness

of non-hyperaccumulating plants was 0.42 of that of the

hyperaccumulators. Selective pressures could favor the evolution

of these plants.

All of these findings suggest that plants accumulating heavy

metals may be utilizing an effective defence against herbivory and

pathogens. They may also be good examples of the methods of

coevolution.