A Taste for Heavy Metal

Researchers call for model to help us understand plants that hoard heavy metals.
28 July 2003

CHRISTOPHER SURRIDGE

The foliage of the yellow-flowered crevice-dweller Alyssum bertolonii can contain as much as 1% nickel - two hundred times the level that would kill most plants.

Understanding how such 'hyperaccumulators' manage metals could help in developing crops to grow on contaminated soils, supplement dietary deficiencies, mop up industrial pollution or even harvest metals from soil.

To achieve such goals, researchers around the world are beginning to realize that they need to be able to compare their work. In other words, they need to be working with the same variety of plant - a model hyperaccumulator.

Wendy Ann Peer and colleagues at Purdue University in West Lafayette, Indiana, have looked at 20 different wild relatives of the cabbage¹. From these they selected the pennycress Thlaspi caerulescens, a choice that has been independently supported by a group at Wageningen University in the Netherlands².

Pennycress grows easily in the lab and hyperaccumulates nickel, zinc and cadmium. What's more, its genome is around 88% identical to the already sequenced DNA of thale cress (Arabidopsis thaliana), making genetic analysis relatively simple.

The fact that pennycress should be proposed as the model is particularly apt. It was the first hyperaccumulating plant identified, in 1865. Its odd habits came to light when incinerated ash from plants growing on zinc- and cadmium-rich soils near the Belgian-German border was found to be 17% zinc.

Since then, many other hyperaccumulating plants have found to suck up metals including arsenic, selenium, manganese and lead. But how and why they manage this feat remains poorly understood.

Tolerance or taste?

Hyperaccumulation allows plants to live on soil that is contaminated, naturally or industrially, by heavy metals. But hyperaccumulation and tolerance do not always go hand in hand.

To take an example, Ana Assunção and colleagues at Vrije University in Amsterdam, the Netherlands, crossed two varieties of zinc-tolerant pennycress, one a hyperaccumulator, the other not³. The offspring had a range of both traits. But the better a plant was at growing on zinc-drenched soil, the less zinc it absorbed.

Another possibility is that a stash of toxic metal could protect a plant from attack by herbivores. This may work against caterpillars and other insects, but new research shows that it is no deterrent to snails.

Caterpillars of the Cabbage White butterfly (Pieris rapae) avoid Indian mustard (Brassica juncea) leaves containing around 0.1% selenium, Elizabet Pilon-Smits' group at Colorado State University in Fort Collins has discovered⁴. Land snails (Mesodon ferrissi) are not so choosey, even seeking out high-selenium leaves.

Garden snails (Helix aspersa) in another experiment at the University of Exeter, UK, ate Arabidopsis plants no matter what their zinc levels⁵. At least these studies confirm every gardener's suspicion: the voracity of snails is almost impossible to deflect.

Use or remove

Working out the advantages of hyperaccumulation by studying plants such as pennycress should help the development of specialized crops. About 1.5 billion people worldwide suffer from zinc deficiency, and 30% of the world's agricultural soil has low levels of this mineral. Crops that use zinc more efficiently could address both problems.

Garden snails eat zinc-loaded leaves. © GettyImages

In general, however, increased levels of toxic chemicals in food crops are not desirable. So plantations of zinc-loving poplar trees, say, might clean zinc-contaminated soils near mines or municipal waste dumps⁶.

Hyperaccumulating plants might even reclaim metals from soils as an alternative to ore mining. Some researchers hope that this ability could be engineered into any plant. Om Parkash Dhankher and colleagues at the University of Georgia in Athens have added an enzyme called arsenate reductase from bacteria to tobacco plants. The resulting plants grow on cadmium-rich soils with increased levels of the metal in their leaves⁷.

1. Peer, W. A. et al. Identifying model metal hyperaccumulating plants: germplasm analysis of 20 Brassicaceae accessions from a wide geographical area. New Phytologist, 159, 421 - 430, (2003). |Article|
2. Assuncao, A. G. L. et al. A cosegregation analysis of zinc (Zn) accumulation and Zn tolerance in the Zn hyperaccumulator Thlaspi caerulescens. New Phytologist, 159, 383 - 390, (2003). |Article|
3. Assuncao, A. G. L., Schat, H. & Aarts, M. G. M. Thlaspi caerulescens, an attractive model species to study heavy metal hyperaccumulation in plants. New Phytologist, 159, 351 - 360, (2003). |Article|
4. Hanson, B. et al. Selenium accumulation protects Brassica juncea from invertebrate herbivory and fungal infection. New Phytologist, 159, 461 - 469, (2003). |Article|
5. Huitson, S. B. & Macnair, M. R. Does zinc protect the zinc hyperaccumulator Arabidopsis halleri from herbivory by snails?. New Phytologist, 159, 453 - 459, (2003). |Article|
6. Di Baccio, D., Tognetti, R., Sebastiani, L. & Vitagliano, C. Responses of Populus deltoides x Populus nigra (Populus x euramericana) clone I-214 to high zinc concentrations. New Phytologist, 159, 443 - 452, (2003). |Article|
7. Dhankher, O. P., Shasti, N. A., Rosen, B. P., Fuhrmann, M. & Meagher, R. B. Increased cadmium tolerance and accumulation by plants expressing bacterial arsenate reductase. New Phytologist, 159, 431 - 441, (2003). |Article|