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Heavy Metal Rocks Plant Cells too

September 22, 2004

Heavy metals can trigger widely varying stress reactions in plants. A team at the Campus Vienna Biocenter was now able to provide evidence for this in a research funded by the Austrian Science Fund (FWF). The results, now awaiting publication, are an important basis to comprehend how plants cope with an increase in heavy metal concentrations in the soil - and how these abilities can be profitably utilised.

Adverse environmental conditions can cause enormous stress in plants. As sedentary beings they are at the absolute mercy of these conditions. Nevertheless, in order to grow and flourish, they have developed a comprehensive series of stress reactions. The recent work by the team of Prof. Heribert Hirt and Dr. Claudia Jonak at the Campus Vienna Biocenter prove how subtly plants can differentiate the various kinds of stress.

Plants easily distinguish heavy metal
Heavy metals can be found naturally in the soil in small concentrations, and thereby constitute no difficulties for plants. But high concentrations have a toxic effect and can occur through environmental loads. Prof. Hirt and his colleagues have now compared for the first time the exact reactions of plants to high concentrations of various heavy metals. Prof. Hirt explains, "Our initial measurements already showed that the heavy metals trigger the activation of four different enzymes, which play a very central role in the stress reactions of plants. These enzymes are the so-called 'MAPKs'." MAPKs is the abbreviation for "mitogen-activated protein kinases", a class of molecular switches which are of great importance for the control of gene expression.

The team made an interesting discovery when the activities of the enzymes were analysed in detail. It found out that different heavy metals activate the same four enzymes, but at varying speeds. The activation through copper took place very fast, but through cadmium at a comparatively much slower rate. "The activation of individual MAPKs through copper already took place after 5-10 minutes, while comparable effects through cadmium occurred only 20 minutes later. This difference is not so crucial for the ability of the plant to cope with the stress, but it points to the fact that different types of stress reactions take place," Prof. Hirt elaborates on the results. Even though the cause for this time difference is still unknown, Prof. Hirt has already developed a hypothesis, which he will put to test in future projects.

Oxygen radicals create stress
The basis for Prof. Hirt's hypothesis is the fact that copper as well as cadmium lead to the production of destructive oxygen radicals in the plant. These radicals can directly activate the MAPKs unlike the heavy metals. Prof. Hirt adds, "Too much copper causes the direct production of oxygen radicals, while cadmium causes their production only indirectly. The reason for this difference is that copper is involved in various vital processes in the plant cell. Oxygen radicals develop only if there is too much copper. Contrary to that, cadmium is not part of any metabolism known to us. Its harmfulness is based on the replacement of other metals participating in the metabolism, but without assuming their function. Even though this eventually also leads to the production of oxygen radicals, this indirect process simply takes more time." But Prof. Hirt also notes that the activation of MAPKs stimulated by heavy metals could be also caused by substances other than oxygen radicals. Additional experiments will clarify these processes in detail.

A better understanding of plant reactions to high concentrations of heavy metals can have a great significance for our environment in the medium-term. For instance, it may become possible to breed plants which have a better chance of survival on soil contaminated by heavy metals. However, the possibilities of the so-called phytoremediation are even more appealing - a technology in which plants are used to extract heavy metals from contaminated soil and thus slowly clean the earth.





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