"Gender-bending" chemicals disrupt plants too

James Randerson / New Scientist 14sep01

This article from New Scientist, below, summarizes new results from John McLachlan's lab at Tulane University which have just been published in the British scientific journal, Nature. Estrogenic compounds interfere with a key ecological process, nitrogen fixation by leguminaceous plants. Nitrogen fixation by legumes is a major component of one of the key biogeochemical cycles, the nitrogen cycle. Without fixed nitrogen, animals would not have access to nitrogen, which is essential for protein synthesis. Nitrogen fixation normally takes place in legumes through the action of bacteria that live symbiotically in the plants' roots. The formation of the symbiosis is facilitated by a phytoestrogen exuded by the roots. The bacteria detects the phytoestrogen with receptors and orients toward the roots. Several years ago, DNA sequencing of the phytoestrogen receptor revealed that it is linked evolutionarily the human estrogen receptor ("homologous by descent"). McLachlan's team, made curious the discovery of the evolutionary link, then did experiments that determine whether or not estrogenic compounds interfere with the establishment of the symbiosis. JP Myers 18sep01

Oestrogen-like compounds in pesticides, herbicides and pollutants can disrupt chemical signals in plants, researchers have shown. Knock-on effects could even be affecting human and livestock fertility.

The scientists demonstrated that the chemicals - such as the pesticide DDT and bisphenol, a by-product of plastic making - disrupt the signalling between leguminous plants and symbiotic bacteria in their roots.

The legumes include soy bean, alfalfa, pea and clover and use the bacteria to convert nitrogen from the air into a form that can be used in protein manufacture.

"This is the first study to show that oestrogen-like compounds disrupt the signalling process," says Jennifer Fox, one of the research team at Tulane University, Louisiana.

It is well established, that oestrogen-like molecules interfere with animal hormones, but this study opens up a new range of potential effects on the environment and on crops.

Impaired growth

Fox assessed the presence of different chemicals on the activity of the bacteria's nitrogen fixation pathway. Oestrogen-like compounds caused a reduction in activity of up to 90 percent.

Paradoxically, this might mean that adding certain pesticides and herbicides to crops will actually inhibit their growth by impairing nitrogen fixation. Farmers might not notice the effect because added fertilisers provide an alternative supply of nitrogen, says Fox. "Maybe people did not get this connection before," she adds.

She suggests that her research might allow farmers to choose pesticides and herbicides that do not have this effect so that they could achieve the same yield for a smaller fertiliser input and hence lower cost.

Potentially more worrying are possible indirect effects on humans and livestock. If the signalling process between plant and bacteria breaks down, the microbe stops converting atmospheric nitrogen. The nitrogen hungry plant then boosts its own signal production.

This, says Fox, might load the plant with natural phyto-oestrogens, which will end up in the person or animal that eats it. Phyto-oestrogens are known to mimic some hormones in mammals, including humans, particularly those related to fertility.

John Hillman, director of the Scottish Crop Research Institute in Dundee says that this is an "interesting idea." However, he added: "I am not aware of evidence for a change in the physiological status of the plant," when oestrogen-mimicking chemicals are present.

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Nitrogen fixation:
Endocrine disrupters and flavonoid signalling

Nature 413, 128 - 129 13sep01

JENNIFER E. FOX*†, MARTA STARCEVIC*, KELVIN Y. KOW*, MATTHEW E. BUROW*‡ & JOHN A. MCLACHLAN*‡

* Environmental Endocrinology Laboratory, Center for Bioenvironmental Research at Tulane and Xavier Universities, New Orleans, Louisiana 70112, USA † Molecular and Cellular Biology Program, Tulane University, New Orleans, Louisiana 70112, USA ‡ Department of Pharmacology, Tulane University Medical School, New Orleans, Louisiana 70112, USA

e-mail: john.mclachlan@tulane.edu

Nitrogen fixation is a symbiotic process initiated by chemical signals from legumes that are recognized by soil bacteria. Here we show that some endocrine-disrupting chemicals (EDCs), so called because of their effect on hormone-signalling pathways in animal cells, also interfere with the symbiotic signalling that leads to nitrogen fixation. Our results raise the possibility that these phytochemically activated pathways may have features in common with hormonal signalling in vertebrates, thereby extending the biological and ecological impact of EDCs.

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