Synopsis by Dr. Pete Myers

Researchers report that chemical contaminants in soil suppress crop yields of alfalfa by interfering with chemical signaling essential for nitrogen fixation.They estimate that over a full growing season crop yields may be decreased by as much as one-third. The contaminants interfere with how nitrogen-fixing bacteria communicate with their plant hosts. When this communication breaks down, the symbiotic relationship between plant and bacteria--essential for nitrogen fixation and thus for soil richness--is impaired and crop yield is reduced.

These contaminants, which included agricultural pesticides like DDT and industrial chemicals like bisphenol A, are already known to disrupt endocrine signaling in animals because of their ability to bind with estrogen receptors. This new research examines disruption of chemical communication between different species, mediated by plant estrogens. The researchers warn that agriculture's ability to deliver sufficient food to meet growing demand may be compromised. While this research focused on alfalfa, similar effects would be expected in other legumes, like beans and soy.

Context: Chemical signals and nitrogen fixation. All organisms require nitrogen to grow. But while nitrogen is abundant on earth, most of it is in the form of an inert gas (N₂) that is chemically unavailable to most organisms. To make it available it has to be 'fixed'-- combined with hydrogen to form ammonia (NH₄) or combined with oxygen to form nitrate (NO₃).

This conversion can happen in several ways, but by far the most important is nitrogen fixation by bacteria. Some of these bacteria are free-living, while others form symbioses with plants or other organisms like termites. The vast majority of nitrogen fixation is carried out by symbiotic bacteria living in legumes like beans, clover and alfalfa.

Farming takes advantage of this, growing legumes in rotation with other crops that require nitrogen. The legumes replenish nitrogen in the soil, which can then be used by crops that don't fix nitrogen. This approach is crucial to increase crop yields while reducing the environmental costs of heavy dependence on agricultural chemicals.

For all this to work, nitrogen-fixing bacteria in the soil must be able to locate the seedling legumes as their roots begin to grow and then initiate the symbiotic relationship with the plants. This begins as the seedlings send out a chemical signal into the soil. In alfalfa, for example, the root exudes a unique mixture of flavonoid signals which attracts the specific species of bacterium that is specialized for symbiosis with alfalfa.

The alfalfa signals bind with bacterial receptors that initiate synthesis of proteins which, in turn, send a signal back to the plant's DNA, instructing it to start making the root nodules which house the bacteria as it fixes nitrogen.

Soil contains a mixture of different nitrogen-fixing bacteria and different species of host plants. The chemical signals sent by the plants and the response by the bacteria help ensure that the bacteria find their proper host. Chemical signals from the wrong plant can actually be antagonistic, delaying or suppressing the process that leads to symbiosis formation, and hence nitrogen fixation. This vulnerability to antagonism by the signals from the wrong plant host is the basis for vulnerability to disruption by synthetic chemicals that Fox et al. have discovered.

In addition to interacting with the bacteria's receptors, flavonoids can also bind with animal estrogen receptors and provoke estrogenic responses. They are one of many estrogenic substances (phytoestrogens) that plants produce naturally.

More on nitrogen fixation.

What did they do? Fox et al. grew alfalfa in the lab and measured aspects of plant growth over a six week period depending upon the treatments to which they were exposed. They measured the number of plants, the number of root nodules, plant yield (dry biomass), and the activity level of the bacterial enzyme, nitrogenase, which fixes nitrogen.

Half of the plants were innoculated with Sinorhizobium meliloti, the species of rhizobial bacterial uniquely adapted to alfalfa. The other half received no nitrogen-fixing bacteria.

Subgroups of these plants were then exposed to five contaminants that previous research had shown were capable of intefering with plant-bacterial signaling: pentachlorophenol, bisphenol A, DDT, methyl parathion and chrysin. The first four are synthetic chemicals used in agriculture and industry. Chrysin is a signaling molecule used by clover to communicate with its own symbiotic bacterium.

In other words, in their experiment that had 10 treatments:

DDT, methyl parathion and pentachlorophenol are insecticides; pentachlorophenol is also a wood preservative. Bisphenol A is a plastic monomer used to make polycarbonate plastic and epoxy resins. Chrysin is a phytoestrogen from clover, another legume.

Each treatment was replicated 5 times and measured at 2, 4 and 6 weeks after Rhizobium innoculation.

What did they find? Alfalfa innoculated with Rhizobium and untreated with chemicals grew best (see graph below). Alfalfa lacking Rhizobium, or alfalfa with Rhizobium and also treated with pentachlorophenol, grew worst. These patterns were seen for numbers of nodules grown per plant, nitrogenase activity and plant yield.

adapted from Fox et al.

Treatment by each of the contaminants decreased plant yield compared to untreated alfalfa innoculated with Rhizobium. This persisted throughout the 6 week assessment period. In contrast, by week 6 Chrysin treated alfalfa (with Rhizobium) did not differ from Rhizobium-innoculated, contaminant free alfalfa.

What does it mean? Based on their experimental results in the lab, and an analysis of alfalfa growing practices, Fox et al. estimate that the inhibitory impacts of contaminants in agricultural soils could introduce a significant lengthening in time to harvest after planting. A delay of 4-6 weeks would reduce the number of harvests a farmer could obtain from three per season to two. That reduction would result in a loss of one-third of overall alfalfa crop yield.

Neither DDT nor pentachlorophenol are still used in North America for agricultural purposes. Both remain in the soil, for the most part at levels beneath those used in these experiments. In 1999 a number of agricultural uses of methyl parathion were stopped by the EPA, but use on alfalfa is still permitted. Bisphenol A is not used agriculturally, but passes through sewage treatment systems and therefore will be found in sewage sludge as it is applied to farmland. Many other estrogenic substances can also be found in post-treatment sewage.

Agricultural studies have shown that nitrogen fixation by legumes is markedly lower in crops that have been treated with nitrogen fertilizer and pesticides compared to untreated legumes. Fox et al. propose that disruption of signaling between bacterium and the legume's roots are contributing to this pattern. Because the use of legumes to maintain soil fertility is a widespreard practice, their findings are of "important practical significance for sustainable agriculture." They suggest that a criteria used to choose pesticides on agricultural fields, particularly those that rotate legumes, should include avoiding chemicals that can disrupt communication between nitrogen fixing bacteria and their host plants.