In a previous post I mentioned that the nitrogen fixation performed by leguminous plants is the result of a symbiosis between the plants and bacteria. In this relationship, the plants create an environment within their roots (called nodules) where the bacteria take up residence. In exchange for energy provided by the plant, the bacteria fix atmospheric nitrogen that is then made available to the plant. Allowing bacteria access to the interior of the root is a potentially risky act on the part of the plant. So, a mechanism to exclude all bacteria except for the desired nitrogen fixer has developed and the symbiotic relationships are species specific. Each plant has specific bacterial symbionts. In the early stages of the establishment of the root nodule, signals are exchanged between the plant roots and bacteria in the soil. First, the plants release phytochemicals that attract the desired bacteria. The target bacteria are attracted by the phytochemicals and these chemicals induce the bacteria to release their own compounds called nod factors. The nod factors, in turn are sensed by the plant. Detection of the appropriate nod factor will induce the plant to initiate the process of allowing the bacteria access to the interior of the root where the nodule will form.
In the latest PNAS, Fox et al. report that many common pesticides inhibit the formation of root nodules by interfering with communications between the plants and bacteria.
While the significance of this in the environment is not known, it has the potential to be of concern because the use of crop rotations that include plants (such as soy or alfalfa) that are capable of enriching the nitrogen content of the soil is one important way to reduce the use of fertilizer (and energy) in modern agriculture.