Although it often seems that bacteria are the aggressors, nodule formation is an example of plants taking the first step in controlling the metabolism of another organism. Roots of alfalfa and other legumes secrete chemicals called flavonoids that bind to and activate a bacterial nodulation gene called nodD. The nodD gene product activates other nod genes, whose products then turn on plant genes that control the growth and function of nodules. Nitrogen-fixing bacteria live inside these nodules in a mutually beneficial relationship with the legumes.
stream banks. Tamarisk trees have spread rapidly, now covering almost 500,000 hectares (over 1,900 square miles) in 15 states. Why has it been so successful? A single tree can produce more than half a million seeds a year, and it has taproots that grow deeper into the dry soils of the west than do the roots of native species. Because of their root system, the trees can outcompete, and replace, most native plants along streams. Thus, most people consider tamarisks to be pests; however, at the bottom of the Grand Canyon, the trees provide habitat for a variety of animals, including the southwestern willow flycatcher, a bird that was on the endangered species list. In one part of Colorado where tamarisk is considered a pest, beavers are helping control the spread of these trees. Beavers use tamarisk trees to build their lodges and dams. The dams create pools of water that provide willow trees with enough water to survive and outcompete the tamarisk. In addition, in their native habitats in Asia, the saltcedar is not a pest because it has natural herbivores that keep the population in check. In the United States, the trees have no natural enemies, which also helps their spread. One plan is to import a Chinese leaf-eating beetle that lives on saltcedar in Asia in the hope that the beetle will reduce the number of saltcedars. However, some ecologists are worried that the beetle may switch to native plants in the community and keep them from returning to their former population size. Thus, the tamarisk is viewed as both a positive and negative member of different communities, and questions remain on how best to control the plants because of complex interactions with other organisms.
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