Rhizobia Essay

The leguminosae family, more commonly known as the legume, pea, or bean family; contain most of the plant species that form a symbiotic relationship with nitrogen fixing bacteria. About 20,000 species of legumes form their symbiosis with rhizobia; a nodule-inducing bacteria. Legumes have emerged as a very valuable resource for many countries for various reasons. The seeds of legumes are very high in protein, and some can also be rich in oil. Some countries donate up to 60% of their arable land in order to plant various types of this plant family. This family of plants is also used to enhance soil fertility and is a valuable source of wood. In various agricultural systems, the need for chemical fertilizer is greatly reduced due to the nitrogen fixation of the symbiotic bacteria that is hosted by the legumes. In some areas, almost 50% of the nitrogen added to the soil is due to this symbiotic relationship (3). Although an exact estimate of this symbiosis is hard to determine, legume-rhizobia relationships remain important to the function and composition of many natural ecosystems throughout the world (2).

The basis for the symbiotic relationship in these species is complex. The infection of the host cell by rhizobia occurs within the plant’s root nodules. Bacteroides, gram-negative anaerobic bacteria, are isolated from the host cell by a peribacteroid membrane; the membrane between the plasma membrane of the cell and the membrane of the bacteroid. The bacteroid contains differentiated rhizobia, which are able to fix nitrogen due to the supply of carbon from the host plant. Sucrose is delivered to the nodules of the root via the phloem, where it is cleaved by suc synthase, and enters the Krebs cycle. The product of glyco...

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...ion amplifies the benefits of nitrogen fixation in legumes (2).

These studies have helped to further the understanding of how legumes respond in elevated carbon dioxide. Researchers have determined that growth at elevated carbon dioxide concentrations stimulates photosynthesis and the increase of carbon to the plant. However this increase requires a higher nutrient supply to account for the increase in photosynthetic output. Based on these findings, legumes have a competitive advantage over nonleguminous plants when grown at elevated level of carbon dioxide. In natural settings however, nutrient availability greatly affects the response of these legumes (2). Legumes are dependent on their symbiotic relationship to nitrogen fixing bacteria, and this relationship and its response to elevated concentrations of carbon dioxide will continue to be studied for years.