Famous nitrogen fixers

Famous nitrogen fixers - Famous Nitrogen Fixers Ann M....

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Famous Nitrogen Fixers Ann M. Hirsch 2008© Farmers have known, probably since the time of the Egyptians, that legumes such as pea, lentil, and clover are important for soil fertility. Such practices as green manuring, crop rotation, and intercropping have been known for millennia and were extensively described by the Romans, but it was not until the 19 th century that an explanation for the success of the legumes in restoring soil fecundity, especially after a crop such as wheat had been grown, was uncovered. In the 19 th century, agriculture in Europe had progressed to the point that both green manuring and intercropping using legume crops was standard procedure. The leguminous plants were known in German as “Stickstoffsammler” or nitrogen accumulators, whereas non-leguminous crops such as wheat were called “Stickstofffreser” or nitrogen consumers. Rhizobia Even though people observed “bumps” on legume roots as early as the 17 th century as evidenced by a drawing published in 1679 by Malpighi (who thought they were insect galls), the mechanism whereby legumes accumulated nitrogen was unknown. It took a German scientist, Hermann Hellriegel, in collaboration with Hermann Wilfarth, to recognize that the legume root nodules themselves were responsible for the conversion of atmospheric nitrogen to ammonia (1888). The organisms inside the nodule were thought by some to be vibrio-like or bacteria-like organisms, but others were of the opinion that they were fungi. The microorganisms were first isolated and cultured from nodules of a number of different legume species by Martinus Beijerinck (1888) of Holland. Over time, modifications to the culture media were made to ensure easy isolation and growth of the nodule bacteria, which were called Rhizobium ( rhiza = root; bios = life). Hermann Hellriegel (1831-1895) Martinus W. Beijerinck (1851-1931) Since that time, the original genus Rhizobium has been found to consist of a number of distinct genera, including Bradyrhizobium , Sinorhizobium , Azorhizobium , Mesorhizobium , and others. Moreover, not only these alpha- proteobacteria, but also members of the beta- proteobacteria have been described as being capable of nodulating legumes and eliciting nitrogen fixation. These so-called beta-rhizobia are members of the genera Burkholderia and Cupriavidus . Although not at all related to the alpha-rhizobia on the basis of 16S RNA, the beta- rhizobial nod genes have sequence similarity with the nod genes of alpha-rhizobia, strongly suggesting that horizontal gene transfer has taken place. The structure of the nodules is identical to that established by the alpha-rhizobia. The first Burkholderia strain discovered ( B. tuberum ) was originally isolated from Aspalathus (Rooibos tea plant) nodules in South Africa, but was described as a Rhizobium in the original publication; sequencing of the 16S RNA DNA region showed that the isolate belong to the beta-rhizobia. The team that discovered this novel
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This note was uploaded on 03/10/2008 for the course MCDB 120L taught by Professor Hirsch during the Winter '07 term at UCLA.

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Famous nitrogen fixers - Famous Nitrogen Fixers Ann M....

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