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

Info icon This preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
1 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
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.
  • Winter '07
  • Hirsch
  • Dr. Johanna Döbereiner, associative nitrogen fixation, John G. Torrey, legume root nodules, Frankia strain CcI3

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern