Physical Map between Marker 8O7 and 146O17 on the Medicago truncatula Linkage Group 1 that Contains the NIP Gene Page: 1
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CHAPTER 1
INTRODUCTION
1.1 Symbiotic Nitrogen- fixing and Legume-Rhizobium Symbiosis
Nitrogen is an essential part of protein and DNA in all living organisms. Although 78%
of the earth's atmosphere is composed of nitrogen, plants and animals cannot use N2 directly.
In order to use nitrogen, organisms need a reduced form of it. One way that N2 is reduced is by
a process called nitrogen fixation. This is the process when nitrogen is taken in the form of N2
from the atmosphere and converted into a reduced form of nitrogen: ammonia.
N2 + 8H+ + 8e + 16Mg-ATP - 2NH3 + H2 + 16Mg -ADP + 16 Pi
(Mylona, Pawlowski et al. 1995)
The nitrogenase enzymes are very susceptible to inactivation by oxygen and these
bacteria cease production of the enzyme in the presence of oxygen. Low oxygen concentration
is achieved by different bacteria by several mechanisms, including living in anaerobic
conditions or respiring to produce lower oxygen levels. In symbiotic nitrogen fixation, the
nodules of the legume plants offer a low oxygen concentration environment for the bacteria.
The nitrogen fixing symbiosis provides legume plants with nitrogen, and thus legume plants
have the ability to grow in barren and infertile soil. By the nodule's nitrogen fixing symbiosis,
legume plants also add reduced nitrogen to the surrounding soil, enriching it and serving as a
natural fertilizer (Graham and Vance 2003).
1.2 Nodule Development
1.2.1 Initiation of Nodulation
Legume plants will enter into the symbiosis and produce nodules with bacteria only
when the environment lacks reduced nitrogen. The initiation of nodulation involves a complexof various chemical compound signals' interaction between plants and bacteria. In the first step
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Lee, Yi-Ching. Physical Map between Marker 8O7 and 146O17 on the Medicago truncatula Linkage Group 1 that Contains the NIP Gene, thesis, December 2007; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc5152/m1/10/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .