Agriculture Reference
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absence of the host plant. In such a symbiosis, in response to some
communications (production of Myc factors by host plant) with the roots of
their host plants, fungal spores start germinating and produce the hyphal
network growing into the cellular root cortex. The extensive hyphal network
can also form arbuscules and vesicels. Arbuscules are branched hyphal
structures exchanging nutrients with the plant roots and vesicles have a high
number of vacuoles and hence can store different ions and compounds (Smith
and Read, 2008).
AM symbiosis is very beneficial for the host plant through the followings:
1) enhanced water and nutrient uptake by host plant because of their extensive
hyphal network, 2) improving soil structure by their hypha and production of a
glycoprotein called glomaline, 3) increasing the solubility of different nutrients
like phosphorous by producing enzymes such as phosphatases 4) alleviation of
soil stresses because of their unique properties and 5) controlling pathogens
through enhancing plant growth (Miransari, 2010a,b).
There are also N-fixing bacteria in soil called rhizobium , which are able to
fix atmospheric N. For the onset of their specific symbiosis, signal molecules
such as flavonoids must be exchanged between the two symbionts.
Accordingly, morphological and physiological alterations in the host plant
roots result in the production of root nodules, in which rhizobium reside and
fix atmospheric N 2 . The productions of some biochemicals by the plant roots
activate the bacterial nodulation ( Nod ) genes, allowing the bacteria to realize
the presence of their host plant. N-fixation process is specific because of these
biochemicals. In other words these signal molecules are just able to express
the Nod genes in the specific rhizobium . For example, soybean ( Glycine max
L.) is able to develop symbiotic association with their specific rhizobium ,
Bradyrhizobium japonicum (Miransari and Smith, 2007; 2008; 2009).
In response to these signal molecules, rhizobium produces the
biochemicals called lipochito- oligosaccharides, which can induce
morphological alterations in the roots of the host plants, especially in the root
hairs. For example, root hair bulging and curling are among such changes
(Miransari et al., 2006). Subsequently, the infection tread is formed, by which
the rhizobium enters the roots of their host plant and by inducing some
morphological and physiological changes, produces root nodules. In the root
nodules, the bacteria, in form of bacteroid, fix atmospheric N 2 and turn it into
available N (NH 3 ) for the use of their host plant by the enzyme nitrogenase
(Long et al., 2001).
There are also some soil bacteria such as Azospirillum and Azotobacter ,
which are able to fix atmospheric N 2 non-symbiotically, although in lower
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