Legume plants establish symbiosis with rhizobia in soils by forming organ called nodules, where symbiotic nitrogen fixation occurs. The initiation of nodule formation by rhizobia is stimulated by a signaling molecule from rhizobia, Nod factor, and nodules are mostly developed from outer cortical cells and form spherical nodules in determinate legumes as Lotus japonicus. This nodule formation process is regulated by several phytohormones, such as auxin, cytokinins, gibberellins, and brassinosteroids as positive regulators of nodule formation, while ethylene, jasmonic acid and abscic acid as negative regulators. Auxin regulates plant growth and development, particularly by the regulated polar movement along the vertical axis, forming concentration gradient through-out the plant. Inhibitors of auxin transport such as l-naphtylphthalamic acid (NPA) and tri-indobenzoic acid (TIBA) have been generally employed to observe the effect of the auxin transport. Another biochemical tool to study auxin functionsis alpha-(phenyl ethyl-2-one)-indole-3-acetic acid (PEO-IAA) as an auxin antagonist. We have investigated the auxin distribution in the determinate nodules of L. japonicus using auxin inhibitors and found that the development of lenticels and nodule vascular bundles were was strongly affected by auxin during the nodulation process.
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