Plant-Rhizobium Interaction and Its Importance to Agriculture
Nodulation begins when a suitable strain of Rhizobium encounters a leguminous root system. The Rhizobium multiplies, in the process producing substantial amounts of IAA and other as yet unidentified but specific substances. These substances cause deformation and curling of the plant root hairs (Fig. 1).
The root hair invaginates at the apical portion, and some rhizobia pass into the invaginated section. This reaction is very localized and in many plants is also acid sensitive (Fig. 2).
A fungus-like infection thread is formed by the plant, and the rhizobia are contained within this thread even as they move down the root hair and into the cortex of the plant (Figs. 3 and 4).
When the infection thread penetrates a tetraploid cell, it begins to break up, and the rhizobia, still surrounded by a coating of plant mucilage, are released into the plant cell cytoplasm (Fig. 5). They begin to multiply, and the IAA produced causes multiplication of the tetraploid and surrounding diploid cells. This multiplication leads to the structure we call a “nodule.” When released into the plant cell, the rhizobia are still bacillary in shape. They quickly lose this cylindrical appearance, becoming “bacteroids,” and in this form begin N2 fixation.
During active fixation, there are usually four distinct nodular zones (Fig. 6): an outer cortical tissue uninfected by bacteria, an inner zone where the cells are infected but the rhizobia are still in the bacillary form, the central tissue containing N2-fixing but generally nonviable Rhizobium “bacteroids,” and an area of degenerating nodule tissue.
KeywordsRoot Hair Nodule Formation Infection Thread Tetraploid Cell Inoculant Strain
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