Question

The cell of a root nodule in a leguminous plant shows a rod-shaped structure called ______.
A. Bacteria
B. Infection thread
C. Bacteroid
D. Ureide

Answer 1

Hint: The leguminous plants have nodules in their roots. These are beads like swollen structures present in rows on roots. These nodules enclose organisms that fix nitrogen for the plant. The infection of nodules starts with attraction by flavonoids produced by plants. The organism that fixes nitrogen belongs to the kingdom Monera.

Complete answer: Leguminous plants are plants that have bead-like nodular structures in their roots. These plants live in a symbiotic relationship with nitrogen-fixing bacteria. The nitrogen-fixing bacteria lives enclosed in the root cells of the roots of the plant. The infection of root cells takes place in steps. The first one is the attraction of bacterial cells by flavonoids produced by plant root cells. Once the bacteria reach the affinity of roots it multiplies and proliferates. Then the bacterial cells produce exopolysaccharides which interact with lectin produced by plants at the surface of the root. This helps in the attachment of bacterial cells to root cells. After this, the bacteria produce indole acetic acid which causes root hairs to curl up. The bacteria then form a rod-shaped structure called an infection thread that causes the root cell infection. This infection thread helps to exchange material between the bacterial cell and the root cell. The bacteria use atmospheric nitrogen and break it down into simpler versions like ammonia and nitrates. The plant uses these nitrogen compounds for growth and development. The root hair cells provide nutrition to the bacterial cells. Thus, it is a healthy symbiotic relationship between the root nodules and the bacteria.
Hence, the correct answer is option B.

Note: The most common symbiotic bacteria that live in nodules of leguminous plant roots is Rhizobium. It is majorly used as a biofertilizer. It gives pink colour to root nodules in which it lives when it is in the active state. Biofertilizers are more beneficial than chemical fertilizers.