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

Root nodules are present at plant roots, especially legumes, which form a symbiosis with microorganisms like Rhizobium. Capable plants form a symbiotic relationship with a host-specific strain of micro-organism under nitrogen-limiting conditions.

Complete answer: Inside legume root nodules, ambient nitrogen gas (N$_2$) is converted into ammonia (NH$_3$) and is then assimilated into amino acids (protein building blocks), nucleotides (DNA and RNA building blocks, as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones. Their ability to absorb gaseous nitrogen makes legumes a perfect farming organism because nitrogen nutrient needs are reduced. In addition, high nitrogen content prevents the production of nodules because there is little advantage for the plant to establish the symbiosis. The energy in the nodule for separating the nitrogen gas comes from sugar translocated from the leaf (a result of photosynthesis). Legume crops include beans, peas, and soybeans.

Therefore, the correct answer is (A), Bacteria.

Note: The nitrogen fixation in the nodule is very sensitive to oxygen. Legume nodules store an iron-rich protein named leghaemoglobin, which is strongly linked to animal myoglobin to promote the absorption of the oxygen gas used in respiration.