Q.36 In leguminous plants, both the rhizobium genes and the plant genes influence nodulation and
nitrogen fixation. Which one of the following functions is NOT encoded by the host plant
genes?
(A) Production of inducers that modify rhizobial cell wall
(B) Production of flavonoid inducers
(C) Establishment of contact between bacteria and legume
(D) Root hair curling
In leguminous plants, nodulation and nitrogen fixation arise from symbiotic interactions between the host plant and Rhizobium bacteria, where plant genes produce signal molecules and structural components while bacterial genes handle infection responses. The correct answer is (D) Root hair curling, as this process relies on rhizobial nodulation genes rather than host plant genes.
Correct Answer
Root hair curling during nodulation is driven by Rhizobium bacteria through Nod factor secretion, not by genes from the legume host plant. Host plants instead provide flavonoids and lectins to initiate signaling and attachment.
Option Explanations
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(A) Production of inducers that modify rhizobial cell wall: Legume roots secrete specific inducers like oligosaccharides that trigger changes in the rhizobial cell wall, enabling infection thread formation; this is encoded by host plant genes.
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(B) Production of flavonoid inducers: Flavonoids from plant roots act as primary signals to induce bacterial nod gene expression, a function directly controlled by host plant genetic pathways.
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(C) Establishment of contact between bacteria and legume: Plant genes encode lectins and surface receptors that recognize rhizobia and facilitate initial adhesion to root hairs.
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(D) Root hair curling: This shepherd’s crook deformation of root hairs results from bacterial Nod factors binding to plant receptors, with the curling mechanism executed via rhizobial genes, not host plant genes.
Symbiosis Process Overview
The interaction starts with plant flavonoids activating rhizobial nod genes, leading to Nod factor production that causes root hair curling and cortical cell division. While plants control signaling and nodule development, bacteria manage infection and curling.
