Host Plant Genes in Legume Nodulation

Nodulation in leguminous plants involves a complex symbiosis between the plant and Rhizobium bacteria, enabling nitrogen fixation. Both the host plant and the bacteria contribute genes that control this process. The question asks to identify a function NOT encoded by the host plant genes.

Analyzing Nodulation Functions

Let's examine the roles of host plant genes versus bacterial genes in the given options:

• Production of flavonoid inducers (Option 2): Legume roots secrete flavonoids. These flavonoids act as signaling molecules, activating specific Rhizobium genes (nod genes). This production is controlled by host plant genes.
• Establishment of contact (Option 3): Initial recognition and adherence between the bacteria and the root surface involve plant surface components and bacterial adhesins. Host plant genes play a role in preparing the root surface and recognizing the bacteria.
• Production of inducers that modify rhizobial cell wall (Option 1): Plant flavonoids trigger the production of bacterial Nod factors. These Nod factors are crucial signaling molecules produced by Rhizobium that directly interact with the plant and lead to downstream events, including modifications related to the bacterial cell surface and signaling. While the plant initiates the signal cascade with flavonoids, the direct production and modification involving bacterial factors are primarily governed by bacterial genes. However, option 4 is a more direct bacterial function.
• Root hair curling (Option 4): This is a characteristic early response of the legume root hair to the presence of compatible Rhizobium. The Rhizobium bacteria produce Nod factors (lipochitooligosaccharides) in response to plant flavonoids. These bacterial Nod factors directly bind to receptors on the root hair cells, triggering a signaling cascade within the plant cell that results in the deformation and curling of the root hair. The direct cause of the curling morphology stems from the action of bacterial Nod factors, encoded by Rhizobium genes. Therefore, root hair curling is primarily a response induced by bacterial products, not directly encoded by host plant genes controlling the curling mechanism itself.

Conclusion on Host Gene Control

While the host plant provides the environment and initial signals (flavonoids) and responds to bacterial signals, the specific morphological change of root hair curling is a direct consequence of bacterial Nod factor activity. The genes responsible for producing these crucial bacterial signaling molecules (Nod factors) and mediating their direct effect on root hair shape reside within the Rhizobium genome.

Therefore, root hair curling is the function NOT encoded by the host plant genes.