Answer: Nod factor signaling triggering root hair curling, infection thread formation, and differentiation of bacteroids expressing nitrogenase.
- A Direct genetic transformation of root cells through integration of bacterial chromosomal DNA segments themselves in most textbook accounts
- B Nod factor signaling triggering root hair curling, infection thread formation, and differentiation of bacteroids expressing nitrogenase
- C Localized changes occurring in root hormone levels alone, with little bacterial entry into the root tissue noted during normal conditions
- D Virus-like insertion of bacterial genes directly and permanently into the host plant genome structure itself as generally observed
Correct answer: B. Nod factor signaling triggering root hair curling, infection thread formation, and differentiation of bacteroids expressing nitrogenase
Explanation: Nod factors (lipochitooligosaccharides) from Rhizobium trigger calcium spiking in root hair cells, curling, infection thread formation, and differentiation into nitrogen-fixing bacteroids.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.