The specificity between legume host and Rhizobium spp. is governed by:
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The symbiotic chemical conversation begins with plant roots releasing Flavonoids (Step 1), which trigger the bacteria to release Nod factors (Step 2) to initiate nodulation.
Step 1: Understanding the Concept:
Establishing a successful symbiotic nitrogen-fixing relationship between a host legume and a *Rhizobium* bacterium requires a highly coordinated, multi-step chemical conversation.
This molecular dialog determines the host-symbiont specificity, ensuring that only compatible partners interact. Step 2: Detailed Explanation:
The initial recognition step that governs this specificity is driven by plant root exudates:
1.Flavonoids (A): The host legume roots exude specific organic compounds called flavonoids (or isoflavonoids) into the surrounding rhizosphere.
2.Nod Gene Activation: These plant flavonoids act as chemoattractants for compatible *Rhizobium* cells and bind to the bacterial regulator protein NodD.
This binding activates the transcription of the bacterial nodulation (*nod*) genes.
3.Nod Factors: In response, the activated bacteria produce lipo-chitin oligosaccharides (known as Nod factors), which trigger root hair curling and infection thread formation in the host plant.
Because different legume species produce unique flavonoids that only activate specific bacterial NodD proteins, flavonoids serve as the primary molecular key governing host-bacterial specificity.
Tryptophan (B) and Indole acetic acid (D) are involved in auxin biosynthesis and plant growth regulation but do not govern host-symbiont specificity.
While cell-surface polysaccharides (C) are important for later stages of infection thread development, they are not the primary initiators of host specificity. Step 2: Final Answer:
The specificity between the legume host and *Rhizobium* species is governed by Flavonoids.
