Q.What are the steps involved in formation of a root nodule?

The formation of a root nodule is a coordinated developmental process involving signalling between the legume host and the Rhizobium bacterium, occurring in the following sequence:

1. Recognition and chemotactic attraction: the roots of leguminous plants secrete specific chemical compounds (flavonoids) into the rhizosphere (soil surrounding the roots). These attract compatible Rhizobium bacteria, which then multiply and colonise the region close to the root, especially near root hairs.

2. Attachment to root hair: the bacteria adhere to the surface of a root hair, and in response, the root hair curls around the attached bacteria (often forming a characteristic "shepherd's crook" curl).

3. Formation of the infection thread: at the point of curling, the root hair's cell wall is locally hydrolysed, and the plasma membrane invaginates to form a tube-like structure called the infection thread. The bacteria travel inward through this thread, moving from the root hair towards the cortex of the root.

4. Invasion of cortical cells and induction of cell division: as the infection thread penetrates deeper, it branches and releases bacteria into the cortical cells. The presence of the bacteria (and associated plant-hormone-like signals) triggers cortical cells (and in many cases pericycle cells) to undergo rapid, localized cell division, producing the swelling/protrusion that becomes the visible root nodule.

5. Differentiation of bacteria into bacteroids: once released into the host cell cytoplasm (enclosed within a plant-derived membrane called the symbiosome), the bacteria stop dividing and differentiate morphologically and physiologically into their active, nitrogen-fixing form called bacteroids. …

Trace the sequence of events step by step: chemical signalling/attraction, root-hair attachment and curling, infection-thread formation, bacterial invasion and induced cortical cell division, bacteroid differentiation, vas …

• Starting the sequence from nitrogen fixation itself, rather than from the earlier recognition/attachment/infection-thread steps that must occur first.
• Omitting the infection thread, which is the specific structure that physically carries the bacteria from the root hair into the cortex. …