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Q9:
What are the steps involved in formation of a root nodule?
Solution :
Step 1: Rhizobial Colonization and Chemotaxis
The process initiates when leguminous roots secrete specific chemical compounds, primarily flavonoids, into the rhizosphere. These compounds act as chemoattractants for nitrogen-fixing bacteria, specifically Rhizobium species [Chemotactic signaling hypothesis]. The bacteria congregate around the root hairs and adhere to the epidermal cell walls.
Step 2: Root Hair Deformation and Infection Thread Formation
Upon attachment, the bacteria release Nod factors (lipochitooligosaccharides), which trigger the root hair to curl around the bacterial colony, forming a structure termed a "shepherd's crook." The plant cell wall degrades locally, allowing the bacteria to enter the root hair through an inward-growing tube known as the infection thread [Mechanism of endocytosis-like internalization].
Step 3: Cortical Cell Division and Nodule Primordium
The infection thread grows inward toward the inner cortex and pericycle of the root. Simultaneously, the chemical signals from the bacteria induce rapid cell division in the cortical and pericycle cells of the host plant. This localized proliferation of host cells results in the formation of a mass of tissue known as the nodule primordium [Mitotic activation via phytohormone signaling].
Step 4: Release of Bacteria and Bacteroid Differentiation
The infection thread reaches the developing nodule primordium and releases the bacteria into the host cell cytoplasm, enclosed within a plant-derived membrane called the peribacteroid membrane. Once inside, the bacteria undergo morphological changes, increasing in size and losing their typical shape to become bacteroids [Transformation phase].
Step 5: Formation of Mature Nodule and Vascular Connection
The bacteroids remain localized within the specialized nodule tissue. The host plant develops vascular bundles that connect the nodule directly to the plant's vascular system. This ensures a steady supply of photosynthates (carbohydrates) from the plant to the bacteria and provides an export pathway for fixed nitrogen (ammonia) from the bacteria to the plant [Source-sink relationship].
Summary of Nodule Development
| Stage | Biological Event |
|---|---|
| Recognition | Flavonoid-Nod factor exchange |
| Invasion | Infection thread penetration |
| Organogenesis | Cortical cell division (Nodule primordium) |
| Symbiosis | Bacteroid differentiation and nitrogen fixation |
Final Solution: The formation of a root nodule involves a precise sequence of chemical communication, root hair curling, infection thread formation, cortical cell proliferation, and the differentiation of Rhizobium into nitrogen-fixing bacteroids within a specialized host-derived membrane structure.
More Questions from Class 11 Biology Mineral Nutrition EXERCISES
- Q1: ‘All elements that are present in a plant need not be essential to its survival’. Comment.
- Q2: Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?
- Q3: Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.
- Q4: Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.
- Q5: If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element?
- Q6: Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?
- Q7: How are the minerals absorbed by the plants?
- Q8: What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in $N_2$-fixation?
- Q10(a): Which of the following statements are true? If false, correct them: (a) Boron deficiency leads to stout axis.
- Q10(b): Which of the following statements are true? If false, correct them: (b) Every mineral element that is present in a cell is needed by the cell.
- Q10(c): Which of the following statements are true? If false, correct them: (c) Nitrogen as a nutrient element, is highly immobile in the plants.
- Q10(d): Which of the following statements are true? If false, correct them: (d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.
CBSE Solutions for Class 11 Biology Mineral Nutrition
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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