18 Questions
What is the fate of the bacteroids as the nodule ages?
They break up into bacteria that digest the remaining cell contents.
When does the period of maximum development and leghaemoglobin content in nodules of annual legumes occur?
Just before flowering.
What is a possible consequence when an infection thread fails to release bacteria?
Nodules become necrotic.
What can lead to the failure of the nodulation process according to the text?
Bacterial defect.
What triggers the release of bacteria from an infection thread during nodulation?
Specific interaction between bacteria and host plant.
At what stage does the functioning of bacteroids become crucial for nodule development?
Following bacteroid formation.
What is the first step in the nodulation process?
Multiplication of Rhizobium in Rhizosphere
What happens after the Rhizobium aggregates at distinct sites adjacent to the root hairs?
Development of bacteroid
Which nutrient-containing component of legume crop plants plays a stimulatory role in attracting and multiplying Rhizobium?
Exudates in Rhizosphere
What characterizes the attachment of Rhizobium on the root surface in the nodulation process?
Polar attachment
Which step involves the formation of an infection thread in the nodulation process?
Formation of infection thread
What stimulates the non-specific interaction between legume crop plants and Rhizobium during the nodulation process?
Root exudates containing potential nutrients
What is the final structure of a nodule in the context of the nodulation process?
Central core containing Rhizobia and a surrounding cortical area with the plant vascular system
What happens shortly prior to or immediately after the release of bacteria into the symbiont cytoplasm?
Rapid cell division in host cells
What is a characteristic feature of the plant cells in the central portion of nodules?
Doubling of chromosome number
Where are rhizobia seen in relation to nodules of polyploid and diploid legumes?
Within Peribacteroid membrane enclosed in a membrane derived from host cells
What is a critical event that occurs within the infection thread during nodule development?
Rhizobial cell division and branching
What is an important aspect of rhizobia multiplication during nodule development?
Occurs after release into the symbiont cytoplasm
Study Notes
Nodule Formation and Rhizobia
- There is an exchange of nutrients between the liquid in the envelop and the membrane, with the membrane excreting energy substances like fumerate and succinate, and absorbing glutamic acid.
- As the nodule ages, cells lose leghaemoglobin, turn brown, and form a large vacuole, causing bacteroids and cell contents to crowd against the cell wall.
Infection and Nodule Formation
- The description of infection and nodule formation only applies if the bacterial strain can form an effective or fully healthy nodule.
- Incompatibility can cause the process to fail at various stages, including primary infection, growth of infection thread, release of bacteria, multiplication, bacteroid formation, and bacteroid persistence.
- Causes of failure can be due to bacterial defects, hereditary defects of the host plant, or specific interactions between bacteria and host plant.
Cross-Inoculation Groups and Rhizobium-Legume Associations
- There are different cross-inoculation groups, including:
- Clover group (R. trifolii) associated with clovers
- Pea group (R. leguminosarum) associated with peas, lentils, and vetch
- Bean group (R. phaseoli) associated with phaseolus and common beans
- Lupin group (R. lupini) associated with lupins
- Soybean group (R. japonicum) associated with soybeans
- Cowpea group (Rhizobium spp.) associated with cowpea, peanut, and limma bean
- Alfalfa group (R. meliloti) associated with alfalfa, sweet clover, and fenugreek
Nodulation Process
- The nodulation process can be classified into the following steps:
- Multiplication of Rhizobium in the rhizosphere
- Attachment of Rhizobium on the root surface
- Curling of root hair
- Formation of infection thread
- Nodule development and release of Rhizobia
- Development of bacteroid
Multiplication and Attachment of Rhizobium
- Rhizobia multiply in the rhizosphere in response to root exudates from legume crops, which contain nutrients that stimulate rhizobia.
- After multiplication, Rhizobium aggregates at distinct sites adjacent to root hairs and attaches to the root surface, characterized by polar attachment.
Infection Thread and Nodule Development
- The infection thread progresses through epidermal cells of the root, causing cortical cells to divide, and Rhizobial cells also divide within the infection thread.
- Rhizobia are released into the symbiont cytoplasm, where they multiply, and a rapid cell division takes place in the host cells.
- The final structure of the nodule consists of a central core containing Rhizobia and a surrounding cortical area with a plant vascular system.
Test your knowledge on Rhizobium-legume associations with this quiz. Identify the correct cross-inoculation group for different legume plants based on the Rhizobium species provided.
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