Plant Microbe Interactions Flashcards
What are the rhizosphere and phyllosphere?
Rhizosphere: The root-associated soil environment.
Phyllosphere: The above-ground environment on plant surfaces, such as leaves.
What are the three types of plant-microbe interactions?
- Commensalism: Microbes benefit, plants unaffected.
- Mutualism: Both benefit (e.g., Rhizobia).
- Parasitism: Microbes harm the plant (pathogens).
How do biotrophic and necrotrophic pathogens differ?
Biotrophic: Invade living cells (e.g., powdery mildew).
Necrotrophic: Invade dead cells (e.g., Botrytis).
What determines plant resistance to pathogens?
Resistance depends on R-genes that recognize pathogen effectors, triggering plant defense responses.
What is the hypersensitive response (HR), and how does it protect plants?
HR triggers localized cell death to limit pathogen spread, often accompanied by production of reactive oxygen species (ROS).
How do effectors and R-genes interact in race-specific resistance?
Pathogen effectors are recognized by plant R-genes, leading to resistance. If no R-gene exists, the pathogen can colonize the plant.
What is systemic acquired resistance (SAR)?
SAR is a broad-spectrum immune response triggered after local infection. It spreads resistance signals throughout the plant.
What role does salicylic acid play in SAR?
Salicylic acid triggers local responses but relies on NHP (N-hydroxy-pipecolic acid) for systemic signal transmission to uninfected tissues.
What are PR proteins, and how are they involved in plant defense?
Pathogenesis-Related (PR) proteins are induced during SAR, overexpressed even in systemic tissues across the plant.
13 known PR protiens, discovered due to comparisons in protiens betweeen non SAR and SAR leaves.
Only known PRs:
PR-2: Glucanase (breaks down fungal glucans).
PR-3: Chitinase (targets fungal chitin).
What role do Rhizobia play in nitrogen fixation?
Rhizobia infect root hairs of legumes, form nodules, and fix atmospheric N2 into NH3 using nitrogenase.
What are Nod factors, and how do they initiate nodulation?
Nod factors are lipochitooligosaccharides secreted by Rhizobia. They trigger root hair curling and the formation of infection threads.
How do flavonoids contribute to Rhizobia attraction?
Plant roots secrete flavonoids, which act as chemoattractants to Rhizobia and activate bacterial nod genes.
How do Rhizobia enter plant roots?
Rhizobia enter via root hair curling. They form infection threads that grow into the cortex, where nodules develop.
What happens to Rhizobia inside root nodules?
Rhizobia differentiate into bacteroids that fix nitrogen. They are enclosed in symbiosomes surrounded by a plant membrane.
Why is leghaemoglobin important in nitrogen fixation?
Leghaemoglobin delivers oxygen to bacteroids at low concentrations, protecting nitrogenase (oxygen-sensitive enzyme) while supporting respiration.
What are some plant immunity responses upon pathogen recognition
Pathogen recognition triggers calcium spiking, ROS production, and activation of defense genes
What experiment demonstrated SAR?
Frank Ross (1961): Tobacco plants pre-infected with an avirulent TMV strain showed resistance to a subsequent virulent TNV infection.
How do pathogens suppress plant immunity?
Pathogens secrete effectors that interfere with R-gene signaling or defense responses, allowing colonization.
What is mycorrhizal symbiosis, and how does it benefit plants?
Symbiotic fungi form associations with plant roots, enhancing nutrient uptake (e.g., phosphorus) and improving stress resistance.
How was the role of Nod factors in root hair curling demonstrated?
Mutants lacking Nod factor receptors showed no root hair curling, proving Nod factors are essential for Rhizobia entry.
What are root nodules, and why are they important?
Root nodules are specialized structures on legume roots where nitrogen-fixing bacteria (Rhizobia) convert atmospheric nitrogen (N2) into ammonia (NH3), providing essential nitrogen for plant growth.
What are the key steps in root nodule formation?
- Flavonoid Secretion: Plant roots release flavonoids to attract Rhizobia.
- Nod Factor Secretion: Rhizobia produce Nod factors in response.
- Root Hair Curling: Nod factors induce curling of root hairs, trapping bacteria.
- Infection Thread Formation: Bacteria invade via infection threads.
- Nodule Development: Bacteria differentiate into bacteroids within nodules.
What role do flavonoids play in nodulation?
Flavonoids are chemical signals released by plant roots to attract specific Rhizobia and activate bacterial nod genes, initiating nodulation.
What are Nod factors, and how do they function?
Nod factors are lipochitooligosaccharides produced by Rhizobia. They bind to plant receptors (e.g., NFR1/NFR5 in legumes) and trigger root hair curling and infection thread formation.
How do Nod factors induce root hair curling?
Nod factors stimulate calcium oscillations in root hair cells, leading to cytoskeletal rearrangements and curling, which traps the Rhizobia.
What is an infection thread, and how does it develop?
Infection threads are tubular structures that form in root hairs, allowing Rhizobia to move into the root cortex. They are initiated by plant cell wall invagination.
What are bacteroids, and how do they function?
Bacteroids are differentiated Rhizobia within root nodules. They express nitrogenase, the enzyme responsible for nitrogen fixation.
What is the role of leghaemoglobin in root nodules?
Leghaemoglobin binds oxygen, maintaining low O2 levels for nitrogenase function while ensuring enough oxygen for bacterial respiration.
Why is nitrogenase critical, and what are its requirements?
Nitrogenase fixes N2 into NH3. It requires:
1. Low oxygen (protected by leghaemoglobin).
2. High energy input (ATP).
3. Electrons from bacterial respiration.
What is a symbiosome, and how does it form?
A symbiosome is a membrane-bound compartment within nodule cells where bacteroids reside. It forms when Rhizobia are engulfed by plant cells during infection.
What is the common SYM pathway, and what does it control?
The SYM pathway is a conserved signalling pathway in legumes that mediates Nod factor recognition and downstream symbiosis processes. It includes calcium oscillations and nuclear signalling.
What are the two main types of root nodules?
- Indeterminate Nodules: Cylindrical, continuously growing , grow from main root (e.g., pea, alfalfa).
- Determinate Nodules: Spherical, fixed growth, grow mainly from lateral root (e.g., soybean).
How was the interaction between flavonoids and Nod factors demonstrated?
Flavonoid mutants failed to attract Rhizobia. Adding flavonoids restored nodulation, proving their role in activating bacterial nod genes.
What role does calcium spiking play in nodulation?
Calcium spiking in root hairs is an early nodulation signal that activates downstream responses leading to nodule formation.
How do plants ensure specificity with Rhizobia?
Plants produce specific flavonoids and recognize corresponding Nod factors, ensuring only compatible Rhizobia form nodules.
How do environmental factors affect nodulation?
- High nitrogen levels inhibit nodulation.
- Stress (e.g., drought) reduces nodulation efficiency.
- Acidic soils can limit Rhizobia survival.
How have plants and Rhizobia co-evolved?
Host plants and Rhizobia evolved mutual recognition systems (e.g., flavonoids, Nod factors) to optimize symbiosis.
What experiment demonstrated the role of leghaemoglobin in nodules?
Mutants lacking leghaemoglobin had dysfunctional nodules, proving its role in maintaining oxygen balance for nitrogen fixation.
Why is nodulation energy-intensive for plants?
Plants provide carbon compounds to Rhizobia and invest resources in nodule maintenance to sustain nitrogen fixation.
What are the benefits of nitrogen fixation for plants?
Fixed nitrogen (NH3) is converted into amino acids and nucleotides, enabling growth in nitrogen-poor soils.
How do symbiosis mutants help study nodulation?
Mutants defective in SYM genes fail to form nodules, revealing key steps in the nodulation pathway.
Can nodulation be introduced into non-legumes?
Attempts to engineer nodulation in crops like wheat involve transferring nodulation genes, but success is limited due to complex signalling.
