Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 37.3 Plant Nutrition Often Involves Relationships with Other Organisms
Most plants engage in mutualistic partnerships with soil microbes—mainly bacteria and fungi—that improve nutrient uptake, growth, and protection. These symbioses are especially vital in nitrogen-deficient soils and are foundational to agriculture.
Rhizobacteria and the Rhizosphere
- Rhizosphere: Soil zone near roots, rich in microbes nourished by root secretions.
- Rhizobacteria can be:
- Free-living in the soil
- Endophytes living between root cells
Benefits to Plants:
- Secrete antibiotics that fight pathogens
- Enhance nutrient availability or detoxify soil
- Produce growth-promoting chemicals
- Can act as biofertilizers to improve crop yields
Bacteria and Nitrogen Fixation
- Plants absorb nitrogen as NO₃⁻ (nitrate) or NH₄⁺ (ammonium).
- Nitrogen-fixing bacteria convert N₂ gas into NH₃ (ammonia) using nitrogenase.
- This process requires high energy (16 ATP per 2 NH₃), fueled by root-derived carbohydrates.
Rhizobium–Legume Mutualism
- Rhizobium infects roots of legumes (e.g., beans), forming nodules.
- Inside, bacteria become bacteroids in vesicles.
- Leghemoglobin buffers oxygen for nitrogenase activity.
- Plants provide sugars; bacteria supply ammonium for amino acid synthesis.
Nodule Development:
- Roots release signals to attract Rhizobium.
- Bacteria enter via infection threads.
- Root cells divide to form nodules.
- Vascular tissue links nodules for nitrogen transport.
Mycorrhizae: Fungal Allies of Roots
- Mycorrhizae = “fungus roots”; symbiosis that improves phosphorus and water uptake.
- Plants provide sugars in return.
Two Main Types:
- Ectomycorrhizae:
- Form fungal sheath around roots
- Hyphae grow between cells
- Common in woody plants
- Arbuscular mycorrhizae (endomycorrhizae):
- Hyphae penetrate cell walls (not membranes)
- Form arbuscules—sites of nutrient exchange
- Present in over 85% of plant species, including most crops
Ecological and Agricultural Importance
- Mycorrhizae helped plants colonize land.
- Fungal inoculants may be needed for crops in new soils.
- Mycorrhizal networks can link multiple plants, influencing ecosystems.
- Invasive plants like garlic mustard disrupt these networks.
Other Nutritional Adaptations
- Epiphytes: grow on plants but don’t take nutrients (e.g., orchids)
- Parasitic plants: absorb nutrients from hosts using haustoria (e.g., mistletoe)
- Carnivorous plants: digest animals for nitrogen in poor soils (e.g., Venus flytrap)
In a Nutshell
Plants depend on microbial allies to obtain scarce nutrients. Rhizobacteria stimulate growth and fight disease. Nitrogen-fixing bacteria form nodules with legumes. Mycorrhizal fungi enhance water and phosphorus uptake. These partnerships support both ecosystems and sustainable farming.