Underground# Flashcards
What is the critical concentration of a mineral?
Minimum concentration that correlates to maxium growth/yield (farmers should aim for0
How do mineral ions move in soil?
By bulk flow (if carried by water) or by diffusion
What is the effect of polyanionic soil particles?
Most soil particles are negativley charged and so adsorb cations to their surface
These ions can be availible to a plant via cation exchange
What pH is best for root growth?
Slightly acidic (5.5-6.5) as most mineral ions are availible for uptake
Plants may pump out H+ to contibute to this
Problems with absorbing phosphate ions
Is extremly immobile and has low abundance (100 times less than nitrogen)
Often a limiter of plant growth
What is the anatomy of the root like?
Root epidermis: trichoblasts. Function of ion uptake
Cortex: Has a high storage capacity
Stele: Contains vasculatur (xylem/phloem)
What is the casparian strip?
Hydrophobic diffusion barrier at endodermal layer of root.
Ions can only enter along apoplastic path
How is potassium uptake optimised to save energy?
Two transporters
One that requires lots of energy has a high K+ affinity
One that requires less energy has a lower K+ affinity
Expression will depend on soil composition
How is nitrate uptake optimised?
Low availibility: CHL1 phosphoylated so it is high affinity
Vise versa at low affinity
What are the two classifications of myocorrhizal associations?
Ectomycorrhiza: extra cellular colonisation of root tisssue
Endomycorrhiza: intra cellular colonisation of root tissue by fungus (includes arbuscular mycorriza)
How do Ectomycorrhiza operate?
Penetrates epidermal layer and outer cortext
Produces network of apoplastically growing hypahe (hartig net)
At same time, fungus forms hyphal mantle around the root
How do arbuscular myocorrhiza operate?
Penertrates outer root cell layers and grows towards inner cortex
Forms branched feeding structures called arbuscules
How is phosphate absorbed by fungi
H+/Pi symporter used to take Pi from the soil
Inside the fungus Pi is coverted to polyphosphate (polyP) and then transferred to the intraradial fungal structures
In the arbusucles, polyP is hydrolysed back to Pi and released to symbiotic interfcae
Plant H+/Pi symporters can take up Pi into the cytoplasm
How many plants species live with mycorrizal associations?
About 90% of land plants (> 80% of these are arbuscular fungi)
What are obligate mycotrophs?
Photosynthetically inactve plants that depend on fungi for all nutrients (including carbon)
What enzyme can reduce atomospheric nitrogen to ammonia?
Nitrogenase
- Inactivated by oxygen (needs anaerobic conditions)
Describe the nitrogenase complex
2 protein complexes
- Homodimeric Fe protein
- Heterodimeric Mo-Fe protein
How do cyanobacteria help plants obtain nitogen?
Cyanobacteria are free living nitrogen fixers
But when they are killed, they release amminia and indirectly fertilise the soil with nitrogen
What are the two types of symbiotic nitrogen fixers?
Rhizobia (more common) and Frankia
- both fix atmospheric N2 when associated with a plant
What are some features of Rhizobia and Frankia?
Evolutionary ‘young’
Rhizobia is most commonly associated with legumes
Both need a high degree of specificity with the host (one strain typically only associates with one plant species)
How is nitrogen fixation carried out by rhizobia?
Requires the development of a highly specialised organ: the nodule
Rhizobia release signals which induce cortical cell division and nodule primorsium formation
Rhizobia attach to the root hair which produce a tube like structure into which the bacteria migrate and proliferate
Bacteria differentiate into bacterioids and begin N2 fixation
What are some adaptations of the nodule for N2 fixation?
Bacteriods need to respire to produce ATP but cannot fix N in the presence of oxygen
Therefore adaptations needed:
- Gas permeability of nodule tissue recused (so O2 concentration is high enough to respire but not to deactivate the enzyme)
- Bacterial cells express cytochrome oxidase, which has a high O2 affinity
How is phosphate taken up from the arbuscules?
TWO TRANSPORTERS as two membranes
- One to excrete from the fungus
- One to be taken up into the plant cell
What is the signal for forming fungal symbiosis?
Strigolactus (?) plant growth hormone
Two things to remember about fungual symbiosis
Very ancient
Very non-specific (opposite of N fixing symbiosis)
