Underground#

Question 1

What is the critical concentration of a mineral?

Answer 1

Minimum concentration that correlates to maxium growth/yield (farmers should aim for0

Question 2

How do mineral ions move in soil?

Answer 2

By bulk flow (if carried by water) or by diffusion

Question 3

What is the effect of polyanionic soil particles?

Answer 3

Most soil particles are negativley charged and so adsorb cations to their surface
These ions can be availible to a plant via cation exchange

Question 4

What pH is best for root growth?

Answer 4

Slightly acidic (5.5-6.5) as most mineral ions are availible for uptake
Plants may pump out H+ to contibute to this

Question 5

Problems with absorbing phosphate ions

Answer 5

Is extremly immobile and has low abundance (100 times less than nitrogen)
Often a limiter of plant growth

Question 6

What is the anatomy of the root like?

Answer 6

Root epidermis: trichoblasts. Function of ion uptake
Cortex: Has a high storage capacity
Stele: Contains vasculatur (xylem/phloem)

Question 7

What is the casparian strip?

Answer 7

Hydrophobic diffusion barrier at endodermal layer of root.
Ions can only enter along apoplastic path

Question 8

How is potassium uptake optimised to save energy?

Answer 8

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

Question 9

How is nitrate uptake optimised?

Answer 9

Low availibility: CHL1 phosphoylated so it is high affinity
Vise versa at low affinity

Question 10

What are the two classifications of myocorrhizal associations?

Answer 10

Ectomycorrhiza: extra cellular colonisation of root tisssue
Endomycorrhiza: intra cellular colonisation of root tissue by fungus (includes arbuscular mycorriza)

Question 11

How do Ectomycorrhiza operate?

Answer 11

Penetrates epidermal layer and outer cortext
Produces network of apoplastically growing hypahe (hartig net)
At same time, fungus forms hyphal mantle around the root

Question 12

How do arbuscular myocorrhiza operate?

Answer 12

Penertrates outer root cell layers and grows towards inner cortex
Forms branched feeding structures called arbuscules

Question 13

How is phosphate absorbed by fungi

Answer 13

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

Question 14

How many plants species live with mycorrizal associations?

Answer 14

About 90% of land plants (> 80% of these are arbuscular fungi)

Question 15

What are obligate mycotrophs?

Answer 15

Photosynthetically inactve plants that depend on fungi for all nutrients (including carbon)

Question 16

What enzyme can reduce atomospheric nitrogen to ammonia?

Answer 16

Nitrogenase
- Inactivated by oxygen (needs anaerobic conditions)

Question 17

Describe the nitrogenase complex

Answer 17

2 protein complexes
- Homodimeric Fe protein
- Heterodimeric Mo-Fe protein

Question 18

How do cyanobacteria help plants obtain nitogen?

Answer 18

Cyanobacteria are free living nitrogen fixers
But when they are killed, they release amminia and indirectly fertilise the soil with nitrogen

Question 19

What are the two types of symbiotic nitrogen fixers?

Answer 19

Rhizobia (more common) and Frankia
- both fix atmospheric N2 when associated with a plant

Question 20

What are some features of Rhizobia and Frankia?

Answer 20

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)

Question 21

How is nitrogen fixation carried out by rhizobia?

Answer 21

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

Question 22

What are some adaptations of the nodule for N2 fixation?

Answer 22

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

Question 23

How is phosphate taken up from the arbuscules?

Answer 23

TWO TRANSPORTERS as two membranes
- One to excrete from the fungus
- One to be taken up into the plant cell

Question 24

What is the signal for forming fungal symbiosis?

Answer 24

Strigolactus (?) plant growth hormone

Question 25

Two things to remember about fungual symbiosis

Answer 25

Very ancient
Very non-specific (opposite of N fixing symbiosis)