lecture 8 - nutrients - mechanisms and strategies Flashcards
How do plants take up diverse nutrients?
Transport against diverse concentration gradients energized by a pH and electrochemical gradient across the plasma membrane, generated by the proton-pumping ATPase
Further possibilities for extracellular-cytoplasmic gradients come from the ability to shuffle things out of the cytoplasm into the vacuole
Can’t rely on diffusion – active gradients need to be created, and if you remember the K+ system for stomata, the same tool (H+ ATPase) can be used to generate pH gradients.
How are the pathways used to take up nutrients?
Apoplast pathway: Between cells Gradient drive Cannot extend beyond Casparian strip Symplast pathway: Within cells Controlled by transporters Controls entry to vascular tissues Remember that what applies to water is also true of mineral ions – the symplastic pathway is the method by which plants regulates how much of each ion is taken up, and how it prevents ions that are not wanted (eg toxic compounds) from being taken up by mass flow and/or diffusion.
What are the problems with NPK?
Main problems: Phosphate, low bioavailability
Not very soluble in soil and hence immobile
P concentration in plants = 2,000 µg/g
Even if total soil P is high – its’s bioavailability is low
Forms complexes with cations (e.g. Al3+, Fe3+, Ca2+) in soil
Not very much in soil solution phase
Rock phosphate (for fertilisers) = finite resource
Main problems: Nitrogen - Plants need a lot of it
N concentration in plants 20000 µg/g
Freely mobile, but particularly hard to get from
Acid bogs
Sandy soils
Heavy inter-plant competition
K- it’s all about high affinity uptake
P and N are more often limiting
P is a particularly huge problem because of imobiility in soil
How do liverworts (early land plants) take up nutrients?
No true “roots”
Rhizoids primarily for anchorage not for water or nutrient uptake
Very low surface area for P absorption
Zone of depletion quickly develops
What are mycorrhizas?
Mycorrhiza literally ‘fungus-root’
Mutualistic symbiosis
Fungal cells penetrate cell wall of root cells - but not the cytoplasm, enhancing carbon and nutrient exchange (endomycorrhiza)
What is an arbuscule?
highly branched structure that maximises surface area for exchange of materials
How does the plant-fungus interface work?
Fungus provides increased phosphorus uptake for the plant
Increased uptake of N and other nutrients too
Plants provide the fungus with a carbon source: the plant may donate up to 20% of fixed carbon to the fungus
What is the hyphal network?
External mycelium in soil
Massively extends root system
Hyphae are very fine and so are cheaper than roots
Connect plants through a common mycelial network
How is the symbiosis established?
In low P (and low N) host roots exude
strigolactone
Strigolactone triggers hyphal growth
Fungus signals back to plant
What are other ways of increasing phosphate capture?
Modifying the ‘rhizosphere’:
Volume of soil influenced by the presence of the plant root
Some plants secrete organic acids into the rhizosphere.
Citrate exchanges for the phosphate
Cluster Roots:
Some plants produce cluster roots under low P conditions
Citrate released in a ‘burst’
Cluster roots also secrete enzymes (e.g acid phosphatases) to release P from organic sources
What are ways of increasing phosphate availability?
Root exudates:
In alkaline soils, phosphate forms insoluble salts
Roots secrete organic acids – reduces local pH, and releases phosphate
Roots secrete mucilage
Habitat for bacteria that mineralise P
improve phosphate mobility in the soil
this is a significant carbon loss
What is ectomycorrhizal symbiosis?
Fungal cells surround but do not penetrate root cells.Carbon and nutrients are exchanged through the plasma membrane
Important in N capture
Soils with complex organic N
Occurs in fewer plant families (often forest trees)
Complex evolutionary history within “higher” fungi
more recent evolutionary development than endomycorrhiza
What is the purpose of nodulation in leguminous plants?
Bacterial Nitrogen Fixation Catalysed by Nitrogenase complex A reductase, provides electrons A nitrogenase, reduces N2 to NH4+ Both are Iron Sulphur proteins Nitrogenase contains Molybdenum Only prokaryotes carry out this reaction
What is root nodule symbiosis?
A highly specialist association between rhizobia and leguminous plants
Rhizobia fix N2 and make it available to the plant as NH4+
Plants provide the rhizobia with a carbon source: can be up to 25% of fixed C
What is leghaemoglobin?
Accumulates in the cytosol of rhizobium containing cells
Contains a haem group- and is red/pink
Regulates O2 transport to the bacteria
Too much O2 inactivates the nitrogenase complex
Too little O2 prevents respiration
