15. Adapting to phosphate availability Flashcards
Main points of the lecture
- The importance of phosphorus in plant nutrition and Pi availability
- Aquiring phosphorus: The importance of PHT1 and PHF1
- Maintaining phosphorus: The importance of PHT2-5
- Adapting to the phosphorus environment: PSRs, RSA and root exudates
What are the 5 main reasons that phosphorus is important for plant nutrition and global biogeochemical cycles?
1
- Phosphorus is important for plant growth, as a energy donor, phospholipid, nucleic acid, starch/sucrose, metabolism etc.,. Depeletion will negatively impact growth (Blank et al., 2012)
- Increasing global resiliance on non-renewale phosphate rock deposits. There is a risk that we are approaching ‘peak phosphate’, with 90% of the world’s Pi from only 5 countries (Cordell et al., 2009
- Most phosphorus is inaccessible to plant roots, as it is immobile and insoluble. Plants can only uptake 20% of phosphorus in the soil (Lewis and Quirk, 1967)
- We need plants with improved phosphorus uptake to meet global food demands. Crops need to have high Phosphorus Acquisition (PAE), and high Phosphorus Use Efficiency (PUE). The amount of phosphorus surplus through leaching and run-off needs to be reduced (Zou et al., 2022)
- There is a highly unequal global phosphorus imbalance. Phosphorus deficits cover 29% of global cropland area, and are leading to immense imbalances in requirement, yield and surpluses
Define PHT1
2.1
A Plant Phosphate Transport that facilitates the uptake of plant phosphate
It is a transporter protein that allows phosphate into/out of the cell.
It is localised in the plasma membrane, and contains 12 membrane domains.
The PHT1 gene family is highly conserved and has immense functional redundancy
The transcriptional regulation and spatial expression of PHT1 in Arabidopsis indicate its importance in the uptake and translocation of Pi from the soil
Define PHF1
2.2
A Plant Phosphate Transporter Traffic Facilitator that allows for the plant ‘assessment’ of phosphate levels, due to its tight post-transcriptional control.
This was discovered by Gonzalez et al., 2005 who observed that phf mutants of Arabidopsis have low Pi uptake
These are facilitator proteins that facilitate the secretion of PHTs to the PM, and determine their presence
How do PHT1 and PHF1 interact under Low Pi?
2.3
Bayle et al., 2011
Under Low Pi, PHT1 secretion from the ER to the PM is mediated by PHF1.
This was measured in phf mutants in which fluorescent PHT1-GFP remains in the ER.
How do PHF1 and PHT1 interact during high Pi levels?
2.4
Bayle et al., 2011
Under high Pi, PM-localised PHT1 transporters will be phosphorylated by PHF1 (Ser-514), thereby preventing its secretion from the ER.
Any PHT1 transporters that are present in the PM will be removed by endosomes and degraded in the vacuole
Can track the progress of protein synthesis using GFP and the use of CHX (cyclohexamide), which can be used to inhibit protein synthesis.
Where are the different PHTs localised?
3.1
Wang et al., 2017
PHT1: PM
PHT2: Chloroplast
PHT3: Mitochondria
PHT4: Golgi
PHT5: Vacuole
The different localisations of PHT potentially has enormous implications for genomic engineering
What is the function of PHT5?
3.2
PHT5 regulates the movement of Pi into the vacuole under high Pi
Bucher and Fabianska, 2016
PHT5 is also called VPT and SPX-MFS3. It is a vacauolar Pi influx transporter
PHT5 localises to the tonoplast, and moves Pi into the vacuole when Pi levels are high, thus providing storage. This allows for cellular Pi homeostasis to be maintained
What is the function of OsVPE1 and OsVPE2?
3.3
Regulates the movement of Pi out of the vacuole under Low Pi
Does the opposite of what PHT5 does
Xu et al., 2019
They are vacuolar Pi efflux transporters that were identified by the use of Proteomics (iTRAQ analysis)
They are also localised to the tonoplast (like PHT5)
They are efflux transporters, that let the phosphate back out
How did Xu et al., 2019 model intracellular Pi homestasis in rice
3.4
Under low Pi, they found a transcriptional increase in PHT1, which is to facilitate Pi uptake. They also found a transcriptional increase in OsVPE1/2, to move Pi out of the vacuole for use
Under high Pi, they found the protesomal degradation of PHT1 to prevent the toxic increase in Pi, and the increase in vacuolar Pi, as mediated by PHT5
What are the 3 ways that plants can adapt to Pi conditions?
4
- The Phosphate Starvation Response (PSR)
- Changes to root system architecture
- Secretion of root exudates
Define the PSR
4.1
**The Phosphate Starvation Response (PSR) underpins local and systemic signalling **
PSR occurs in shoots through systemic signalling, and is delivered through the xylem/phloem. It is characterised by increased Pi recycling, reduced photosynthesis, increases in sugar concentrations and lipid remodelling
It can also occur locally in the roots, in which it is characterised by Root System Architecture, changes in metabolism, and increases in Pi uptake
How does Root System Architecture allow for adaptive responses to low pi?
4.2
Changes in traits such as reduced gravitropism, increased elongation of lateral roots, and root hairs, and production of aerenchyma and cluster roots
The model to study RSA was created by Peret et al., 2011 by observing the activation and repression of high and low Pi in A. thaliana
Liu et al., 2021 also showed that this response was common in beans. rice and white lupin
How does the secretion of root exudates allow for improved Pi adaptation?
4.3
Increases the availability of Pi
This can occur via the expression and secretion of organic acids (i.e., malate, citrate(, the exudation of extracellular acid phosphatas by roots, and the exudation of flavonoids and strigolactones
