16. Phosphate signalling and sensing in plants Flashcards
Structure of the lecture
**The SPX-PHR complex **
1. PHRs regulate the PSR by regulating PSR-associated genes
2. The SPX complex allows PHRs to sense the external Pi levels
3. Low Pi conditions: PHRs regulate SPX expression
4. High Pi conditions: SPX inhibits PHR function, thus creating a negative feedback loop
5. The PHR-SPX module
Systemic Pi signalling
1. PHO1: Phosphate loading under low Pi
2. PHO2: Degradation of PHO1 via ubiquitination under high Pi
3. miR399: Degradation of PHO2 in the shoots under low Pi
What is PHR1?
1.1.1
A MYB-coiled Transcription Factor that regulates PSR-associated genes
PHR1 functions as a dimer to bind a cis-element in the promoter of several low Pi-induced genes via its MYB domain
The MYB domain will localise to the nucleus under both high and low Pi to induce different genes.
How was PHR1 identified?
1.1.2
It was found that AtPHR1 in Arabidopsis and OsPHR2 in Rice act as the Master Regulator of the Phosphate Starvation Response (PSR)
These were identified via forward genetic screens that identified the mutants that failed to induce the PSR marker gene, IPS1.
IPS1 stands for Induced under Phosphate Starvation, and it regulates PHO1 and PHO2, but is mostly just used as a marker that PSR is occurring
IPS1 has a P1BS promoter
What is some evidence for the importance of PHR?
1.1.3
- PHR1 binds to the P1BS promoter, which is present in a very large range of Pi-Starvation-Induced genes (PSI), which includes PHT1, PHF1, miR399, IPS1, PHO1 and SPX
- The knockout of AtPHR1 leads to a completely defective PSR on all levels, which anthocyanins, starches and sugar accumulation impaired.
- The overexpression of PHR1 leads to increased Pi levels, and transcriptional increase in several PSR genes
How has the PHR family evolved?
1.1.4
Highly conserved across plants
Master regulators of the plant PSR response
Maintenance of Pi levels
What is SPX?
1.2.1
A phosphate sensor that is regulated by PHR. It is highly evolutionarily conserved
What occurs to SPX under low phosphate conditions
1.3
Under low phosphate conditions, SPX is downregulated
PHR2 is constitutively expressed, and is binding to P1BS via its MYB domain to activate PHT transporters, and allow for the uptake of Pi
PHR2 regulates the SPX expression during the PSR by binding to the P1BS promoter region of the SPX. SPX will mostly be being degraded.
The degradation of SPX means that PHR2 can be transported to the nucleus, in order to activate to the proteins involve in the PSR response.
SPX is a sensor, and therefore can detect the PPi clusters. Therefore, a little bit will still be produced, in order to enact these changes.
This was shown in rice, in which the OsSPX1 and OsSPX2 gene promoters both contain the P1BS OsPHR2 binding site
What does PHR2 do during low phosphate conditions?
1.3.2
Binds to the P1BS promoters of various other proteins via its MYB domain
Activation of PHF, which controls PHT and therefore Pi uptake
Activation of miR399 to degrade PHO2
Under high Pi conditions, how do SPX and PHR interact?
1.4
Under high Pi, SPX1/2 are negative regulators of the PSR.
SPX1/2 binds to PHR2 via the MYB domain, leading to its inhibition
SPX ‘senses’ high phosphate levels, and acts like a ‘sponge’ to bind to PHR2 via its P1BS cis-promoters
PHR2 therefore is not being transported to the nucleus and cannot act to upregulate PSR, and all following proteins/responses are downregulated
What is PHO1?
1.6.1
PHO1 is a protein that regulates systemic Pi transport, and is induced under low Pi conditions
Stefanovic et al., 2011
How is PHO1 regulated?
1.6.2
PHO1 is transcriptionally regulated by PHR1 (MYB binding via the P1BS element in PHO1)
This mediates the loading and translocation of of the Pi from roots to shoots
What is the action of PHO1?
1.6.3
Stefanovic et al., 2011
PHO1 leads to reduced shoot growth, but elevated shoot Pi levels - increasing focus of energy on Pi uptake as opposed to rgowth
High Pi levels in the xylem exudates
In pho1 mutants, there is excess phospahte in the xylem, leads to a toxic effect
What is PHO2?
1.7.1
PHO2 is a ubiquitin-conjugating enzyme, E2. It functions in the roots to modulate shoot Pi levels
Leads to the proteolytic degradation of PHO1. PHR1 and PHF1 under higher Pi
PHO2 degrades these transporters to prevent the accumulation of phosphate transporters and lead to Pi toxicity
Liu et al., 2012
Huang et al., 2013
How does PHO2 function?
1.6.2
Functions in the ROOT to modulate Pi levels
Plays a role in systemic homeostasis in order to prevent excess Pi uptake. pho2 mutants hyperaccumulate shoot Pi and show Pi toxicity
Delhaize and Randall, 1995
What is miR399?
1.7.1
A microRNA that leads to the degradation of PHO2 in the shoots during low Pi conditions
What does miR399 do under Low Pi?
1.7.2
miR399 produced
Transcriptionally induced inthe shoots
Regulated by PHR1 (P1BS binding)
mi399 moves from shoot to root and silences PHO2 post-transcriptionally
What does mi399 do in High Pi conditions?
1.7.3
miR399 not activated, since PHR2 function is suppressed by SPX. This means that PHO2 degrades PHO1, and prevents the root-shoot transport of Pi
