Team:HKUST-Rice/Phosphate Sensor
Phosphate Sensor - PphoA , PphoBR
Introduction
Phosphorus plays an essential role in plant growth. It associates with various growth factors for root development, seed production, etc. Deficiency in phosphorus leads to stunted plant growth, yet the symptoms are not obvious. Therefore, it is important to monitor the levels of phosphate in the soil for healthy plant growth.
Phosphate sensor Mechanism
Escherichia coli detects inorganic phosphate from the environment by the PhoR/PhoB two-component system (Hsieh & Wanner, 2010). As illustrated in Figure 1, PphoA and PphoBR is cross-regulated by PhoB and PhoR. The sensory histidine kinase PhoR behaves either as an activator or inactivator for PhoB depending on different states (inhibition state, activation state, deactivation state). When phosphate is limited, PhoR acts as a phospho-donor for the phosphorylation of PhoB. The phosphorylated PhoB will directly activate PphoA and PphoBR. In contrast, when there is high phosphate concentration, PhoR interferes with phosphorylation of PhoB which in turn inactivates both PphoA and PphoBR.
Phosphate sensor Design
Figure 2. Phosphate sensor constructs. (a) PphoA with GFP generator and (b) PphoBR with GFP generator. |
Native PphoA and PphoBR from E. coli can be utilized as our phosphate promoter. They were cloned and ligated with the GFP generator (pSB1C3-BBa_I13504) respectively. GFP sign is used as a promoter activity readout under different phosphate concentrations. |
Experiments performed
Characterization on the constructs (BBa_K1682013) was done using M9 minimal medium (without phosphate). Quantitative characterization on the promoters were done by measuring the fluorescence signal intensity using an EnVision® multilabel reader.
Please visit Phosphate sensor Experiment Protocol for more details.
Results
After measuring the GFP signal intensity using an EnVision® multilabel reader, the fluorescence signals were normalized against the cell density.
Figure 3. Characterization of PphoA and PphoBR in M9 minimal medium. GFP expression of pSB1C3-PphoA-BBa_I13504 and pSB1C3-PphoBR-BBa_I13504 in 0, 10, 30, 50, 100, 150, 200, 250 and 300 µM phosphate concentration are shown. Error bars were presented in SEM. .
As shown in Figure 3a, PphoA is induced under phosphate limitation and repressed under high phosphate concentration. The fluorescence intensity dropped by 2.99 folds between 0 to 200μM concentration of phosphate. Furthermore, a plateau is observed starting from the 200 μM phosphate concentration point, suggesting that the dynamic range of PphoA is from 0-200 μM of phosphate.
As shown in Figure 3b, PphoBR is induced under phosphate limitation and repressed under high phosphate concentration. The fluorescence intensity dropped by 1.88 folds between 0 to 250 μM concentration of phosphate. Furthermore, a plateau is observed starting from the 250μM of phosphate concentration point, suggesting that the dynamic range of PphoBR is from 0-250μM of phosphate.
Discussion
To select the promoter with better resolution of different phosphate concentrations, we compared the activity difference of two promoters between 0 and
250 μM of phosphate.
Figure 4. Comparison between PphoA and PphoBR GFP expression.The fold change of PphoA and PphoBR GFP expression between 0 and 250 μM of phosphate. For PphoA, the fluorescence intensity from 0 to 250 μM of phosphate dropped by 2.99 folds. For PphoBR, it dropped by 1.88 folds. Error bars were presented in SEM.
The fold change of PphoA GFP expression is greater than that of PphoBR between 0 and 250μM of phosphate. PphoA is suggested to be used if a better resolution of phosphate concentration is desired.
References
Hsieh, Y. J., & Wanner, B. L. (2010). Global regulation by the seven-component P i signaling system. Current opinion in microbiology, 13(2), 198-203.