High salinity, low temperature, and drought cause osmotic stresses, which is a major environmental factor that affects plant productivity1,2. Protein phosphorylation is one of the most significant post-translational modifications mediating signal perception and transduction in plant response to osmotic stress3,4,5. SNF1-related protein kinase 2s (SnRK2s) are involved in the osmotic stress signaling6. Nine of ten members of the SnRK2 family show significant activation in response to osmotic stress7,8. The snrk2.1/2/3/4/5/6/7/8/9/10 decuple (snrk2-dec) mutant having mutations in all ten SnRK2 displayed hypersensitivity to osmotic stress. In snrk2-dec mutant, the osmotic stress-induced accumulation of inositol 1,4,5-trisphosphate (IP3), abscisic acid (ABA) biosynthesis, and gene expressions are strongly reduced, highlighting the vital role of SnRK2s in osmotic stress responses6. However, it is still unclear how SnRK2s kinases regulate these biological processes. Profiling the phosphoproteomic changes in response to osmotic stress is an efficient way to bridge this gap and to delineate the osmotic stress-triggered defense mechanisms in plants.
Mass spectrometry (MS) is a powerful technique for mapping plant phosphoproteome9. Characterization of plant phosphoproteomics, however, remain a challenge due to the dynamic range of plant proteome and the complexity of plant lysate4. To overcome these challenges, we developed a universal plant phosphoproteomic workflow, which eliminates unwanted interferences such as from photosynthetic pigments and secondary metabolites, and enabling the deep coverage of plant phosphoproteome10. Several phosphopeptide enrichment methods such as immobilized metal ion affinity chromatography (IMAC) and metal oxide chromatography (MOC) have been developed for enriching phosphopeptides prior to MS analysis11,12,13,14,15,16. Acidic non-phosphopeptides co-purifying with phosphopeptides are the major interferences for phosphopeptide detection. Previously, we standardized the pH value and organic acid concentration of IMAC loading buffer to eliminate the binding of non-phosphopeptides, to obtain more than 90% enrichment specificity bypassing the pre-fractionation step11.
Sample loss in the multi-step process of phosphopeptide enrichment and fractionation hampers the sensitivity of phosphopeptide identification and the depth of phosphoproteomic coverage. Stop-and-go-extraction tips (stage tips) are pipette tips that contain small disks to cap the end of the tip, which can be incorporated with chromatography for peptide fractionation and cleaning17. Sample loss during the stage tip procedure can be minimized by avoiding sample transfer between the tubes. We have successfully implemented stage tip in Ga3+-IMAC and Fe3+-IMAC to separate low abundant multiple phosphorylated peptides from singly phosphorylated peptides, which improved the depth of human phosphoproteome15. In addition, the use of high pH reversed-phase (Hp-RP) stage tip has demonstrated the wider coverage of human membrane proteome compared to that of strong cation exchange (SCX) and strong anion exchange (SAX) chromatography18. Therefore, integrating IMAC and Hp-RP stage tip techniques can increase plant phosphoproteome coverage with simplicity, high specificity, and high throughput. We have demonstrated that this strategy identified more than 20,000 phosphorylation sites from Arabidopsis seedlings, representing an enhanced depth of plant phosphoproteome19.
Here, we report a stage tip-based phosphoproteomic protocol for phosphoproteomic profiling in Arabidopsis. This workflow was applied to study the phosphoproteomic perturbation of wild-type and snrk2-dec mutant seedlings in response to osmotic stress. The phosphoproteomic analysis revealed the phosphorylation sites implicated in kinase activation and early osmotic stress signaling. Comparative analysis of wild-type and snrk2-dec mutant phosphoproteome data leaded the discovery of a Raf-like kinase (RAF)-SnRK2 kinase cascade which plays a key role in osmore stress signaling in high plants.