Ubiquitination Proteomics of Sugar Beets Under Salt Stress Response Revealed Roles of Photosynthesis, Ca 2+ Signaling, and the Proteasome System

Salt stress seriously affects crop growth and agricultural productivity. The sugar beet M14 line is known for its salt tolerance and thus becomes a valuable material for studying plant response to salt stress. In this study, we tested the hypothesis that ubiquitination plays an important role in the M14 salt tolerance using label-free proteomics of leaves of the M14 seedlings under salt stress. Through qRT-PCR and Western blot analysis, we determined the time points for the response of leaf ubiquitinated proteins to 200 mM NaCl stress for 1 h and to 400 mM NaCl for 9 h. At these two time points, we identified a total of 101 differentially ubiquitinated proteins through LC-MS/MS analysis, of which 15 were differentially abundant under both salt concentrations. Furthermore, we selected 19 genes encoding the differentially ubiquitinated proteins involved in metabolism, photosynthesis, ion transport, stress response, transcription and translation, and post-translational modifications for qRT-PCR analysis. The trends of changes at the transcriptional, protein, and ubiquitination levels were not consistent, revealing the complexity of the regulatory mechanisms of ubiquitination in sugar beet salt stress response. In addition, we identified seven differentially ubiquitinated proteins in both leaves and roots, as well as eight proteins that underwent both ubiquitination and phosphorylation modifications. The ubiquitinated proteins were enriched in photosynthesis, Ca²⁺ signaling, stress response, and the ubiquitin-proteasome system. The results support the hypothesis, and future research needs to focus on the functional characterization of the specific ubiquitination event and its crosstalk with phosphorylation.