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Updated: Jul 26, 2025

Phosphoproteomic Strategy for Profiling Osmotic Stress Signaling in Arabidopsis
Published on: June 25, 2020
Arabidopsis PLANT U-BOX44 down-regulates osmotic stress signaling by mediating Ca2+-DEPENDENT PROTEIN KINASE4
Wei Fan1, Xiliang Liao1, Yanqiu Tan1
1Joint Center for Single Cell Biology, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China.
Abstract:
Calcium (Ca2+)-dependent protein kinases (CPKs) are essential regulators of plant responses to diverse environmental stressors, including osmotic stress. CPKs are activated by an increase in intracellular Ca2+ levels triggered by osmotic stress. However, how the levels of active CPK protein are dynamically and precisely regulated has yet to be determined. Here, we demonstrate that NaCl/mannitol-induced osmotic stress promoted the accumulation of CPK4 protein by disrupting its 26S proteasome-mediated CPK4 degradation in Arabidopsis (Arabidopsis thaliana). We isolated PLANT U-BOX44 (PUB44), a U-box type E3 ubiquitin ligase that ubiquitinates CPK4 and triggers its degradation. A calcium-free or kinase-inactive CPK4 variant was preferentially degraded compared to the Ca2+-bound active form of CPK4. Furthermore, PUB44 exhibited a CPK4-dependent negative role in the response of plants to osmotic stress. Osmotic stress induced the accumulation of CPK4 protein by inhibiting PUB44-mediated CPK4 degradation. The present findings reveal a mechanism for regulating CPK protein levels and establish the relevance of PUB44-dependent CPK4 regulation in modulating plant osmotic stress responses, providing insights into osmotic stress signal transduction mechanisms.
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