ABA promotes sulfite stress tolerance by ABF4-mediated upregulation of SOX expression

With the development of industry, atmospheric SO2 is rapidly increased, seriously affecting plant growth and crop production. However, the mechanism of how plants respond to high SO2-induced sulfite stress is poorly characterized. Here we report that ABA signaling is involved in plant tolerance to sulfite stress. Our results indicate that the sulfite-stressed plants accumulated higher ABA with promoted expression of ABA-responsive genes compared with unstressed control and the ABA-biosynthetic mutant ABA DEFICIENT2 (aba2–1) had less sulfite tolerance than wild-type plants, supporting a role of ABA in plant sulfite stress response. ABA functions through its downstream transcription factor ABF4 because the abf4–1 mutant was more sensitive but the 35 S::ABF4 plants more tolerant to the sulfite stress than wild-type plants. Further, ABF4 upregulates SOX expression by directly binding to its promoter and overexpression of SOX can rescue increased stress sensitivity of abf4–1 mutant, showing that SOX acts downstream of ABF4. Together, our study reveals that the sulfite-stressed plants can activate ABA/ABF4 signaling to enhance SOX expression in plant sulfite stress tolerance.