PHYTOCHROME-INTERACTING FACTOR 3 represses plant salt tolerance through MADS-box transcription factor AGAMOUS-LIKE 21

Salt stress poses a threat to plant water and nutrient uptake and leads to multiple forms of damages in plants, making it a major challenge to global crop production. PHYTOCHROME-INTERACTING FACTOR 3 (PIF3) is a key transcription factor in light signaling and it has been reported to play a critical role in plant responses to salt stress. Here, we demonstrated that PIF3 negatively regulates plant salt tolerance in Arabidopsis, and salt stress significantly enhances the interaction between PIF3 and light-activated PHYTOCHROME B (PHYB), leading to accelerated degradation of PIF3 in light, thus alleviating its negative regulation on plant salt tolerance. Additionally, we identified AGAMOUS-LIKE 21 (AGL21) as a downstream target gene of PIF3. PIF3 directly binds to the promoter of the AGL21 gene to promote its expression. The PIF3-AGL21 module transcriptionally modulates the expression a battery of downstream genes, including those involved in redox homeostasis regulation, thereby leading to accumulation of reactive oxygen species (ROS) and disruption of redox homeostasis within plants in response to salt stress. PHYB-mediated degradation of PIF3 attenuates the PIF3-AGL21 module to restore redox homeostasis and enhance plant tolerance to salt stress.