The SlbZIP30-SlbZIP51 Module Safeguards Photosynthesis under Chilling Stress by Coordinating Nuclear Transcriptional Activation and Chloroplast Protein Stabilization in Tomato

Chilling stress severely restricts plant growth and productivity by impairing photosynthetic performance. However, the molecular mechanisms underlying chilling-induced photoinhibition and its mitigation remain incompletely understood. In this study, we functionally characterized two basic leucine zipper (bZIP) transcription factors, SlbZIP30 and SlbZIP51, from tomato (Solanum lycopersicum), and demonstrated that they are important regulators involved in photosynthetic protection in response to chilling stress. SlbZIP30 is localized in both chloroplasts and the nucleus with distinct compartment-specific functions. In chloroplasts, SlbZIP30 interacts with and stabilizes triose phosphate isomerase 1 (SlTPI1) and light-harvesting complex protein SlLHCA1 to preserve photosynthetic machinery integrity. In the nucleus, chilling induces SlbZIP51 translocation from the cytoplasm, where it forms a heterodimer with SlbZIP30 and activates transcription of SlCPN60α1, SlTPI2, and SlRBCS1, which encode chaperonin 60 subunit α1, triose phosphate isomerase isoform, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit 1, respectively. These proteins support carbon assimilation, sustaining photosynthetic capacity under chilling stress. Collectively, our findings uncover a photoprotective mechanism mediated by dual-localized SlbZIP30, which coordinates chloroplastic protein stabilization and nuclear transcriptional regulation to mitigate chilling-induced photodamage, providing a practical framework for engineering chilling-resilient crops.