Functional characterization of class I SlHSP17.7 gene responsible for tomato cold-stress tolerance

Small heat shock proteins (sHSPs) increase stress tolerance in a wide variety of organisms and enable them to endure changes in their environment. However, the molecular mechanism by which sHSPs protect plants against cold stress is unknown. Here, the sHSP of tomato named SlHSP17.7 (Solyc06g076540.1.1) has the characteristic of low temperature induced expression in BL21(DE3) E. coli and a molecular chaperone function in vitro. Overexpression of SlHSP17.7 showed a tolerant response to cold stress treatment due to an induce intracellular sucrose and less accumulation of ROS. Yeast two-hybrid assays showed that SlHSP17.7 is a binding partner of the cation/Ca²⁺ exchanger (SlCCX1-like; Solyc07g006370.1.1). This interaction was confirmed by pull down and bimolecular fluorescence complementation (BiFC) assays. High SlHSP17.7 and low SlCCX1-like levels alleviated programed cell death (PCD) under cold stress. Thus, SlHSP17.7 might be a cofactor of SlCCX1-like targeting endoplasmic reticulum (ER) membrane proteins, retaining intracellular Ca²⁺ homeostasis, and decreasing cold stress sensitivity. These findings provide a sound basis for genetic engineering of cold stress tolerance in tomato.