Identification of the SlCBL4-SlCIPK24 Calcium Signaling Pathway Involved in Salt Response in Tomato (Solanum lycopersicum L.)

Soil salinity is one of the major environmental factors that hinder the growth and productivity of tomato. Therefore, it is very important to develop tomato plants tolerant to salt stress. In Arabidopsis, it is well known that calcineurin B-like protein 4 (CBL4) calcium sensor and its target CBL-interacting protein kinase 24 (CIPK24) are involved in mediating salt response. In this study, we isolated tomato cDNA clones (SjCBL4 and SjCIPK24), which encode a polypeptide very similar to the Arabidopsis CBL4 and CIPK24, respectively. Deduced amino acid sequence analysis revealed that SjCBL4 contain four canonical EF-hand calcium-binding motifs. In fact, our electrophoresis mobility shift assays showed that SjCBL4 protein purified from E.coli via the GST-fusion expression system possesses the ability to bind with Ca²⁺ in vitro. We also discovered that SjCBL4, like AtCBL4, can interact with the Arabidopsis CIPK24 in a yeast two-hybrid system. Furthermore, we demonstrated that SjCBL4 interacts with SjCIPK24 in vitro and in vivo. Taken together, these findings strongly suggest that SjCBL4 and SjCIPK24 are very likely to be the tomato orthologs of the Arabidopsis genes, which can be utilized to develop salt tolerant tomato plants.