CAMTA5 suppresses blossom-end rot in tomato by regulating the expression of the functional CBP60A variant

Low-calcium (Ca) stress induces blossom-end rot (BER), a physiological disorder that seriously reduces the yield and quality of tomato (Solanum lycopersicum L.). However, the mechanisms underlying low-Ca-induced BER remain insufficiently understood. Calmodulin-binding transcriptional activators (CAMTAs) are evolutionarily conserved CaM-binding proteins that play crucial roles in plant responses to Ca deficiency. In this study, we found that the expression of SlCAMTA5 increases in tomato under Ca-deficient conditions. Knockout of SlCAMTA5 markedly elevates BER incidence, accompanied by reactive oxygen species (ROS) accumulation and reduced Ca uptake capacity in the fruit peel under low-Ca conditions, whereas SlCAMTA5 overexpression results in the opposite phenotypes, demonstrating its function in suppressing BER. Transcriptome analysis showed that the expression of calcium ion (Ca2+) transport-related genes (CML37, CML38, and CDPK28) is upregulated in wild-type (WT) plants but downregulated in camta5 mutants under low-Ca stress. Further assays revealed that CAMTA5 interacts with CML38 and CDPK28 via its P1 region containing ankyrin (ANK) repeats, several IQ motifs, and calmodulin-binding (CAMBD) domains. Moreover, the Ca2+ transport-related gene calmodulin-binding protein 60 A (CBP60A) was identified as a gene undergoing differential alternative splicing in WT under low-Ca stress. It was also confirmed as differentially expressed in camta5 mutants. Under Ca deficiency, new nonfunctional variants of CBP60A are generated from its pre-mRNA. CAMTA5 directly binds to the promoter of the functional CBP60A transcript, activating its expression. Moreover, the interaction between CAMTA5 and CML38/CDPK28 enhances this transcriptional activation. Collectively, our findings demonstrated that the CML38/CDPK28-CAMTA5 module suppresses BER occurrence by regulating the functional alternatively spliced variant of CBP60A.