The Effect of the CBL-CIPK Signaling Pathway on Cucumber Plants Growth under High CO₂ and Low Nitrogen Conditions

Rising atmospheric CO₂ exposes plants to high-CO₂ environments, while excessive nitrogen fertilizer use degrades soil, highlighting the need to reduce nitrogen input and cultivate vigorous cucumber seedlings under HC-LN conditions. Calcineurin B-like proteins (CBLs) sense calcium signals and regulate carbon/nitrogen metabolism via CBL-interacting protein kinases (CIPKs), though their roles in cucumber under HC-LN conditions are unclear. Here, we identified seven CsCBL and 19 CsCIPK genes. Transcriptome analysis of two cucumber varieties with differing HC-LN sensitivities revealed that CsCBL4.2 and six CsCIPKs respond to HC-LN. Downstream genes of CsCBL-CsCIPK complexes were enriched in carbon/nitrogen metabolic pathways and linked to leaf area, root mass, photosynthesis, carbohydrate, and nitrate accumulation. Protein interaction assays showed that CsCBL4.2 interacts with four CsCIPKs. Notably, silencing of CsCBL4.2 or CsCIPK14 enhanced cucumber adaptation to HC-LN, while overexpression reduced tobacco adaptation. These results offer insights for cucumber cultivation and breeding under HC-LN conditions.