DcCAMTA6-DcCML8 promotes GABA accumulation in fresh-cut carrots via transcriptional activation of polyamine metabolism genes

Polyamine (PAs) degradation serves as a key pathway for γ-aminobutyric acid (GABA) synthesis in plants. While calcium chloride (CaCl₂) treatment is known to induce GABA accumulation in fresh-cut carrots, the underlying molecular mechanism regulating PAs metabolism remains unclear. In this study, CaCl₂ treatment increased calmodulin (CaM), GABA, [Ca²⁺ ]_cyt, ornithine, arginine and histidine contents in fresh-cut carrots. It concurrently enhanced the degradation of PAs by upregulating the transcript levels and enzyme activities of arginine decarboxylase (ADC), polyamine oxidase (PAO), and diamine oxidase (DAO). Conversely, treatment with the CaM antagonist trifluoperazine resulted in lower GABA content, which was achieved by reducing the activities and expression levels of ADC and PAO. Furthermore, CaCl₂ treatment induced the expression of DcCAMTA3, DcCAMTA4, DcCAMTA6, and DcCML8. Moreover, DcCAMTA3, DcCAMTA4, and DcCAMTA6 up-regulated the transcription of the DcADC and DcPAO1 genes, thereby enhancing GABA biosynthesis. Further analysis revealed that DcCAMTA6 protein directly binds to the CGTG cis-element in the promoters of DcADC and DcPAO1 and cooperates with DcCML8 to activate their expression. Therefore, our findings indicate that CaCl₂ treatment enhances the conversion of PAs to GABA through the DcCAMTA6-DcCML8-mediated upregulation of DcADC and DcPAO1 expression. This finding expands the theoretical basis for understanding GABA accumulation induced by CaCl₂ treatment in fresh-cut carrots.