The effect of calcium chloride (CaCl2) treatment on γ-aminobutyric acid (GABA) accumulation in fresh-cut cantaloupe and the involved mechanisms were investigated. The result showed that 1% (w/v) CaCl2 treatment increased GABA content and activities of glutamate decarboxylase (GAD) and succinate semialdehyde dehydrogenase (SSADH), while decreased glutamate (Glu) content and GABA transaminase (GABA-T) activities in fresh-cut cantaloupe. CmCML11 and CmCAMTA5 expressions of CaCl2-treated fruit increased by 187.4% and 165.6% than control fruit in the initial 6 h. Besides, expressions of GABA shunt genes, including CmGAD1, CmGAD2, CmGABA-T and CmSSADH were also up-regulated by CaCl2 treatment during early storage. Moreover, acting as a transcriptional activator, CmCAMTA5 could bind to the CG-box in promoters of CmGAD1, CmGABA-T and CmSSADH and activate their transcription. Furthermore, the interaction between CmCML11 and CmCAMTA5 could enhance the transcriptional activation on GABA shunt genes which were regulated by CmCAMTA5. Collectively, our findings revealed that CaCl2 treatment promoted GABA accumulation in fresh-cut cantaloupe via the combined effect of CmCML11 and CmCAMTA5 in the regulation of expressions of CmGAD1, CmGABA-T, and CmSSADH in GABA shunt.
GABA is able to increase resistance to biotic and abiotic stresses in fresh-cut fruits and vegetables. Therefore, the objective of this research was to explore the potential regulatory mechanisms of γ-aminobutyric acid (GABA) accumulation in fresh-cut stem lettuce following GABA treatment. The evidence showed that exogenous GABA stimulated the GABA shunt by elevating glutamate levels, the activities of GABA transaminase (GABA-T) and glutamate decarboxylase (GAD). Similarly, GABA stimulated polyamine metabolism by increasing the activities of 4-amino aldehyde dehydrogenase (AMADH), polyamine oxidase (PAO) and diamine oxidase (DAO), as well as elevating free polyamines, arginine and ornithine levels. Subsequently, GABA application up-regulated the expression of GABA shunt genes and polyamine metabolism genes. Additionally, GABA treatment resulted in the down-regulation of LsMYB44 and LsWRKY12 expressions. Notably, LsMYB44 bound to MYB binding sites in the LsGAD, LsGABAT1, LsADC1, LsPAO2, LsALDH7B4 promoters and repressed transcription of these genes. The interaction between LsMYB44 and LsWRKY12 was associated with the transcriptional repression of polyamine metabolism and GABA shunt genes by LsMYB44. In conclusion, LsMYB44 and LsWRKY12 downregulated the transcription of key genes of GABA shunt and polyamine metabolism in fresh-cut lettuce. This downregulation, however, was alleviated by the application of GABA, thereby promoting endogenous GABA accumulation.