Correlation Analysis Reveals That CnGAD2 May Regulate Salt Tolerance and Aroma Metabolism in Aromatic Coconut via GABA-Dependent Signaling

The distinct aroma of the coconut (Cocos nucifera L.) is attributed to the presence of 2-acetyl-1-pyrroline (2AP), although the molecular and genetic pathway associated with its stress tolerance and aroma properties is largely unexplored. The glutamate decarboxylase (GAD) enzyme is involved in the formation of γ-aminobutyric acid (GABA), which is implicated in both adaptation to abiotic stress and 2AP synthesis. In this study, we comprehensively characterized the GAD gene family in coconuts and related Arecaceae species, revealing three highly conserved paralogs that diverged before speciation. It is established that CnGAD2 harbors a typical structure for the glutamate decarboxylase domain with palm-specific long-intron structure and the crucial C-terminal residues required for CaM binding. Notably, CnGAD2 was highly expressed in the middle leaves of aromatic germplasm compared with non-aromatic germplasm. Its expression level was also strongly associated with salt-stress severity and the accumulation of GABA and 2AP. An allele-based haplotype association study revealed a dominant haplotype that has higher levels of 2AP content than other haplotypes. Collectively, these findings suggest that CnGAD2 may function as an important link between salt-stress signaling and aroma biosynthesis. The identified dominant haplotype could therefore serve as a promising genetic resource for improving stress tolerance and flavor quality in coconut, an economically important tropical crop.