Integrated Transcriptomic and Metabolomic Analysis Reveals Metabolic Associations Underlying Oil Accumulation in Macadamia Cultivars

The kernel oil content of macadamia can reach up to 80%, yet the molecular basis underlying cultivar-dependent variation in lipid accumulation remains insufficiently understood. In this study, four macadamia cultivars with contrasting oil traits (O.C, QA1, QA2, and QA3) were investigated using integrated transcriptomic, metabolomic, and nutritional analyses to explore the metabolic pathways associated with kernel oil accumulation. Phenotypic evaluation revealed significant differences in fruit characteristics and nutritional composition among cultivars, with O.C exhibiting the highest crude fat content and QA1 showing the highest crude protein content. Transcriptomic analysis identified numerous differentially expressed genes (DEGs), which were mainly enriched in glycerolipid metabolism, α-linolenic acid metabolism, and amino acid biosynthesis pathways. Metabolomic profiling detected 778 metabolites, among which lipid-related compounds represented a major proportion. Integrated analysis revealed three major metabolic pathways associated with oil accumulation, including amino acid biosynthesis, glycerolipid metabolism, and linoleic acid metabolism. In the amino acid biosynthesis pathway, differential accumulation of amino acids and expression changes of genes including ASNS, GLUL, and MAT were observed among cultivars. In glycerolipid metabolism, differential expression of TAG biosynthesis-related genes (DGAT and PDAT) and variation in phospholipid-related metabolites, including LysoPC and LysoPE, were detected among cultivars. In linoleic acid metabolism, differential expression of fatty acid desaturation and elongation genes (SAD, FAD2, FAD3, and FAE1/KCS) was accompanied by cultivar-dependent differences in the abundance of several fatty acid-related metabolites. Overall, this study provides a comprehensive characterization of transcriptomic and metabolic variation associated with crude fat content and fatty acid-related metabolites in macadamia kernels and identifies candidate genes for further functional validation and molecular breeding studies.