Amino acids are important component of fruit nutrition and quality. In this study, three longan cultivars, including non-aroma types 'Shixia' (SX), 'Lidongben' (LDB), and strong aroma type 'Xiangcui' (XC), were selected to analyze free amino acids (FAAs) variations at six distinct growth stages (S1-S6). The genome-wide identification and expression analysis of genes related to the branched-chain amino acids (BCAA) synthesis pathway were carried out. Results showed that 36 FAAs were identified, and the total FAAs content ranged from 2601.0 to 9073.5 mg/kg, which increased drastically with fruit development until ripening. L-glutamic acid (Glu), L-alanine (Ala), L-arginine (Arg), γ-Aminobutyric acid (GABA), L-aspartic acid (Asp), L-leucine (Leu), hydroxyl-proline (Hypro), and L-serine (Ser) were the predominant FAAs (1619.9-7213.9 mg/kg) in pulp, accounting for 62.28-92.05% of the total amino acids. During the period of rapid fruit expansion (S2-S4), the aroma of XC changed from light to strong, and the contents of L-alanine (Ala) and L-leucine (Leu) were significantly higher than those of SX and LDB. Furthermore, a total of two 2-isopropyl malate synthase (IPMS), two 3-isopropyl malate dehydrogenase (IPMD), and 16 BCAA transferase (BCAT) genes were identified. The expression levels of DilBCAT1, -6, and -9 genes in XC were significantly higher than those in SX and LDB, while DilBCAT16 in XC was lower. The content of Leu was negatively correlated with the expression of DilBCAT1, -6, and -9 in three varieties, but positively correlated with DilBCAT16, indicating that these four genes may be responsible for the different synthesis and degradation of Leu among cultivars.
Keywords: BCAT; IPMD; IPMS; L-alanine; L-leucine; fruit quality.