Ectoine[(S)-2-methyl-1,4,5,6-tetrahydropyrimidine-4-carboxylic acid] and its hydroxyectoine[(S,S)-2-methyl-5-hydroxy-1,4,5,6-tetrahydropyrimidine-4-carboxylic acid] play a crucial role in the salt adaptation mechanism in halophilic bacteria. The biosynthesis of ectoine is catalyzed by three enzymes:L-diaminobutyric acid aminotransferase (EctB), L-diaminobutyric acid acetyltransferase (EctA), and ectoine synthase (EctC). Using L-Aspartate-β-semialdehyde (ASA) as the substrate, L-2,4-diaminobutyrate (DABA), N-γ-Acetyl-L-2,4-diaminobutyrate (ADABA) and ectoine are biosynthesized successively. Hydroxyectoine is produced by the hydroxylation of ectoine with ectoine hydroxylase (EctD). In normal, the ectoine synthesis genes exist in the form of ectABC gene cluster, while the hydroxyectoine synthesis gene ectD exists alone. The functional expression of ectoine synthesis genes in microbe and genetically modified cash crops can improve their resistance to extreme conditions such as hyperosmotic stress, alkali stress, and drought. The expression of hydroxyectoine synthesis gene also can enhance the host's ability to resist heat and drought. The biological functions and potential applications of ectoines have become a frontier hotspot.