New advances in genetic engineering for l-theanine biosynthesis

l-theanine, the most represented non-proteinogenic amino acid in tea plants (Camellia sinensis), is extensively utilized as a food supplement and a pharmaceutical, nutraceutical, and cosmetic ingredient. Tea plants are major sources of l-theanine, however, successfully extracting l-theanine from plants is limited by environmental conditions. Furthermore, the chemical synthesis of l-theanine is inefficient owing to the coproduction of undesirable stereoisomers that affect downstream applications. To overcome these problems, microbial engineering processes based on enzymatic catalysis and fermentation have emerged as sustainable manufacturing approaches for l-theanine biosynthesis. Advantages of these processes include the availability of well-established microbial engineering tools and strategies and high production rates. In this review, we first introduced the natural biosynthetic pathway of l-theanine production and discussed the detailed microbial or enzymatic processes involved in l-theanine production. We then summarized the information from recently published studies on the production of l-theanine using microbial platforms, emphasizing the latest advances and most relevant process parameters in l-theanine biosynthesis. As an alternative approach to the non-environmentally friendly and largely energy-consuming traditional methods, the microbial production of l-theanine is expected to be successfully utilized in various industrial applications.