Engineering stress resistance: advances in glycine betaine production for sustainable agriculture

With the rapid development of industry and frequent extreme weather events, sustainable agricultural production is increasingly confronted with various adversities. Improving the resistance of plants and eco-friendly microbial fertilizers to abiotic stresses is essential for sustainable agricultural development. Glycine betaine (GB), a trimethyl derivative of glycine, plays a crucial role as a cellular osmotic pressure regulator, enhancing the resistance of agricultural plants and microorganisms to stress. Betaine is typically obtained through natural extraction or chemical synthesis; however, the former faces seasonal and geographical limitations, whereas the latter is associated with environmental pollution. Additionally, the post-extraction processes for both methods can increase production costs. Biosynthesis via plants or microorganisms represents a more sustainable approach to betaine production, aligning with the principles of green agriculture. Genetically engineered plants incorporating the betaine synthesis pathway can directly increase GB production, improving plant stress resistance and eliminating the need for costly post-extraction processes. Similarly, genetically engineered microorganisms that construct or enhance the GB synthesis pathway can effectively increase GB production and associated resistance, with relatively simple post-extraction processes that further reduce costs and can be produced continuously. However, the GB synthesis pathway in microorganisms has been less studied. This review summarizes metabolic engineering studies of the GB synthesis pathway in plants and microorganisms and proposes strategies to further increase GB production in engineered microorganisms. In this paper, we review the methods of GB production, its protective effects, its synthesis pathways in organisms, and advancements in genetic engineering research. We also discuss the potential for developing genetically engineered plants or microorganisms for GB synthesis technologies, aiming to lay the groundwork for future applications in sustainable agricultural development.