Introduction: Global climate change has significantly increased the frequency and intensity of abiotic stresses, thereby limiting plant growth, development, and overall yield by damaging physiological systems. In response to environmental stressors, plants have evolved various adaptive mechanisms, including the accumulation of compatible solutes such as glycine betaine (GB), which plays a pivotal role in protecting cellular functions under adverse conditions. Also known simply as betaine, this compound is a methylated glycine derivative recognized across plant species for its ability to mitigate the deleterious effects of stressful environments. Its zwitterionic structure; comprising a positively charged trimethylammonium group and a negatively charged carboxyl group, confers high solubility and chemical stability to the molecule.

Owing to its excellent biocompatibility, favorable carbon-to-nitrogen ratio, and high-concentration accumulation, glycine betaine can enhance plant tolerance against a wide spectrum of abiotic stresses. Specifically, GB contributes to photosynthetic recovery and the alleviation of oxidative stress by reducing the accumulation and facilitating the detoxification of reactive oxygen species (ROS). Furthermore, it plays a crucial role in stabilizing membranes and macromolecules, while protecting key components of the photosynthetic apparatus, such as the Rubisco enzyme, Photosystem II (PSII), quaternary enzymes, and complex protein structures. Notably, glycine betaine can accumulate at high concentrations within plant cells without interfering with normal metabolic processes, thereby significantly increasing resilience to various osmotic stresses, extreme temperatures (heat and cold), and oxidative damage. The biosynthesis of glycine betaine occurs through distinct metabolic pathways, including the choline oxidation pathway (prevalent in plants and mammals), the direct glycine methylation pathway (specific to certain bacteria and halophytes), the choline dehydrogenase pathway, and the serine metabolism pathway. Such diversity underscores the vital importance of this osmolyte in mediating responses to environmental stresses. Recently, biotechnological interventions, such as Agrobacterium-mediated transformation, have successfully enhanced stress tolerance in susceptible species by overexpressing key genes, most notably codA, BADH, GSMT, and SDMT. Given the functional diversity of genes involved in the glycine betaine biosynthetic pathway, extensive efforts have been made to develop transgenic plants capable of effective accumulation of this metabolite; however, serious challenges such as unstable and weak transgene expression remain as key obstacles in this path. Factors including promoter type, genomic integration site, and epigenetic factors can influence the final performance. Furthermore, the overexpression of enzymes in the glycine betaine biosynthetic pathway may potentially impair growth by disrupting metabolic stability. Therefore, future research should focus on decoding the molecular networks regulating the biosynthesis, signaling, and transport of glycine betaine, particularly its crosstalk with phytohormones, transcription factors, and the identification of stress-inducible promoters. Additionally, optimizing transformation protocols and synchronizing glycine betaine gene expression with the overall plant metabolism are essential.

Aims: This review examines the biosynthesis, physiological functions, and molecular regulation of glycine betaine (GB). It highlights genetic engineering techniques to boost GB production, offering sustainable strategies for enhancing crop tolerance to environmental stresses.

Conclusion: In-planta biosynthesis of glycine betaine (GB) offers a more sustainable alternative to exogenous application, aligning closely with the principles of green agriculture. By integrating GB synthesis pathways, genetically engineered crops can autonomously boost metabolite production and bolster stress resilience, thereby eliminating the logistical costs of external treatments. Furthermore, a more profound investigation into these biosynthetic pathways will facilitate the identification and cloning of novel target genes, ultimately maximizing GB accumulation and enhancing environmental tolerance.