Extreme weather events, including drought, have occurred worldwide with increasing frequency and severity in recent years. Drought stress is the main limiting factor for agricultural development in many regions, including tobacco—an important economic crop and a model plant for scientific research. As it is adapted to the tropics, tobacco is highly susceptible to drought stress, with resultant decreases in yield and quality. Glycine betaine (GB) is an osmoregulatory substance that can enhance plant resistance to various abiotic stresses. Here, we investigate the protective mechanism of genetically engineered glycine betaine (GB) on tobacco photosynthesis under drought stress induced by 30% PEG-6000. This study used transgenic tobacco (T) accumulating GB and wild-type tobacco (WT) to investigate the protective effects conferred by the genetic engineering of GB synthesis on tobacco photosynthesis under drought stress (induced by 30% PEG-6000). The results showed that the net photosynthetic rate of the tobacco plants significantly decreased under drought stress, and the degree of decrease was significantly lower in the T line than in the WT line. GB accumulation improved the resistance of photosynthesis to drought stress. Furthermore, under drought stress, the photosynthesis improvement in the T line was related to the accumulation of GB, leading to maintenance of the water status, the promotion of osmotic regulation, and an enhancement in antioxidant enzyme activities, which reduced membrane peroxidation and thereby increased the thylakoid membrane’s protein content and function, especially the photosystem II (PSII) function. The results provide a theoretical basis for further research on genetic engineering related to GB synthesis and the field application of exogenous GB.
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