**7. Conclusions and Prospects**

Under the changing climate threat, strategies are needed to alleviate the adverse impacts of harsh environmental stresses such as HS on plants. The most expedient strategy is to use the plant's natural defense system for withstanding these stresses. Under HS, many plants naturally accumulate GB, a compound known to mediate HS tolerance via osmoregulation, photosynthetic mechanisms, and signaling molecules, such as CDPKs, MAPKs, nitric oxide (NO), and sugars, which activate stress-responsive genes and HSF genes.

As discussed above, GB biosynthesis has different roles in different organelles; for example, chloroplastic GB is actively involved in stress tolerance, while cytosolic GB lacks such functionality. As a result, high levels of GB in plants are not the only factor enhancing stress tolerance. The principal substrate for GB synthesis is choline, which occurs in the cytosol. Choline transport to the chloroplast takes place via choline transporters. Several problems occur in GB-non-accumulating plants: (1) a limited amount of endogenous choline exists and (2) choline transporters present on the chloroplast membrane do not transport the required level of choline to chloroplasts. Thus, molecular biologists require this information for different crops to develop transgenic lines/cultivars with enhanced GB-accumulating ability.

Plants that do not naturally accumulate GB under HS have less HS tolerance than those that do. Various strategies have been used to increase GB accumulation in these plants to improve their tolerance against HS. Exogenous supplementation of GB in different forms, such as seed priming or plant priming, has enhanced endogenous GB levels and thus improved plant growth and development under HS. Genetic engineering could be used to introduce biosynthetic pathway-related genes into GB-deficient species from plants or microorganisms. While various studies have demonstrated the importance of GB in thermotolerance, very few have revealed the molecular roles of GB in thermotolerance. Moreover, transgenic lines generated for different crops have been based on a single gene transformation, with marked progress in terms of enhanced GB accumulation coupled with improved thermotolerance. However, all these studies have been undertaken under semi-controlled or controlled conditions, and the developed transgenic lines have not been tested under natural field conditions. Thus, further research is needed to generate thermotolerant lines/cultivars for different crops threatened by the rising temperatures of climate change. The effectiveness of exogenous GB application should be tested at the field level.

**Author Contributions:** Conceptualization, F.Z. and M.A.; writing—original draft preparation; F.Z. writing—review and editing, F.Z., M.A. and K.H.M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

