**4. Mechanisms of GB-Mediated Thermotolerance**

Glycine betaine is a compatible osmolyte that likely plays an important role in osmoregulation in plants subjected to extreme environmental cues, including high-temperature stress) [21,49]. Additionally, it is likely that GB activates signaling molecules such as

calcium-dependent protein kinases (CDPKs) and mitogen-activated protein kinases (MAPKs) [50], which could activate stress-responsive and heat-shock transcription factor (HSF) genes [51,52]. The activated stress-responsive genes may boost the natural defense system by enhancing the activities of enzymatic antioxidants, such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), which may alleviate the negative impact of uncontrolled ROS causing oxidative damage triggered by heat stress)[53] (Figure 1). The elimination/reduction of ROS may keep biological membranes intact [54]. Furthermore, activated HSF genes may lead to the synthesis and activation of HSPs [55]. Most HSPs can also act as chaperones, which can prevent heat-induced aggregation of proteins [56]. The role of HSPs in plant thermotolerance has been elucidated in several comprehensive reviews [56,57]. GB can also significantly prevent photoinhibition by stabilizing the structure of the O2-evolving center (PSII) [19,58]. Thus, overall, GB can stabilize photosynthesis in heat-stressed plants, promoting growth under heat stress.

It is now evident that high temperatures cause many metabolic changes in plants that involve intricate reprogramming of cellular activities to safeguard organellar ultrastructures and functions under heat stress [59]. Although some promising roles of GB are depicted in Figure 1 for counteracting heat-induced physiological disorders, intensive research is needed to elucidate how and to what extent GB can regulate some key processes involved in plant thermotolerance, other than those highlighted in Figure 2. For example, very little information is available on the crosstalk between GB and other biomolecules, including various plant growth regulators.

**Figure 2.** Proposed mechanism of glycine betaine-mediated thermotolerance in plants.
