1. Introduction
Heat stress is an increasingly prevalent environmental constraint for plants. Rising average global temperatures, and more frequent temperature extremes have a negative impact on world crop yield (Kang et al., 2009; Lobell & Field, 2007). Warm temperatures can impair plant growth, fertility, development, metabolism, photosynthesis and immunity (Hatfield & Prueger, 2015; Howarth & Ougham, 1993; Janda et al., 2019; Wolf et al., 1991; Xu et al., 1995). In natural environments, plants experience daily and seasonal temperature fluctuations that vary in range, rate and duration. Whether a temperature becomes stressful depends on these variables, as well as coincident stress factors such as drought and salinity. At the cellular level, heat stress perturbs protein folding, membrane fluidity, cytoskeletal organization, transport, and enzymatic reactions, which leads to metabolic imbalances and pernicious accumulation of by-products such as reactive oxygen species (ROS). It is therefore of primary interest for plants to sense temperature alterations and initiate timely adaptive strategies to preserve cell function and viability. Plants respond to different temperature ranges with widely divergent physiological and developmental responses. However much less is known about the sensing mechanisms involved.
At temperatures above the optimum ambient growth temperature, but still within the physiological range (i.e. up to around 28°C for Arabidopsis), many plants undergo a process known as thermomorphogenesis, in which they alter morphology and development (for example, through expanded leaf structure, deeper roots and early flowering) to reduce exposure to potentially damaging temperatures (Fig. 1) (Casal & Balasubramanian, 2019; Crawford et al., 2012; Park & Park, 2019). At higher temperatures, i.e 28-37°C for Arabidopsis, there is still some growth, but several adverse effects become visible. Reproductive development and photosynthesis are affected, and root and shoot growth rates are compromised. At these temperatures, plants employ a variety of acclimation strategies to enhance temperature tolerance, including the production of molecular chaperones within minutes, and modulating the composition of cell membranes over a period of days. As temperatures rise above 40°C, severe heat stress is experienced, which can result in global injury, malfunction and ultimately, cell death (Fig. 1).
Molecular plant scientists have long questioned how heat is actually perceived and converted into a cellular signal. Since macromolecules are generally affected by heat, many have the potential to serve as thermosensors. The concept of thermosensor needs to incorporate processes akin to ligand-receptor binding coupled to downstream signaling, but also include less well-demarcated processes such as heat-induced increases in membrane fluidity followed by changes in membrane structure and function. This makes it difficult to identify macromolecules that actually perceive temperature and elicit specific signalling events. In recent years, several potential thermosensors and sensing mechanisms have emerged, both of ambient warm and stressful hot temperatures. Here, we summarize that knowledge, focussing on their mode of action, and provide a perspective for future research in this exciting field.