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.