Ambient thermometers in plants: from physiological outputs towards mechanisms of thermal sensing

Plants respond to ambient temperature changes over a series of timescales. Genetic and physiological studies over the last decades have revealed myriad thermally sensitive pathways in plants. A recent study provides a genetic and biochemical mechanistic description of how thermal changes can be transduced to influence gene expression. What remains to be revealed in this, and other thermally controlled responses, is a description of the primary temperature-sensing event. Cooling and warming alter membrane fluidity and elicit intracellular free-calcium elevations, a process that has been considered the primary event controlling plant responses to temperature. Such direct thermal sensors appear to process temperature information. Future efforts will be required to identify the effector proteins linking perception to response. This review considers the evidence for plant thermometers to date, provides a description of several notable physiological and developmental processes under ambient temperature control, and outlines major questions that remain to be addressed in the understanding of thermometers in plants.