Extracellular Sensors and Extracellular Induction Components in Stress Tolerance Induction

This chapter reviews how physical stresses and extracellular chemical stresses are sensed and how sensing triggers stress responses. Intracellular sensors switch on many inducible systems, but, for many stress responses, detection of extracellular chemical stresses is by extracellular sensors (extracellular sensing components [ESCs]), and these ESCs are activated by stress, in the absence of organisms, to extracellular induction components (EICs). As well as inducing stress responses in the producing organisms, EICs can (because they are small molecules), act as alarmones, giving advanced warning to unstressed organisms of impending stress challenges and preparing them to resist such challenges. EICs bring about these effects by diffusing to regions not yet subjected to stress, and warning organisms there of impending stress. Thus, they act pheromonally, and this intercellular communication (cross-talk) gives early warning of stress. The ESC—EIC pair for a specific response are of very similar sizes and properties, but ESCs cannot induce stress tolerance, unless activated. All ESCs examined to date occur in more than one structural form, the nature of this depends on conditions prevailing during synthesis; each form shows a distinct activation profile, leading to a second type of early warning against stress. Distinct ESCs sense physical stresses, e.g., thermal stress and UV stress. Such physical stresses activate ESCs to EICs, and the latter can give early warning to unstressed organisms of impending physical stresses and prepare these organisms to resist such stresses by inducing them to stress tolerance. It should be noted that at least five ESCs act as biological thermometers, detecting increasing temperature, with gradual increases leading to gradual ESC activation.