Sensors Make Sense of Signaling

Abstract

Signaling between and within cells involves reversible changes in the activity of chemicals, ions, metabolites and proteins. In this Special Focus Issue we have collected new articles investigating the function of biological sensors that detect these changes that occur during signaling. The Editors were keen also to seek the contribution of articles describing the development and use of man-made sensors to measure the in vivo dynamic changes in metabolites and second messengers. Sensors are components that detect, through binding, alterations in the environment, and transduce those alterations to an output. Endogenous cellular sensors that evoke biological responses and man-made sensors used by the experimentalist to measure signaling events should be capable of quantitative measurement of dynamic changes that can occur in milliseconds and could last for several hours. These sensors must be able to respond to the large fold changes in the concentration of hormones, second messengers, ions and metabolites that can occur in the apoplast and the symplast. The endogenous and man-made sensors also need to be capable of responding to and reporting spatially delimited signaling processes that might be restricted to specific organs, tissues, organelles or a subregion of the cytosol. Spatially delimited sensing can be achieved by cell type expression of endogenous and man-made sensors and subcellular targeting of proteins. Endogenous sensors are often restricted to specific regions of the cytosol by tethering to membranes or other signaling components. Man-made sensors are often based on fluorescent proteins to maintain the spatial fidelity of the reported output of the signal. There are strong parallels in the study of the endogenous sensors honed by evolution and those made by man as tools for the experimentalist because the nature of the signal transduced by the plant, or that detected by the experimentalist, will depend on both the pattern of the signal and the properties of the sensor. This is exemplified by a simple thought experiment in which a cellular gradient of a signal (e.g. second messenger or metabolite; Fig. 1A) is detected by a sensor (e.g. a protein receptor or a fluorescent indicator of biological activity; Fig. 1B). The sensor provides an output dependent on the sensor’s binding affinity for the signal (Fig. 1). The same is true for the measurement of temporal dynamics of changes in signals (Fig. 2A). The full dynamic pattern and temporal extent of the signal might not be accurately reflected by the reporter, depending on the sensor properties (Fig. 2B). Thus, the output from a signaling pathway or experimental analysis will depend on both the dynamics of the signal and the properties of the sensor. This presents challenges for the experimentalist, because one might not be certain of the true concentration and dynamic range of a signal. Experimentalists will have a better chance of correctly interpreting the temporal and spatial dynamics of the signal if they have access to a suite of sensors with a variety of properties. We are pleased that this issue contains reports of several new in vivo man-made sensor technologies. (c) The Author 2017. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists.