Genetically encoded biosensors as gateways to retrograde redox signalling in live plants

In eukaryotic cells, protein supply to organelles varies depending on the stage of development and, in particular, exposure to environmental challenges. Adequate protein supply in terms of quality and quantity relies on sophisticated retrograde signalling systems that enable appropriate responses to the respective stress situations. Among many other retrograde signals, reactive oxygen species, which are generated during the initial stress response, are thought to be involved in transduction of redox-related signals that may also involve multiple redox pairs such as NAD(P)H/NAD(P)+ and redox-active metabolites such as glutathione. Deciphering such signals requires detailed knowledge of their amplitudes and temporal and spatial dynamics. Genetically encoded biosensors based on fluorescent proteins have been developed for a number of different redox-related physiological parameters and can be monitored in living cells, tissues, and even whole plants using a variety of instruments adapted to the respective resolution requirements, thus opening gateways to retrograde signalling in plant cells. This review summarizes and critically evaluates current probes and devices used to monitor sensor fluorescence. It also outlines how biosensors can be used in combination with genetic and pharmacological approaches, to extract meaningful information and dissect the retrograde redox signalling systems in living plants.