Illuminating plant metabolism with genetically encoded biosensors

The metabolic flexibility of plants enables them to cope particularly well with changing environmental conditions. This flexibility is achieved by cellular processes that require tight coordination in space and time and constant balancing to maximise plant fitness. If we want to identify crops with higher yields and improved resistance to abiotic and biotic stresses, then we need to unravel these metabolic processes experimentally, and genetically encoded biosensors (GEBs) seem ideal for this. They allow non-invasive monitoring of metabolic processes in living cells over time and with high spatial and temporal resolution. The list of sensors and sensor variants that have been developed or established in plants continues to grow, providing insights into more and more parameters of plant metabolism. This, together with technological advances, also facilitates paraplexing and multiplexing experiments, where several processes are monitored simultaneously by GEBs. Despite these advantages, GEBs need to be used carefully and users must fully understand their characteristics in the chosen experimental plant system in order to draw meaningful conclusions from the spectroscopic changes of a sensor. Here, we aim to provide a list of fluorescent GEBs that can be selected for in planta use and highlight recent biological insights gained from them, focusing on advances where multiple GEBs have been used. We also discuss criteria for selecting an appropriate sensor and aspects of the field that remain challenging, in the hope of helping plant scientists to generate and interpret plant metabolism data using GEBs in a meaningful way.