Design of a biosensor for direct visualisation of auxin

In plants, one of the most important regulative small molecules is indole-3-acetic acid (IAA) known as auxin. Its dynamic redistribution plays an essential role in virtually every aspect of plant life, ranging from cell shape and division to organogenesis and responses to light and gravity. So far, the spatial and temporal distribution of auxin at cellular resolution could not be determined directly. Instead it has been inferred from visualisation of irreversible processes involving the endogenous auxin response machinery. This detection system failed to record transient changes. Here we report on a genetically encoded biosensor for quantitative in vivo visualisation of auxin distributions. The sensor is based on the E. coli tryptophan repressor (TrpR) whose binding pocket was engineered for specific IAA binding and coupled to fluorescent proteins to employ FRET as readout. This sensor, unlike previous systems, enables direct monitoring of the fast uptake and clearance of auxin by individual cells in the plant as well as the graded spatial distribution along the root axis and its perturbation by transport inhibitors. Thus, our auxin sensor enables mapping of auxin concentrations at (sub)cellular resolution and their changes in time and space during plant life.