Cytosolic hormone levels must be tightly controlled at the level of influx, efflux, synthesis, degradation and compartmentation. To determine ABA dynamics at the single cell level, FRET sensors (ABACUS) covering a range ∼0.2–800 µM were engineered using structure-guided design and a high-throughput screening platform. When expressed in yeast, ABACUS1 detected concentrative ABA uptake mediated by the AIT1/NRT1.2 transporter. Arabidopsis roots expressing ABACUS1-2µ (Kd∼2 µM) and ABACUS1-80µ (Kd∼80 µM) respond to perfusion with ABA in a concentration-dependent manner. The properties of the observed ABA accumulation in roots appear incompatible with the activity of known ABA transporters (AIT1, ABCG40). ABACUS reveals effects of external ABA on homeostasis, that is, ABA-triggered induction of ABA degradation, modification, or compartmentation. ABACUS can be used to study ABA responses in mutants and quantitatively monitor ABA translocation and regulation, and identify missing components. The sensor screening platform promises to enable rapid fine-tuning of the ABA sensors and engineering of plant and animal hormone sensors to advance our understanding of hormone signaling. DOI: http://dx.doi.org/10.7554/eLife.01741.001
eLife digest Plants are able to respond to detrimental changes in their environment—when, for example, water becomes scarce or the soil becomes too salty—in ways that minimize stress and damage caused by these changes. Hormones are chemicals that trigger the plant’s response under these circumstances. Abscisic acid is the hormone that regulates how plants respond to drought and salt stress, and also controls growth and development. In the past, it was possible to measure the average level of this hormone in a given tissue, but not the level in individual cells in a living plant, nor in specific compartments within a cell. Moreover, it was difficult to follow directly how abscisic acid moved between the plant cells, tissues or organs. Now, Jones et al. (and independently Waadt et al.) have developed tools that can measure the levels of abscisic acid within defined compartments of individual cells in living plants and in real time. The plants were genetically engineered to produce sensor proteins with two properties: they can bind to abscisic acid in a reversible manner, and they contain two ‘reporters’ that fluoresce at different wavelengths. Shining light onto the plant at a specific wavelength that is only absorbed by one of the reporters causes both of the reporters on the sensor proteins to fluoresce. However, the two reporters fluoresce differently when the sensor binds to abscisic acid. Specifically, one reporter fluoresces more and the other less. Hence, measuring the ratio of these two wavelengths in the light that is given off by the sensor proteins can be used as a measure of the concentration of abscisic acid in a plant cell. Jones et al. used a high-throughput platform to engineer five sensor proteins that detect abscisic acid over a wide range of concentrations. Using these ‘ABACUS’ sensors in living plants could track the uptake of abscisic acid into root cells, and revealed that the concentration of the hormone inside the cell stayed below the levels provided on the outside. Since known abscisic acid-transporters are capable of raising the hormone concentration inside a cell above that provided on the outside, abscisic acid transport into plant roots may occur via as-yet-undiscovered transporter proteins. Jones et al. also show that root cells rapidly eliminate abscisic acid, and that adding extra abscisic acid to the roots increases the rate of elimination within minutes. Plants were also engineered to target the sensor proteins specifically to the cell nucleus. In the future, targeting these sensors to the cell wall should allow tracking of the cell-to-cell movement of this hormone. Further aims include using ABACUS to track abscisic acid in plants undergoing stress, and to use the high-throughput platform to develop new sensors to track other hormones in living organisms (including animals). DOI: http://dx.doi.org/10.7554/eLife.01741.002