The allocation of sugars from photosynthetic leaves to storage tissues in seeds, fruits, and tubers is an essential determinant of crop yields. In plants, transporters play critical roles in allocating carbon to different organs. Transgenic modifications of plant membrane transporters have been shown to enhance crop yield and increase plants' resistance to biotic and abiotic stresses. Yet, quantitative, systems-level models to support this effort are lacking.
Recently, biosensors gained traction for collecting spatio-temporally resolved information on cell physiology and to validate computational models. In this talk, we report the design and use of genetically encoded biosensors to measure the activity of SWEETs, the only known family of sugar transporters that facilitate the cellular release of sugar in plants. We created a SweetTrac sensor by inserting circularly-permutated GFP into a SWEET transporter, resulting in a chimera that translate substrate-triggered conformational rearrangements during the transport cycle into detectable changes in fluorescence intensity. We demonstrate that a combination of cell sorting and bioinformatics can be applied as a general approach to accelerate the design of biosensors for in vivo biochemistry.
Finally, mass action kinetics analysis of the sensor's response suggests that SWEETs are low-affinity, near-symmetric transporters that can rapidly equilibrate intra- and extracellular concentrations of sugars. These types of models provide new insight into the working of sugar transporters and can help realize multiscale, dynamic simulations of metabolite allocation to guide crop improvement.