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S ummary Plants use electrical and chemical signals for systemic communication. Herbivory, for instance, appears to trigger local apoplasmic glutamate accumulation, systemic electrical signals and calcium waves that travel to report insect damage to neighboring leaves and initiate defense. To monitor extra- and intracellular glutamate concentrations in plants, we generated lines expressing genetically encoded fluorescent glutamate sensors. In contrast to cytosolically localized sensors, extracellularly displayed variants inhibited plant growth and proper development. Phenotypic analyses of high-affinity display sensor lines revealed that root meristem development, particularly the quiescent center (QC), the number of lateral roots, vegetative growth and flower architecture were affected. Notably, the severity of the phenotypes was proportional to the affinity of the displayed glutamate sensors, intimating that their ability to bind extracellular glutamate caused the observed defects. Congruously, root growth defects were suppressed by supplementing culture media with low levels of glutamate. Overall, our data indicate sensor noxiousness was dependent on its membrane-tethering and likely caused by sequestration of extracellular glutamate, specifically at the cell surface, thereby either disrupting the supply of glutamate to meristematic cells and/or by impairing local glutamatergic signaling during development.