A eukaryotic input circuit: computationally designed receptors, synthetic eukaryotic signal transduction pathways, and a synthetic signal sensitive promoter that allow highly specific transcriptional induction in response to an externally provided ligand is disclosed. The input circuit is able to specifically bind a targeted substance and transmit a signal to the nucleus where transcription of a gene is activated. An output circuit serves as a simple readout system of the substance detected by the input circuit. The readout circuit exemplified here is a degreening circuit which causes plants to turn white. Activation of the degreening circuit can be detected by eye, or remotely with a variety of machines (hand-held, aircraft or satellite based) and is also resettable. When linked the input circuit if operably linked to the output circuit, produces a functional plant detector.