In the August issue of Nature Biotechnology, Rajagopal Subramaniam and colleagues from the Université de Montreal describe a system for visualizing protein-protein interactions in living plant cells (Nature Biotechnology 2001, 19:769-772). Their technique is based on an in vivo protein-fragment complementation assay in which protein interactions between fusion proteins induce folding and reassembly of fragments of the murine dihydrofolate reductase (DHFR) enzyme. Protein-protein interactions are monitored by the DHFR inhibitor fluorescein-conjugated methotrexate (fMTX). Subramaniam et al. used their assay to examine the interaction between Arabidopsis disease-resistance protein NPR1/NIM1 and the basic leucine-zipper protein TGA2. They show that salicylic acid induced NPR1-TGA2 interaction in tobacco or potato cells. This plant two-hybrid system should prove useful for the functional annotation of plant genomes.