Fig. 1. Two-input logic gates controlling tissue-specific gene expression in roots.

Native promoters such as the a SMB promoter (columella and lateral root cap) and b PIN4 promoter (columella and stele) can be used to drive tissue-specific expression of genes. By combining these promoters with synthetic activators and repressors, Brophy et al. generated circuits that can perform Boolean logic operations, creating novel patterns of gene expression not found in nature. c In the NOR gate, GFP is expressed only in tissues where both the PIN4 and SMB promoter are not active. d In the NIMPLY gate, GFP is expressed only where SMB is active, but PIN4 is not.