A HIGHER-PLANT 7-TRANSMEMBRANE RECEPTOR THAT INFLUENCES SENSITIVITY TO CYTOKININS

Background: All organisms perceive and respond to a profusion of envir onmental and endogenous signals that influence growth, development and behaviour. The G-protein signalling pathway is a highly conserved mec hanism for transducing extracellular signals, and the superfamily of r eceptors that have seven transmembrane (7TM) domains is a primary elem ent of this pathway. Evidence that heterotrimeric G proteins are invol ved in signal transduction in plants is accumulating, prompting specul ation that plant 7TM receptors might exist. Results: Using information in the dbEST database of expressed sequence tags, we isolated an Arab idopsis thaliana gene (GCR1) that encodes a protein with seven predict ed membrane-spanning domains and other features characteristic of 7TM receptors. The protein shows 18-23% amino-acid identity (46-53% simila rity) to, and good colinear alignment with, 7TM receptors from three d ifferent families. Its highest sequence identity is with the Dictyoste lium cAMP receptors. GCR1 is expressed at very low levels in the roots , stems and leaves of Arabidopsis; it is a single-copy gene which maps close to the restriction fragment length polymorphism marker m291 on chromosome 5. Transgenic Arabidopsis expressing antisense GCR1 under t he control of the constitutive cauliflower mosaic virus 35S promoter h ave reduced sensitivity to cytokinins in roots and shoots, yet respond normally to all other plant hormones. This suggests a functional role for GCR1 in cytokinin signal transduction. Conclusions: GCR1 encodes the first 7TM receptor homologue identified in higher plants and is in volved in cytokinin signal transduction. This discovery suggests that 7TM receptors are ancient and predate the divergence of plants and ani mals. (C) Current Biology Ltd ISSN 0960-9822.