Shoot growth depends on meristems, pools of stem cells that are maintained by a negative feedback loop between the CLAVATA pathway and the WUSCHEL homeobox gene(1). CLAVATA signalling involves a secreted peptide, CLAVATA3 (CLV3)(2), and its perception by cell surface leucine-rich repeat (LRR) receptors, including the CLV1 receptor kinase(3) and a LRR receptor-like protein, CLV2 (ref. 4). However, the signalling mechanisms downstream of these receptors are poorly understood, especially for LRR receptor-like proteins, which lack a signalling domain(5). Here we show that maize COMPACT PLANT2 (CT2) encodes the predicted alpha-subunit (G alpha) of a heterotrimeric GTP binding protein. Maize ct2 phenotypes resemble Arabidopsis thaliana clavata mutants, and genetic, biochemical and functional assays indicate that CT2/G alpha transmits a stem-cell-restrictive signal from a CLAVATA LRR receptor, suggesting a new function for G alpha signalling in plants. Heterotrimeric GTP-binding proteins are membrane-associated molecular switches that are commonly activated by ligand binding to an associated seven-pass transmembrane (7TM) G-protein-coupled receptor (GPCR)(6). Recent studies have questioned the idea that plant heterotrimeric G proteins interact with canonical GPCRs(7), and our findings suggest that single pass transmembrane receptors act as GPCRs in plants, challenging the dogma that GPCRs are exclusively 7TM proteins.