Shoot branching in plants is regulated by many environmental cues and by specific hormones such as strigolactone (SL). We show that the GAT1_2.1 gene (At1g15040) is repressed over 50-fold by nitrogen stress, and is also involved in branching control. At1g15040 is predicted to encode a class I glutamine amidotransferase (GAT1), a superfamily for which Arabidopsis (Arabidopsis thaliana) has 30 potential members. Most members can be categorized into known biosynthetic pathways, for the amidation of known acceptor molecules (e.g. CTP synthesis). Some members, like GAT1_2.1, are of unknown function, likely involved in amidation of unknown acceptors. A gat1_2.1 mutant exhibits a significant increase in shoot branching, similar to mutants in SL biosynthesis. The results suggest that GAT1_2.1 is not involved in SL biosynthesis since exogenously applied GR24 (a synthetic SL) does not correct the mutant phenotype. The subfamily of GATs (GATase1_2), with At1g15040 as the founding member, appears to be present in all plants (including mosses), but not other organisms. This suggests a plant-specific function such as branching control. We discuss the possibility that the GAT1_2.1 enzyme may activate SLs (e.g. GR24) by amidation, or more likely could embody a new pathway for repression of branching.