Summary Many differentiated animal cells, and all higher plant cells, build interphase microtubule arrays of specific architectures without benefit of a central organizer, such as a centrosome, to control the location and geometry of microtubule nucleation. These acentrosomal arrays support essential cell functions such as morphogenesis [1, 2], but the mechanisms by which the new microtubules are positioned and oriented are poorly understood. In higher plants, nucleation of microtubules arises from distributed γ-tubulin ring complexes (γ-TuRC) at the cell cortex that are associated primarily with existing microtubules [3-5] and from which new microtubules are nucleated in a geometrically bimodal fashion, either in parallel to the mother microtubule, or as a branching event at a mean angle of approximately 40° to the mother microtubule. By imaging the dynamics of individual nucleation events in Arabidopsis, we found that a conserved peripheral protein of the γ-TuRC complex, GCP-WD/NEDD1 [6-8], associated with motile γ-TuRCs and localized to nucleation events. Knock down of this essential protein resulted in reduction of γ-TuRC recruitment to cortical microtubules and total nucleation frequency, showing that GCP-WD controls γ-TuRC positioning and function in these interphase arrays. Further, we discovered an unexpected role for GCP-WD in determining the geometry of microtubule-dependent microtubule nucleation, where it acts to increase the likelihood of branching over parallel nucleation. Cells with normally complex patterns of cortical array organization constructed simpler arrays with cell-wide ordering, suggesting that control of nucleation frequency, positioning and geometry by GCP-WD allows plant cells to build alternative cortical array architectures.