Shown are tobacco leaf epidermal cells. Two of these express a Golgi apparatus­targeted green fluorescent protein fusion as a result of Agrobacterium-mediated transient expression. The plant Golgi apparatus exists typically as a population of discrete and highly mobile Golgi stacks, which are visible in the cell at bottom right as green punctuate structures (the plastids in the underlying mesophyll cells are red). When this Golgi marker was coexpressed with an inhibitory mutant of Arabidopsis Rab GTPase, AtRab1b, membrane traffic between the endoplasmic reticulum and Golgi apparatus was impaired, and the Golgi marker accumulated in the endoplasmic reticulum in addition to the Golgi stacks, as shown by the cell at top left. Unexpectedly, in the presence of the Rab mutant, vectorial movement of the Golgi stacks was reduced and in most cases abolished. These observations, reported by Batoko et al. on pages 2201­2218 of this issue, suggest that plant Golgi motility and endoplasmic reticulum­Golgi membrane traffic may be mechanistically coupled