The mechanisms underlying the organization and dynamics of plant endomembranes are largely unknown. Arabidopsis RHD3, a distant member of the dynamin superfamily, has recently been implicated in plant ER morphology and Golgi movement through analyses of dominant-negative mutants of the putative GTPase domain in a heterologous system. Whether RHD3 is indispensable for ER architecture and what role regions other than the putative GTPase domain play in RHD3 function are unanswered questions. Here we characterized an EMS mutant,gom8, with disrupted Golgi movement and positioning and compromised ER shape and dynamics.gom8mapped to a missense mutation in the RHD3 hairpin loop domain, causing accumulation of the mutant protein into large structures, a markedly different distribution compared with wild-type RHD3 over the ER network. Despite the GOM8 distribution, tubules fused in the peripheral ER of thegom8mutant. These data imply that integrity of the hairpin region is important for the subcellular distribution of RHD3, and that reduced availability of RHD3 over the ER can cause ER morphology defects, but does not prevent peripheral fusion between tubules. This was confirmed by evidence thatgom8was phenocopied in anRHD3null background. Furthermore, we established that the region encompassing the RHD3 hairpin domain and the C-terminal cytosolic domain is necessary for RHD3 function. We conclude that RHD3 is important in ER morphology, but is dispensable for peripheral ER tubulation in an endogenous context, and that its activity relies on the C-terminal region in addition to the GTPase domain.