AbstractThe endoplasmic reticulum (ER) is composed of tubules, sheets, and three-way junctions, resulting in a highly conserved polygonal network in all eukaryotes. The molecular mechanisms responsible for the organization of these structures are obscure. To identify novel factors responsible for ER morphology, we employed a forward genetic approach using a transgenic Arabidopsis thaliana plant (GFP-h) with fluorescently labeled ER. We isolated two mutants with defects in ER morphology and designated them endoplasmic reticulum morphology1 (ermo1) and ermo2. The cells of both mutants developed a number of ER-derived spherical bodies, ∼1 μm in diameter, in addition to the typical polygonal network of ER. The spherical bodies were distributed throughout the ermo1 cells, while they formed a large aggregate in ermo2 cells. We identified the responsible gene for ermo1 to be GNOM-LIKE1 (GNL1) and the gene for ermo2 to be SEC24a. Homologs of both GNL1 and SEC24a are involved in membrane trafficking between the ER and Golgi in yeast and animal cells. Our findings, however, suggest that GNL1/ERMO1 and SEC24a/ERMO2 have a novel function in ER morphology in higher plants.