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Controlling the site of secretion is an essential aspect of cell biology. In walled eukaryotic cells, such as plants, growth is dictated by secreting growth material and wall remodeling enzymes to specific subcellular sites. Thus, plants are excellent model systems to decipher the molecular basis of regulating the site of secretion. We have been using the juvenile, filamentous tissues of mosses to explore the mechanisms driving polarized growth, or tip growth, in plants. Recently we demonstrated that proteins associated with endoplasmic reticulum structure and function, including members of the COPII complex of proteins that forms vesicles on the ER as well as the enigmatic SABRE protein that localizes to the ER, impact polarized growth. In plants the genes that encode COPII protein subunits have undergone gene family expansion compared to in other opisthokonts. We showed that increased gene expansion may underlie distinct ER exit sites that are differentially required for polarized growth. Additionally, we have found that an evolutionarily conserved protein, SABRE, previously thought to impact the cytoskeleton instead affects ER structure ultimately impacting cell expansion. These findings shed light and identify molecules linking the ER to cell expansion.