The COPII components Sec23 and Sec24 form isoform specific subcomplexes with Sec23D/E and Sec24C/D essential for tip growth

COPII, a coat of proteins that form vesicles on the ER, mediates vesicle traffic from the ER to the Golgi. In contrast to metazoans that have few genes encoding each COPII component, plants have expanded these gene families leading to the hypothesis that plant COPII has functionally diversified. Here, we analyzed the gene families encoding for the Sec23/24 heterodimer in the moss Physcomitrium (Physcomitrella) patens. In P. patens, Sec23 and Sec24 gene families are each comprised of seven genes. We discovered that Sec23D is functionally redundant with Sec23E and together they are essential for protonemal development. Sec23D is critical for secretion to the Golgi and the plasma membrane in tip growing protonemata. Sec23D preferentially associates with Sec24C/D, also essential for protonemata. In contrast, the remaining five Sec23 genes are dispensable for tip growth, but are redundant with Sec23D/E for gametophore formation. While Sec23A/B/C/F/G do not quantitatively affect Golgi secretion in protonemata, they do contribute to secretion to the plasma membrane. Furthermore, the three highly expressed Sec23 genes in protonemata, Sec23B/D/G form morphologically distinct ER exit sites. These data suggest that Sec23D/E form unique ER exit sites contributing to secretion that is essential for tip growing protonemata.