Catalyzes the biosynthesis of sphingolipids with very long-chain fatty acid (VLCFA) (PubMed:31182845). Required for the formation of plasmodesmal cytoplasmic sleeve during the transition from type I to type II plasmodesmata to modulate post-sieve elements (SE) unloading and symplastic cell-to-cell molecular trafficking at the phloem pole pericycle (PPP)-endodermis interface in roots (PubMed:31182845). Sphingolipid metabolism. Enhanced plasmodesmata-mediated symplastic transport through the phloem pole pericycle (PPP)-endodermis interface due to a defect in the formation of the endoplasmic reticulum (ER)-plasma membrane tethers during plasmodesmal morphogenesis and associated with pores lacking cytoplasmic sleeve. Belongs to the sphingomyelin synthase family.