Sphingolipid biosynthesis modulates plasmodesmal ultrastructure and phloem unloading

During phloem unloading, multiple cell-to-cell transport events move organic substances to the root meristem. Whereas the primary unloading event from the sieve elements (SE) to the phloem pole pericycle (PPP) has been characterized to some extent, little is known about post-SE unloading. Here, we report a novel gene, PLM (PHLOEM UNLOADING MODULATOR), in the absence of which plasmodesmata-mediated symplastic transport through the PPP-endodermis interface is specifically enhanced. Increased unloading is attributable to a defect in the formation of the ER-plasma membrane tethers during plasmodesmal morphogenesis, resulting in the majority of pores lacking a visible cytoplasmic sleeve. PLM encodes a putative enzyme required for the biosynthesis of sphingolipids with very long-chain fatty acid (VLCFA). Taken together, our results indicate that post-SE unloading involves sphingolipid metabolism which impacts plasmodesmal ultrastructure. They also raise the question of how and why plasmodesmata with no cytoplasmic sleeve facilitate molecular trafficking.