  During phloem unloading, multiple cell-to-cell transport events move organic
substances to the root meristem. Although the primary unloading event from the
sieve elements to the phloem pole pericycle has been characterized to some
extent, little is known about post-sieve element unloading. Here, we report a
novel gene, PHLOEM UNLOADING MODULATOR (PLM), in the absence of which
plasmodesmata-mediated symplastic transport through the phloem pole
pericycle--endodermis interface is specifically enhanced. Increased unloading
is attributable to a defect in the formation of the endoplasmic
reticulum--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. Taken together, our results indicate that
post-sieve element unloading involves sphingolipid metabolism, which affects
plasmodesmal ultrastructure. They also raise the question of how and why
plasmodesmata with no cytoplasmic sleeve facilitate molecular trafficking.
