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Abstract
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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.
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