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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This work was supported by the Finnish Centre of Excellence in Molecular Biology of Primary Producers (Academy of Finland CoE program 2014–2019, decision no. 271832), the Gatsby Foundation (GAT3395/PR3), the National Science Foundation Biotechnology and Biological Sciences Research Council grant (BB/N013158/1), the University of Helsinki (award 799992091), the European Research Council Advanced Investigator Grant SYMDEV (no. 323052) and the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant no. 772103-BRIDGING to E.M.B.). J.D.P. is funded by a PhD fellowship from the Formation à la Recherche dans l’Industrie et l’Agriculture (grant no. 1.E.096.18), Belgium. This work was also funded as part of the US Department of Energy Joint BioEnergy Institute (http://www.jbei.org) supported by the US Department of Energy, Office of Science, Office of Biological and Environmental Research, through contract DE-AC02-05CH11231 between the Lawrence Berkeley National Laboratory and the US Department of Energy. We thank R. Wightman for assistance with confocal imaging, M. Bourdon for help in conducting immunolocalization and K. Abley for help with the seed germination assay. The SB-EM imaging was supported by the Biocenter Finland (I.B. and E.J.), and we thank M. Lindman, A. Salminen and M. Veikkolainen for sample preparation for SB-EM. We thank M. Roth and R. Welti for polar lipid analysis, A. Shevchenko and K. Schuchmann for sphingomyelin analysis and P. Dupree for data interpretation. Electron tomography was performed at the plant pole of the Bordeaux Imaging Centre (http://www.bic.u-bordeaux.fr/). The Region Aquitaine also supported the acquisition of the electron microscope (grant no. 2011 13 04 007 PFM), and FranceBioImaging Infrastructure supported the acquisition of the AFS2 and ultra-microtome. The sterol analyses were performed at the Functional Genomic Center of Bordeaux, Metabolome/Lipidome platform (https://metabolome.cgfb.u-bordeaux.fr/en), funded by Grant MetaboHUB-ANR-11-INBS-0010.