Sphingolipid biosynthesis modulates plasmodesmal ultrastructure and phloem unloading

Abstract

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.

Remorin interacting with PCaP1 impairs Turnip mosaic virus intercellular movement but is antagonised by VPg

“…PD are viewed as signalling hub (Lee, 2015) and special membrane contact sites (MCSs) (Tilsner & Torrance, 2011;P erez-Sancho et al, 2016;Tilsner et al, 2016;Bayer et al, 2017;Brault et al, 2019), with lipid rafts or nanodomains (NDs) enriched in sterol and sphingolipids resident on the PD-PM (Raffaele et al, 2009;Mongrand et al, 2010;Tilsner & Torrance, 2011;Grison et al, 2015;Yan et al, 2019). Several functional proteins and signalling lipids are recruited to these NDs by lateral segregation to regulate PD permeability.…”

“…The homeostasis of NDs is very important for the resident molecules or complexes to carry out their biological functions. REMs and flotillins are important scaffold proteins of NDs (Mongrand et al, 2010;Jarsch et al, 2014;Ott, 2017;Gronnier et al, 2018;Liang et al, 2018), while the actin cytoskeleton is involved in ND assembly and stabilisation (Szymanski et al, 2015;Ott, 2017;Liang et al, 2018;Yan et al, 2019). Therefore, REMs, flotillins and actin filaments jointly coordinate ND homeostasis.…”

Computational tools for serial block EM reveal differences in plasmodesmata distributions and wall environments

“…Electron microscopy (EM) resolved the structure of these channels, identifying a continuous plasma membrane (PM) and a constricted form of the endoplasmic reticulum (ER), the desmotubule, running across the pore between the two cells (Lopez-Saez et al, 1966;Robards, 1971). More recently, with the application of electron tomography, variable apposition of PM-ER membranes was shown (Yan et al, 2019;Nicolas et al, 2017b). Classical electron microscopy can also be used to study PD occurrence.…”

“…Importantly, the positions of cellular objects are retained relative to one another and the datasets are good starting points for 3D reconstruction (reviewed in Kittelmann et al, 2016). SB-EM technology has been successfully employed to study PD, demonstrating defects in sieve pore (a modified form of PD) structure and distribution (Dettmer et al, 2014), and allowing quantification of PD densities at the interfaces of the sieve element (SE) (Ross-Eliott et al 2018) and at the endodermal (EN) face of phloem pole pericycle (PPP) cells (Yan et al, 2019). Both SE and PPP cells are key players in the process of phloem unloading, largely mediated by PD (reviewed in Truernit, 2017).…”

“…To extract the relevant information from SB-EM datasets, we developed novel computational and visualization tools dedicated to PD analysis. We deployed the SB-EM datasets from Yan et al (2019) as a study case. We first address the spatial distribution of PD.…”