Iswanto, A. B. B. | bioRxiv Science

bioRxiv2021

α1-COP delivers sphingolipid modifiers and controls plasmodesmal callose deposition in Arabidopsis

Callose is a plant cell wall polymer in the form of {beta}-1,3-glucan, which regulates symplasmic channel size at plasmodesmata (PD). It plays a crucial role in a variety of processes in plants through the regulation of intercelluar symplasmic continuity. However, how to maintain callose homeostasis at PD in the molecular levels is poorly understood. To further elucidate the mechanism of PD callose homeostasis, we screened and identified an Arabidopsis mutant plant that exhibited excessive callose deposition at PD. Based on the Next-generation sequencing (NGS)-based mapping, other mutant allele analysis, and complementation assay, the mutated gene was shown to be 1-COP, which encodes a member of the COPI coatomer complex comprised of , {beta}, {beta}', {gamma}, {delta}, {varepsilon}, and {zeta} subunits. Since there is no report on the link between COPI and callose/PD, it was extremely curious to know the roles of 1-COP or COPI in PD regulation through callose deposition. Here, we report that loss-of-function of 1-COP directly elevates the callose accumulation at PD by affecting subcellular protein localization of callose degradation enzyme PdBG2. This process is linked to ERH1, an inositol phosphoryl ceramide synthase (IPCS), and glucosylceramide synthase (GCS) functions through physical interactions with the 1-COP protein. In addition, the loss-of-function of 1-COP also alters the subcellular localization of ERH1 and GCS proteins, results in a reduction of GlcCers and GlcHCers molecules, which are the key SL species for lipid raft formation. According to our findings, we propose that 1-COP protein, together with the SL modifiers controlling lipid raft compositions, regulates the function of GPI-anchored PD proteins and hence the callose turnover at PD and symplastic movement of biomolecules. Our findings provide the first key clue to link the COPI-mediated intracellular trafficking pathway to the callose-mediated intercellular signaling pathway through PD. One-sentence summaryPlant-specific coatomer protein functions as a negative regulator of callose accumulation by regulating the translocation of sphingolipid enzymes.

cell biology↗

bioRxiv2020

Glucosylhydroxyceramides Modulate Secretion Machinery of a Subset of Plasmodesmata Proteins and a Change in the Callose Accumulation

The plasma membranes encapsulated in the plasmodesmata (PDs) with symplasmic nano-channels contain abundant lipid rafts, which are enriched by sphingolipids and sterols. The attenuation of sterol compositions has demonstrated the role played by lipid raft integrity in the intercellular trafficking of glycosylphosphatidylinositol (GPI)-anchored PD proteins, particularly affecting in the callose enhancement. The presence of callose at PD is tightly attributed to the callose metabolic enzymes, callose synthases (CalSs) and {beta}-1,3-glucanases (BGs) in regulating callose accumulation and callose degradation, respectively. Sphingolipids have been implicated in signaling and membrane protein trafficking, however the underlying processes linking sphingolipid compositions to the control of symplasmic apertures remain unknown. A wide variety of sphingolipids in plants prompts us to investigate which sphingolipid molecules are important in regulating symplasmic apertures. Here, we demonstrate that perturbations of sphingolipid metabolism by introducing several potential sphingolipid (SL) pathway inhibitors and genetically modifying SL contents from two independent SL pathway mutants are able to modulate callose deposition to control symplasmic connectivity. Our data from pharmacological and genetic approaches show that the alteration in glucosylhydroxyceramides (GlcHCers) particularly disturb the secretory machinery for GPI-anchored PdBG2 protein, resulting in an over accumulated callose. Moreover, our results reveal that SL-enriched lipid rafts link symplasmic channeling to PD callose homeostasis by controlling the targeting of GPI-anchored PdBG2. This study elevates our understanding of the molecular linkage underlying intracellular trafficking and precise targeting to specific destination of GPI-anchored PD proteins incorporated with GlcHCers contents.

plant biology↗