ER assembly of SNARE complexes mediating formation of partitioning membrane in Arabidopsis cytokinesis

Investigating vesicle-mediated regulation of pollen tube growth through BFA inhibition and AS-ODN targeting of TfRABA4D in Torenia fournieri

In flowering plants, pollen tube growth is essential for delivering immotile sperm cells during double fertilization, directly influencing seed yield. This process relies on vesicle-mediated trafficking to sustain tip growth and fertility. However, investigating pollen tube growth is challenging in non-model plants due to the lack of transgenic tools. Here, we developed a method to transiently inhibit vesicle activity in pollen tubes of the wishbone flower (Torenia fournieri), a classic plant for sexual reproduction studies, using brefeldin A (BFA) and antisense oligodeoxynucleotides (AS-ODNs) targeting key genes. BFA broadly disrupted vesicle gradient homeostasis in T. fournieri pollen tubes, leading to widespread changes in cell wall deposition, ROS distribution, and pollen tube morphology. To assess the role of specific genes, we designed AS-ODNs against TfANX, the sole ANXUR homolog in T. fournieri, which successfully penetrated cell membranes and suppressed TfANX expression. This inhibition impaired pollen tube tip growth, causing pollen tube leakage at the shank region and, in some cases, multiple leakages. Similarly, AS-ODN targeting TfRABA4D, a pollen-specific vesicle regulator, induced a bulging phenotype and disrupted pectin deposition and reduced ROS distribution, mirroring BFA effects. These findings elucidate vesicle-mediated regulation in pollen tube tip growth and introduce an accessible method for genetic manipulation in reproductive research of non-model plants.

Where R-SNAREs like to roam - the vesicle-associated membrane proteins VAMP721 & VAMP722 in trafficking hotspots

VAMP721 and VAMP722, play crucial roles in membrane fusion at post-Golgi compartments. They are involved in cell plate formation, recycling, endocytosis, and secretion. While individual SNARE actors and regulators exhibit significant overlap, specificity is achieved through distinct combinations of these components. Cytokinesis-related SNAREs traffic as preformed CIS-complexes, which require disassembly by the NSF/αSNAP chaperoning complex to facilitate subsequent homotypic fusion at the cell plate. Recent findings suggest a similar mechanism may operate during secretion. Regulation of VAMP721 activity involves interactions with tethers, GTPases, and Sec1/Munc18 proteins, along with a newly discovered phosphorylation at Tyrosine residue 57. These advances provide valuable insights into the fascinating world of cellular trafficking and membrane fusion.

Distinct ADP-ribosylation factor-GTP exchange factors govern the opposite polarity of 2 receptor kinases

Polarity of plasma membrane proteins is essential for cell morphogenesis and control of cell division and, thus, influences organ and whole plant development. In Arabidopsis (Arabidopsis thaliana) root endodermal cells, 2 transmembrane kinases, INFLORESCENCE AND ROOT APICES RECEPTOR KINASE (IRK) and KINASE ON THE INSIDE (KOIN), accumulate at opposite lateral domains. Their polarization is tightly linked to their activities regulating cell division and ground tissue patterning. The polarization of IRK and KOIN relies solely on the secretion of newly synthesized protein. However, the secretion machinery by which their opposite, lateral polarity is achieved remains largely unknown. Here, we show that different sets of ADP-ribosylation factor (ARF)-guanine-nucleotide exchange factors (ARF-GEFs) mediate their secretion. ARF-GEF GNOM-like-1 (GNL1) regulates KOIN secretion to the inner polar domain, thereby directing KOIN sorting early in the secretion pathway. For IRK, combined chemical and genetic analyses showed that the ARG-GEF GNL1, GNOM, and the BREFELDIN A-INHIBITED-GUANINE NUCLEOTIDE-EXCHANGE FACTORs 1 to 4 (BIG1-BIG4) collectively regulate its polar secretion. The ARF-GEF-dependent mechanisms guiding IRK or KOIN lateral polarity were active across different root cell types and functioned regardless of the protein's inner/outer polarity in those cells. Therefore, we propose that specific polar trafficking of IRK and KOIN occurs via distinct mechanisms that are not constrained by cell identity or polar axis and likely rely on individual protein recognition.

A tyrosine phospho-switch within the Longin domain of VAMP721 modulates SNARE functionality

The final step in secretion is membrane fusion facilitated by SNARE proteins that reside in opposite membranes. The formation of a trans-SNARE complex between one R and three Q coiled-coiled SNARE domains drives the final approach of the membranes providing the mechanical energy for fusion. Biological control of this mechanism is exerted by additional domains within some SNAREs. For example, the N-terminal Longin domain (LD) of R-SNAREs (also called Vesicle-associated membrane proteins, VAMPs) can fold back onto the SNARE domain blocking interaction with other cognate SNAREs. The LD may also determine the subcellular localization via interaction with other trafficking-related proteins. Here, we provide cell-biological and genetic evidence that phosphorylation of the Tyrosine57 residue regulates the functionality of VAMP721. We found that an aspartate mutation mimics phosphorylation, leading to protein instability and subsequent degradation in lytic vacuoles. The mutant SNARE also fails to rescue the defects of vamp721vamp722 loss-of-function lines in spite of its wildtype-like localization within the secretory pathway and the ability to interact with cognate SNARE partners. Most importantly, it imposes a dominant negative phenotype interfering with root growth, normal secretion and cytokinesis in wildtype plants generating large aggregates that mainly contain secretory vesicles. Non-phosphorylatable VAMP721Y57F needs higher gene dosage to rescue double mutants in comparison to native VAMP721 underpinning that phosphorylation modulates SNARE function. We propose a model where short-lived phosphorylation of Y57 serves as a regulatory step to control VAMP721 activity, favoring its open state and interaction with cognate partners to ultimately drive membrane fusion.

The interplay between cell wall integrity and cell cycle progression in plants

Plant cell walls are dynamic structures that play crucial roles in growth, development, and stress responses. Despite our growing understanding of cell wall biology, the connections between cell wall integrity (CWI) and cell cycle progression in plants remain poorly understood. This review aims to explore the intricate relationship between CWI and cell cycle progression in plants, drawing insights from studies in yeast and mammals. We provide an overview of the plant cell cycle, highlight the role of endoreplication in cell wall composition, and discuss recent findings on the molecular mechanisms linking CWI perception to cell wall biosynthesis and gene expression regulation. Furthermore, we address future perspectives and unanswered questions in the field, such as the identification of specific CWI sensing mechanisms and the role of CWI maintenance in the growth-defense trade-off. Elucidating these connections could have significant implications for crop improvement and sustainable agriculture.

D, Noriega A, Drakakaki G, Sylvester AW, Rasmussen CG. Subcellular positioning during cell division and cell plate formation in maize

Introduction

During proliferative plant cell division, the new cell wall, called the cell plate, is first built in the middle of the cell and then expands outward to complete cytokinesis. This dynamic process requires coordinated movement and arrangement of the cytoskeleton and organelles.

Methods

Here we use live-cell markers to track the dynamic reorganization of microtubules, nuclei, endoplasmic reticulum, and endomembrane compartments during division and the formation of the cell plate in maize leaf epidermal cells.

Results

The microtubule plus-end localized protein END BINDING1 (EB1) highlighted increasing microtubule dynamicity during mitosis to support rapid changes in microtubule structures. The localization of the cell-plate specific syntaxin KNOLLE, several RAB-GTPases, as well as two plasma membrane localized proteins was assessed after treatment with the cytokinesis-specific callose-deposition inhibitor Endosidin7 (ES7) and the microtubule-disrupting herbicide chlorpropham (CIPC). While ES7 caused cell plate defects in Arabidopsis thaliana, it did not alter callose accumulation, or disrupt cell plate formation in maize. In contrast, CIPC treatment of maize epidermal cells occasionally produced irregular cell plates that split or fragmented, but did not otherwise disrupt the accumulation of cell-plate localized proteins.

Discussion

Together, these markers provide a robust suite of tools to examine subcellular trafficking and organellar organization during mitosis and cell plate formation in maize.

NSF/αSNAP2-mediated cis-SNARE complex disassembly precedes vesicle fusion in Arabidopsis cytokinesis

Eukaryotic membrane fusion requires trans-SNARE complexes bridging the gap between adjacent membranes1. Fusion between a transport vesicle and its target membrane transforms the trans- into a cis-SNARE complex. The latter interacts with the hexameric AAA+-ATPase N-ethylmaleimide-sensitive factor (NSF) and its co-factor alpha-soluble NSF attachment protein (αSNAP), forming a 20S complex2,3. ATPase activity disassembles the SNAP receptor (SNARE) complex into Qa-SNARE, which folds back onto itself, and its partners4,5. The fusion of identical membranes has a different sequence of events6. The fusion partners each have cis-SNARE complexes to be broken up by NSF and αSNAP. The Qa-SNARE monomers are then stabilized by interaction with Sec1/Munc18-type regulators (SM proteins) to form trans-SNARE complexes, as shown for the yeast vacuole7. Membrane fusion in Arabidopsis cytokinesis is formally akin to vacuolar fusion8. Membrane vesicles fuse with one another to form the partitioning membrane known as the cell plate. Cis-SNARE complexes of cytokinesis-specific Qa-SNARE KNOLLE and its SNARE partners are assembled at the endoplasmic reticulum and delivered by traffic via the Golgi/trans-Golgi network to the cell division plane9. The SM protein KEULE is required for the formation of trans-SNARE complexes between adjacent membrane vesicles10. Here we identify NSF and its adaptor αSNAP2 as necessary for the disassembly of KNOLLE cis-SNARE complexes, which is a prerequisite for KNOLLE-KEULE interaction in cytokinesis. In addition, we show that NSF is required for other trafficking pathways and interacts with the respective Q-SNAREs. The SNARE complex disassembly machinery is conserved in plants and plays a unique essential role in cytokinesis.

Rab GTPases, tethers, and SNAREs work together to regulate Arabidopsis cell plate formation

Cell plates are transient structures formed by the fusion of vesicles at the center of the dividing plane; furthermore, these are precursors to new cell walls and are essential for cytokinesis. Cell plate formation requires a highly coordinated process of cytoskeletal rearrangement, vesicle accumulation and fusion, and membrane maturation. Tethering factors have been shown to interact with the Ras superfamily of small GTP binding proteins (Rab GTPases) and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which are essential for cell plate formation during cytokinesis and are fundamental for maintaining normal plant growth and development. In Arabidopsis thaliana, members of the Rab GTPases, tethers, and SNAREs are localized in cell plates, and mutations in the genes encoding these proteins result in typical cytokinesis-defective phenotypes, such as the formation of abnormal cell plates, multinucleated cells, and incomplete cell walls. This review highlights recent findings on vesicle trafficking during cell plate formation mediated by Rab GTPases, tethers, and SNAREs.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.

A rich and bountiful harvest: Key discoveries in plant cell biology

The field of plant cell biology has a rich history of discovery, going back to Robert Hooke's discovery of cells themselves. The development of microscopes and preparation techniques has allowed for the visualization of subcellular structures, and the use of protein biochemistry, genetics, and molecular biology has enabled the identification of proteins and mechanisms that regulate key cellular processes. In this review, seven senior plant cell biologists reflect on the development of this research field in the past decades, including the foundational contributions that their teams have made to our rich, current insights into cell biology. Topics covered include signaling and cell morphogenesis, membrane trafficking, cytokinesis, cytoskeletal regulation, and cell wall biology. In addition, these scientists illustrate the pathways to discovery in this exciting research field.

Cell biology of primary cell wall synthesis in plants

Building a complex structure such as the cell wall, with many individual parts that need to be assembled correctly from distinct sources within the cell, is a well-orchestrated process. Additional complexity is required to mediate dynamic responses to environmental and developmental cues. Enzymes, sugars, and other cell wall components are constantly and actively transported to and from the plasma membrane during diffuse growth. Cell wall components are transported in vesicles on cytoskeletal tracks composed of microtubules and actin filaments. Many of these components, and additional proteins, vesicles, and lipids are trafficked to and from the cell plate during cytokinesis. In this review, we first discuss how the cytoskeleton is initially organized to add new cell wall material or to build a new cell wall, focusing on similarities during these processes. Next, we discuss how polysaccharides and enzymes that build the cell wall are trafficked to the correct location by motor proteins and through other interactions with the cytoskeleton. Finally, we discuss some of the special features of newly formed cell walls generated during cytokinesis.

SNAREs Regulate Vesicle Trafficking During Root Growth and Development

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins assemble to drive the final membrane fusion step of membrane trafficking. Thus, SNAREs are essential for membrane fusion and vesicular trafficking, which are fundamental mechanisms for maintaining cellular homeostasis. In plants, SNAREs have been demonstrated to be located in different subcellular compartments and involved in a variety of fundamental processes, such as cytokinesis, cytoskeleton organization, symbiosis, and biotic and abiotic stress responses. In addition, SNAREs can also contribute to the normal growth and development of Arabidopsis. Here, we review recent progress in understanding the biological functions and signaling network of SNAREs in vesicle trafficking and the regulation of root growth and development in Arabidopsis.

Sphingolipids Modulate Secretion of Glycosylphosphatidylinositol-Anchored Plasmodesmata Proteins and Callose Deposition

Plasma membranes encapsulated in the symplasmic nanochannels of plasmodesmata (PD) contain abundant lipid rafts, which are enriched with sphingolipids (SLs) and sterols. Reduction of sterols has highlighted the role played by lipid raft integrity in the intercellular trafficking of glycosylphosphatidylinositol (GPI)-anchored PD proteins, particularly in affecting callose enhancement. The presence of callose at PD is strongly attributed to the regulation of callose accumulation and callose degradation by callose synthases and β-1,3-glucanases (BGs), respectively. SLs are implicated in signaling and membrane protein trafficking; however, the underlying processes linking SL composition to the control of symplasmic apertures remain unknown. The wide variety of SLs in plants prompted us to investigate which SL molecules are important for regulating symplasmic apertures in Arabidopsis (Arabidopsis thaliana). We introduced several potential SL pathway inhibitors and genetically modified SL contents using two independent SL pathway mutants. We were able to modulate callose deposition to control symplasmic connectivity through perturbations of SL metabolism. Alteration in glucosylhydroxyceramides or related SL composition particularly disturbed the secretory machinery for the GPI-anchored PdBG2 protein, resulting in an overaccumulation of callose. Moreover, our results revealed 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 of GPI-anchored PD proteins incorporating glucosyl SLs.

SNARE Complexity in Arbuscular Mycorrhizal Symbiosis

How cells control the proper delivery of vesicles and their associated cargo to specific plasma membrane (PM) domains upon internal or external cues is a major question in plant cell biology. A widely held hypothesis is that expansion of plant exocytotic machinery components, such as SNARE proteins, has led to a diversification of exocytotic membrane trafficking pathways to function in specific biological processes. A key biological process that involves the creation of a specialized PM domain is the formation of a host-microbe interface (the peri-arbuscular membrane) in the symbiosis with arbuscular mycorrhizal fungi. We have previously shown that the ability to intracellularly host AM fungi correlates with the evolutionary expansion of both v- (VAMP721d/e) and t-SNARE (SYP132α) proteins, that are essential for arbuscule formation in Medicago truncatula. Here we studied to what extent the symbiotic SNAREs are different from their non-symbiotic family members and whether symbiotic SNAREs define a distinct symbiotic membrane trafficking pathway. We show that all tested SYP1 family proteins, and most of the non-symbiotic VAMP72 members, are able to complement the defect in arbuscule formation upon knock-down/-out of their symbiotic counterparts when expressed at sufficient levels. This functional redundancy is in line with the ability of all tested v- and t-SNARE combinations to form SNARE complexes. Interestingly, the symbiotic t-SNARE SYP132α appeared to occur less in complex with v-SNAREs compared to the non-symbiotic syntaxins in arbuscule-containing cells. This correlated with a preferential localization of SYP132α to functional branches of partially collapsing arbuscules, while non-symbiotic syntaxins accumulate at the degrading parts. Overexpression of VAMP721e caused a shift in SYP132α localization toward the degrading parts, suggesting an influence on its endocytic turn-over. These data indicate that the symbiotic SNAREs do not selectively interact to define a symbiotic vesicle trafficking pathway, but that symbiotic SNARE complexes are more rapidly disassembled resulting in a preferential localization of SYP132α at functional arbuscule branches.

Endoplasmic reticulum stress-induced accumulation of VAMP721/722 requires CALRETICULIN 1 and CALRETICULIN 2 in Arabidopsis

Excessive demand for translation and protein folding in the endoplasmic reticulum (ER) can cause ER stress in plants. Here, we show that CALRETICULIN 1 (CRT1) and CRT2 are critical components in the accumulation of VESICLE-ASSOCIATED MEMBRANE PROTEIN 721 (VAMP721) and VAMP722 during ER stress responses. We show that CRT2 interacts with VAMP722 and that CRT1/2 post-translationally maintain elevated VAMP721/722 levels under ER stress. The greater growth inhibition in VAMP721/722-deficient plants, induced by tunicamycin, suggests that plants under ER stress maintain physiological homeostasis, at least in part, by regulating VAMP721/722 levels, as VAMP721/722 are known to participate in various biological processes.

Division Plane Establishment and Cytokinesis

Plant cells divide their cytoplasmic content by forming a new membrane compartment, the cell plate, via a rerouting of the secretory pathway toward the division plane aided by a dynamic cytoskeletal apparatus known as the phragmoplast. The phragmoplast expands centrifugally and directs the cell plate to the preselected division site at the plasma membrane to fuse with the parental wall. The division site is transiently decorated by the cytoskeletal preprophase band in preprophase and prophase, whereas a number of proteins discovered over the last decade reside continuously at the division site and provide a lasting spatial reference for phragmoplast guidance. Recent studies of membrane fusion at the cell plate have revealed the contribution of functionally conserved eukaryotic proteins to distinct stages of cell plate biogenesis and emphasize the coupling of cell plate formation with phragmoplast expansion. Together with novel findings concerning preprophase band function and the setup of the division site, cytokinesis and its spatial control remain an open-ended field with outstanding and challenging questions to resolve.

Exocyst subunit Sec6 is positioned by microtubule overlaps in the moss phragmoplast prior to cell plate membrane arrival

During plant cytokinesis a radially expanding membrane-enclosed cell plate is formed from fusing vesicles that compartmentalizes the cell in two. How fusion is spatially restricted to the site of cell plate formation is unknown. Aggregation of cell-plate membrane starts near regions of microtubule overlap within the bipolar phragmoplast apparatus of the moss Physcomitrella patens Since vesicle fusion generally requires coordination of vesicle tethering and subsequent fusion activity, we analyzed the subcellular localization of several subunits of the exocyst, a tethering complex active during plant cytokinesis. We found that the exocyst complex subunit Sec6 but not the Sec3 or Sec5 subunits localized to microtubule overlap regions in advance of cell plate construction in moss. Moreover, Sec6 exhibited a conserved physical interaction with an ortholog of the Sec1/Munc18 protein KEULE, an important regulator for cell-plate membrane vesicle fusion in Arabidopsis Recruitment of the P. patens protein KEULE and vesicles to the early cell plate was delayed upon Sec6 gene silencing. Our findings, thus, suggest that vesicle-vesicle fusion is, in part, enabled by a pool of exocyst subunits at microtubule overlaps, which is recruited independently of vesicle delivery.

Specificity of plant membrane trafficking - ARFs, regulators and coat proteins

Approximately one-third of all eukaryotic proteins are delivered to their destination by trafficking within the endomembrane system. Such cargo proteins are incorporated into forming membrane vesicles on donor compartments and delivered to acceptor compartments by vesicle fusion. How cargo proteins are sorted into forming vesicles is still largely unknown. Here we review the roles of small GTPases of the ARF/SAR1 family, their regulators designated ARF guanine-nucleotide exchange factors (ARF-GEFs) and ARF GTPase-activating proteins (ARF-GAPs) as well as coat protein complexes during membrane vesicle formation. Although conserved across eukaryotes, these four functional groups of proteins display plant-specific modifications in composition, structure and function.