V-ATPase activity in the TGN/EE is required for exocytosis and recycling in Arabidopsis

Leucine-rich-repeat-containing variable lymphocyte receptors as modules to target plant-expressed proteins

BACKGROUND

The ability to target and manipulate protein-based cellular processes would accelerate plant research; yet, the technology to specifically and selectively target plant-expressed proteins is still in its infancy. Leucine-rich repeats (LRRs) are ubiquitously present protein domains involved in mediating protein-protein interactions. LRRs confer the binding specificity to the highly diverse variable lymphocyte receptor (VLR) antibodies (including VLRA, VLRB and VLRC types) that jawless vertebrates make as the functional equivalents of jawed vertebrate immunoglobulin-based antibodies.

RESULTS

In this study, VLRBs targeting an effector protein from a plant pathogen, HopM1, were developed by immunizing lampreys and using yeast surface display to select for high-affinity VLRBs. HopM1-specific VLRBs (VLR_M1) were expressed in planta in the cytosol, the trans-Golgi network, and the apoplast. Expression of VLR_M1 was higher when the protein localized to an oxidizing environment that would favor disulfide bridge formation (when VLR_M1 was not localized to the cytoplasm), as disulfide bonds are necessary for proper VLR folding. VLR_M1 specifically interacted in planta with HopM1 but not with an unrelated bacterial effector protein while HopM1 failed to interact with a non-specific VLRB.

CONCLUSIONS

In the future, VLRs may be used as flexible modules to bind proteins or carbohydrates of interest in planta, with broad possibilities for their use by binding directly to their targets and inhibiting their action, or by creating chimeric proteins with new specificities in which endogenous LRR domains are replaced by those present in VLRs.

BRASSINOSTEROID INSENSITIVE2 negatively regulates cellulose synthesis in Arabidopsis by phosphorylating cellulose synthase 1

The deposition of cellulose is a defining aspect of plant growth and development, but regulation of this process is poorly understood. Here, we demonstrate that the protein kinase BRASSINOSTEROID INSENSITIVE2 (BIN2), a key negative regulator of brassinosteroid (BR) signaling, can phosphorylate Arabidopsis cellulose synthase A1 (CESA1), a subunit of the primary cell wall cellulose synthase complex, and thereby negatively regulate cellulose biosynthesis. Accordingly, point mutations of the BIN2-mediated CESA1 phosphorylation site abolished BIN2-dependent regulation of cellulose synthase activity. Hence, we have uncovered a mechanism for how BR signaling can modulate cellulose synthesis in plants.

OsPRA2 fine-tunes rice brassinosteroid receptor

Brassinosteroids (BRs) are a kind of plant hormones that regulate many important agronomic traits. Although the framework of the BR signaling pathway has been outlined, detailed working mechanisms at several steps of this pathway, especially at the BR receptor level, need to be further elucidated. Recently we have reported that a rice small G protein, OsPRA2, bound to the BR receptor OsBRI1 at the plasma membrane (PM) and inhibited its kinase activity and its interaction with the co-receptor OsBAK1, leading to a lower sensitivity to BR treatment and dephosphorylation of OsBZR1. In this follow-up study, we identified an OsPRA2-interacting protein through yeast 2 hybrid system, which is annotated as a C2-domain containing GTPase activating protein (OsGAP1). Overexpression of OsGAP1 in Arabidopsis resulted in a phenotype mimic to BR-deficiency, implying that the GTPase activity of OsPRA2 is also involved in regulating BR signaling. In addition, we observed that, besides colocalizing at the PM, OsPRA2 was also colocalized with OsBRI1 in small granules near PM, suggesting that the function of OsPRA2 in vesicle traffic may contribute to the regulation of OsBRI1.

Cellulose and callose synthesis and organization in focus, what's new?

Plant growth and development are supported by plastic but strong cell walls. These walls consist largely of polysaccharides that vary in content and structure. Most of the polysaccharides are produced in the Golgi apparatus and are then secreted to the apoplast and built into the growing walls. However, the two glucan polymers cellulose and callose are synthesized at the plasma membrane by cellulose or callose synthase complexes, respectively. Cellulose is the most common cell wall polymer in land plants and provides strength to the walls to support directed cell expansion. In contrast, callose is integral to specialized cell walls, such as the cell plate that separates dividing cells and growing pollen tube walls, and maintains important functions during abiotic and biotic stress responses. The last years have seen a dramatic increase in our understanding of how cellulose and callose are manufactured, and new factors that regulate the synthases have been identified. Much of this knowledge has been amassed via various microscopy-based techniques, including various confocal techniques and super-resolution imaging. Here, we summarize and synthesize recent findings in the fields of cellulose and callose synthesis in plant biology.

Brassinosteroid signaling and BRI1 dynamics went underground

Brassinosteroids (BRs) are a group of steroid molecules perceived at the cell surface and that act as plant hormones. Since their discovery as crucial growth substances, BRs were mainly studied for their action in above ground organs and the BR signaling pathway was largely uncovered in the context of hypocotyl elongation. However, for the past two years, most of the exciting findings on BR signaling have been made using roots as a model. The Arabidopsis root is a system of choice for cell biology and allowed detailed characterization of BR perception at the cell membrane. In addition, a series of elegant articles dissected how BRs act in tissue specific manners to control root growth and development.

Receptor-mediated sorting of soluble vacuolar proteins: myths, facts, and a new model

To prevent their being released to the cell exterior, acid hydrolases are recognized by receptors at some point in the secretory pathway and diverted towards the lytic compartment of the cell (lysosome or vacuole). In animal cells, the receptor is called the mannosyl 6-phosphate receptor (MPR) and it binds hydrolase ligands in the trans-Golgi network (TGN). These ligands are then sequestered into clathrin-coated vesicles (CCVs) because of motifs in the cytosolic tail of the MPR which interact first with monomeric adaptors (Golgi-localized, Gamma-ear-containing, ARF-binding proteins, GGAs) and then with tetrameric (adaptin) adaptor complexes. The CCVs then fuse with an early endosome, whose more acidic lumen causes the ligands to dissociate. The MPRs are then recycled back to the TGN via retromer-coated carriers. Plants have vacuolar sorting receptors (VSRs) which were originally identified in CCVs isolated from pea (Pisum sativum L.) cotyledons. It was therefore assumed that VSRs would have an analogous function in plants to MPRs in animals. Although this dogma has enjoyed wide support over the last 20 years there are many inconsistencies. Recently, results have been published which are quite contrary to it. It now emerges that VSRs and their ligands can interact very early in the secretory pathway, and dissociate in the TGN, which, in contrast to its mammalian counterpart, has a pH of 5.5. Multivesicular endosomes in plants lack proton pump complexes and consequently have an almost neutral internal pH, which discounts them as organelles of pH-dependent receptor-ligand dissociation. These data force a critical re-evaluation of the role of CCVs at the TGN, especially considering that vacuolar cargo ligands have never been identified in them. We propose that one population of TGN-derived CCVs participate in retrograde transport of VSRs from the TGN. We also present a new model to explain how secretory and vacuolar cargo proteins are effectively separated after entering the late Golgi/TGN compartments.

Cellulose-Microtubule Uncoupling Proteins Prevent Lateral Displacement of Microtubules during Cellulose Synthesis in Arabidopsis

Cellulose is the most abundant biopolymer on Earth and is the major contributor to plant morphogenesis. Cellulose is synthesized by plasma membrane-localized cellulose synthase complexes (CSCs). Nascent cellulose microfibrils become entangled in the cell wall, and further catalysis therefore drives the CSC forward through the membrane: a process guided by cortical microtubules via the protein CSI1/POM2. Still, it is unclear how the microtubules can withstand the forces generated by the motile CSCs to effectively direct CSC movement. Here, we identified a family of microtubule-associated proteins, the cellulose synthase-microtubule uncouplings (CMUs), that located as static puncta along cortical microtubules. Functional disruption of the CMUs caused lateral microtubule displacement and compromised microtubule-based guidance of CSC movement. CSCs that traversed the microtubules interacted with the microtubules via CSI1/POM2, which prompted the lateral microtubule displacement. Hence, we have revealed how microtubules can withstand the propulsion of the CSCs during cellulose biosynthesis and thus sustain anisotropic plant cell growth.

Functional Characterization of Propeptides in Plant Subtilases as Intramolecular Chaperones and Inhibitors of the Mature Protease

Subtilisin-like serine proteases (SBTs) are extracellular proteases that depend on their propeptides for zymogen maturation and activation. The function of propeptides in plant SBTs is poorly understood and was analyzed here for the propeptide of tomato subtilase 3 (SBT3PP). SBT3PP was found to be required as an intramolecular chaperone for zymogen maturation and secretion of SBT3 in vivo Secretion was impaired in a propeptide-deletion mutant but could be restored by co-expression of the propeptide in trans SBT3 was inhibited by SBT3PP with a Kd of 74 nm for the enzyme-inhibitor complex. With a melting point of 87 °C, thermal stability of the complex was substantially increased as compared with the free protease suggesting that propeptide binding stabilizes the structure of SBT3. Even closely related propeptides from other plant SBTs could not substitute for SBT3PP as a folding assistant or autoinhibitor, revealing high specificity for the SBT3-SBT3PP interaction. Separation of the chaperone and inhibitor functions of SBT3PP in a domain-swap experiment indicated that they are mediated by different regions of the propeptide and, hence, different modes of interaction with SBT3. Release of active SBT3 from the autoinhibited complex relied on a pH-dependent cleavage of the propeptide at Asn-38 and Asp-54. The remarkable stability of the autoinhibited complex and pH dependence of the secondary cleavage provide means for stringent control of SBT3 activity, to ensure that the active enzyme is not released before it reaches the acidic environment of the trans-Golgi network or its final destination in the cell wall.

Golgi-localized STELLO proteins regulate the assembly and trafficking of cellulose synthase complexes in Arabidopsis

As the most abundant biopolymer on Earth, cellulose is a key structural component of the plant cell wall. Cellulose is produced at the plasma membrane by cellulose synthase (CesA) complexes (CSCs), which are assembled in the endomembrane system and trafficked to the plasma membrane. While several proteins that affect CesA activity have been identified, components that regulate CSC assembly and trafficking remain unknown. Here we show that STELLO1 and 2 are Golgi-localized proteins that can interact with CesAs and control cellulose quantity. In the absence of STELLO function, the spatial distribution within the Golgi, secretion and activity of the CSCs are impaired indicating a central role of the STELLO proteins in CSC assembly. Point mutations in the predicted catalytic domains of the STELLO proteins indicate that they are glycosyltransferases facing the Golgi lumen. Hence, we have uncovered proteins that regulate CSC assembly in the plant Golgi apparatus.

Proteomic profiling of cellulase-aid-extracted membrane proteins for functional identification of cellulose synthase complexes and their potential associated- components in cotton fibers

Cotton fibers are an excellent model for understanding of cellulose biosynthesis in higher plants. In this study, we determined a high cellulose biosynthesis activity in vitro by optimizing biochemical reaction conditions in cotton fibers. By adding a commercial cellulase enzyme into fibers extraction process, we extracted markedly higher levels of GhCESA1 and GhCESA8 proteins and observed an increase in β-1,4-glucan and β-1,3-glucan products in vitro. LC-MS/MS analysis of anti-GhCESA8-immunoprecipitated proteins showed that 19 proteins could be found in three independent experiments including four CESAs (GhCESA1,2,7,8), five well-known non-CESA proteins, one callose synthase (CALS) and nine novel proteins. Notably, upon the cellulase treatment, four CESAs, one CALS and four novel proteins were measured at relatively higher levels by calculating total peptide counts and distinct peptide numbers, indicating that the cellulase-aid-extracted proteins most likely contribute to the increase in β-glucan products in vitro. These results suggest that the cellulase treatment may aid to release active cellulose synthases complexes from growing glucan chains and make them more amenable to extraction. To our knowledge, it is the first time report about the functional identification of the potential proteins that were associated with plant cellulose and callose synthases complexes by using the cellulase-aided protein extraction.

Receptor-mediated sorting of soluble vacuolar proteins ends at the trans-Golgi network/early endosome

The sorting of soluble proteins for degradation in the vacuole is of vital importance in plant cells, and relies on the activity of vacuolar sorting receptors (VSRs). In the plant endomembrane system, VSRs bind vacuole-targeted proteins and facilitate their transport to the vacuole. Where exactly these interactions take place has remained controversial, however. Here, we examine the potential for VSR-ligand interactions in all compartments of the vacuolar transport system in tobacco mesophyll protoplasts. To do this, we developed compartment-specific VSR sensors that assemble as a result of a nanobody-epitope interaction, and monitored the degree of ligand binding by analysing Förster resonance energy transfer using fluorescence lifetime imaging microscopy (FRET-FLIM). We show that VSRs bind ligands in the endoplasmic reticulum (ER) and in the Golgi, but not in the trans-Golgi network/early endosome (TGN/EE) or multivesicular late endosomes, suggesting that the post-TGN/EE trafficking of ligands towards the vacuole is VSR independent. We verify this by showing that non-VSR-ligands are also delivered to the vacuole from the TGN/EE after endocytic uptake. We conclude that VSRs are required for the transport of ligands from the ER and the Golgi to the TGN/EE, and suggest that the onward transport to the vacuole occurs by default.

The biosensor toolbox for plant developmental biology

Plant development is highly interconnected with the metabolic state of tissues and cells. Current research efforts focus on the identification of the links and mechanisms that govern the interplay between metabolic and gene-regulatory networks. Genetically encoded sensors that allow detection of small molecules in vivo and at high spatio-temporal resolution promise to be the tools of choice for quantifying and visualizing the dynamics of metabolite flux in plants. We provide an overview about current approaches to measure signaling molecules, such as hormones, calcium and sugars, as well as for monitoring the metabolic state via energy equivalents and pH. Biosensors show great potential to address questions of plant development but there are also limitations where alternative approaches are needed.

The impact of abiotic factors on cellulose synthesis

As sessile organisms, plants require mechanisms to sense and respond to changes in their environment, including both biotic and abiotic factors. One of the most common plant adaptations to environmental changes is differential regulation of growth, which results in growth either away from adverse conditions or towards more favorable conditions. As cell walls shape plant growth, this differential growth response must be accompanied by alterations to the plant cell wall. Here, we review the impact of four abiotic factors (osmotic conditions, ionic stress, light, and temperature) on the synthesis of cellulose, an important component of the plant cell wall. Understanding how different abiotic factors influence cellulose production and addressing key questions that remain in this field can provide crucial information to cope with the need for increased crop production under the mounting pressures of a growing world population and global climate change.

Differential Gene Expression and Protein Phosphorylation as Factors Regulating the State of the Arabidopsis SNX1 Protein Complexes in Response to Environmental Stimuli

Endosomal recycling of plasma membrane proteins contributes significantly to the regulation of cellular transport and signaling processes. Members of the Arabidopsis (Arabidopsis thaliana) SORTING NEXIN (SNX) protein family were shown to mediate the endosomal retrieval of transporter proteins in response to external challenges. Our aim is to understand the possible ways through which external stimuli influence the activity of SNX1 in the root. Several proteins are known to contribute to the function of SNX1 through direct protein-protein interaction. We, therefore, compiled a list of all Arabidopsis proteins known to physically interact with SNX1 and employed available gene expression and proteomic data for a comprehensive analysis of the transcriptional and post-transcriptional regulation of this interactome. The genes encoding SNX1-interaction partners showed distinct expression patterns with some, like FAB1A, being uniformly expressed, while others, like MC9 and BLOS1, were expressed in specific root zones and cell types. Under stress conditions known to induce SNX1-dependent responses, two genes encoding SNX1-interacting proteins, MC9 and NHX6, showed major gene-expression variations. We could also observe zone-specific transcriptional changes of SNX1 under iron deficiency, which are consistent with the described role of the SNX1 protein. This suggests that the composition of potential SNX1-containing protein complexes in roots is cell-specific and may be readjusted in response to external stimuli. On the level of post-transcriptional modifications, we observed stress-dependent changes in the phosphorylation status of SNX1, FAB1A, and CLASP. Interestingly, the phosphorylation events affecting SNX1 interactors occur in a pattern which is largely complementary to transcriptional regulation. Our analysis shows that transcriptional and post-transcriptional regulation play distinct roles in SNX1-mediated endosomal recycling under external stress.

Cellulose biosynthesis inhibitors - a multifunctional toolbox

In the current review, we examine the growing number of existing Cellulose Biosynthesis Inhibitors (CBIs) and based on those that have been studied with live cell imaging we group their mechanism of action. Attention is paid to the use of CBIs as tools to ask fundamental questions about cellulose biosynthesis.

The cellulose synthase companion proteins act non-redundantly with CELLULOSE SYNTHASE INTERACTING1/POM2 and CELLULOSE SYNTHASE 6

Cellulose is a cell wall constituent that is essential for plant growth and development, and an important raw material for a range of industrial applications. Cellulose is synthesized at the plasma membrane by massive cellulose synthase (CesA) complexes that track along cortical microtubules in elongating cells of Arabidopsis through the activity of the protein CELLULOSE SYNTHASE INTERACTING1 (CSI1). In a recent study we identified another family of proteins that also are associated with the CesA complex and microtubules, and that we named COMPANIONS OF CELLULOSE SYNTHASE (CC). The CC proteins protect the cellulose synthesising capacity of Arabidopsis seedlings during exposure to adverse environmental conditions by enhancing microtubule dynamics. In this paper we provide cell biology and genetic evidence that the CSI1 and the CC proteins fulfil distinct functions during cellulose synthesis. We also show that the CC proteins are necessary to aid cellulose synthesis when components of the CesA complex are impaired. These data indicate that the CC proteins have a broad role in aiding cellulose synthesis during environmental changes and when core complex components are non-functional.

Job Sharing in the Endomembrane System: Vacuolar Acidification Requires the Combined Activity of V-ATPase and V-PPase

The presence of a large central vacuole is one of the hallmarks of a prototypical plant cell, and the multiple functions of this compartment require massive fluxes of molecules across its limiting membrane, the tonoplast. Transport is assumed to be energized by the membrane potential and the proton gradient established by the combined activity of two proton pumps, the vacuolar H(+)-pyrophosphatase (V-PPase) and the vacuolar H(+)-ATPase (V-ATPase). Exactly how labor is divided between these two enzymes has remained elusive. Here, we provide evidence using gain- and loss-of-function approaches that lack of the V-ATPase cannot be compensated for by increased V-PPase activity. Moreover, we show that increased V-ATPase activity during cold acclimation requires the presence of the V-PPase. Most importantly, we demonstrate that a mutant lacking both of these proton pumps is conditionally viable and retains significant vacuolar acidification, pointing to a so far undetected contribution of the trans-Golgi network/early endosome-localized V-ATPase to vacuolar pH.