Live cell imaging of FM4-64, a tool for tracing the endocytic pathways in Arabidopsis root cells

Amplified growth and heavy metal toxicity of Chlorococcum sp. from exposure to low-dose lanthanum(III)

Rare earth elements (REEs) are extensively utilized in industry, agriculture, advanced materials and other fields, leading to their dispersion in water bodies as emerging contaminants. Meanwhile, the coexistence of REEs and heavy metals (HMs) has become a novel form of water contamination (REE-HM co-contamination), though scientists have limited understanding of its hazards. Here, Chlorococcum sp. cultured in Taihu Lake water was selected to examine the effects of low-dose lanthanum(III) [La(III)] on its growth and HM accumulation. Low-dose La(III) (0.5-30 μg/L) promoted algal growth and increased the contents of Cd (136.7 %), Pb (92.0 %), and Cr (84.3 %), along with the bioconcentration factor of Cd (135.5 %), Pb (91.7 %), and Cr (84.0 %) in Chlorococcum sp. These changes were attributed to La(III)-induced adaptive physiological regulations, including essential element uptake, photosynthesis, and antioxidant enzyme activities, achieved through La(III)-enhanced clathrin-mediated endocytosis. In summary, low-dose La(III) increased the growth and HM accumulation of Chlorococcum sp. in REE-HM co-contaminated water. This phenomenon amplified the toxicity of Chlorococcum sp., causing the HM accumulation in predators in the grazing food chain and posing a new threat to aquatic ecosystems.

PIN2-mediated self-organizing transient auxin flow contributes to auxin maxima at the tip of Arabidopsis cotyledons

Directional auxin transport and formation of auxin maxima are critical for embryogenesis, organogenesis, pattern formation, and growth coordination in plants, but the mechanisms underpinning the initiation and establishment of these auxin dynamics are not fully understood. Here we show that a self-initiating and -terminating transient auxin flow along the marginal cells (MCs) contributes to the formation of an auxin maximum at the tip of Arabidopsis cotyledon that globally coordinates the interdigitation of puzzle-shaped pavement cells in the cotyledon epidermis. Prior to the interdigitation, indole butyric acid (IBA) is converted to indole acetic acid (IAA) to induce PIN2 accumulation and polarization in the marginal cells, leading to auxin flow toward and accumulation at the cotyledon tip. Once IAA levels at the cotyledon tip reaches a maximum, it activates pavement cell interdigitation as well as the accumulation of the IBA transporter TOB1 in MCs, which sequesters IBA to the vacuole and reduces IBA availability and IAA levels. The reduction of IAA levels results in PIN2 down-regulation and cessation of the auxin flow. Hence, our results elucidate a self-activating and self-terminating transient polar auxin transport system in cotyledons, contributing to the formation of localized auxin maxima that spatiotemporally coordinate pavement cell interdigitation.

Lysozyme-coated nanoparticles for active uptake and delivery of synthetic RNA and plasmid-encoded genes in plants

Nanoparticle-mediated delivery of nucleic acids and proteins into intact plants has the potential to modify metabolic pathways and confer desirable traits in crops. Here we show that layered double hydroxide (LDH) nanosheets coated with lysozyme are actively taken up into the root tip, root hairs and lateral root junctions by endocytosis, and translocate via an active membrane trafficking pathway in plants. Lysozyme coating enhanced nanosheet uptake by (1) loosening the plant cell wall and (2) stimulating the expression of endocytosis and other membrane trafficking genes. The lysozyme-coated nanosheets efficiently delivered synthetic mRNA, double-stranded RNA, small interfering RNA and plasmid DNA up to 15 kb in size into tobacco roots, and also functional nucleic acids into leaves, callus, flowers and developing pollen of dicot and monocot species. Thus, lysozyme-coated LDH nanoparticles are a versatile tool for efficiently delivering functional nucleic acids into plants.

Tethering of cellulose synthase complex to the plasma membrane relies on the isoform of EXO70A1 in Arabidopsis

In yeast and mammals, the EXO70 subunit of the exocyst complex plays a key role in mediating the tethering of exocytic vesicles to the plasma membrane (PM). In plants, however, the role of EXO70 in regulating vesicle tethering during exocytosis remains unclear. In land plants, EXO70 has undergone significant evolutionary expansion, resulting in multiple EXO70 paralogues that may allow the exocyst to form various isoforms with specific functions. Previous research in Arabidopsis has shown that generally disrupting exocyst function leads to various defects in cellulose synthase (CESA) complex (CSC) trafficking. In this study, we utilized real-time imaging combined with genetic approaches to explore the role of EXO70A1, a member of the EXO70 family in Arabidopsis, in CSC trafficking. The exo70a1 mutant exhibited a decrease in crystalline cellulose content and a reduced density of functional CSCs in the PM. Moreover, the delivery of tdTomato-CESA6 from the cortex to the PM was compromised in the mutant, leading to the accumulation of CSC vesicles at the cell cortex. However, the velocity of tdTomato-CESA6 in the PM was unaffected in exo70a1. These findings suggest that EXO70A1 has a specific role in tethering CSCs to the PM.

The Vacuolar H+-ATPase subunit C is involved in oligogalacturonide (OG) internalization and OG-triggered immunity

In plants, the perception of cell wall fragments initiates signal transduction cascades that activate the immune response. Previous research on early protein dynamics induced by oligogalacturonides (OGs), pectin fragments acting as damage-associated molecular patterns (DAMPs), revealed significant phosphorylation changes in several proteins. Among them, the subunit C of the vacuolar H+-ATPase, known as DE-ETIOLATED 3 (DET3), was selected to elucidate its role in the OG-triggered immune response. The Arabidopsis det3 knockdown mutant exhibited defects in H2O2 accumulation, mitogen-activated protein kinases (MAPKs) activation, and induction of defense marker genes in response to OG treatment. Interestingly, the det3 mutant showed a higher basal resistance to the fungal pathogen Botrytis cinerea that, in turn, was completely reversed by the pre-treatment with OGs. Our results suggest a compromised ability of the det3 mutant to maintain a primed state over time, leading to a weaker defense response when the plant is later exposed to the fungal pathogen. Using fluorescently labelled OGs, we demonstrated that endocytosis of OGs was less efficient in the det3 mutant, implicating DET3 in the internalization process of OGs. This impairment aligns with the observed defect in the priming response in the det3 mutant, underscoring that proper internalization and signaling of OGs are crucial for initiating and maintaining a primed state in plant defense responses.

The wheat pathogenesis-related protein (TdPR1.2) enhanced tolerance to abiotic and biotic stresses in transgenic Arabidopsis plants

In plants, the pathogenesis-related (PR) proteins have been identified as important regulators of biotic and abiotic stresses. PR proteins branch out into 19 different classes (PR1-PR19). Basically, all PR proteins display a well-established method of action, with the notable exception of PR1, which is a member of a large superfamily of proteins with a common CAP domain. We have previously isolated and characterized the first PR1 from durum wheat, called TdPR-1.2. In the current research work, TdPR1.2 gene was used to highlight its functional activities under various abiotic (sodium chloride (100 mM NaCl) and oxidative stresses (3 mM H2O2), hormonal salicylic acid (SA), abscisic acid (ABA) and jasmonic acid (JA), and abiotic stresses (Botrytis cinerea and Alternaria solani). Enhancement survival index was detected in Arabidopsis transgenic plants expressing TdPR1.2 gene. Moreover, quantitative real-time reverse transcription PCR (qRT-PCR) analysis demonstrated induction of antioxidant enzymes such as catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD). It equally revealed a decrease of malondialdehyde (MDA) as well as hydrogen peroxide (H2O2) levels in transgenic Arabidopsis plants compared to control lines, confirming the role of TdPR1.2 in terms of alleviating biotic and abiotic stresses in transgenic Arabidopsis plants. Eventually, RT-qPCR results showed a higher expression of biotic stress-related genes (PR1 and PDF1.2) in addition to a downregulation of the wound-related gene (LOX3 and VSP2) in transgenic lines treated with jasmonic acid (JA). Notably, these findings provide evidence for the outstanding functions of PR1.2 from durum wheat which can be further invested to boost tolerance in crop plants to abiotic and biotic stresses.

The S-acylation cycle of transcription factor MtNAC80 influences cold stress responses in Medicago truncatula

S-acylation is a reversible post-translational modification catalyzed by protein S-acyltransferases (PATs), and acyl protein thioesterases (APTs) mediate de-S-acylation. Although many proteins are S-acylated, how the S-acylation cycle modulates specific biological functions in plants is poorly understood. In this study, we report that the S-acylation cycle of transcription factor MtNAC80 is involved in the Medicago truncatula cold stress response. Under normal conditions, MtNAC80 localized to membranes through MtPAT9-induced S-acylation. In contrast, under cold stress conditions, MtNAC80 translocated to the nucleus through de-S-acylation mediated by thioesterases such as MtAPT1. MtNAC80 functions in the nucleus by directly binding the promoter of the glutathione S-transferase gene MtGSTU1 and promoting its expression, which enables plants to survive under cold stress by removing excess malondialdehyde and H2O2. Our findings reveal an important function of the S-acylation cycle in plants and provide insight into stress response and tolerance mechanisms.

PIN2-mediated self-organizing transient auxin flow contributes to auxin maxima at the tip of Arabidopsis cotyledons

Directional auxin transport and formation of auxin maxima are critical for embryogenesis, organogenesis, pattern formation, and growth coordination in plants, but the mechanisms underpinning the initiation and establishment of these auxin dynamics are not fully understood. Here we show that a self-initiating and -terminating transient auxin flow along the marginal cells (MCs) contributes to the formation of an auxin maximum at the tip of Arabidopsis cotyledon that globally coordinates the interdigitation of puzzle-shaped pavement cells in the cotyledon epidermis. Prior to the interdigitation, indole butyric acid (IBA) is converted to indole acetic acid (IAA) to induce PIN2 accumulation and polarization in the marginal cells, leading to auxin flow toward and accumulation at the cotyledon tip. When IAA levels at the cotyledon tip reaches a maximum, it activates pavement cell interdigitation as well as the accumulation of the IBA transporter TOB1 in MCs, which sequesters IBA to the vacuole and reduces IBA availability and IAA levels. The reduction of IAA levels results in PIN2 down-regulation and cessation of the auxin flow. Hence, our results elucidate a self-activating and self-terminating transient polar auxin transport system in cotyledons, contributing to the formation of localized auxin maxima that spatiotemporally coordinate pavement cell interdigitation.

CDC48A, an interactor of WOX2, is required for embryonic patterning in Arabidopsis thaliana

KEY MESSAGE

Interactor of WOX2, CDC48A, is crucial for early embryo patterning and shoot meristem stem cell initiation, but is not required for WOX2 protein turnover or subcellular localization. During Arabidopsis embryo patterning, the WUSCHEL HOMEOBOX 2 (WOX2) transcription factor is a major regulator of protoderm and shoot stem cell initiation. Loss of WOX2 function results in aberrant protodermal cell divisions and, redundantly with its paralogs WOX1, WOX3, and WOX5, compromised shoot meristem formation. To elucidate the molecular basis for WOX2 function, we searched for protein interactors by IP-MS/MS from WOX2-overexpression roots displaying reprogramming toward shoot-like cell fates. Here, we report that WOX2 directly interacts with the type II AAA ATPase molecular chaperone CELL DIVISION CYCLE 48A (CDC48A). We confirmed this interaction with bimolecular fluorescence complementation and co-immunoprecipitation and found that both proteins co-localize in the nucleus. We show that CDC48A loss of function results in protoderm and shoot meristem stem cell initiation defects similar to WOX2 loss of function. We also provide evidence that CDC48A promotes WOX2 activity independently of proteolysis or the regulation of nuclear localization, common mechanisms of CDC48A function in other processes. Our results point to a new role of CDC48A in potentiating WOX2 function during early embryo patterning.

The TRAPPIII subunit, Trs85, has a dual role in the trafficking of cellulose synthase complexes in Arabidopsis

Plant cell walls are essential for defining plant growth and development, providing structural support to the main body and responding to abiotic and biotic cues. Cellulose, the main structural polymer of plant cell walls, is synthesized at the plasma membrane by cellulose synthase complexes (CSCs). The construction and transport of CSCs to and from the plasma membrane is poorly understood but is known to rely on the coordinated activity of cellulose synthase-interactive protein 1 (CSI1), a key regulator of CSC trafficking. In this study, we found that Trs85, a TRAPPIII complex subunit, interacted with CSI1 in vitro. Using functional genetics and live-cell imaging, we have shown that trs85-1 mutants have reduced cellulose content, stimulated CSC delivery, an increased population of static CSCs and deficient clathrin-mediated endocytosis in the primary cell wall. Overall, our findings suggest that Trs85 has a dual role in the trafficking of CSCs, by negatively regulating the exocytosis and clathrin-mediated endocytosis of CSCs.

Cytoplasmic calcium influx mediated by plant MLKLs confers TNL-triggered immunity

The plant homolog of vertebrate necroptosis inducer mixed-lineage kinase domain-like (MLKL) contributes to downstream steps in Toll-interleukin-1 receptor domain NLR (TNL)-receptor-triggered immunity. Here, we show that Arabidopsis MLKL1 (AtMLKL1) clusters into puncta at the plasma membrane upon TNL activation and that this sub-cellular reorganization is dependent on the TNL signal transducer, EDS1. We find that AtMLKLs confer TNL-triggered immunity in parallel with RPW8-type HeLo-domain-containing NLRs (RNLs) and that the AtMLKL N-terminal HeLo domain is indispensable for both immunity and clustering. We show that the AtMLKL HeLo domain mediates cytoplasmic Ca2+ ([Ca2+]cyt) influx in plant and human cells, and AtMLKLs are responsible for sustained [Ca2+]cyt influx during TNL-triggered, but not CNL-triggered, immunity. Our study reveals parallel immune signaling functions of plant MLKLs and RNLs as mediators of [Ca2+]cyt influx and a potentially common role of the HeLo domain fold in the Ca2+-signal relay of diverse organisms.

Lanthanum promotes Solanum nigrum L. growth and phytoremediation of cadmium and lead through endocytosis: Physiological and biochemical response, heavy metal uptake and visualization

Rare earth elements (REEs) are important to enhance agricultural productivity. The utilization of phytoremediation as a green technology for addressing heavy metal (HMs) contamination in soil and wastewater has gained significant attention. In our research, we conducted indoor hydroponic experiments to examine the impacts of lanthanum (La) on the growth and enrichment capacity of Solanum nigrum L. (S. nigrum). S. nigrum was cultivated in 10 mg·L-1 of cadmium (Cd), 25 mg·L-1 of lead (Pb), and a mixture of both (5 mg·L-1 Cd + 15 mg·L-1 Pb). Additionally, S. nigrum were subjected to foliar spray or hydroponic supplementation of La(III). The treatment with La(III) significantly increased total fresh weight by 17.82 % to 42.20 %, compared to the treatment without La(III). Furthermore, La(III) facilitated the endocytosis of roots and enhanced Cd2+ flux ranging from 15.64 % to 75.99 % when compared to the treatment without La(III). Foliar and hydroponic application of La(III) resulted in an increase in the translocation factors (TF) in plants of Cd and Pb compared to treatments without La(III). These findings can offer valuable insights into the potential of La(III) to enhance the phytoremediation of soil or wastewater polluted with compounds.

Extracellular pectin-RALF phase separation mediates FERONIA global signaling function

The FERONIA (FER)-LLG1 co-receptor and its peptide ligand RALF regulate myriad processes for plant growth and survival. Focusing on signal-induced cell surface responses, we discovered that intrinsically disordered RALF triggers clustering and endocytosis of its cognate receptors and FER- and LLG1-dependent endocytosis of non-cognate regulators of diverse processes, thus capable of broadly impacting downstream responses. RALF, however, remains extracellular. We demonstrate that RALF binds the cell wall polysaccharide pectin. They phase separate and recruit FER and LLG1 into pectin-RALF-FER-LLG1 condensates to initiate RALF-triggered cell surface responses. We show further that two frequently encountered environmental challenges, elevated salt and temperature, trigger RALF-pectin phase separation, promiscuous receptor clustering and massive endocytosis, and that this process is crucial for recovery from stress-induced growth attenuation. Our results support that RALF-pectin phase separation mediates an exoskeletal mechanism to broadly activate FER-LLG1-dependent cell surface responses to mediate the global role of FER in plant growth and survival.

Monitoring the Intracellular Trafficking of Virus-Induced Structures and Intercellular Spread of Viral Infection in Plants Using Endomembrane Trafficking Pathway-Specific Chemical Inhibitor and Organelle-Selective Fluorescence Dye

Infection by positive-strand RNA viruses induces extensive remodeling of the host endomembrane system in favor of viral replication and movement. The integral membrane protein 6K2 of potyviruses induces the formation of membranous virus replication vesicles at the endoplasmic reticulum exit site (ERES). The intracellular trafficking of 6K2-induced vesicles along with microfilaments requires the vesicular transport pathway, actomyosin motility system, and possibly post-Golgi compartments such as endosomes as well. Recent studies have shown that endocytosis is essential for the intracellular movement of potyviruses from the site of viral genome replication/assembly site to plasmodesmata (PD) to enter neighboring cells. In this chapter, we describe a detailed protocol of how to use endomembrane trafficking pathway-specific chemical inhibitors and organelle-selective fluorescence dye to study the trafficking of potyviral proteins and potyvirus-induced vesicles and to unravel the role of endocytosis and the endocytic pathway in potyvirus infection in Nicotiana benthamiana plants.

CRISPR/Cas9 mutated p-coumaroyl shikimate 3'-hydroxylase 3 gene in Populus tomentosa reveals lignin functioning on supporting tree upright

The lignin plays one of the most important roles in plant secondary metabolism. However, it is still unclear how lignin can contribute to the impressive height of wood growth. In this study, C3'H, a rate-limiting enzyme of the lignin pathway, was used as the target gene. C3'H3 was knocked out by CRISPR/Cas9 in Populus tomentosa. Compared with wild-type popular trees, c3'h3 mutants exhibited dwarf phenotypes, collapsed xylem vessels, weakened phloem thickening, decreased hydraulic conductivity and photosynthetic efficiency, and reduced auxin content, except for reduced total lignin content and significantly increased H-subunit lignin. In the c3'h3 mutant, the flavonoid biosynthesis genes CHS, CHI, F3H, DFR, ANR, and LAR were upregulated, and flavonoid metabolite accumulations were detected, indicating that decreasing the lignin biosynthesis pathway enhanced flavonoid metabolic flux. Furthermore, flavonoid metabolites, such as naringenin and hesperetin, were largely increased, while higher hesperetin content suppressed plant cell division. Thus, studying the c3'h3 mutant allows us to deduce that lignin deficiency suppresses tree growth and leads to the dwarf phenotype due to collapsed xylem and thickened phloem, limiting material exchanges and transport.

S-nitrosylation of RABG3E positively regulates vesicle trafficking to promote salt tolerance

Nitric oxide (NO) is a key signaling molecule affecting the response of plants to salt stress; however, the underlying molecular mechanism is poorly understood. In this study, we conducted a phenotype analysis and found that the small GTPase RABG3E (RAB7) promotes salt tolerance in Arabidopsis thaliana. NO promotes the S-nitrosylation of RAB7 at Cys-171, which in turn helps maintain the ion balance in salt-stressed plants. Furthermore, the S-nitrosylation of RAB7 at Cys-171 enhances the enzyme's GTPase activity, thereby promoting vesicle trafficking and increasing its interaction with phosphatidylinositol phosphates-especially phosphatidylinositol-4-phosphate (PI4P). Exogenously applied PI4P increases vesicle trafficking and promotes salt tolerance depending on the S-nitrosylation of RAB7 at Cys-171. These findings illustrate a unique mechanism in salt tolerance, by which NO regulates vesicle trafficking and ion homeostasis through the S-nitrosylation of RAB7 and its interaction with PI4P.

Endocytosis-mediated entry of a caterpillar effector into plants is countered by Jasmonate

Insects and pathogens release effectors into plant cells to weaken the host defense or immune response. While the imports of some bacterial and fungal effectors into plants have been previously characterized, the mechanisms of how caterpillar effectors enter plant cells remain a mystery. Using live cell imaging and real-time protein tracking, we show that HARP1, an effector from the oral secretions of cotton bollworm (Helicoverpa armigera), enters plant cells via protein-mediated endocytosis. The entry of HARP1 into a plant cell depends on its interaction with vesicle trafficking components including CTL1, PATL2, and TET8. The plant defense hormone jasmonate (JA) restricts HARP1 import by inhibiting endocytosis and HARP1 loading into endosomes. Combined with the previous report that HARP1 inhibits JA signaling output in host plants, it unveils that the effector and JA establish a defense and counter-defense loop reflecting the robust arms race between plants and insects.

Strigolactones promote the localization of the ABA exporter ABCG25 at the plasma membrane in root epidermal cells of Arabidopsis thaliana

The phytohormones strigolactones crosstalk with abscisic acid (ABA) in acclimation to osmotic stress, as ascertained in leaves. However, our knowledge about underground tissues is limited, and lacking in Arabidopsis: whether strigolactones affect ABA transport across plasma membranes has never been addressed. We evaluated the effect of strigolactones on the localization of ATP BINDING CASSETTE G25 (ABCG25), an ABA exporter in Arabidopsis thaliana. Wild-type, strigolactone-insensitive, and strigolactone-depleted seedlings expressing a green fluorescent protein:ABCG25 construct were treated with ABA or strigolactones, and green fluorescent protein was quantified by confocal microscopy in different subcellular compartments of epidermal root cells. We show that strigolactones promote the localization of an ABA transporter at the plasma membrane by enhancing its endosomal recycling. Genotypes altered in strigolactone synthesis or perception are not impaired in ABCG25 recycling promotion by ABA, which acts downstream or independent of strigolactones in this respect. Additionally, we confirm that osmotic stress decreases strigolactone synthesis in A. thaliana root cells, and that this decrease may support local ABA retention under low water availability by allowing ABCG25 internalization. Thus, we propose a new mechanism for ABA homeostasis regulation in the context of osmotic stress acclimation: the fine-tuning by strigolactones of ABCG25 localization in root cells.

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.

Cyclosporine A (CsA) prevents synaptic impairment caused by truncated tau by caspase-3

During Alzheimer's (AD), tau protein suffers from abnormal post-translational modifications, including cleaving by caspase-3. These tau forms affect synaptic plasticity contributing to the cognitive decline observed in the early stages of AD. In addition, caspase-3 cleaved tau (TauC3) impairs mitochondrial dynamics and organelles transport, which are both relevant processes for synapse. We recently showed that the absence of tau expression reverts age-associated cognitive and mitochondrial failure by blocking the mitochondrial permeability transition pore (mPTP). mPTP is a mitochondrial complex involved in calcium regulation and apoptosis. Therefore, we studied the effects of TauC3 against the dendritic spine and synaptic vesicle formation and the possible role of mPTP in these alterations. We used mature hippocampal mice neurons to express a reporter protein (GFP, mCherry), coupled to full-length human tau protein (GFP-T4, mCherry-T4), and coupled to human tau protein cleaved at D421 by caspase-3 (GFP-T4C3, mCherry-T4C3) and synaptic elements were evaluated. Treatment with cyclosporine A (CsA), an immunosuppressive drug with inhibitory activity on mPTP, prevented ROS increase and mitochondrial depolarization induced by TauC3 in hippocampal neurons. These results were corroborated with immortalized cortical neurons in which ROS increase and ATP loss induced by this tau form were prevented by CsA. Interestingly, TauC3 expression significantly reduced dendritic spine density (filopodia type) and synaptic vesicle number in hippocampal neurons. Also, neurons transfected with TauC3 showed a significant accumulation of synaptophysin protein in their soma. More importantly, all these synaptic alterations were prevented by CsA, suggesting an mPTP role in these negative changes derived from TauC3 expression.

Ice plant root plasma membrane aquaporins are regulated by clathrin-coated vesicles in response to salt stress

The regulation of root Plasma membrane (PM) Intrinsic Protein (PIP)-type aquaporins (AQPs) is potentially important for salinity tolerance. However, the molecular and cellular details underlying this process in halophytes remain unclear. Using free-flow electrophoresis and label-free proteomics, we report that the increased abundance of PIPs at the PM of the halophyte ice plant (Mesembryanthemum crystallinum L.) roots under salinity conditions is regulated by clathrin-coated vesicles (CCV). To understand this regulation, we analyzed several components of the M. crystallinum CCV complexes: clathrin light chain (McCLC) and subunits μ1 and μ2 of the adaptor protein (AP) complex (McAP1μ and McAP2μ). Co-localization analyses revealed the association between McPIP1;4 and McAP2μ and between McPIP2;1 and McAP1μ, observations corroborated by mbSUS assays, suggesting that AQP abundance at the PM is under the control of CCV. The ability of McPIP1;4 and McPIP2;1 to form homo- and hetero-oligomers was tested and confirmed, as well as their activity as water channels. Also, we found increased phosphorylation of McPIP2;1 only at the PM in response to salt stress. Our results indicate root PIPs from halophytes might be regulated through CCV trafficking and phosphorylation, impacting their localization, transport activity, and abundance under salinity conditions.

Plant autophagosomes mature into amphisomes prior to their delivery to the central vacuole

Autophagosomes are double-membraned vesicles that traffic harmful or unwanted cellular macromolecules to the vacuole for recycling. Although autophagosome biogenesis has been extensively studied, autophagosome maturation, i.e., delivery and fusion with the vacuole, remains largely unknown in plants. Here, we have identified an autophagy adaptor, CFS1, that directly interacts with the autophagosome marker ATG8 and localizes on both membranes of the autophagosome. Autophagosomes form normally in Arabidopsis thaliana cfs1 mutants, but their delivery to the vacuole is disrupted. CFS1's function is evolutionarily conserved in plants, as it also localizes to the autophagosomes and plays a role in autophagic flux in the liverwort Marchantia polymorpha. CFS1 regulates autophagic flux by bridging autophagosomes with the multivesicular body-localized ESCRT-I component VPS23A, leading to the formation of amphisomes. Similar to CFS1-ATG8 interaction, disrupting the CFS1-VPS23A interaction blocks autophagic flux and renders plants sensitive to nitrogen starvation. Altogether, our results reveal a conserved vacuolar sorting hub that regulates autophagic flux in plants.

ADP-ribosylation factor D1 modulates Golgi morphology, cell plate formation, and plant growth in Arabidopsis

ADP-ribosylation factor (ARF) family proteins, one type of small guanine-nucleotide-binding (G) proteins, play a central role in regulating vesicular traffic and organelle structures in eukaryotes. The Arabidopsis (Arabidopsis thaliana) genome contains more than 21 ARF proteins, but relatively little is known about the functional heterogeneity of ARF homologs in plants. Here, we characterized the function of a unique ARF protein, ARFD1B, in Arabidopsis. ARFD1B exhibited both cytosol and punctate localization patterns, colocalizing with a Golgi marker in protoplasts and transgenic plants. Distinct from other ARF1 homologs, overexpression of a dominant-negative mutant form of ARFD1B did not alter the localization of the Golgi marker mannosidase I (ManI)-RFP in Arabidopsis cells. Interestingly, the ARFD1 artificial microRNA knockdown mutant arfd1 displayed a deleterious growth phenotype, while this phenotype was restored in complemented plants. Further, confocal imaging and transmission electron microscopy analyses of the arfd1 mutant revealed defective cell plate formation and abnormal Golgi morphology. Pull-down and liquid chromatography-tandem mass spectrometry analyses identified Coat Protein I (COPI) components as interacting partners of ARFD1B, and subsequent bimolecular fluorescence complementation, yeast (Saccharomyces cerevisiae) two-hybrid, and co-immunoprecipitation assays further confirmed these interactions. These results demonstrate that ARFD1 is required for cell plate formation, maintenance of Golgi morphology, and plant growth in Arabidopsis.

The Arabidopsis SAC9 enzyme is enriched in a cortical population of early endosomes and restricts PI(4,5)P2 at the plasma membrane

Membrane lipids, and especially phosphoinositides, are differentially enriched within the eukaryotic endomembrane system. This generates a landmark code by modulating the properties of each membrane. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] specifically accumulates at the plasma membrane in yeast, animal, and plant cells, where it regulates a wide range of cellular processes including endocytic trafficking. However, the functional consequences of mispatterning PI(4,5)P2 in plants are unknown. Here, we functionally characterized the putative phosphoinositide phosphatase SUPPRESSOR OF ACTIN9 (SAC9) in Arabidopsis thaliana (Arabidopsis). We found that SAC9 depletion led to the ectopic localization of PI(4,5)P2 on cortical intracellular compartments, which depends on PI4P and PI(4,5)P2 production at the plasma membrane. SAC9 localizes to a subpopulation of trans-Golgi Network/early endosomes that are enriched in a region close to the cell cortex and that are coated with clathrin. Furthermore, it interacts and colocalizes with Src Homology 3 Domain Protein 2 (SH3P2), a protein involved in endocytic trafficking. In the absence of SAC9, SH3P2 localization is altered and the clathrin-mediated endocytosis rate is reduced. Together, our results highlight the importance of restricting PI(4,5)P2 at the plasma membrane and illustrate that one of the consequences of PI(4,5)P2 misspatterning in plants is to impact the endocytic trafficking.

Characterization of the WAK Gene Family Reveals Genes for FHB Resistance in Bread Wheat (Triticum aestivum L.)

Wall-associated kinases (WAKs) are important receptor-like proteins that play major roles in plant defense against pathogens. Fusarium head blight (FHB), one of the most widespread and devastating crop diseases, reduces wheat yield and leads to quality deterioration. Although WAK gene families have been studied in many plants, systematic research on bread wheat (Triticum aestivum) and its role in FHB resistance, in particular, is lacking. In this study, we identified and characterized 320 genes of the TaWAK family in wheat distributed across all chromosomes except 4B and divided them into three phylogenetic groups. Duplication and synteny analyses provided valuable information on the evolutionary characteristics of the TaWAK genes. The gene expression pattern analysis suggested that TaWAK genes play diverse roles in plant biological processes and that at least 30 genes may be involved in the response to Fusarium infection in wheat spikes, with most of the genes contributing to pectin- and chitin-induced defense pathways. Furthermore, 45 TaWAK genes were identified within 17 hcmQTLs that are related to wheat FHB resistance. Our findings provide potential candidate genes for improving FHB resistance and insights into the future functional analysis of TaWAK genes in wheat.

Crumbs complex-directed apical membrane dynamics in epithelial cell ingression

Epithelial cells often leave their tissue context and ingress to form new cell types or acquire migratory ability to move to distant sites during development and tumor progression. Cells lose their apical membrane and epithelial adherens junctions during ingression. However, how factors that organize apical-basal polarity contribute to ingression is unknown. Here, we show that the dynamic regulation of the apical Crumbs polarity complex is crucial for normal neural stem cell ingression. Crumbs endocytosis and recycling allow ingression to occur in a normal timeframe. During early ingression, Crumbs and its complex partner the RhoGEF Cysts support myosin and apical constriction to ensure robust ingression dynamics. During late ingression, the E3-ubiquitin ligase Neuralized facilitates the disassembly of the Crumbs complex and the rapid endocytic removal of the apical cell domain. Our findings reveal a mechanism integrating cell fate, apical polarity, endocytosis, vesicle trafficking, and actomyosin contractility to promote cell ingression, a fundamental morphogenetic process observed in animal development and cancer.

High-Throughput 3D Phenotyping of Plant Shoot Apical Meristems From Tissue-Resolution Data

Confocal imaging is a well-established method for investigating plant phenotypes on the tissue and organ level. However, many differences are difficult to assess by visual inspection and researchers rely extensively on ad hoc manual quantification techniques and qualitative assessment. Here we present a method for quantitatively phenotyping large samples of plant tissue morphologies using triangulated isosurfaces. We successfully demonstrate the applicability of the approach using confocal imaging of aerial organs in Arabidopsis thaliana. Automatic identification of flower primordia using the surface curvature as an indication of outgrowth allows for high-throughput quantification of divergence angles and further analysis of individual flowers. We demonstrate the throughput of our method by quantifying geometric features of 1065 flower primordia from 172 plants, comparing auxin transport mutants to wild type. Additionally, we find that a paraboloid provides a simple geometric parameterisation of the shoot inflorescence domain with few parameters. We utilise parameterisation methods to provide a computational comparison of the shoot apex defined by a fluorescent reporter of the central zone marker gene CLAVATA3 with the apex defined by the paraboloid. Finally, we analyse the impact of mutations which alter mechanical properties on inflorescence dome curvature and compare the results with auxin transport mutants. Our results suggest that region-specific expression domains of genes regulating cell wall biosynthesis and local auxin transport can be important in maintaining the wildtype tissue shape. Altogether, our results indicate a general approach to parameterise and quantify plant development in 3D, which is applicable also in cases where data resolution is limited, and cell segmentation not possible. This enables researchers to address fundamental questions of plant development by quantitative phenotyping with high throughput, consistency and reproducibility.

Synthesis, Crystal Structure, and Insecticidal Activity of Steroidal N-Piperidone

A series of steroidal piperidone derivatives were synthesized, and their agricultural activities were evaluated against Myzus persicae, Aphis citricola, Brevicoryne brassicae Linn., and Bemisia tabaci (Gennadius). Most of the tested compounds exhibited potent insecticidal activity against these four pests. Compound I-9 displayed the highest activity against M. persicae, A. citricola, and Brevicoryne brassicae, with LC₅₀ values of 11.3, 10.4, and 8.68 μg/mL, respectively. The mode of action test indicated that these derivatives had superior contact and systemic insecticidal activity against M. persicae. In addition, we initially explored whether the foregut and midgut might be the action sites of the target derivatives against M. persicae. Furthermore, a field trial showed that the control of compound I-9 was similar to that of acetamiprid against M. persicae, at a dose of 50 μg/mL; the control rates were 97.8 and 99.2% after 14 and 21 days, respectively. The structure-activity relationship of these analogues provided some important insights for the discovery and development of new insecticides to solve the current pesticide resistance crisis.

Imaging the living plant cell: From probes to quantification

At the center of cell biology is our ability to image the cell and its various components, either in isolation or within an organism. Given its importance, biological imaging has emerged as a field of its own, which is inherently highly interdisciplinary. Indeed, biologists rely on physicists and engineers to build new microscopes and imaging techniques, chemists to develop better imaging probes, and mathematicians and computer scientists for image analysis and quantification. Live imaging collectively involves all the techniques aimed at imaging live samples. It is a rapidly evolving field, with countless new techniques, probes, and dyes being continuously developed. Some of these new methods or reagents are readily amenable to image plant samples, while others are not and require specific modifications for the plant field. Here, we review some recent advances in live imaging of plant cells. In particular, we discuss the solutions that plant biologists use to live image membrane-bound organelles, cytoskeleton components, hormones, and the mechanical properties of cells or tissues. We not only consider the imaging techniques per se, but also how the construction of new fluorescent probes and analysis pipelines are driving the field of plant cell biology.

Connected function of PRAF/RLD and GNOM in membrane trafficking controls intrinsic cell polarity in plants

Cell polarity is a fundamental feature underlying cell morphogenesis and organismal development. In the Arabidopsis stomatal lineage, the polarity protein BASL controls stomatal asymmetric cell division. However, the cellular machinery by which this intrinsic polarity site is established remains unknown. Here, we identify the PRAF/RLD proteins as BASL physical partners and mutating four PRAF members leads to defects in BASL polarization. Members of PRAF proteins are polarized in stomatal lineage cells in a BASL-dependent manner. Developmental defects of the praf mutants phenocopy those of the gnom mutants. GNOM is an activator of the conserved Arf GTPases and plays important roles in membrane trafficking. We further find PRAF physically interacts with GNOM in vitro and in vivo. Thus, we propose that the positive feedback of BASL and PRAF at the plasma membrane and the connected function of PRAF and GNOM in endosomal trafficking establish intrinsic cell polarity in the Arabidopsis stomatal lineage.

Turnip mosaic virus co-opts the vacuolar sorting receptor VSR4 to promote viral genome replication in plants by targeting viral replication vesicles to the endosome

Accumulated experimental evidence has shown that viruses recruit the host intracellular machinery to establish infection. It has recently been shown that the potyvirus Turnip mosaic virus (TuMV) transits through the late endosome (LE) for viral genome replication, but it is still largely unknown how the viral replication vesicles labelled by the TuMV membrane protein 6K2 target LE. To further understand the underlying mechanism, we studied the involvement of the vacuolar sorting receptor (VSR) family proteins from Arabidopsis in this process. We now report the identification of VSR4 as a new host factor required for TuMV infection. VSR4 interacted specifically with TuMV 6K2 and was required for targeting of 6K2 to enlarged LE. Following overexpression of VSR4 or its recycling-defective mutant that accumulates in the early endosome (EE), 6K2 did not employ the conventional VSR-mediated EE to LE pathway, but targeted enlarged LE directly from cis-Golgi and viral replication was enhanced. In addition, VSR4 can be N-glycosylated and this is required for its stability and for monitoring 6K2 trafficking to enlarged LE. A non-glycosylated VSR4 mutant enhanced the dissociation of 6K2 from cis-Golgi, leading to the formation of punctate bodies that targeted enlarged LE and to more robust viral replication than with glycosylated VSR4. Finally, TuMV hijacks N-glycosylated VSR4 and protects VSR4 from degradation via the autophagy pathway to assist infection. Taken together, our results have identified a host factor VSR4 required for viral replication vesicles to target endosomes for optimal viral infection and shed new light on the role of N-glycosylation of a host factor in regulating viral infection.

A key feature of the replication of positive-strand RNA viruses is the rearrangement of the host endomembrane system to produce a membranous replication organelle. Recent reports suggest that the late endosome (LE) serves as a replication site for the potyvirus Turnip mosaic virus (TuMV), but the mechanism(s) by which TuMV replication vesicles target LE are far from being fully elucidated. Identification of the host factors involved in this transport process could lead to new strategies to combat TuMV infection. In this report, we provide evidence that TuMV replication depends on functional vesicle transport from cis-Golgi to the enlarged LE pathway that is mediated by a specific VSR family member, VSR4, from Arabidopsis. Knock out of VSR4 impaired the targeting of TuMV replication vesicles to enlarged LE and suppressed viral infection, and this process depends on the specific interaction between VSR4 and the viral replication vesicle-forming protein 6K2. We also showed that N-glycosylation of VSR4 modulates the targeting of TuMV replication vesicles to enlarged LE and enhances viral infection, thus contributing to our understanding of how TuMV manipulates host factors in order to establish optimal infection. These results may have implications for the role of VSR in other positive-strand RNA viruses.

Pectin Modification in Seed Coat Mucilage by In Vivo Expression of Rhamnogalacturonan-I- and Homogalacturonan-Degrading Enzymes

The cell wall is essential for plant survival. Determining the relationship between cell wall structure and function using mutant analysis or overexpressing cell wall-modifying enzymes has been challenging due to the complexity of the cell wall and the appearance of secondary, compensatory effects when individual polymers are modified. In addition, viability of the plants can be severely impacted by wall modification. A useful model system for studying structure-function relationships among extracellular matrix components is the seed coat epidermal cells of Arabidopsis thaliana. These cells synthesize relatively simple, easily accessible, pectin-rich mucilage that is not essential for plant viability. In this study, we expressed enzymes predicted to modify polysaccharide components of mucilage in the apoplast of seed coat epidermal cells and explored their impacts on mucilage. The seed coat epidermal-specific promoter TESTA ABUNDANT2 (TBA2) was used to drive expression of these enzymes to avoid adverse effects in other parts of the plant. Mature transgenic seeds expressing Rhamnogalacturonate lyase A (RglA) or Rhamnogalacturonate lyase B (RglB) that degrade the pectin rhamnogalacturonan-I (RG-I), a major component of mucilage, had greatly reduced mucilage capsules surrounding the seeds and concomitant decreases in the monosaccharides that comprise the RG-I backbone. Degradation of the minor mucilage component homogalacturonan (HG) using the HG-degrading enzymes Pectin lyase A (PLA) or ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE2 (ADPG2) resulted in developing seed coat epidermal cells with disrupted cell-cell adhesion and signs of early cell death. These results demonstrate the feasibility of manipulating the seed coat epidermal cell extracellular matrix using a targeted genetic engineering approach.

Tools for Assessing Cell-Cycle Progression in Plants

Estimation of cell-cycle parameters is crucial for understanding the developmental programs established during the formation of an organism. A number of complementary approaches have been developed and adapted to plants to assess the cell-cycle status in different proliferative tissues. The most classical methods relying on metabolic labeling are still very much employed and give valuable information on cell-cycle progression in fixed tissues. However, the growing knowledge of plant cell-cycle regulators with defined expression pattern together with the development of fluorescent proteins technology enabled the generation of fusion proteins that function individually or in conjunction as cell-cycle reporters. Together with the improvement of imaging techniques, in vivo live imaging to monitor plant cell-cycle progression in normal growth conditions or in response to different stimuli has been possible. Here, we review these tools and their specific outputs for plant cell-cycle analysis.

Plant cell adhesion and growth on artificial fibrous scaffolds as an in vitro model for plant development

Mechanistic studies of plant development would benefit from an in vitro model that mimics the endogenous physical interactions between cells and their microenvironment. Here, we present artificial scaffolds to which both solid- and liquid-cultured tobacco BY-2 cells adhere without perturbing cell morphology, division, and cortical microtubule organization. Scaffolds consisting of polyvinylidene tri-fluoroethylene (PVDF-TrFE) were prepared to mimic the cell wall’s fibrillar structure and its relative hydrophobicity and piezoelectric property. We found that cells adhered best to scaffolds consisting of nanosized aligned fibers. In addition, poling of PVDF-TrFE, which orients the fiber dipoles and renders the scaffold more piezoelectric, increased cell adhesion. Enzymatic treatments revealed that the plant cell wall polysaccharide, pectin, is largely responsible for cell adhesion to scaffolds, analogous to pectin-mediated cell adhesion in plant tissues. Together, this work establishes the first plant biomimetic scaffolds that will enable studies of how cell-cell and cell-matrix interactions affect plant developmental pathways.

ClCd and ClCf act redundantly at the trans-Golgi network/early endosome and prevent acidification of the Golgi stack

The trans-Golgi network/early endosome (TGN/EE) serves as the central hub in which exocytic and endocytic trafficking pathways converge and specificity of cargo routing needs to be achieved. Acidification is a hallmark of the TGN/EE and is maintained by the vacuolar H+-ATPase (V-ATPase) with support of proton-coupled antiporters. We show here that ClCd and ClCf, two distantly related members of the Arabidopsis Cl- channel (ClC) family, colocalize in the TGN/EE, where they act redundantly, and are essential for male gametophyte development. Combining an inducible knockdown approach and in vivo pH measurements, we show here that reduced ClC activity does not affect pH in the TGN/EE but causes hyperacidification of trans-Golgi cisternae. Taken together, our results show that ClC-mediated anion transport into the TGN/EE is essential and affects spatiotemporal aspects of TGN/EE maturation as well as its functional separation from the Golgi stack.

Phosphorylation of NONPHOTOTROPIC HYPOCOTYL3 affects photosensory adaptation during the phototropic response

Photosensory adaptation, which can be classified as sensor or effector adaptation, optimizes the light sensing of living organisms by tuning their sensitivity to changing light conditions. During the phototropic response in Arabidopsis (Arabidopsis thaliana), the light-dependent expression controls of blue-light (BL) photoreceptor phototropin 1 (phot1) and its modulator ROOT PHOTOTROPISM2 (RPT2) are known as the molecular mechanisms underlying sensor adaptation. However, little is known about effector adaption in plant phototropism. Here, we show that control of the phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 (NPH3) leads to effector adaptation in hypocotyl phototropism. We generated unphosphorable and phosphomimetic NPH3 proteins on seven phosphorylation sites in the etiolated seedlings of Arabidopsis. Unphosphorable NPH3 showed a shortening of its retention time in the cytosol and caused an inability to adapt to very low fluence rates of BL (∼10-5 µmol m-2 s-1) during the phototropic response. In contrast, the phosphomimetic NPH3 proteins had a lengthened retention time in the cytosol and could not enable the adaptation to BL at fluence rates of 10-3 µmol m-2 s-1 or more. Our results indicate that the activation level of phot1 and the corresponding phosphorylation level of NPH3 determine the dissociation rate and the reassociation rate of NPH3 on the plasma membrane, respectively. These mechanisms may moderately maintain the active state of phot1 signaling across a broad range of BL intensities and contribute to the photosensory adaptation of phot1 signaling during the phototropic response in hypocotyls.

Identification and Analysis of GhEXO Gene Family Indicated That GhEXO7_At Promotes Plant Growth and Development Through Brassinosteroid Signaling in Cotton (Gossypium hirsutum L.)

Brassinosteroids (BRs), an efficient plant endogenous hormone, significantly promotes plant nutrient growth adapting to biological and abiotic adversities. BRs mainly promote plant cell elongation by regulating gene expression patterns. EXORDIUM (EXO) genes have been characterized as the indicators of BR response genes. Cotton, an ancient crop, is of great economic value and its fibers can be made into all kinds of fabrics. However, EXO gene family genes have not been full identified in cotton. 175 EXO genes were identified in nine plant species, of which 39 GhEXO genes in Gossypium hirsutum in our study. A phylogenetic analysis grouped all of the proteins encoded by the EXO genes into five major clades. Sequence identification of conserved amino acid residues among monocotyledonous and dicotyledonous species showed a high level of conservation across the N and C terminal regions. Only 25% the GhEXO genes contain introns besides conserved gene structure and protein motifs distribution. The 39 GhEXO genes were unevenly distributed on the 18 At and Dt sub-genome chromosomes. Most of the GhEXO genes were derived from gene duplication events, while only three genes showed evidence of tandem duplication. Homologous locus relationships showed that 15 GhEXO genes are located on collinear blocks and that all orthologous/paralogous gene pairs had Ka > Ks values, indicating purifying selection pressure. The GhEXO genes showed ubiquitous expression in all eight tested cotton tissues and following exposure to three phytohormones, IAA, GA, and BL. Furthermore, GhEXO7_At was mainly expressed in response to BL treatment, and was predominantly expressed in the fibers. GhEXO7_At was found to be a plasma membrane protein, and its ectopic expression in Arabidopsis mediated BR-regulated plant growth and development with altered expression of DWF4, CPD, KCS1, and EXP5. Additionally, the functions of GhEXO7_At were confirmed by virus-induced gene silencing (VIGS) in cotton. This study will provide important genetic resources for future cotton breeding programs.

The plasma membrane-associated Ca2+ -binding protein, PCaP1, is required for oligogalacturonide and flagellin-induced priming and immunity

Early signalling events in response to elicitation include reversible protein phosphorylation and re-localization of plasma membrane (PM) proteins. Oligogalacturonides (OGs) are a class of damage-associated molecular patterns (DAMPs) that act as endogenous signals to activate the plant immune response. Previous data on early phosphoproteome changes in Arabidopsis thaliana upon OG perception uncovered the immune-related phospho-regulation of several membrane proteins, among which PCaP1, a PM-anchored protein with actin filament-severing activity, was chosen for its potential involvement in OG- and flagellin-triggered responses. Here, we demonstrate that PCaP1 is required for late, but not early, responses induced by OGs and flagellin. Moreover, pcap1 mutants, unlike the wild type, are impaired in the recovery of full responsiveness to a second treatment with OGs performed 24 h after the first one. Localization studies on PCaP1 upon OG treatment in plants expressing a functional PCaP1-GFP fusion under the control of PCaP1 promoter revealed fluorescence on the PM, organized in densely packed punctate structures, previously reported as microdomains. Fluorescence was found to be associated also with endocytic vesicles, the number of which rapidly increased after OG treatment, suggesting both an endocytic turnover of PCaP1 for maintaining its homeostasis at the PM and an OG-induced endocytosis.

Astrocyte arborization enhances Ca2+ but not cAMP signaling plasticity

The plasticity of astrocytes is fundamental for their principal function, maintaining homeostasis of the central nervous system throughout life, and is associated with diverse exposomal challenges. Here, we used cultured astrocytes to investigate at subcellular level basic cell processes under controlled environmental conditions. We compared astroglial functional and signaling plasticity in standard serum‐containing growth medium, a condition mimicking pathologic conditions, and in medium without serum, favoring the acquisition of arborized morphology. Using opto−/electrophysiologic techniques, we examined cell viability, expression of astroglial markers, vesicle dynamics, and cytosolic Ca²⁺ and cAMP signaling. The results revealed altered vesicle dynamics in arborized astrocytes that was associated with increased resting [Ca²⁺]_i and increased subcellular heterogeneity in [Ca²⁺]_i, whereas [cAMP]_i subcellular dynamics remained stable in both cultures, indicating that cAMP signaling is less prone to plastic remodeling than Ca²⁺ signaling, possibly also in in vivo contexts.

Subcellular Localizations of Catalase and Exogenously Added Fatty Acid in Chlamydomonas reinhardtii

Fatty acids are important biological components, yet the metabolism of fatty acids in microalgae is not clearly understood. Previous studies found that Chlamydomonas reinhardtii, the model microalga, incorporates exogenously added fatty acids but metabolizes them differently from animals and yeast. Furthermore, a recent metabolic flux analysis found that the majority of lipid turnover in C. reinhardtii is the recycling of acyl chains from and to membranes, rather than β -oxidation. This indicates that for the alga, the maintenance of existing acyl chains may be more valuable than their breakdown for energy. To gain cell-biological knowledge of fatty acid metabolism in C. reinhardtii, we conducted microscopy analysis with fluorescent probes. First, we found that CAT1 (catalase isoform 1) is in the peroxisomes while CAT2 (catalase isoform 2) is localized in the endoplasmic reticulum, indicating the alga is capable of detoxifying hydrogen peroxide that would be produced during β-oxidation in the peroxisomes. Second, we compared the localization of exogenously added FL-C16 (fluorescently labelled palmitic acid) with fluorescently marked endosomes, mitochondria, peroxisomes, lysosomes, and lipid droplets. We found that exogenously added FL-C16 are incorporated and compartmentalized via a non-endocytic route within 10 min. However, the fluorescence signals from FL-C16 did not colocalize with any marked organelles, including peroxisomes. During triacylglycerol accumulation, the fluorescence signals from FL-C16 were localized in lipid droplets. These results support the idea that membrane turnover is favored over β-oxidation in C. reinhardtii. The knowledge gained in these analyses would aid further studies of the fatty acid metabolism.

Cytokinin Modulates Cellular Trafficking and the Cytoskeleton, Enhancing Defense Responses

The plant hormone cytokinin (CK) plays central roles in plant development and throughout plant life. The perception of CKs initiating their signaling cascade is mediated by histidine kinase receptors (AHKs). Traditionally thought to be perceived mostly at the endoplasmic reticulum (ER) due to receptor localization, CK was recently reported to be perceived at the plasma membrane (PM), with CK and its AHK receptors being trafficked between the PM and the ER. Some of the downstream mechanisms CK employs to regulate developmental processes are unknown. A seminal report in this field demonstrated that CK regulates auxin-mediated lateral root organogenesis by regulating the endocytic recycling of the auxin carrier PIN1, but since then, few works have addressed this issue. Modulation of the cellular cytoskeleton and trafficking could potentially be a mechanism executing responses downstream of CK signaling. We recently reported that CK affects the trafficking of the pattern recognition receptor LeEIX2, influencing the resultant defense output. We have also recently found that CK affects cellular trafficking and the actin cytoskeleton in fungi. In this work, we take an in-depth look at the effects of CK on cellular trafficking and on the actin cytoskeleton in plant cells. We find that CK influences the actin cytoskeleton and endomembrane compartments, both in the context of defense signaling-where CK acts to amplify the signal-as well as in steady state. We show that CK affects the distribution of FLS2, increasing its presence in the plasma membrane. Furthermore, CK enhances the cellular response to flg22, and flg22 sensing activates the CK response. Our results are in agreement with what we previously reported for fungi, suggesting a fundamental role for CK in regulating cellular integrity and trafficking as a mechanism for controlling and executing CK-mediated processes.

SlERF52 regulates SlTIP1;1 expression to accelerate tomato pedicel abscission

Abscission of plant organs is induced by developmental signals and diverse environmental stimuli and involves multiple regulatory networks, including biotic or abiotic stress-impaired auxin flux in the abscission zone (AZ). Depletion of auxin activates AZ ethylene (ETH) production and triggers acceleration of abscission, a process that requires hydrogen peroxide (H2O2). However, the interaction between these networks and the underlying mechanisms that control abscission are poorly understood. Here, we found that expression of tonoplast intrinsic proteins, which belong to the aquaporin (AQP) family in the AZ was important for tomato (Solanum lycopersicum) pedicel abscission. Liquid chromatography-tandem mass spectrometry and in situ hybridization revealed that SlTIP1;1 was most abundant and specifically present in the tomato pedicel AZ. SlTIP1;1 localized in the plasma membrane and tonoplast. Knockout of SlTIP1;1 resulted in delayed abscission, whereas overexpression of SlTIP1;1 accelerated abscission. Further analysis indicated that SlTIP1;1 mediated abscission via gating of cytoplasmic H2O2 concentrations and osmotic water permeability (Pf). Elevated cytoplasmic levels of H2O2 caused a suppressed auxin signal in the early abscission stage and enhanced ETH production during abscission. Furthermore, we found that increasing Pf was required to enhance the turgor pressure to supply the break force for AZ cell separation. Moreover, we observed that SlERF52 bound directly to the SlTIP1;1 promoter to regulate its expression, demonstrating a positive loop in which cytoplasmic H2O2 activates ETH production, which activates SlERF52. This, in turn, induces SlTIP1;1, which leads to elevated cytoplasmic H2O2 and water influx.

Nanobody-Dependent Delocalization of Endocytic Machinery in Arabidopsis Root Cells Dampens Their Internalization Capacity

Plant cells perceive and adapt to an ever-changing environment by modifying their plasma membrane (PM) proteome. Whereas secretion deposits new integral membrane proteins, internalization by endocytosis removes membrane proteins and associated ligands, largely with the aid of adaptor protein (AP) complexes and the scaffolding molecule clathrin. Two AP complexes function in clathrin-mediated endocytosis at the PM in plant cells, the heterotetrameric AP-2 complex and the hetero-octameric TPLATE complex (TPC). Whereas single subunit mutants in AP-2 develop into viable plants, genetic mutation of a single TPC subunit causes fully penetrant male sterility and silencing single subunits leads to seedling lethality. To address TPC function in somatic root cells, while minimizing indirect effects on plant growth, we employed nanobody-dependent delocalization of a functional, GFP-tagged TPC subunit, TML, in its respective homozygous genetic mutant background. In order to decrease the amount of functional TPC at the PM, we targeted our nanobody construct to the mitochondria and fused it to TagBFP2 to visualize it independently of its bait. We furthermore limited the effect of our delocalization to those tissues that are easily accessible for live-cell imaging by expressing it from the PIN2 promoter, which is active in root epidermal and cortex cells. With this approach, we successfully delocalized TML from the PM. Moreover, we also show co-recruitment of TML-GFP and AP2A1-TagRFP to the mitochondria, suggesting that our approach delocalized complexes, rather than individual adaptor complex subunits. In line with the specific expression domain, we only observed minor effects on root growth, yet realized a clear reduction of endocytic flux in epidermal root cells. Nanobody-dependent delocalization in plants, here exemplified using a TPC subunit, has the potential to be widely applicable to achieve specific loss-of-function analysis of otherwise lethal mutants.

The AGCVIII kinase Dw2 modulates cell proliferation, endomembrane trafficking, and MLG/xylan cell wall localization in elongating stem internodes of Sorghum bicolor

Stems of bioenergy sorghum (Sorghum bicolor L. Moench.), a drought-tolerant C4 grass, contain up to 50 nodes and internodes of varying length that span 4-5 m and account for approximately 84% of harvested biomass. Stem internode growth impacts plant height and biomass accumulation and is regulated by brassinosteroid signaling, auxin transport, and gibberellin biosynthesis. In addition, an AGCVIII kinase (Dw2) regulates sorghum stem internode growth, but the underlying mechanism and signaling network are unknown. Here we provide evidence that mutation of Dw2 reduces cell proliferation in internode intercalary meristems, inhibits endocytosis, and alters the distribution of heteroxylan and mixed linkage glucan in cell walls. Phosphoproteomic analysis showed that Dw2 signaling influences the phosphorylation of proteins involved in lipid signaling (PLDδ), endomembrane trafficking, hormone, light, and receptor signaling, and photosynthesis. Together, our results show that Dw2 modulates endomembrane function and cell division during sorghum internode growth, providing insight into the regulation of monocot stem development.

Nanobody-Dependent Delocalization of Endocytic Machinery in Arabidopsis Root Cells Dampens Their Internalization Capacity

Plant cells perceive and adapt to an ever-changing environment by modifying their plasma membrane (PM) proteome. Whereas secretion deposits new integral membrane proteins, internalization by endocytosis removes membrane proteins and associated ligands, largely with the aid of adaptor protein (AP) complexes and the scaffolding molecule clathrin. Two AP complexes function in clathrin-mediated endocytosis at the PM in plant cells, the heterotetrameric AP-2 complex and the hetero-octameric TPLATE complex (TPC). Whereas single subunit mutants in AP-2 develop into viable plants, genetic mutation of a single TPC subunit causes fully penetrant male sterility and silencing single subunits leads to seedling lethality. To address TPC function in somatic root cells, while minimizing indirect effects on plant growth, we employed nanobody-dependent delocalization of a functional, GFP-tagged TPC subunit, TML, in its respective homozygous genetic mutant background. In order to decrease the amount of functional TPC at the PM, we targeted our nanobody construct to the mitochondria and fused it to TagBFP2 to visualize it independently of its bait. We furthermore limited the effect of our delocalization to those tissues that are easily accessible for live-cell imaging by expressing it from the PIN2 promoter, which is active in root epidermal and cortex cells. With this approach, we successfully delocalized TML from the PM. Moreover, we also show co-recruitment of TML-GFP and AP2A1-TagRFP to the mitochondria, suggesting that our approach delocalized complexes, rather than individual adaptor complex subunits. In line with the specific expression domain, we only observed minor effects on root growth, yet realized a clear reduction of endocytic flux in epidermal root cells. Nanobody-dependent delocalization in plants, here exemplified using a TPC subunit, has the potential to be widely applicable to achieve specific loss-of-function analysis of otherwise lethal mutants.

AtSEC22 Regulates Cell Morphogenesis via Affecting Cytoskeleton Organization and Stabilities

The plant cytoskeleton forms a stereoscopic network that regulates cell morphogenesis. The cytoskeleton also provides tracks for trafficking of vesicles to the target membrane. Fusion of vesicles with the target membrane is promoted by SNARE proteins, etc. The vesicle-SNARE, Sec22, regulates membrane trafficking between the ER and Golgi in yeast and mammals. Arabidopsis AtSEC22 might also regulate early secretion and is essential for gametophyte development. However, the role of AtSEC22 in plant development is unclear. To clarify the role of AtSEC22 in the regulation of plant development, we isolated an AtSEC22 knock-down mutant, atsec22-4, and found that cell morphogenesis and development were seriously disturbed. atsec22-4 exhibited shorter primary roots (PRs), dwarf plants, and partial abortion. More interestingly, the atsec22-4 mutant had less trichomes with altered morphology, irregular stomata, and pavement cells, suggesting that cell morphogenesis was perturbed. Further analyses revealed that in atsec22-4, vesicle trafficking was blocked, resulting in the trapping of proteins in the ER and collapse of structures of the ER and Golgi apparatus. Furthermore, AtSEC22 defects resulted in impaired organization and stability of the cytoskeleton in atsec22-4. Our findings revealed essential roles of AtSEC22 in membrane trafficking and cytoskeleton dynamics during plant development.

Imaging of the Entry Pathway of a Cell-Penetrating Peptide-DNA Complex From the Extracellular Space to Chloroplast Nucleoids Across Multiple Membranes in Arabidopsis Leaves

Each plant cell has hundreds of copies of the chloroplast genome and chloroplast transgenes do not undergo silencing. Therefore, chloroplast transformation has many powerful potential agricultural and industrial applications. We previously succeeded in integrating exogenous genes into the chloroplast genome using peptide-DNA complexes composed of plasmid DNA and a fusion peptide consisting of a cell-penetrating peptide (CPP) and a chloroplast transit peptide (cpPD complex). However, how cpPD complexes are transported into the chloroplast from outside the cell remains unclear. Here, to characterize the route by which these cpPD complexes move into chloroplasts, we tracked their movement from the extracellular space to the chloroplast stroma using a fluorescent label and confocal laser scanning microscopy (CLSM). Upon infiltration of cpPD complexes into the extracellular space of Arabidopsis thaliana leaves, the complexes reached the chloroplast surface within 6h. The cpPD complexes reached were engulfed by the chloroplast outer envelope membrane and gradually integrated into the chloroplast. We detected several cpPD complexes localized around chloroplast nucleoids and observed the release of DNA from the cpPD. Our results thus define the route taken by the cpPD complexes for gene delivery from the extracellular space to the chloroplast stroma.

Visualising endocytosis in plants: past, present, and future

Chris Hawes had a lively fascination for the immensely complex organisation of the endomembrane system, including the process of endocytosis. This is the method by which eukaryotic cells internalise membrane proteins, lipids, carbohydrates, and cell wall enzymes from the cell surface through membrane bound vesicles. Endocytosis occurs progressively, starting with early membrane deformation, scission, and finally the release of the vesicle into the cytoplasm. Next to secretion, endocytosis allows the cell to control the proteome composition of its inner and outer surface membrane and as such, its communication with the outside world. Whereas endocytosis was initially considered theoretically impossible in plants due to their high turgor pressure, it is now established as essential for plant life. Furthermore, endocytosis remains a highly active field of research, both in yeast, animal, and plant model systems. Over the past three decades, the tools and techniques used to visualise, quantify, and characterise endocytosis have resulted in an increasingly higher spatiotemporal understanding of this process. Here we provide a brief history of plant endocytosis research from the time when Chris Hawes was investigating the process, to the current state-of-the-art in the field. We will end this chapter with a discussion on some promising future developments for plant endocytosis research. LAY DESCRIPTION: Endocytosis is a key process whereby eukaryotic cells can selectively take up membrane proteins, extracellular material and lipids. As this process controls the abundance and protein composition of the plasma membrane, it also controls the communication of the cell with the outside world. Whereas endocytosis was initially considered theoretically impossible in plants due to their high turgor pressure, it is now established as essential for plant life. Today, endocytosis remains a highly active field of research, both in yeast, animal, and plant model systems. Endocytosis was one of the favourite research topics of Chris Hawes, which is why this mini-review is part of the Festschrift issue in his honour. We provide here a brief history of plant endocytosis research from the time when Chris Hawes was investigating the process, to the current state-of-the-art in the field. Over the past three decades, the tools and techniques that were developed to visualise, quantify, and characterise endocytosis have allowed to achieve an increasingly higher spatiotemporal understanding of this process. We end this chapter with a discussion on some promising future developments for plant endocytosis research.

3D morphological analysis of Arabidopsis sepals

How complicated cell activities produce characteristic tissue and organ morphologies is an important question in plant morphogenesis. To address this question, 3D morphometry of plant organs on multiscales is indispensable. In recent years, advances in confocal microscopy with fluorescent probes that mark the cell wall or plasma membrane enable the visualization of organ morphology with submicron precision. In parallel, new quantitative and correlative imaging pipelines realize 3D image processing on 2D curved surface, facilitating the study of cell and tissue behaviors in plant organogenesis. Here, we describe methods for 3D morphometry of Arabidopsis sepals, focusing on live imaging coupled with MorphoGraphX-based 3D image processing for cellular growth analysis.