Auxin-dependent control of a plasmodesmal regulator creates a negative feedback loop modulating lateral root emergence

Hypoxia reshapes Arabidopsis root architecture by integrating ERF-VII factor response and abscisic acid homoeostasis

Oxygen limitation (hypoxia), arising as a key stress factor due to flooding, negatively affects plant development. Consequently, maintaining root growth under such stress is crucial for plant survival, yet we know little about the root system's adaptions to low-oxygen conditions and its regulation by phytohormones. In this study, we examine the impact of hypoxia and, herein, the regulatory role of group VII ETHYLENE-RESPONSE FACTOR (ERFVII) transcription factors on root growth in Arabidopsis. We found lateral root (LR) elongation to be actively maintained by hypoxia via ERFVII factors, as erfVII seedlings possess hypersensitivity towards hypoxia regarding their LR growth. Pharmacological inhibition of abscisic acid (ABA) biosynthesis revealed ERFVII-driven counteraction of hypoxia-induced inhibition of LR formation in an ABA-dependent manner. However, postemergence LR growth under hypoxia mediated by ERFVIIs was independent of ABA. In roots, ERFVIIs mediate, among others, the induction of ABA-degrading ABA 8'-hydroxylases CYP707A1 expression. RAP2.12 could activate the pCYC707A1:LUC reporter gene, indicating, combined with single mutant analyses, that this transcription factor regulates ABA levels through corresponding transcript upregulation. Collectively, hypoxia-induced adaptation of the Arabidopsis root system is shaped by developmental reprogramming, whereby ERFVII-dependent promotion of LR emergence, but not elongation, is partly executed through regulation of ABA degradation.

Plasmodesmata and intercellular molecular traffic control

Plasmodesmata are plasma membrane-lined connections that join plant cells to their neighbours, establishing an intercellular cytoplasmic continuum through which molecules can travel between cells, tissues, and organs. As plasmodesmata connect almost all cells in plants, their molecular traffic carries information and resources across a range of scales, but dynamic control of plasmodesmal aperture can change the possible domains of molecular exchange under different conditions. Plasmodesmal aperture is controlled by specialised signalling cascades accommodated in spatially discrete membrane and cell wall domains. Thus, the composition of plasmodesmata defines their capacity for molecular trafficking. Further, their shape and density can likewise define trafficking capacity, with the cell walls between different cell types hosting different numbers and forms of plasmodesmata to drive molecular flux in physiologically important directions. The molecular traffic that travels through plasmodesmata ranges from small metabolites through to proteins, and possibly even larger mRNAs. Smaller molecules are transmitted between cells via passive mechanisms but how larger molecules are efficiently trafficked through plasmodesmata remains a key question in plasmodesmal biology. How plasmodesmata are formed, the shape they take, what they are made of, and what passes through them regulate molecular traffic through plants, underpinning a wide range of plant physiology.

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.

Control of phloem unloading and root development

Root growth and development need proper carbon partitioning between sources and sinks. Photosynthesis products are unloaded from the phloem and enter the root meristem cell by cell. While sugar transporters play a major role in phloem loading, phloem unloading occurs via the plasmodesmata in growing root tips. The aperture and permeability of plasmodesmata strongly influence symplastic unloading. Recent research has dissected the symplastic path for phloem unloading and identified several genes that regulate phloem unloading in the root. Callose turnover and membrane lipid composition alter the shape of plasmodesmata, allowing fine-tuning to adapt phloem unloading to the environmental and developmental conditions. Unloaded sugars act both as an energy supply and as signals to coordinate root growth and development. Increased knowledge of how phloem unloading is regulated enhances our understanding of carbon allocation in plants. In the future, it may be possible to modulate carbon allocation between sources and sinks in a manner that would contribute to increased plant biomass and carbon fixation.

Plasmodesmata callose binding protein 2 contributes to the regulation of cambium/phloem formation and auxin response during the tissue reunion process in incised Arabidopsis stem

Plants are exposed to a variety of biotic and abiotic stresses, including wounding at the stem. The healing process (tissue reunion) begins immediately after stem wounding. The plant hormone auxin plays an important role during tissue reunion. In decapitated stems, auxin transport from the shoot apex is reduced and tissue reunion does not occur but is restored by application of indole-3-acetic acid (IAA). In this study, we found that plasmodesmata callose binding protein 2 (PDCB2) affects the expansion of the cambium/phloem region via changes in auxin response during the process of tissue reunion. PDCB2 was expressed in the cortex and endodermis on the incised side of stems 1-3 days after incision. PDCB2-knockout plants showed reduced callose deposition at plasmodesmata and DR5::GUS activity in the endodermis/cortex in the upper region of the incision accompanied by an increase in size of the cambium/phloem region during tissue reunion. In addition, PIN(PIN-FORMED)3, which is involved in lateral auxin transport, was induced by auxin in the cambium/phloem and endodermis/cortex in the upper part of the incision in wild type, but its expression of PIN3 was decreased in pdcb2 mutant. Our results suggest that PDCB2 contributes to the regulation of cambium/phloem development via auxin response.

The Germin-like protein gene OsGER4 is involved in heat stress response in rice root development

Rice (Oryza sativa L.) is one of the most important dietary carbohydrate sources for half of the world's population. However, it is not well adapted to environmental stress conditions, necessitating to create new and improved varieties to help ensure sufficient rice production in the face of rising populations and shrinking arable land. Recently, the development of the CRISPR/Cas9 gene editing system has allowed researchers to study functional genomics and engineer new rice varieties with great efficiency compared to conventional methods. In this study, we investigate the involvement of OsGER4, a germin-like protein identified by a genome-wide association study that is associated with rice root development under a stress hormone jasmonic acids treatment. Analysis of the OsGER4 promoter region revealed a series of regulatory elements that connect this gene to ABA signaling and water stress response. Under heat stress, osger4 mutant lines produce a significantly lower crown root than wild-type Kitaake rice. The loss of OsGER4 also led to the reduction of lateral root development. Using the GUS promoter line, OsGER4 expression was detected in the epidermis of the crown root primordial, in the stele of the crown root, and subsequently in the primordial of the lateral root. Taken together, these results illustrated the involvement of OsGER4 in root development under heat stress by regulating auxin transport through plasmodesmata, under control by both ABA and auxin signaling.

Structure-function study of a Ca2+-independent metacaspase involved in lateral root emergence

Metacaspases are part of an evolutionarily broad family of multifunctional cysteine proteases, involved in disease and normal development. As the structure-function relationship of metacaspases remains poorly understood, we solved the X-ray crystal structure of an Arabidopsis thaliana type II metacaspase (AtMCA-IIf) belonging to a particular subgroup not requiring calcium ions for activation. To study metacaspase activity in plants, we developed an in vitro chemical screen to identify small molecule metacaspase inhibitors and found several hits with a minimal thioxodihydropyrimidine-dione structure, of which some are specific AtMCA-IIf inhibitors. We provide mechanistic insight into the basis of inhibition by the TDP-containing compounds through molecular docking onto the AtMCA-IIf crystal structure. Finally, a TDP-containing compound (TDP6) effectively hampered lateral root emergence in vivo, probably through inhibition of metacaspases specifically expressed in the endodermal cells overlying developing lateral root primordia. In the future, the small compound inhibitors and crystal structure of AtMCA-IIf can be used to study metacaspases in other species, such as important human pathogens, including those causing neglected diseases.

Revisiting the relationship between turgor pressure and plant cell growth

Growth is central to plant morphogenesis. Plant cells are encased in rigid cell walls, and they must overcome physical confinement to grow to specific sizes and shapes. Cell wall tension and turgor pressure are the main mechanical components impacting plant cell growth. Cell wall mechanics has been the focus of most plant biomechanical studies, and turgor pressure was often considered as a constant and largely passive component. Nevertheless, it is increasingly accepted that turgor pressure plays a significant role in plant growth. Numerous theoretical and experimental studies suggest that turgor pressure can be both spatially inhomogeneous and actively modulated during morphogenesis. Here, we revisit the pressure-growth relationship by reviewing recent advances in investigating the interactions between cellular/tissular pressure and growth.

Callose metabolism and the regulation of cell walls and plasmodesmata during plant mutualistic and pathogenic interactions

Cell walls are essential for plant growth and development, providing support and protection from external environments. Callose is a glucan that accumulates in specialized cell wall microdomains including around intercellular pores called plasmodesmata. Despite representing a small percentage of the cell wall (~0.3% in the model plant Arabidopsis thaliana), callose accumulation regulates important biological processes such as phloem and pollen development, cell division, organ formation, responses to pathogenic invasion and to changes in nutrients and toxic metals in the soil. Callose accumulation modifies cell wall properties and restricts plasmodesmata aperture, affecting the transport of signaling proteins and RNA molecules that regulate plant developmental and environmental responses. Although the importance of callose, at and outside plasmodesmata cell walls, is widely recognized, the underlying mechanisms controlling changes in its synthesis and degradation are still unresolved. In this review, we explore the most recent literature addressing callose metabolism with a focus on the molecular factors affecting callose accumulation in response to mutualistic symbionts and pathogenic elicitors. We discuss commonalities in the signaling pathways, identify research gaps and highlight opportunities to target callose in the improvement of plant responses to beneficial versus pathogenic microbes.

Organ Patterning at the Shoot Apical Meristem (SAM): The Potential Role of the Vascular System

Auxin, which is transported in the outermost cell layer, is one of the major players involved in plant organ initiation and positioning at the shoot apical meristem (SAM). However, recent studies have recognized the role of putative internal signals as an important factor collaborating with the well-described superficial pathway of organogenesis regulation. Different internal signals have been proposed; however, their nature and transport route have not been precisely determined. Therefore, in this mini-review, we aimed to summarize the current knowledge regarding the auxin-dependent regulation of organ positioning at the SAM and to discuss the vascular system as a potential route for internal signals. In addition, as regular organ patterning is a universal phenomenon, we focus on the role of the vasculature in this process in the major lineages of land plants, i.e., bryophytes, lycophytes, ferns, gymnosperms, and angiosperms.

Intercellular Communication during Stomatal Development with a Focus on the Role of Symplastic Connection

Stomata are microscopic pores on the plant epidermis that serve as a major passage for the gas and water exchange between a plant and the atmosphere. The formation of stomata requires a series of cell division and cell-fate transitions and some key regulators including transcription factors and peptides. Monocots have different stomatal patterning and a specific subsidiary cell formation process compared with dicots. Cell-to-cell symplastic trafficking mediated by plasmodesmata (PD) allows molecules including proteins, RNAs and hormones to function in neighboring cells by moving through the channels. During stomatal developmental process, the intercellular communication between stomata complex and adjacent epidermal cells are finely controlled at different stages. Thus, the stomata cells are isolated or connected with others to facilitate their formation or movement. In the review, we summarize the main regulation mechanism underlying stomata development in both dicots and monocots and especially the specific regulation of subsidiary cell formation in monocots. We aim to highlight the important role of symplastic connection modulation during stomata development, including the status of PD presence at different cell-cell interfaces and the function of relevant mobile factors in both dicots and monocots.

Plasmodesmata play a key role in leaf vein patterning

Leaf veins provide a vital transport route in plants, and the formation of vein patterns has fascinated many scientists over the years. This Primer explores a new PLOS Biology study which reveals how transport through plasmodesmata plays a key role in vein patterning.

Leaf vein patterning is regulated by the aperture of plasmodesmata intercellular channels

To form tissue networks, animal cells migrate and interact through proteins protruding from their plasma membranes. Plant cells can do neither, yet plants form vein networks. How plants do so is unclear, but veins are thought to form by the coordinated action of the polar transport and signal transduction of the plant hormone auxin. However, plants inhibited in both pathways still form veins. Patterning of vascular cells into veins is instead prevented in mutants lacking the function of the GNOM (GN) regulator of auxin transport and signaling, suggesting the existence of at least one more GN-dependent vein-patterning pathway. Here we show that in Arabidopsis such a pathway depends on the movement of auxin or an auxin-dependent signal through plasmodesmata (PDs) intercellular channels. PD permeability is high where veins are forming, lowers between veins and nonvascular tissues, but remains high between vein cells. Impaired ability to regulate PD aperture leads to defects in auxin transport and signaling, ultimately leading to vein patterning defects that are enhanced by inhibition of auxin transport or signaling. GN controls PD aperture regulation, and simultaneous inhibition of auxin signaling, auxin transport, and regulated PD aperture phenocopies null gn mutants. Therefore, veins are patterned by the coordinated action of three GN-dependent pathways: auxin signaling, polar auxin transport, and movement of auxin or an auxin-dependent signal through PDs. Such a mechanism of tissue network formation is unprecedented in multicellular organisms.

How do plants form vein networks, in the absence of cellular migration or direct cell-cell interaction? This study shows that a GNOM-dependent combination of polar auxin transport, auxin signal transduction, and movement of an auxin signal through plasmodesmata patterns leaf vascular cells into veins.

The receptor kinase SRF3 coordinates iron-level and flagellin dependent defense and growth responses in plants

Iron is critical for host–pathogen interactions. While pathogens seek to scavenge iron to spread, the host aims at decreasing iron availability to reduce pathogen virulence. Thus, iron sensing and homeostasis are of particular importance to prevent host infection and part of nutritional immunity. While the link between iron homeostasis and immunity pathways is well established in plants, how iron levels are sensed and integrated with immune response pathways remains unknown. Here we report a receptor kinase SRF3, with a role in coordinating root growth, iron homeostasis and immunity pathways via regulation of callose synthases. These processes are modulated by iron levels and rely on SRF3 extracellular and kinase domains which tune its accumulation and partitioning at the cell surface. Mimicking bacterial elicitation with the flagellin peptide flg22 phenocopies SRF3 regulation upon low iron levels and subsequent SRF3-dependent responses. We propose that SRF3 is part of nutritional immunity responses involved in sensing external iron levels.

Iron homeostasis is known to influence plant immune signaling. Here the authors characterize SRF3, a receptor kinase that acts as a negative regulator of callose synthesis, that is required for root responses to iron deficiency and pathogen signals.

Auxin analog-induced Ca2+ signaling is independent of inhibition of endosomal aggregation in Arabidopsis roots

Much of what we know about the role of auxin in plant development derives from exogenous manipulations of auxin distribution and signaling, using inhibitors, auxins, and auxin analogs. In this context, synthetic auxin analogs, such as 1-naphthalene acetic acid (1-NAA), are often favored over the endogenous auxin, indole-3-acetic acid (IAA), in part due to their higher stability. While such auxin analogs have proven instrumental in revealing the various faces of auxin, they display in some cases bioactivities distinct from IAA. Here, we focused on the effect of auxin analogs on the accumulation of PIN proteins in brefeldin A-sensitive endosomal aggregations (BFA bodies), and correlation with the ability to elicit Ca2+ responses. For a set of commonly used auxin analogs, we evaluated if auxin analog-induced Ca2+ signaling inhibits PIN accumulation. Not all auxin analogs elicited a Ca2+ response, and their differential ability to elicit Ca2+ responses correlated partially with their ability to inhibit BFA-body formation. However, in tir1/afb and cngc14, 1-NAA-induced Ca2+ signaling was strongly impaired, yet 1-NAA still could inhibit PIN accumulation in BFA bodies. This demonstrates that TIR1/AFB-CNGC14-dependent Ca2+ signaling does not inhibit BFA body formation in Arabidopsis roots.

Cell-to-Cell Communication During Plant-Pathogen Interaction

Being sessile, plants are continuously challenged by changes in their surrounding environment and must survive and defend themselves against a multitude of pathogens. Plants have evolved a mode for pathogen recognition that activates signaling cascades such as reactive oxygen species, mitogen-activated protein kinase, and Ca²⁺ pathways, in coordination with hormone signaling, to execute the defense response at the local and systemic levels. Phytopathogens have evolved to manipulate cellular and hormonal signaling and exploit hosts' cell-to-cell connections in many ways at multiple levels. Overall, triumph over pathogens depends on how efficiently the pathogens are recognized and how rapidly the plant response is initiated through efficient intercellular communication via apoplastic and symplastic routes. Here, we review how intercellular communication in plants is mediated, manipulated, and maneuvered during plant-pathogen interaction.[Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.

Cellular and molecular bases of lateral root initiation and morphogenesis

Lateral root development is essential for the establishment of the plant root system. Lateral root initiation is a multistep process that impacts early primordium morphogenesis and is linked to the formation of a morphogenetic field of pericycle founder cells. Gradual recruitment of founder cells builds this morphogenetic field in an auxin-dependent manner. The complex process of lateral root primordium morphogenesis includes several subprocesses, which are presented in this review. The underlying cellular and molecular mechanisms of these subprocesses are examined.

Fifteen compelling open questions in plant cell biology

As scientists, we are at least as excited about the open questions-the things we do not know-as the discoveries. Here, we asked 15 experts to describe the most compelling open questions in plant cell biology. These are their questions: How are organelle identity, domains, and boundaries maintained under the continuous flux of vesicle trafficking and membrane remodeling? Is the plant cortical microtubule cytoskeleton a mechanosensory apparatus? How are the cellular pathways of cell wall synthesis, assembly, modification, and integrity sensing linked in plants? Why do plasmodesmata open and close? Is there retrograde signaling from vacuoles to the nucleus? How do root cells accommodate fungal endosymbionts? What is the role of cell edges in plant morphogenesis? How is the cell division site determined? What are the emergent effects of polyploidy on the biology of the cell, and how are any such "rules" conditioned by cell type? Can mechanical forces trigger new cell fates in plants? How does a single differentiated somatic cell reprogram and gain pluripotency? How does polarity develop de-novo in isolated plant cells? What is the spectrum of cellular functions for membraneless organelles and intrinsically disordered proteins? How do plants deal with internal noise? How does order emerge in cells and propagate to organs and organisms from complex dynamical processes? We hope you find the discussions of these questions thought provoking and inspiring.

Plasmodesmata Structural Components and Their Role in Signaling and Plant Development

Plasmodesmata are plant intercellular channels that mediate the transport of small and large molecules including RNAs and transcription factors (TFs) that regulate plant development. In this review, we present current research on plasmodesmata form and function and discuss the main regulatory pathways. We show the progress made in the development of approaches and tools to dissect the plasmodesmata proteome in diverse plant species and discuss future perspectives and challenges in this field of research.

Auxin fluxes through plasmodesmata

Characterising the processes that control auxin dynamics is essential to understanding how auxin regulates plant development. Over recent years, several studies have investigated auxin diffusion through plasmodesmata, characterising this cell-to-cell diffusion and demonstrating that it affects auxin distributions. Furthermore, studies have shown that plasmodesmatal auxin diffusion affects developmental processes, including phototropism, lateral root emergence and leaf hyponasty. This short Tansley Insight review describes how these studies have contributed to our understanding of auxin dynamics and discusses potential future directions in this area.

A Tale of Two Domains Pushing Lateral Roots

Successful plant organ development depends on well-coordinated intercellular communication between the cells of the organ itself, as well as with surrounding cells. Intercellular signals often move via the symplasmic pathway using plasmodesmata. Intriguingly, brief periods of symplasmic isolation may also be necessary to promote organ differentiation and functionality. Recent findings suggest that symplasmic isolation of a subset of parental root cells and newly forming lateral root primordia (LRPs) plays a vital role in modulating lateral root development and emergence. In this opinion article we discuss how two symplasmic domains may be simultaneously established within an LRP and its overlying cells, and the significance of plasmodesmata in this process.

SmSPL6 Induces Phenolic Acid Biosynthesis and Affects Root Development in Salvia miltiorrhiza

Salvia miltiorrhiza is a renowned model medicinal plant species for which 15 SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) family genes have been identified; however, the specific functions of SmSPLs have not been well characterized as of yet. For this study, the expression patterns of SmSPL6 were determined through its responses to treatments of exogenous hormones, including indole acetic acid (IAA), gibberellic acid (GA₃), methyl jasmonic acid (MeJA), and abscisic acid (ABA). To characterize its functionality, we obtained SmSPL6-ovexpressed transgenic S. miltiorrhiza plants and found that overexpressed SmSPL6 promoted the accumulation of phenolic acids and repressed the biosynthesis of anthocyanin. Meanwhile, the root lengths of the SmSPL6-overexpressed lines were significantly longer than the control; however, both the fresh weights and lateral root numbers decreased. Further investigations indicated that SmSPL6 regulated the biosynthesis of phenolic acid by directly binding to the promoter regions of the enzyme genes Sm4CL9 and SmCYP98A14 and activated their expression. We concluded that SmSPL6 regulates not only the biosynthesis of phenolic acids, but also the development of roots in S. miltiorrhiza.

Do Plasmodesmata Play a Prominent Role in Regulation of Auxin-Dependent Genes at Early Stages of Embryogenesis?

Plasmodesmata form intercellular channels which ensure the transport of various molecules during embryogenesis and postembryonic growth. However, high permeability of plasmodesmata may interfere with the establishment of auxin maxima, which are required for cellular patterning and the development of distinct tissues. Therefore, diffusion through plasmodesmata is not always desirable and the symplastic continuum must be broken up to induce or accomplish some developmental processes. Many data show the role of auxin maxima in the regulation of auxin-responsive genes and the establishment of various cellular patterns. However, still little is known whether and how these maxima are formed in the embryo proper before 16-cell stage, that is, when there is still a nonpolar distribution of auxin efflux carriers. In this work, we focused on auxin-dependent regulation of plasmodesmata function, which may provide rapid and transient changes of their permeability, and thus take part in the regulation of gene expression.

Organelles-nucleus-plasmodesmata signaling (ONPS): an update on its roles in plant physiology, metabolism and stress responses

Plasmodesmata allow movement of metabolites and signaling molecules between plant cells and are, therefore, critical players in plant development and physiology, and in responding to environmental signals and stresses. There is emerging evidence that plasmodesmata are controlled by signaling originating from other organelles, primarily the chloroplasts and mitochondria. These signals act in the nucleus to alter expression of genetic pathways that control both trafficking via plasmodesmata and the plasmodesmatal pores themselves. This control circuit was dubbed organelle-nucleus-plasmodesmata signaling (ONPS). Here we discuss how ONPS arose during plant evolution and highlight the discovery of an ONPS-like module for regulating stomata. We also consider recent findings that illuminate details of the ONPS circuit and its roles in plant physiology, metabolism, and defense.

Uncharted routes: exploring the relevance of auxin movement via plasmodesmata

Auxin is an endogenous small molecule with an incredibly large impact on growth and development in plants. Movement of auxin between cells, due to its negative charge at most physiological pHs, strongly relies on families of active transporters. These proteins import auxin from the extracellular space or export it into the same. Mutations in these components have profound impacts on biological processes. Another transport route available to auxin, once the substance is inside the cell, are plasmodesmata connections. These small channels connect the cytoplasms of neighbouring plant cells and enable flow between them. Interestingly, the biological significance of this latter mode of transport is only recently starting to emerge with examples from roots, hypocotyls and leaves. The existence of two transport systems provides opportunities for reciprocal cross-regulation. Indeed, auxin levels influence proteins controlling plasmodesmata permeability, while cell-cell communication affects auxin biosynthesis and transport. In an evolutionary context, transporter driven cell-cell auxin movement and plasmodesmata seem to have evolved around the same time in the green lineage. This highlights a co-existence from early on and a likely functional specificity of the systems. Exploring more situations where auxin movement via plasmodesmata has relevance for plant growth and development, and clarifying the regulation of such transport, will be key aspects in coming years.This article has an associated Future Leader to Watch interview with the author of the paper.

Intercellular trafficking via plasmodesmata: molecular layers of complexity

Plasmodesmata are intercellular pores connecting together most plant cells. These structures consist of a central constricted form of the endoplasmic reticulum, encircled by some cytoplasmic space, in turn delimited by the plasma membrane, itself ultimately surrounded by the cell wall. The presence and structure of plasmodesmata create multiple routes for intercellular trafficking of a large spectrum of molecules (encompassing RNAs, proteins, hormones and metabolites) and also enable local signalling events. Movement across plasmodesmata is finely controlled in order to balance processes requiring communication with those necessitating symplastic isolation. Here, we describe the identities and roles of the molecular components (specific sets of lipids, proteins and wall polysaccharides) that shape and define plasmodesmata structural and functional domains. We highlight the extensive and dynamic interactions that exist between the plasma/endoplasmic reticulum membranes, cytoplasm and cell wall domains, binding them together to effectively define plasmodesmata shapes and purposes.

Evaluating molecular movement through plasmodesmata

Plasmodesmata are membrane-lined cytoplasmic passageways that facilitate the movement of nutrients and various types of molecules between cells in the plant. They are highly dynamic channels, opening or closing in response to physiological and developmental stimuli or environmental challenges such as biotic and abiotic stresses. Accumulating evidence supports the idea that such dynamic controls occur through integrative cellular mechanisms. Currently, a few fluorescence-based methods are available that allow monitoring changes in molecular movement through plasmodesmata. In this chapter, following a brief introduction to those methods, we provide a detailed step-by-step protocol for the Drop-ANd-See (DANS) assay, which is advantageous when it is desirable to measure plasmodesmal permeability non-invasively, in situ and in real-time. We discuss the experimental conditions one should consider to produce reliable and reproducible DANS results along with troubleshooting ideas.

The canalization hypothesis - challenges and alternatives

The 'canalization hypothesis' was suggested 50 years ago by Tsvi Sachs to account for the formation of vascular strands in response to wounding or auxin application. The hypothesis proposes that positive feedback between auxin movement through a cell and the cell's auxin conductivity leads to the gradual selection of narrow 'canals' of polar auxin transport that will differentiate into vascular strands. Though the hypothesis has provided an invaluable conceptual framework to understand the patterned formation of vascular strands, evidence has been accumulating that seems to be incompatible with the hypothesis. We suggest that the challenging evidence is incompatible with current interpretations of the hypothesis but not with the concept at the core of the hypothesis' original formulation.

An evolutionarily conserved motif is required for Plasmodesmata-located protein 5 to regulate cell-to-cell movement

Numerous cell surface receptors and receptor-like proteins (RLPs) undergo activation or deactivation via a transmembrane domain (TMD). A subset of plant RLPs distinctively localizes to the plasma membrane-lined pores called plasmodesmata. Those RLPs include the Arabidopsis thaliana Plasmodesmata-located protein (PDLP) 5, which is well known for its vital function regulating plasmodesmal gating and molecular movement between cells. In this study, we report that the TMD, although not a determining factor for the plasmodesmal targeting, serves essential roles for the PDLP5 function. In addition to its role for membrane anchoring, the TMD mediates PDLP5 self-interaction and carries an evolutionarily conserved motif that is essential for PDLP5 to regulate cell-to-cell movement. Computational modeling-based analyses suggest that PDLP TMDs have high propensities to dimerize. We discuss how a specific mode(s) of TMD dimerization might serve as a common mechanism for PDLP5 and other PDLP members to regulate cell-to-cell movement.

Wang, Robles-Luna et al demonstrate that in Arabidopsis, the transmembrane domain (TMD) of plasmodesmata-located protein 5 (PDLP5) is required for PDPL5 dimerisation and membrane anchoring. This study suggests the importance of the TMD in the role for PDPL5 in regulating pasmodesmal opening.

Metabolic Cellular Communications: Feedback Mechanisms between Membrane Lipid Homeostasis and Plant Development

Membrane lipids are often viewed as passive building blocks of the endomembrane system. However, mounting evidence suggests that sphingolipids, sterols, and phospholipids are specifically targeted by developmental pathways, notably hormones, in a cell- or tissue-specific manner to regulate plant growth and development. Targeted modifications of lipid homeostasis may act as a way to execute a defined developmental program, for example, by regulating other signaling pathways or participating in cell differentiation. Furthermore, these regulations often feed back on the very signaling pathway that initiates the lipid metabolic changes. Here, we review several recent examples highlighting the intricate feedbacks between membrane lipid homeostasis and plant development. In particular, these examples illustrate how all aspects of membrane lipid metabolic pathways are targeted by these feedback regulations. We propose that the time has come to consider membrane lipids and lipid metabolism as an integral part of the developmental program needed to build a plant.