Glycosylphosphatidylinositol (GPI) Modification Serves as a Primary Plasmodesmal Sorting Signal

JAGGER localization and function are dependent on GPI anchor addition

KEY MESSAGE

GPI anchor addition is important for JAGGER localization and in vivo function. Loss of correct GPI anchor addition in JAGGER, negatively affects its localization and function. In flowering plants, successful double fertilization requires the correct delivery of two sperm cells to the female gametophyte inside the ovule. The delivery of a single pair of sperm cells is achieved by the entrance of a single pollen tube into one female gametophyte. To prevent polyspermy, Arabidopsis ovules avoid the attraction of multiple pollen tubes to one ovule-polytubey block. In Arabidopsis jagger mutants, a significant number of ovules attract more than one pollen tube to an ovule due to an impairment in synergid degeneration. JAGGER encodes a putative arabinogalactan protein which is predicted to be anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. Here, we show that JAGGER fused to citrine yellow fluorescent protein (JAGGER-cYFP) is functional and localizes mostly to the periphery of ovule integuments and transmitting tract cells. We further investigated the importance of GPI-anchor addition domains for JAGGER localization and function. Different JAGGER proteins with deletions in predicted ω-site regions and GPI attachment signal domain, expected to compromise the addition of the GPI anchor, led to disruption of JAGGER localization in the cell periphery. All JAGGER proteins with disrupted localization were also not able to rescue the polytubey phenotype, pointing to the importance of GPI-anchor addition to in vivo function of the JAGGER protein.

Promoter and domain structures regulate FLA12 function during Arabidopsis secondary wall development

Introduction

Fasciclin-like arabinogalactan-proteins (FLAs) are a family of multi-domain glycoproteins present at the cell surface and walls of plants. Arabidopsis thaliana FLA12 and homologs in cotton, Populus, and flax have been shown to play important functions regulating secondary cell wall (SCW) development. FLA12 has been shown to have distinct roles from the closely related FLA11 that also functions during SCW development. The promoter and domain features of FLA12 that regulate functional specificity have not been well characterized.

Methods

In this study, promoter swap experiments of FLA11 and FLA12 were investigated. Mutation of proposed functional regions within FLA12 were used to investigate the role of post-translational modifications on sub-cellular location and trafficking. Domain swap experiments between FLA11 and FLA12 were performed to identify regions of functional specificity.

Results

Promote swap experiments showed that FLA12 is differentially expressed in both stem and rosette leaves compared to FLA11. Post-translational modifications, in particular addition of the glycosylphosphatidylinositol-anchor (GPI-anchor), were shown to be important for FLA12 location at the plasma membrane (PM)/cell wall interface. Domain swap experiments between FLA11 and FLA12 showed that the C-terminal arabinogalactan (AG) glycan motif acts as a key regulatory region differentiating FLA12 functions from FLA11.

Discussion

Understanding of FLA12 promoter and functional domains has provided new insights into the regulation of SCW development and functional specificity of FLAs for plant growth and development.

Temporal transcriptome and metabolite analyses provide insights into the biochemical and physiological processes underlying endodormancy release in pistachio (Pistacia vera L.) flower buds

Pistachio (Pistacia vera L.), an economically and nutritionally important tree crop, relies on winter chill for bud endodormancy break and subsequent blooming and nut production. However, insufficient winter chill poses an increasing challenge in pistachio growing regions. To gain a better understanding of the physiological and biochemical responses of endodormant pistachio buds to chilling accumulation, we investigated the global gene expression changes in flower buds of pistachio cv. Kerman that were cultivated at three different orchard locations and exposed to increasing durations of winter chill. The expression of genes encoding β-1,3-glucanase and β-amylase, enzymes responsible for breaking down callose (β-1,3-glucan) and starch (α-1,4-glucan), respectively, increased during the endodormancy break of pistachio buds. This result suggested that the breakdown of callose obstructing stomata as well as the release of glucose from starch enables symplasmic trafficking and provides energy for bud endodormancy break and growth. Interestingly, as chilling accumulation increased, there was a decrease in the expression of nine-cis-epoxycarotenoid dioxygenase (NCED), encoding an enzyme that uses carotenoids as substrates and catalyzes the rate-limiting step in abscisic acid (ABA) biosynthesis. The decrease in NCED expression suggests ABA biosynthesis is suppressed, thus reducing inhibition of endodormancy break. The higher levels of carotenoid precursors and a decrease in ABA content in buds undergoing endodormancy break supports this suggestion. Collectively, the temporal transcriptome and biochemical analyses revealed that the degradation of structural (callose) and non-structural (starch) carbohydrates, along with the attenuation of ABA biosynthesis, are critical processes driving endodormancy break in pistachio buds.

Individual lipid transfer proteins from Tanacetum parthenium show different specificity for extracellular accumulation of sesquiterpenes

A highly specialized function for individual LTPs for different products from the same terpenoid biosynthesis pathway is described and the function of an LTP GPI anchor is studied. Sequiterpenes produced in glandular trichomes of the medicinal plant Tanacetum parthenium (feverfew) accumulate in the subcuticular extracellular space. Transport of these compounds over the plasma membrane is presumably by specialized membrane transporters, but it is still not clear how these hydrophobic compounds are subsequently transported over the hydrophilic cell wall. Here we identified eight so-called non-specific Lipid transfer proteins (nsLTPs) genes that are expressed in feverfew trichomes. A putative function of these eight nsLTPs in transport of the lipophilic sesquiterpene lactones produced in feverfew trichomes, was tested in an in-planta transport assay using transient expression in Nicotiana benthamiana. Of eight feverfew nsLTP candidate genes analyzed, two (TpLTP1 and TpLTP2) can specifically improve extracellular accumulation of the sesquiterpene costunolide, while one nsLTP (TpLTP3) shows high specificity towards export of parthenolide. The specificity of the nsLTPs was also tested in an assay that test for the exclusion capacity of the nsLTP for influx of extracellular substrates. In such assay, TpLTP3 was identified as most effective in blocking influx of both costunolide and parthenolide, when these substrates are infiltrated into the apoplast. The TpLTP3 is special in having a GPI-anchor domain, which is essential for the export activity of TpLTP3. However, addition of the TpLTP3 GPI-anchor domain to TpLTP1 resulted in loss of TpLTP1 export activity. These novel export and exclusion assays thus provide new means to test functionality of plant nsLTPs.

Glycosylphosphatidylinositol anchor lipid remodeling directs proteins to the plasma membrane and governs cell wall mechanics

Glycosylphosphatidylinositol (GPI) anchoring is a common protein modification that targets proteins to the plasma membrane (PM). Knowledge about the GPI lipid tail, which guides the secretion of GPI-anchored proteins (GPI-APs), is limited in plants. Here, we report that rice (Oryza sativa) BRITTLE CULM16 (BC16), a membrane-bound O-acyltransferase (MBOAT) remodels GPI lipid tails and governs cell wall biomechanics. The bc16 mutant exhibits fragile internodes, resulting from reduced cell wall thickness and cellulose content. BC16 is the only MBOAT in rice and is located in the endoplasmic reticulum and Golgi apparatus. Yeast gup1Δ mutant restoring assay and GPI lipid composition analysis demonstrated BC16 as a GPI lipid remodelase. Loss of BC16 alters GPI lipid structure and disturbs the targeting of BC1, a GPI-AP for cellulose biosynthesis, to the PM lipid nanodomains. Atomic force microscopy revealed compromised deposition of cellulosic nanofibers in bc16, leading to an increased Young's modulus and abnormal mechanical properties. Therefore, BC16-mediated lipid remodeling directs the GPI-APs, such as BC1, to the cell surface to fulfill multiple functions, including cellulose organization. Our work unravels a mechanism by which GPI lipids are remodeled in plants and provides insights into the control of cell wall biomechanics, offering a tool for breeding elite crops with improved support strength.

Function of Plasmodesmata in the Interaction of Plants with Microbes and Viruses

Plasmodesmata (PD) are gated plant cell wall channels that allow the trafficking of molecules between cells and play important roles during plant development and in the orchestration of cellular and systemic signaling responses during interactions of plants with the biotic and abiotic environment. To allow gating, PD are equipped with signaling platforms and enzymes that regulate the size exclusion limit (SEL) of the pore. Plant-interacting microbes and viruses target PD with specific effectors to enhance their virulence and are useful probes to study PD functions.

Proper protein folding in the endoplasmic reticulum is required for attachment of a glycosylphosphatidylinositol anchor in plants

Endoplasmic reticulum (ER) quality control processes recognize and eliminate misfolded proteins to maintain cellular protein homeostasis and prevent the accumulation of defective proteins in the secretory pathway. Glycosylphosphatidylinositol (GPI)-anchored proteins carry a glycolipid modification, which provides an efficient ER export signal and potentially prevents the entry into ER-associated degradation (ERAD), which is one of the major pathways for clearance of terminally misfolded proteins from the ER. Here, we analyzed the degradation routes of different misfolded glycoproteins carrying a C-terminal GPI-attachment signal peptide in Arabidopsis thaliana. We found that a fusion protein consisting of the misfolded extracellular domain from Arabidopsis STRUBBELIG and the GPI-anchor attachment sequence of COBRA1 was efficiently targeted to hydroxymethylglutaryl reductase degradation protein 1 complex-mediated ERAD without the detectable attachment of a GPI anchor. Non-native variants of the GPI-anchored lipid transfer protein 1 (LTPG1) that lack a severely misfolded domain, on the other hand, are modified with a GPI anchor and targeted to the vacuole for degradation. Impaired processing of the GPI-anchoring signal peptide by mutation of the cleavage site or in a GPI-transamidase-compromised mutant caused ER retention and routed the non-native LTPG1 to ERAD. Collectively, these results indicate that for severely misfolded proteins, ER quality control processes are dominant over ER export. For less severely misfolded proteins, the GPI anchor provides an efficient ER export signal resulting in transport to the vacuole.

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.

Targeted misexpression of NAC052, acting in H3K4 demethylation, alters leaf morphological and anatomical traits in Arabidopsis thaliana

In an effort to identify genetic regulators for the cell ontogeny around the veins in Arabidopsis thaliana leaves, an activation-tagged mutant line with altered leaf morphology and altered bundle sheath anatomy was characterized. This mutant had a small rosette area with wrinkled leaves and chlorotic leaf edges, as well as enhanced chloroplast numbers in the (pre-)bundle sheath tissue. It had a bundle-specific promoter from the gene GLYCINE DECARBOXYLASE SUBUNIT-T from the C4 species Flaveria trinervia (GLDTFt promoter) inserted in the coding region of the transcriptional repressor NAC052, functioning in H3K4 demethylation, in front of an alternative start codon in-frame with the natural start codon. Reconstruction of the mutation event of our activation-tagged line by creating a line expressing an N-terminally truncated sequence of NAC052 under control of the GLDTFt promoter confirmed the involvement of NAC052 in leaf development. Our study not only reveals leaf anatomic and transcriptomic effects of an N-terminally truncated NAC052 under control of the GLDTFt promoter, but also identifies NAC052 as a novel genetic regulator of leaf development.

On the Potential Function of Type II Arabinogalactan O-Glycosylation in Regulating the Fate of Plant Secretory Proteins

In a plant-specific mode of protein glycosylation, various sugars and glycans are attached to hydroxyproline giving rise to a variety of diverse O-glycoproteins. The sub-family of arabinogalactan proteins is implicated in a multitude of biological functions, however, the mechanistic role of O-glycosylation on AGPs by type II arabinogalactans is largely elusive. Some models suggest roles of the O-glycans such as in ligand-receptor interactions and as localized calcium ion store. Structurally different but possibly analogous types of protein O-glycosylation exist in animal and yeast models and roles for O-glycans were suggested in determining the fate of O-glycoproteins by affecting intracellular sorting or proteolytic activation and degradation. At present, only few examples exist that describe how the fate of artificial and endogenous arabinogalactan proteins is affected by O-glycosylation with type II arabinogalactans. In addition to other roles, these glycans might act as a molecular determinant for cellular localization and protein lifetime of many endogenous proteins.

Plasmodesmata Conductivity Regulation: A Mechanistic Model

Plant cells form a multicellular symplast via cytoplasmic bridges called plasmodesmata (Pd) and the endoplasmic reticulum (ER) that crosses almost all plant tissues. The Pd proteome is mainly represented by secreted Pd-associated proteins (PdAPs), the repertoire of which quickly adapts to environmental conditions and responds to biotic and abiotic stresses. Although the important role of Pd in stress-induced reactions is universally recognized, the mechanisms of Pd control are still not fully understood. The negative role of callose in Pd permeability has been convincingly confirmed experimentally, yet the roles of cytoskeletal elements and many PdAPs remain unclear. Here, we discuss the contribution of each protein component to Pd control. Based on known data, we offer mechanistic models of mature leaf Pd regulation in response to stressful effects.

Salicylic acid-mediated plasmodesmal closure via Remorin-dependent lipid organization

Plasmodesmata (PD) create cytoplasmic and membrane continuities between adjacent cells to facilitate cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity against plant pathogens, including the accumulation of the defense hormone, salicylic acid (SA). However, the mechanism of how this occurs is not well understood. Here, we uncover a mechanism by which SA triggers Remorin-dependent membrane lipid nanodomain assembly, leading to enhancement of the liquid-ordered phase. The higher-ordered lipids, which are particularly enriched at PD membrane, decreased PD membrane plasticity, and thus restricted PD opening and impeded virus spreading. Our findings address a knowledge gap in plant defense mechanisms at the membrane level that rely on SA-controlled lipid order and PD closure.

Plasmodesmata (PD) are plant-specific membrane-lined channels that create cytoplasmic and membrane continuities between adjacent cells, thereby facilitating cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity in response to numerous environmental stimuli. In particular, during defense against plant pathogens, the defense hormone, salicylic acid (SA), plays a crucial role in the regulation of PD permeability in a callose-dependent manner. Here, we uncover a mechanism by which plants restrict the spreading of virus and PD cargoes using SA signaling by increasing lipid order and closure of PD. We showed that exogenous SA application triggered the compartmentalization of lipid raft nanodomains through a modulation of the lipid raft-regulatory protein, Remorin (REM). Genetic studies, superresolution imaging, and transmission electron microscopy observation together demonstrated that Arabidopsis REM1.2 and REM1.3 are crucial for plasma membrane nanodomain assembly to control PD aperture and functionality. In addition, we also found that a 14-3-3 epsilon protein modulates REM clustering and membrane nanodomain compartmentalization through its direct interaction with REM proteins. This study unveils a molecular mechanism by which the key plant defense hormone, SA, triggers membrane lipid nanodomain reorganization, thereby regulating PD closure to impede virus spreading.

Multiple C2 domains and transmembrane region proteins (MCTPs) tether membranes at plasmodesmata

In eukaryotes, membrane contact sites (MCS) allow direct communication between organelles. Plants have evolved a unique type of MCS, inside intercellular pores, the plasmodesmata, where endoplasmic reticulum (ER)-plasma membrane (PM) contacts coincide with regulation of cell-to-cell signalling. The molecular mechanism and function of membrane tethering within plasmodesmata remain unknown. Here, we show that the multiple C2 domains and transmembrane region protein (MCTP) family, key regulators of cell-to-cell signalling in plants, act as ER-PM tethers specifically at plasmodesmata. We report that MCTPs are plasmodesmata proteins that insert into the ER via their transmembrane region while their C2 domains dock to the PM through interaction with anionic phospholipids. A Atmctp3/Atmctp4 loss of function mutant induces plant developmental defects, impaired plasmodesmata function and composition, while MCTP4 expression in a yeast Δtether mutant partially restores ER-PM tethering. Our data suggest that MCTPs are unique membrane tethers controlling both ER-PM contacts and cell-to-cell signalling.

Plasmodesmata-Related Structural and Functional Proteins: The Long Sought-After Secrets of a Cytoplasmic Channel in Plant Cell Walls

Plant cells are separated by cellulose cell walls that impede direct cell-to-cell contact. In order to facilitate intercellular communication, plant cells develop unique cell-wall-spanning structures termed plasmodesmata (PD). PD are membranous channels that link the cytoplasm, plasma membranes, and endoplasmic reticulum of adjacent cells to provide cytoplasmic and membrane continuity for molecular trafficking. PD play important roles for the development and physiology of all plants. The structure and function of PD in the plant cell walls are highly dynamic and tightly regulated. Despite their importance, plasmodesmata are among the few plant cell organelles that remain poorly understood. The molecular properties of PD seem largely elusive or speculative. In this review, we firstly describe the general PD structure and its protein composition. We then discuss the recent progress in identification and characterization of PD-associated plant cell-wall proteins that regulate PD function, with particular emphasis on callose metabolizing and binding proteins, and protein kinases targeted to and around PD.

Glycosylphosphatidylinositol-Anchored Proteins in Arabidopsis and One of Their Common Roles in Signaling Transduction

Diverse proteins are found modified with glycosylphosphatidylinositol (GPI) at their carboxyl terminus in eukaryotes, which allows them to associate with membrane lipid bilayers and anchor on the external surface of the plasma membrane. GPI-anchored proteins (GPI-APs) play crucial roles in various processes, and more and more GPI-APs have been identified and studied. In this review, previous genomic and proteomic predictions of GPI-APs in Arabidopsis have been updated, which reveal their high abundance and complexity. From studies of individual GPI-APs in Arabidopsis, certain GPI-APs have been found associated with partner receptor-like kinases (RLKs), targeting RLKs to their subcellular localization and helping to recognize extracellular signaling polypeptide ligands. Interestingly, the association might also be involved in ligand selection. The analyses suggest that GPI-APs are essential and widely involved in signal transduction through association with RLKs.

Dynamic regulation of plasmodesmatal permeability and its application to horticultural research

Effective cell-to-cell communication allows plants to fine-tune their developmental processes in accordance with the prevailing environmental stimuli. Plasmodesmata (PD) are intercellular channels that span the plant cell wall and serve as cytoplasmic bridges to facilitate efficient exchange of signaling molecules between neighboring cells. The identification of PD-associated proteins and the subsequent elucidation of the regulation of PD structure have provided vital insights into the role of PD architecture in enforcing crucial cellular processes, including callose deposition, ER-Golgi-based secretion, cytoskeleton dynamics, membrane lipid raft organization, chloroplast metabolism, and cell wall formation. In this review, we summarize the emerging discoveries from recent studies that elucidated the regulatory mechanisms involved in PD biogenesis and the dynamics of PD opening-closure. Retrospectively, PD-mediated cell-to-cell communication has been implicated in diverse cellular and physiological processes that are fundamental for the development of horticultural plants. The potential application of PD biotechnological engineering represents a powerful approach for improving agronomic traits in horticultural crops in the future.

REM1.3's phospho-status defines its plasma membrane nanodomain organization and activity in restricting PVX cell-to-cell movement

Plants respond to pathogens through dynamic regulation of plasma membrane-bound signaling pathways. To date, how the plant plasma membrane is involved in responses to viruses is mostly unknown. Here, we show that plant cells sense the Potato virus X (PVX) COAT PROTEIN and TRIPLE GENE BLOCK 1 proteins and subsequently trigger the activation of a membrane-bound calcium-dependent kinase. We show that the Arabidopsis thaliana CALCIUM-DEPENDENT PROTEIN KINASE 3-interacts with group 1 REMORINs in vivo, phosphorylates the intrinsically disordered N-terminal domain of the Group 1 REMORIN REM1.3, and restricts PVX cell-to-cell movement. REM1.3's phospho-status defines its plasma membrane nanodomain organization and is crucial for REM1.3-dependent restriction of PVX cell-to-cell movement by regulation of callose deposition at plasmodesmata. This study unveils plasma membrane nanodomain-associated molecular events underlying the plant immune response to viruses.

Viruses propagate in plants through membranous channels, called plasmodesmata, linking each cell to its neighboring cell. In this work, we challenge the role of the plasma membrane in the regulation of virus propagation. By studying the dynamics and the activation of a plant-specific protein called REMORIN, we found that the way this protein is organized inside the membrane is crucial to fulfill its function in the immunity of plants against viruses.

Fascinating Fasciclins: A Surprisingly Widespread Family of Proteins that Mediate Interactions between the Cell Exterior and the Cell Surface

The Fasciclin 1 (FAS1) domain is an ancient structural motif in extracellular proteins present in all kingdoms of life and particularly abundant in plants. The FAS1 domain accommodates multiple interaction surfaces, enabling it to bind different ligands. The frequently observed tandem FAS1 arrangement might both positively and negatively regulate ligand binding. Additional protein domains and post-translational modifications are partially conserved between different evolutionary clades. Human FAS1 family members are associated with multiple aspects of health and disease. At the cellular level, mammalian FAS1 proteins are implicated in extracellular matrix structure, cell to extracellular matrix and cell to cell adhesion, paracrine signaling, intracellular trafficking and endocytosis. Mammalian FAS1 proteins bind to the integrin family of receptors and to protein and carbohydrate components of the extracellular matrix. FAS1 protein encoding plant genes exert effects on cellulosic and non-cellulosic cell wall structure and cellular signaling but to establish the modes of action for any plant FAS1 protein still requires biochemical experimentation. In fungi, eubacteria and archaea, the differential presence of FAS1 proteins in closely related organisms and isolated biochemical data suggest functions in pathogenicity and symbiosis. The inter-kingdom comparison of FAS1 proteins suggests that molecular mechanisms mediating interactions between cells and their environment may have evolved at the earliest known stages of evolution.

Identification of Plasmodesmal Localization Sequences in Proteins In Planta

Plasmodesmata (Pd) are cell-to-cell connections that function as gateways through which small and large molecules are transported between plant cells. Whereas Pd transport of small molecules, such as ions and water, is presumed to occur passively, cell-to-cell transport of biological macromolecules, such proteins, most likely occurs via an active mechanism that involves specific targeting signals on the transported molecule. The scarcity of identified plasmodesmata (Pd) localization signals (PLSs) has severely restricted the understanding of protein-sorting pathways involved in plant cell-to-cell macromolecular transport and communication. From a wealth of plant endogenous and viral proteins known to traffic through Pd, only three PLSs have been reported to date, all of them from endogenous plant proteins. Thus, it is important to develop a reliable and systematic experimental strategy to identify a functional PLS sequence, that is both necessary and sufficient for Pd targeting, directly in the living plant cells. Here, we describe one such strategy using as a paradigm the cell-to-cell movement protein (MP) of the Tobacco mosaic virus (TMV). These experiments, that identified and characterized the first plant viral PLS, can be adapted for discovery of PLS sequences in most Pd-targeted proteins.

Lipid Raft, Regulator of Plasmodesmal Callose Homeostasis

Abstract: The specialized plasma membrane microdomains known as lipid rafts are enriched by sterols and sphingolipids. Lipid rafts facilitate cellular signal transduction by controlling the assembly of signaling molecules and membrane protein trafficking. Another specialized compartment of plant cells, the plasmodesmata (PD), which regulates the symplasmic intercellular movement of certain molecules between adjacent cells, also contains a phospholipid bilayer membrane. The dynamic permeability of plasmodesmata (PDs) is highly controlled by plasmodesmata callose (PDC), which is synthesized by callose synthases (CalS) and degraded by β-1,3-glucanases (BGs). In recent studies, remarkable observations regarding the correlation between lipid raft formation and symplasmic intracellular trafficking have been reported, and the PDC has been suggested to be the regulator of the size exclusion limit of PDs. It has been suggested that the alteration of lipid raft substances impairs PDC homeostasis, subsequently affecting PD functions. In this review, we discuss the substantial role of membrane lipid rafts in PDC homeostasis and provide avenues for understanding the fundamental behavior of the lipid raft-processed PDC.

Plasmodesmata enable multicellularity: new insights into their evolution, biogenesis, and functions in development and immunity

Plant cells are connected by plasmodesmata (PD), cytosolic bridges that allow molecules to freely move across the cell wall. Recently resolved relationships among land plants and their algal relatives reveal that land plants evolved PD independently from algae. Proteomic and genetic screens illuminate new dimensions of the structural and regulatory pathways that control PD biogenesis. Biochemical studies demonstrate that immunological signals induce systemic defenses by moving from diseased cells through PD; subsequently, PD transport is restricted to quarantine diseased cells. Here, we review our expanding knowledge of the roles of PD in plant development, physiology, and immunity.