A Sec14p-nodulin domain phosphatidylinositol transfer protein polarizes membrane growth of Arabidopsis thaliana root hairs

Mammalian START-like phosphatidylinositol transfer proteins - Physiological perspectives and roles in cancer biology

PtdIns and its phosphorylated derivatives, the phosphoinositides, are the biochemical components of a major pathway of intracellular signaling in all eukaryotic cells. These lipids are few in terms of cohort of unique positional isomers, and are quantitatively minor species of the bulk cellular lipidome. Nevertheless, phosphoinositides regulate an impressively diverse set of biological processes. It is from that perspective that perturbations in phosphoinositide-dependent signaling pathways are increasingly being recognized as causal foundations of many human diseases - including cancer. Although phosphatidylinositol transfer proteins (PITPs) are not enzymes, these proteins are physiologically significant regulators of phosphoinositide signaling. As such, PITPs are conserved throughout the eukaryotic kingdom. Their biological importance notwithstanding, PITPs remain understudied. Herein, we review current information regarding PITP biology primarily focusing on how derangements in PITP function disrupt key signaling/developmental pathways and are associated with a growing list of pathologies in mammals.

OsSNDP4, a Sec14-nodulin Domain Protein, is Required for Pollen Development in Rice

Pollen is encased in a robust wall that shields the male gametophyte from various stresses and aids in pollination. The pollen wall consists of gametophyte-derived intine and sporophyte-derived exine. The exine is mainly composed of sporopollenin, which is biopolymers of aliphatic lipids and phenolics. The process of exine formation has been the subject of extensive research, yet the underlying molecular mechanisms remain elusive. In this study, we identified a rice mutant of the OsSNDP4 gene that is impaired in pollen development. We demonstrated that OsSNDP4, a putative Sec14-nodulin domain protein, exhibits a preference for binding to phosphatidylinositol (3)-phosphate [PI(3)P], a lipid primarily found in endosomal and vacuolar membranes. The OsSNDP4 protein was detected in association with the endoplasmic reticulum (ER), vacuolar membranes, and the nucleus. OsSNDP4 expression was detected in all tested organs but was notably higher in anthers during exine development. Loss of OsSNDP4 function led to abnormal vacuole dynamics, inhibition in Ubisch body development, and premature degradation of cellular contents and organelles in the tapetal cells. Microspores from the ossndp4 mutant plant displayed abnormal exine formation, abnormal vacuole enlargement, and ultimately, pollen abortion. RNA-seq assay revealed that genes involved in the biosynthesis of fatty acid and secondary metabolites, the biosynthesis of lipid polymers, and exosome formation were enriched among the down-regulated genes in the mutant anthers, which correlated with the morphological defects observed in the mutant anthers. Base on these findings, we propose that OsSNDP4 regulates pollen development by binding to PI(3)P and influencing the dynamics of membrane systems. The involvement of membrane systems in the regulation of sporopollenin biosynthesis, Ubisch body formation, and exine formation provides a novel mechanism regulating pollen wall development.

SEC14-like condensate phase transitions at plasma membranes regulate root growth in Arabidopsis

Protein function can be modulated by phase transitions in their material properties, which can range from liquid- to solid-like; yet, the mechanisms that drive these transitions and whether they are important for physiology are still unknown. In the model plant Arabidopsis, we show that developmental robustness is reinforced by phase transitions of the plasma membrane-bound lipid-binding protein SEC14-like. Using imaging, genetics, and in vitro reconstitution experiments, we show that SEC14-like undergoes liquid-like phase separation in the root stem cells. Outside the stem cell niche, SEC14-like associates with the caspase-like protease separase and conserved microtubule motors at unique polar plasma membrane interfaces. In these interfaces, SEC14-like undergoes processing by separase, which promotes its liquid-to-solid transition. This transition is important for root development, as lines expressing an uncleavable SEC14-like variant or mutants of separase and associated microtubule motors show similar developmental phenotypes. Furthermore, the processed and solidified but not the liquid form of SEC14-like interacts with and regulates the polarity of the auxin efflux carrier PINFORMED2. This work demonstrates that robust development can involve liquid-to-solid transitions mediated by proteolysis at unique plasma membrane interfaces.

Identification and Expression Analysis of Phosphatidylinositol Transfer Proteins Genes in Rice

The family of phosphatidylinositol transfer proteins (PITPs) is able to bind specific lipids to carry out various biological functions throughout different stages of plant life. But the function of PITPs in rice plant is unclear. In this study, 30 PITPs were identified from rice genome, which showed differences in physicochemical properties, gene structure, conservation domains, and subcellular localization. The promoter region of the OsPITPs genes included at least one type of hormone response element, such as methyl jasmonate (Me JA) and salicylic acid (SA). Furthermore, the expression level of OsML-1, OsSEC14-3, OsSEC14-4, OsSEC14-15, and OsSEC14-19 genes were significantly affected by infection of rice blast fungus Magnaporthe oryzae. Based on these findings, it is possible that OsPITPs may be involved in rice innate immunity in response to M. oryzae infection through the Me JA and SA pathway.

Role of SEC14-like phosphatidylinositol transfer proteins in membrane identity and dynamics

Membrane identity and dynamic processes, that act at membrane sites, provide important cues for regulating transport, signal transduction and communication across membranes. There are still numerous open questions as to how membrane identity changes and the dynamic processes acting at the surface of membranes are regulated in diverse eukaryotes in particular plants and which roles are being played by protein interaction complexes composed of peripheral and integral membrane proteins. One class of peripheral membrane proteins conserved across eukaryotes comprises the SEC14-like phosphatidylinositol transfer proteins (SEC14L-PITPs). These proteins share a SEC14 domain that contributes to membrane identity and fulfills regulatory functions in membrane trafficking by its ability to sense, bind, transport and exchange lipophilic substances between membranes, such as phosphoinositides and diverse other lipophilic substances. SEC14L-PITPs can occur as single-domain SEC14-only proteins in all investigated organisms or with a modular domain structure as multi-domain proteins in animals and streptophytes (comprising charales and land plants). Here, we present an overview on the functional roles of SEC14L-PITPs, with a special focus on the multi-domain SEC14L-PITPs of the SEC14-nodulin and SEC14-GOLD group (PATELLINs, PATLs in plants). This indicates that SEC14L-PITPs play diverse roles from membrane trafficking to organism fitness in plants. We concentrate on the structure of SEC14L-PITPs, their ability to not only bind phospholipids but also other lipophilic ligands, and their ability to regulate complex cellular responses through interacting with proteins at membrane sites.

Mechanisms by which small molecules of diverse chemotypes arrest Sec14 lipid transfer activity

Phosphatidylinositol (PtdIns) transfer proteins (PITPs) enhance the activities of PtdIns 4-OH kinases that generate signaling pools of PtdIns-4-phosphate. In that capacity, PITPs serve as key regulators of lipid signaling in eukaryotic cells. Although the PITP phospholipid exchange cycle is the engine that stimulates PtdIns 4-OH kinase activities, the underlying mechanism is not understood. Herein, we apply an integrative structural biology approach to investigate interactions of the yeast PITP Sec14 with small-molecule inhibitors (SMIs) of its phospholipid exchange cycle. Using a combination of X-ray crystallography, solution NMR spectroscopy, and atomistic MD simulations, we dissect how SMIs compete with native Sec14 phospholipid ligands and arrest phospholipid exchange. Moreover, as Sec14 PITPs represent new targets for the development of next-generation antifungal drugs, the structures of Sec14 bound to SMIs of diverse chemotypes reported in this study will provide critical information required for future structure-based design of next-generation lead compounds directed against Sec14 PITPs of virulent fungi.

OsSNDP3 Functions for the Polar Tip Growth in Rice Pollen Together with OsSNDP2, a Paralog of OsSNDP3

Understanding pollen tube growth is critical for crop yield maintenance. The pollen tube provides a path for sperm cells for fertilization with egg cells. Cells must be subdivided into functionally and structurally distinct compartments for polar tip growth, and phosphoinositides are thought to be one of the facilitators for polarization during pollen tube growth. OsSNDP3 encodes Sec14-nodulin domain-containing protein and localizes in the nucleus and the microdomains of the plasma membrane in tobacco leaf epidermis cells. OsSNDP3 is thought to bind with phosphatidylinositol 4,5-bisphosphate based on the data including the information of basic amino acids in the C-terminal and colocalization with 2X Pleckstrin homology domain of Phospholipase C delta-1. OsSNDP3 interacts with a protein that contains a class I nodulin domain. We discovered that OsSNDP3 plays a significant role in pollen tube germination using CRISPR/Cas9 systems, whereas another pollen-preferential Sec14-nodulin domain-containing protein, OsSNDP2, additively functions with OsSNDP3 during pollen tube germination. Gene Ontology analysis using downregulated genes in ossndp3 indicated that the expression of genes involved in the phosphatidylinositol metabolic process and tip growth was significantly altered in ossndp3. OsSNDP3 aids pollen polar tip growth by binding with phosphatidylinositol 4,5-bisphosphate. We can better understand the roles of phosphoinositides during pollen tube growth by studying the functions of OsSNDP3 and OsSNDP2. And downregulated genes in ossndp3 might be useful targets for future research on polar tip growth.

Chloroplast-localized PITP7 is essential for plant growth and photosynthetic function in Arabidopsis

Recent studies of chloroplast-localized Sec14-like protein (CPSFL1, also known as phosphatidylinositol transfer protein 7, PITP7) showed that CPSFL1 is necessary for photoautotropic growth and chloroplast vesicle formation in Arabidopsis (Arabidopsis thaliana). Here, we investigated the functional roles of CPSFL1/PITP7 using two A. thaliana mutants carrying a putative null allele (pitp7-1) and a weak allele (pitp7-2), respectively. PITP7 transcripts were undetectable in pitp7-1 and less abundant in pitp7-2 than in the wild-type (WT). The severity of mutant phenotypes, such as plant developmental abnormalities, levels of plastoquinone-9 (PQ-9) and chlorophylls, photosynthetic protein complexes, and photosynthetic performance, were well related to PITP7 transcript levels. The pitp7-1 mutation was seedling lethal and was associated with significantly lower levels of PQ-9 and major photosynthetic proteins. pitp7-2 plants showed greater susceptibility to high-intensity light stress than the WT, attributable to defects in nonphotochemical quenching and photosynthetic electron transport. PITP7 is specifically bound to phosphatidylinositol phosphates (PIPs) in lipid-binding assays in vitro, and the point mutations R82, H125, E162, or K233 reduced the binding affinity of PITP7 to PIPs. Further, constitutive expression of PITP7^H125Q or PITP7^E162K in pitp7-1 homozygous plants restored autotrophic growth in soil but without fully complementing the mutant phenotypes. Consistent with a previous study, our results demonstrate that PITP7 is essential for plant development, particularly the accumulation of PQ-9 and photosynthetic complexes. We propose a possible role for PITP7 in membrane trafficking of hydrophobic ligands such as PQ-9 and carotenoids through chloroplast vesicle formation or direct binding involving PIPs.

The functions of phospholipases and their hydrolysis products in plant growth, development and stress responses

Cell membranes are the initial site of stimulus perception from environment and phospholipids are the basic and important components of cell membranes. Phospholipases hydrolyze membrane lipids to generate various cellular mediators. These phospholipase-derived products, such as diacylglycerol, phosphatidic acid, inositol phosphates, lysophopsholipids, and free fatty acids, act as second messengers, playing vital roles in signal transduction during plant growth, development, and stress responses. This review focuses on the structure, substrate specificities, reaction requirements, and acting mechanism of several phospholipase families. It will discuss their functional significance in plant growth, development, and stress responses. In addition, it will highlight some critical knowledge gaps in the action mechanism, metabolic and signaling roles of these phospholipases and their products in the context of plant growth, development and stress responses.

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.

Genome-wide analyses of the Nodulin-like gene family in bread wheat revealed its potential roles during arbuscular mycorrhizal symbiosis

Nodulin-like (NL) genes are involved in transporting of various substances and may play key roles during the establishment of symbiosis in legumes plants. However, basic biological information of NL genes in the wheat genome is still largely unknown. Here, we identified and characterized NL genes in wheat via integrating genomic information, collinearity analysis, co-expression network analysis (WGCNA) and transcriptome analysis. In addition, we analyzed the polymorphisms and the roles of NL genes during arbuscular mycorrhizal (AM) symbiosis using a large wheat panel consists of 259 wheat genotypes. We identified 181 NL genes in the wheat genome, which were classified into SWEET, Early Nodulin-Like (ENODL), Major Facilitator Superfamily-Nodulin (MFS), Vacuolar Iron Transporter (VIT) and Early nodulin 93 (ENOD93) subfamily. The expansion of NL genes was mainly driven by segmental duplication. The bHLH genes are potential unrecognized transcription factors regulating NL genes. Moreover, two NL genes were more sensitive than other NL genes to AM colonization. The polymorphisms of NL genes are mainly due to random drift, and the natural mutation of NL genes led to significant differences in the mycorrhizal dependence of wheat in phosphorus uptake. The results concluded that NL genes potentially play important roles during AM symbiosis with wheat.

Advanced genomics identifies growth effectors for proteotoxic ER stress recovery in Arabidopsis thaliana

Adverse environmental and pathophysiological situations can overwhelm the biosynthetic capacity of the endoplasmic reticulum (ER), igniting a potentially lethal condition known as ER stress. ER stress hampers growth and triggers a conserved cytoprotective signaling cascade, the unfolded protein response (UPR) for ER homeostasis. As ER stress subsides, growth is resumed. Despite the pivotal role of the UPR in growth restoration, the underlying mechanisms for growth resumption are yet unknown. To discover these, we undertook a genomics approach in the model plant species Arabidopsis thaliana and mined the gene reprogramming roles of the UPR modulators, basic leucine zipper28 (bZIP28) and bZIP60, in ER stress resolution. Through a network modeling and experimental validation, we identified key genes downstream of the UPR bZIP-transcription factors (bZIP-TFs), and demonstrated their functional roles. Our analyses have set up a critical pipeline for functional gene discovery in ER stress resolution with broad applicability across multicellular eukaryotes.

Ko and Brandizzi use Arabidopsis thaliana to investigate the downstream regulators of two major endoplasmic reticulum (ER) stress-related transcription factors, bZIP60 and bZIP28. Their results provide further insight on how two modulators of the unfolded protein response contribute to growth recovery from ER stress.

Emerging Prospects for Combating Fungal Infections by Targeting Phosphatidylinositol Transfer Proteins

The emergence of fungal “superbugs” resistant to the limited cohort of anti-fungal agents available to clinicians is eroding our ability to effectively treat infections by these virulent pathogens. As the threat of fungal infection is escalating worldwide, this dwindling response capacity is fueling concerns of impending global health emergencies. These developments underscore the urgent need for new classes of anti-fungal drugs and, therefore, the identification of new targets. Phosphoinositide signaling does not immediately appear to offer attractive targets due to its evolutionary conservation across the Eukaryota. However, recent evidence argues otherwise. Herein, we discuss the evidence identifying Sec14-like phosphatidylinositol transfer proteins (PITPs) as unexplored portals through which phosphoinositide signaling in virulent fungi can be chemically disrupted with exquisite selectivity. Recent identification of lead compounds that target fungal Sec14 proteins, derived from several distinct chemical scaffolds, reveals exciting inroads into the rational design of next generation Sec14 inhibitors. Development of appropriately refined next generation Sec14-directed inhibitors promises to expand the chemical weaponry available for deployment in the shifting field of engagement between fungal pathogens and their human hosts.

Plasma membrane nano-organization specifies phosphoinositide effects on Rho-GTPases and actin dynamics in tobacco pollen tubes

Pollen tube growth requires coordination of cytoskeletal dynamics and apical secretion. The regulatory phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) is enriched in the subapical plasma membrane of pollen tubes of Arabidopsis thaliana and tobacco (Nicotiana tabacum) and can influence both actin dynamics and secretion. How alternative PtdIns(4,5)P2 effects are specified is unclear. In tobacco pollen tubes, spinning disc microscopy (SD) reveals dual distribution of a fluorescent PtdIns(4,5)P2-reporter in dynamic plasma membrane nanodomains vs. apparent diffuse membrane labeling, consistent with spatially distinct coexisting pools of PtdIns(4,5)P2. Several PI4P 5-kinases (PIP5Ks) can generate PtdIns(4,5)P2 in pollen tubes. Despite localizing to one membrane region, the PIP5Ks AtPIP5K2-EYFP and NtPIP5K6-EYFP display distinctive overexpression effects on cell morphologies, respectively related to altered actin dynamics or membrane trafficking. When analyzed by SD, AtPIP5K2-EYFP associated with nanodomains, whereas NtPIP5K6-EYFP localized diffusely. Chimeric AtPIP5K2-EYFP and NtPIP5K6-EYFP variants with reciprocally swapped membrane-associating domains evoked reciprocally shifted effects on cell morphology upon overexpression. Overall, active PI4P 5-kinase variants stabilized actin when targeted to nanodomains, suggesting a role of nanodomain-associated PtdIns(4,5)P2 in actin regulation. This notion is further supported by interaction and proximity of nanodomain-associated AtPIP5K2 with the Rho-GTPase NtRac5, and by its functional interplay with elements of Rho of plants signaling. Plasma membrane nano-organization may thus aid the specification of PtdIns(4,5)P2 functions to coordinate cytoskeletal dynamics and secretion.

Attracted to membranes: lipid-binding domains in plants

Membranes are essential for cells and organelles to function. As membranes are impermeable to most polar and charged molecules, they provide electrochemical energy to transport molecules across and create compartmentalized microenvironments for specific enzymatic and cellular processes. Membranes are also responsible for guided transport of cargoes between organelles and during endo- and exocytosis. In addition, membranes play key roles in cell signaling by hosting receptors and signal transducers and as substrates and products of lipid second messengers. Anionic lipids and their specific interaction with target proteins play an essential role in these processes, which are facilitated by specific lipid-binding domains. Protein crystallography, lipid-binding studies, subcellular localization analyses, and computer modeling have greatly advanced our knowledge over the years of how these domains achieve precision binding and what their function is in signaling and membrane trafficking, as well as in plant development and stress acclimation.

Anionic phospholipid gradients: an uncharacterized frontier of the plant endomembrane network

Anionic phospholipids include phosphatidic acid (PA), phosphatidylserine (PS), phosphatidylinositol (PI), and its phosphorylated derivatives the phosphoinositides (e.g. phosphatidylinositol-4-phosphate [PI4P] and phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2]). Although anionic phospholipids are low-abundant lipids, they are particularly important for membrane functions. In particular, anionic lipids act as biochemical and biophysical landmarks that contribute to the establishment of membrane identity, signaling activities, and compartment morphodynamics. Each anionic lipid accumulates in different endomembranes according to a unique subcellular pattern, where they locally provide docking platforms for proteins. As such, they are mostly believed to act in the compartments in which they accumulate. However, mounting evidence throughout eukaryotes suggests that anionic lipids are not as compartment-specific as initially thought and that they are instead organized as concentration gradients across different organelles. In this update, we review the evidence for the existence of anionic lipid gradients in plants. We then discuss the possible implication of these gradients in lipid dynamics and homeostasis, and also in coordinating subcellular activities. Finally, we introduce the notion that anionic lipid gradients at the cellular scale may translate into gradients at the tissue level, which could have implications for plant development.

Interactions between calcium and ABA signaling pathways in the regulation of fruit ripening

Fruit ripening and senescence are finely controlled by plant hormones such as ethylene and abscisic acid (ABA) but also by calcium ions and by calcium-dependent signaling pathways. Although there are extensive data supporting an interaction between ethylene and calcium in fruit ripening, the regulatory mechanisms resulting from the interaction between ABA and calcium have not yet been fully clarified. In this article, we have reviewed the full roles of calcium and its sensors (CaM, CMLs, CDPKs, CBLs) as well as ABA and the interactions between the two signaling pathways in the regulation of stress responses and in fruit ripening. To illustrate the possible interaction between calcium sensors and ABA signaling components in the control of fruit ripening, we propose an interaction model between the calcium and ABA signaling pathways.

A phosphoinositide 5-phosphatase from Solanum tuberosum is activated by PAMP-treatment and may antagonize phosphatidylinositol 4,5-bisphosphate at Phytophthora infestans infection sites

Potato (Solanum tuberosum) plants susceptible to late blight disease caused by the oomycete Phytophthora infestans display enhanced resistance upon infiltration with the pathogen-associated molecular pattern (PAMP), Pep-13. Here, we characterize a potato gene similar to Arabidopsis 5-phosphatases which was identified in transcript arrays performed to identify Pep-13 regulated genes, and termed StIPP. Recombinant StIPP protein specifically dephosphorylated the D5-position of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂ ) in vitro. Other phosphoinositides or soluble inositolpolyphosphates were not converted. When transiently expressed in tobacco (Nicotiana tabacum) pollen tubes, a StIPP-YFP fusion localized to the subapical plasma membrane and antagonized PtdIns(4,5)P₂ -dependent effects on cell morphology, indicating in vivo functionality. Phytophthora infestans-infection of N. benthamiana leaf epidermis cells resulted in relocalization of StIPP-GFP from the plasma membrane to the extra-haustorial membrane (EHM). Colocalizion with the effector protein RFP-AvrBlb2 at infection sites is consistent with a role of StIPP in the plant-oomycete interaction. Correlation analysis of fluorescence distributions of StIPP-GFP and biosensors for PtdIns(4,5)P₂ or phosphatidylinositol 4-phosphate (PtdIns4P) indicate StIPP activity predominantly at the EHM. In Arabidopsis protoplasts, expression of StIPP resulted in the stabilization of the PAMP receptor, FLAGELLIN-SENSITIVE 2, indicating that StIPP may act as a PAMP-induced and localized antagonist of PtdIns(4,5)P₂ -dependent processes during plant immunity.

Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

Cellular membranes are critical platforms for intracellular signaling that involve complex interfaces between lipids and proteins, and a web of interactions between a multitude of lipid metabolic pathways. Membrane lipids impart structural and functional information in this regulatory circuit that encompass biophysical parameters such as membrane thickness and fluidity, as well as chaperoning the interactions of protein binding partners. Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play key roles in intracellular membrane signaling, and these involvements are translated into an impressively diverse set of biological outcomes. The phosphatidylinositol transfer proteins (PITPs) are key regulators of phosphoinositide signaling. Found in a diverse array of organisms from plants, yeast and apicomplexan parasites to mammals, PITPs were initially proposed to be simple transporters of lipids between intracellular membranes. It now appears increasingly unlikely that the soluble versions of these proteins perform such functions within the cell. Rather, these serve to facilitate the activity of intrinsically biologically insufficient inositol lipid kinases and, in so doing, promote diversification of the biological outcomes of phosphoinositide signaling. The central engine for execution of such functions is the lipid exchange cycle that is a fundamental property of PITPs. How PITPs execute lipid exchange remains very poorly understood. Molecular dynamics simulation approaches are now providing the first atomistic insights into how PITPs, and potentially other lipid-exchange/transfer proteins, operate.

Phylogenetic analysis of plant multi-domain SEC14-like phosphatidylinositol transfer proteins and structure-function properties of PATELLIN2

SEC14L-PITPs guide membrane recognition and signaling. An increasingly complex modular structure of SEC14L-PITPs evolved in land plants compared to green algae. SEC14/CRAL-TRIO and GOLD domains govern membrane binding specificity. SEC14-like phosphatidylinositol transfer proteins (SEC14L-PITPs) provide cues for membrane identity by exchanging lipophilic substrates, ultimately governing membrane signaling. Flowering plant SEC14L-PITPs often have modular structure and are associated with cell division, development, and stress responses. Yet, structure-function relationships for biochemical-cellular interactions of SEC14L-PITPs are rather enigmatic. Here, we evaluate the phylogenetic relationships of the SEC14L-PITP superfamily in the green lineage. Compared to green algae, land plants have an extended set of SEC14L-PITPs with increasingly complex modular structure. SEC14-GOLD PITPs, present in land plants but not Chara, diverged to three functional subgroups, represented by the six PATELLIN (PATL) proteins in Arabidopsis. Based on the example of Arabidopsis PATL2, we dissect the functional domains for in vitro binding to phosphoinositides and liposomes and for plant cell membrane association. While the SEC14 domain and its CRAL-TRIO-N-terminal extension serve general membrane attachment of the protein, the C-terminal GOLD domain directs it to the plasma membrane by recognizing specific phosphoinositides. We discuss that the different domains of SEC14L-PITPs integrate developmental and environmental signals to control SEC14L-PITP-mediated membrane identity, important to initiate dynamic membrane events.

A Sec14-like phosphatidylinositol transfer protein paralog defines a novel class of heme-binding proteins

Yeast Sfh5 is an unusual member of the Sec14-like phosphatidylinositol transfer protein (PITP) family. Whereas PITPs are defined by their abilities to transfer phosphatidylinositol between membranes in vitro, and to stimulate phosphoinositide signaling in vivo, Sfh5 does not exhibit these activities. Rather, Sfh5 is a redox-active penta-coordinate high spin Fe^III hemoprotein with an unusual heme-binding arrangement that involves a co-axial tyrosine/histidine coordination strategy and a complex electronic structure connecting the open shell iron d-orbitals with three aromatic ring systems. That Sfh5 is not a PITP is supported by demonstrations that heme is not a readily exchangeable ligand, and that phosphatidylinositol-exchange activity is resuscitated in heme binding-deficient Sfh5 mutants. The collective data identify Sfh5 as the prototype of a new class of fungal hemoproteins, and emphasize the versatility of the Sec14-fold as scaffold for translating the binding of chemically distinct ligands to the control of diverse sets of cellular activities.

Functions of Anionic Lipids in Plants

Anionic phospholipids, which include phosphatidic acid, phosphatidylserine, and phosphoinositides, represent a small percentage of membrane lipids. They are able to modulate the physical properties of membranes, such as their surface charges, curvature, or clustering of proteins. Moreover, by mediating interactions with numerous membrane-associated proteins, they are key components in the establishment of organelle identity and dynamics. Finally, anionic lipids also act as signaling molecules, as they are rapidly produced or interconverted by a set of dedicated enzymes. As such, anionic lipids are major regulators of many fundamental cellular processes, including cell signaling, cell division, membrane trafficking, cell growth, and gene expression. In this review, we describe the functions of anionic lipids from a cellular perspective. Using the localization of each anionic lipid and its related metabolic enzymes as starting points, we summarize their roles within the different compartments of the endomembrane system and address their associated developmental and physiological consequences.

Phospholipids across scales: lipid patterns and plant development

Phospholipids are major building blocks of cell membranes and as such they have a key structural role in maintaining their integrity as a hydrophobic barrier. However, phospholipids not only have structural but also regulatory functions that are involved in a myriad of signaling pathways. Integrative approaches in plants recently revealed that certain phospholipids have distinct patterns of accumulation at the tissue or organ scales, which turned out to be important cues in a developmental context. Using examples on different phospholipid classes, including phosphatidylinositol-4,5-bisphosphate, phosphatidylserine, phosphatidylcholine, and phosphatidic acid, we review how spatio-temporal lipid patterns arise at the organismal level and what are their downstream consequences on plant development.

Functional diversification of the chemical landscapes of yeast Sec14-like phosphatidylinositol transfer protein lipid-binding cavities

Phosphatidylinositol-transfer proteins (PITPs) are key regulators of lipid signaling in eukaryotic cells. These proteins both potentiate the activities of phosphatidylinositol (PtdIns) 4-OH kinases and help channel production of specific pools of phosphatidylinositol 4-phosphate (PtdIns(4)P) dedicated to specific biological outcomes. In this manner, PITPs represent a major contributor to the mechanisms by which the biological outcomes of phosphoinositide are diversified. The two-ligand priming model proposes that the engine by which Sec14-like PITPs potentiate PtdIns kinase activities is a heterotypic lipid-exchange cycle where PtdIns is a common exchange substrate among the Sec14-like PITP family, but the second exchange ligand varies with the PITP. A major prediction of this model is that second-exchangeable ligand identity will vary from PITP to PITP. To address the heterogeneity in the second exchange ligand for Sec14-like PITPs, we used structural, computational, and biochemical approaches to probe the diversities of the lipid-binding cavity microenvironments of the yeast Sec14-like PITPs. The collective data report that yeast Sec14-like PITP lipid-binding pockets indeed define diverse chemical microenvironments that translate into differential ligand-binding specificities across this protein family.

Arabidopsis PCaP2 modulates the phosphatidylinositol 4,5-bisphosphate signal on the plasma membrane and attenuates root hair elongation

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂ ] serves as a subcellular signal on the plasma membrane, mediating various cell-polarized phenomena including polar cell growth. Here, we investigated the involvement of Arabidopsis thaliana PCaP2, a plant-unique plasma membrane protein with phosphoinositide-binding activity, in PtdIns(4,5)P₂ signaling for root hair tip growth. The long-root-hair phenotype of the pcap2 knockdown mutant was found to stem from its higher average root hair elongation rate compared with the wild type and to counteract the low average rate caused by a defect in the PtdIns(4,5)P₂ -producing enzyme gene PIP5K3. On the plasma membrane of elongating root hairs, the PCaP2 promoter-driven PCaP2-green fluorescent protein (GFP), which complemented the pcap2 mutant phenotype, overlapped with the PtdIns(4,5)P₂ marker 2xCHERRY-2xPH^PLC in the subapical region, but not at the apex, suggesting that PCaP2 attenuates root hair elongation via PtdIns(4,5)P₂ signaling on the subapical plasma membrane. Consistent with this, a GFP fusion with the PCaP2 phosphoinositide-binding domain PCaP2^N23 , root hair-specific overexpression of which caused a low average root hair elongation rate, localized more intense to the subapical plasma membrane than to the apical plasma membrane similar to PCaP2-GFP. Inducibly overexpressed PCaP2-GFP, but not its derivative lacking the PCaP2^N23 domain, replaced 2xCHERRY-2xPH^PLC on the plasma membrane in root meristematic epidermal cells, and suppressed FM4-64 internalization in elongating root hairs. Moreover, inducibly overexpressed PCaP2 arrested an endocytic process of PIN2-GFP recycling. Based on these results, we conclude that PCaP2 functions as a negative modulator of PtdIns(4,5)P₂ signaling on the subapical plasma membrane probably through competitive binding to PtdIns(4,5)P₂ and attenuates root hair elongation.

Functional characterization of genes mediating cell wall metabolism and responses to plant cell wall integrity impairment

BACKGROUND

Plant cell walls participate in all plant-environment interactions. Maintaining cell wall integrity (CWI) during these interactions is essential. This realization led to increased interest in CWI and resulted in knowledge regarding early perception and signalling mechanisms active during CWI maintenance. By contrast, knowledge regarding processes mediating changes in cell wall metabolism upon CWI impairment is very limited.

RESULTS

To identify genes involved and to investigate their contributions to the processes we selected 23 genes with altered expression in response to CWI impairment and characterized the impact of T-DNA insertions in these genes on cell wall composition using Fourier-Transform Infrared Spectroscopy (FTIR) in Arabidopsis thaliana seedlings. Insertions in 14 genes led to cell wall phenotypes detectable by FTIR. A detailed analysis of four genes found that their altered expression upon CWI impairment is dependent on THE1 activity, a key component of CWI maintenance. Phenotypic characterizations of insertion lines suggest that the four genes are required for particular aspects of CWI maintenance, cell wall composition or resistance to Plectosphaerella cucumerina infection in adult plants.

CONCLUSION

Taken together, the results implicate the genes in responses to CWI impairment, cell wall metabolism and/or pathogen defence, thus identifying new molecular components and processes relevant for CWI maintenance.

Genome-wide identification, phylogeny, and expression analysis of Sec14-like PITP gene family in sugarcane

Six Sec14-like PITP genes from sugarcane were identified, two of them were cloned, and their biological functions were characterized indicating their involvement in plant defense against biotic and abiotic stresses. Sec14, a phosphatidylinositol transfer protein (PITP) is widely present in eukaryotes. In this study, the structure and expression patterns of six Sec14-like PITP genes (ScSEC14-1, ScSEC14p, ScSFH1, ScSFH2, ScPATL1, and ScPATL2) from sugarcane were analyzed, and two of them (ScSEC14-1 and ScSEC14p) were cloned and functionally verified. Phylogenetic analysis divided these genes into four groups, including group I (ScSFH1 and ScSFH2), group II (ScPATL1 and ScPATL2), Group III (ScSEC14p), and group V (ScSEC14-1). qRT-PCR analysis showed tissue-specific expression of these genes, primarily in the root, leaf, and bud tissues. They responded differently to SA, MeJA, and ABA stresses. ScSEC14-1, ScSEC14p, and ScSFH2 were upregulated by CuCl₂ and CdCl₂, while ScSEC14-1, ScSFH1, ScSFH2, and ScPATL1 were upregulated by PEG and NaCl. When infected by Sporisorium scitamineum, the transcripts of ScSFH1, ScSFH2, ScPATL1, and ScPATL2 were upregulated in the resistant genotype Yacheng 05-179, while those of ScSEC14-1 and ScSEC14p were upregulated in the susceptible genotype ROC22. Subcellular localization showed that ScSEC14-1 and ScSEC14p were mainly localized in the plasma membrane and cytoplasm. Enhanced growth of Escherichia coli BL21 cells expressing ScSEC14-1 and ScSEC14p showed high tolerance to NaCl and mannitol stresses. The transient overexpression of ScSEC14-1 and ScSEC14p in Nicotiana benthamiana leaves enhanced its resistance to the infection of tobacco pathogens Ralstonia solanacearum and Fusarium solani var. coeruleum. We can conclude the involvement of ScSEC14-1 and ScSEC14p in the defense against biotic and abiotic stresses, which should facilitate further research on Sec14-like PITP gene family, especially its regulatory mechanisms in sugarcane.

An equal opportunity collaboration between lipid metabolism and proteins in the control of membrane trafficking in the trans-Golgi and endosomal systems

Recent years have witnessed the evolution of the cell biology of lipids into an extremely active area of investigation. Deciphering the involvement of lipid metabolism and lipid signaling in membrane trafficking pathways defines a major nexus of contemporary experimental activity on this front. Significant effort in that direction is invested in understanding the trans-Golgi network/endosomal system where unambiguous connections between membrane trafficking and inositol lipid and phosphatidylcholine metabolism were first discovered. However, powered by new advances in contemporary cell biology, the march of science is rapidly expanding that window of inquiry to include ever more diverse arms of the lipid metabolome, and to include other compartments of the secretory pathway as well.

Patellin protein family functions in plant development and stress response

The plant patellin (PATL) proteins are yeast Sec14 protein (Sec14p)-like phosphatidylinositol transfer proteins (PITPs), which are widely distributed across the plant kingdom. The model plant Arabidopsis has six PATL members (designated as PATL1-PATL6). Accumulated evidence has indicated the involvement of Arabidopsis PATLs in various biological processes. This mini-review briefly summarizes our current knowledge on individual PATLs regarding their roles in plant development and stress tolerance regulation. The elucidation of PATLs' biological function in plants will provide new insights on plant membrane trafficking and its regulatory roles in either plant growth or environmental stress response signaling networks.

The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes

Phosphoinositides are key regulators of a large number of diverse cellular processes that include membrane trafficking, plasma membrane receptor signaling, cell proliferation, and transcription. How a small number of chemically distinct phosphoinositide signals are functionally amplified to exert specific control over such a diverse set of biological outcomes remains incompletely understood. To this end, a novel mechanism is now taking shape, and it involves phosphatidylinositol (PtdIns) transfer proteins (PITPs). The concept that PITPs exert instructive regulation of PtdIns 4-OH kinase activities and thereby channel phosphoinositide production to specific biological outcomes, identifies PITPs as central factors in the diversification of phosphoinositide signaling. There are two evolutionarily distinct families of PITPs: the Sec14-like and the StAR-related lipid transfer domain (START)-like families. Of these two families, the START-like PITPs are the least understood. Herein, we review recent insights into the biochemical, cellular, and physiological function of both PITP families with greater emphasis on the START-like PITPs, and we discuss the underlying mechanisms through which these proteins regulate phosphoinositide signaling and how these actions translate to human health and disease.

A Golgi Lipid Signaling Pathway Controls Apical Golgi Distribution and Cell Polarity during Neurogenesis

Phosphatidylinositol (PtdIns) transfer proteins (PITPs) stimulate PtdIns-4-P synthesis and signaling in eukaryotic cells, but to what biological outcomes such signaling circuits are coupled remains unclear. Herein, we show that two highly related StART-like PITPs, PITPNA and PITPNB, act in a redundant fashion to support development of the embryonic mammalian neocortex. PITPNA/PITPNB do so by driving PtdIns-4-P-dependent recruitment of GOLPH3, and likely ceramide transfer protein (CERT), to Golgi membranes with GOLPH3 recruitment serving to promote MYO18A- and F-actin-directed loading of the Golgi network to apical processes of neural stem cells (NSCs). We propose the primary role for PITP/PtdIns-4-P/GOLPH3/CERT signaling in NSC Golgi is not in regulating bulk membrane trafficking but in optimizing apically directed membrane trafficking and/or apical membrane signaling during neurogenesis.

PATELLINS are regulators of auxin-mediated PIN1 relocation and plant development in Arabidopsis thaliana

Coordinated cell polarization in developing tissues is a recurrent theme in multicellular organisms. In plants, a directional distribution of the plant hormone auxin is at the core of many developmental programs. A feedback regulation of auxin on the polarized localization of PIN auxin transporters in individual cells has been proposed as a self-organizing mechanism for coordinated tissue polarization, but the molecular mechanisms linking auxin signalling to PIN-dependent auxin transport remain unknown. We used a microarray-based approach to find regulators of the auxin-induced PIN relocation in Arabidopsis thaliana root, and identified a subset of a family of phosphatidylinositol transfer proteins (PITPs), the PATELLINs (PATLs). Here, we show that PATLs are expressed in partially overlapping cell types in different tissues going through mitosis or initiating differentiation programs. PATLs are plasma membrane-associated proteins accumulated in Arabidopsis embryos, primary roots, lateral root primordia and developing stomata. Higher order patl mutants display reduced PIN1 repolarization in response to auxin, shorter root apical meristem, and drastic defects in embryo and seedling development. This suggests that PATLs play a redundant and crucial role in polarity and patterning in Arabidopsis.

Genome-Wide Transcriptional Changes and Lipid Profile Modifications Induced by Medicago truncatula N5 Overexpression at an Early Stage of the Symbiotic Interaction with Sinorhizobium meliloti

Plant lipid-transfer proteins (LTPs) are small basic secreted proteins, which are characterized by lipid-binding capacity and are putatively involved in lipid trafficking. LTPs play a role in several biological processes, including the root nodule symbiosis. In this regard, the Medicago truncatula nodulin 5 (MtN5) LTP has been proved to positively regulate the nodulation capacity, controlling rhizobial infection and nodule primordia invasion. To better define the lipid transfer protein MtN5 function during the symbiosis, we produced MtN5-downregulated and -overexpressing plants, and we analysed the transcriptomic changes occurring in the roots at an early stage of Sinorhizobium meliloti infection. We also carried out the lipid profile analysis of wild type (WT) and MtN5-overexpressing roots after rhizobia infection. The downregulation of MtN5 increased the root hair curling, an early event of rhizobia infection, and concomitantly induced changes in the expression of defence-related genes. On the other hand, MtN5 overexpression favoured the invasion of the nodules by rhizobia and determined in the roots the modulation of genes that are involved in lipid transport and metabolism as well as an increased content of lipids, especially galactolipids that characterize the symbiosome membranes. Our findings suggest the potential participation of LTPs in the synthesis and rearrangement of membranes occurring during the formation of the infection threads and the symbiosome membrane.

MAPKs Influence Pollen Tube Growth by Controlling the Formation of Phosphatidylinositol 4,5-Bisphosphate in an Apical Plasma Membrane Domain

An apical plasma membrane domain enriched in the regulatory phospholipid phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] is critical for polar tip growth of pollen tubes. How the biosynthesis of PtdIns(4,5)P2 by phosphatidylinositol 4-phosphate 5-kinases (PI4P 5-kinases) is controlled by upstream signaling is currently unknown. The pollen-expressed PI4P 5-kinase PIP5K6 is required for clathrin-mediated endocytosis and polar tip growth in pollen tubes. Here, we identify PIP5K6 as a target of the pollen-expressed mitogen-activated protein kinase MPK6 and characterize the regulatory effects. Based on an untargeted mass spectrometry approach, phosphorylation of purified recombinant PIP5K6 by pollen tube extracts could be attributed to MPK6. Recombinant MPK6 phosphorylated residues T590 and T597 in the variable insert of the catalytic domain of PIP5K6, and this modification inhibited PIP5K6 activity in vitro. PIP5K6 interacted with MPK6 in yeast two-hybrid tests, immuno-pull-down assays, and by bimolecular fluorescence complementation at the apical plasma membrane of pollen tubes. In vivo, MPK6 expression resulted in reduced plasma membrane association of a fluorescent PtdIns(4,5)P2 reporter and decreased endocytosis without impairing membrane association of PIP5K6. Effects of PIP5K6 expression on pollen tube growth and cell morphology were attenuated by coexpression of MPK6 in a phosphosite-dependent manner. Our data indicate that MPK6 controls PtdIns(4,5)P2 production and membrane trafficking in pollen tubes, possibly contributing to directional growth.

Precision targeting by phosphoinositides: how PIs direct endomembrane trafficking in plants

Each phosphoinositide (PI, also known as phosphatidylinositol phosphate, polyphosphoinositide, PtdInsP or PIP) species is partitioned in the endomembrane system and thereby contributes to the identity of membrane compartments. However, membranes are in constant flux within this system, which raises the questions of how the spatiotemporal pattern of phosphoinositides is established and maintained within the cell. Here, we review the general mechanisms by which phosphoinositides and membrane trafficking feedbacks on each other to regulate cellular patterning. We then use the specific examples of polarized trafficking, endosomal sorting and vacuolar biogenesis to illustrate these general concepts.

The regulation of cell polarity by lipid transfer proteins of the SEC14 family

SEC14 lipid transfer proteins are important regulators of phospholipid metabolism. Structural, genetic and cell biological studies in yeast suggest that they help phosphatidylinositol (PtdIns)/phosphoinositide (PIP) kinases to overcome their intrinsic inefficiency to recognize membrane-embedded substrate, thereby playing a key role in PIP homeostasis. Genomes of higher plants encode a high number and diversity of SEC14 proteins, often in combination with other domains. The Arabidopsis SEC14-Nlj16 protein AtSFH1, an important regulator of root hair development, plays an important role in the establishment of PIP microdomains. Key to this mechanism is a highly specific interaction of the Nlj16 domain with PtdIns(4,5)P₂ and an interaction-triggered oligomerization of the protein. Nlj16/PtdIns(4,5)P₂ interaction depends on a polybasic motif similar to those identified in other regulatory proteins.

Phosphoinositide signaling in plant development

The membranes of eukaryotic cells create hydrophobic barriers that control substance and information exchange between the inside and outside of cells and between cellular compartments. Besides their roles as membrane building blocks, some membrane lipids, such as phosphoinositides (PIs), also exert regulatory effects. Indeed, emerging evidence indicates that PIs play crucial roles in controlling polarity and growth in plants. Here, I highlight the key roles of PIs as important regulatory membrane lipids in plant development and function.

Dynamics and energetics of the mammalian phosphatidylinositol transfer protein phospholipid exchange cycle

Phosphatidylinositol-transfer proteins (PITPs) regulate phosphoinositide signaling in eukaryotic cells. The defining feature of PITPs is their ability to exchange phosphatidylinositol (PtdIns) molecules between membranes, and this property is central to PITP-mediated regulation of lipid signaling. However, the details of the PITP-mediated lipid exchange cycle remain entirely obscure. Here, all-atom molecular dynamics simulations of the mammalian StART-like PtdIns/phosphatidylcholine (PtdCho) transfer protein PITPα, both on membrane bilayers and in solvated systems, informed downstream biochemical analyses that tested key aspects of the hypotheses generated by the molecular dynamics simulations. These studies provided five key insights into the PITPα lipid exchange cycle: (i) interaction of PITPα with the membrane is spontaneous and mediated by four specific protein substructures; (ii) the ability of PITPα to initiate closure around the PtdCho ligand is accompanied by loss of flexibility of two helix/loop regions, as well as of the C-terminal helix; (iii) the energy barrier of phospholipid extraction from the membrane is lowered by a network of hydrogen bonds between the lipid molecule and PITPα; (iv) the trajectory of PtdIns or PtdCho into and through the lipid-binding pocket is chaperoned by sets of PITPα residues conserved throughout the StART-like PITP family; and (v) conformational transitions in the C-terminal helix have specific functional involvements in PtdIns transfer activity. Taken together, these findings provide the first mechanistic description of key aspects of the PITPα PtdIns/PtdCho exchange cycle and offer a rationale for the high conservation of particular sets of residues across evolutionarily distant members of the metazoan StART-like PITP family.

In Vivo Imaging of Diacylglycerol at the Cytoplasmic Leaflet of Plant Membranes

Diacylglycerol (DAG) is an important intermediate in lipid biosynthesis and plays key roles in cell signaling, either as a second messenger itself or as a precursor of phosphatidic acid. Methods to identify distinct DAG pools have proven difficult because biochemical fractionation affects the pools, and concentrations are limiting. Here, we validate the use of a genetically encoded DAG biosensor in living plant cells. The sensor is composed of a fusion between yellow fluorescent protein and the C1a domain of protein kinase C (YFP-C1aPKC) that specifically binds DAG, and was stably expressed in suspension-cultured tobacco BY-2 cells and whole Arabidopsis thaliana plants. Confocal imaging revealed that the majority of the YFP-C1aPKC fluorescence did not locate to membranes but was present in the cytosol and nucleus. Treatment with short-chain DAG or PMA (phorbol-12-myristate-13-acetate), a phorbol ester that binds the C1a domain of PKC, caused the recruitment of the biosensor to the plasma membrane. These results indicate that the biosensor works and that the basal DAG concentration in the cytoplasmic leaflet of membranes (i.e. accessible to the biosensor) is in general too low, and confirms that the known pools in plastids, the endoplasmic reticulum and mitochondria are located at the luminal face of these compartments (i.e. inaccessible to the biosensor). Nevertheless, detailed further analysis of different cells and tissues discovered four novel DAG pools, namely at: (i) the trans-Golgi network; (ii) the cell plate during cytokinesis; (iii) the plasma membrane of root epidermal cells in the transition zone, and (iv) the apex of growing root hairs. The results provide new insights into the spatiotemporal dynamics of DAG in plants and offer a new tool to monitor this in vivo.

Distinctive features and differential regulation of the DRTS genes of Arabidopsis thaliana

In plants and protists, dihydrofolate reductase (DHFR) and thymidylate synthase (TS) are part of a bifunctional enzyme (DRTS) that allows efficient recycling of the dihydrofolate resulting from TS activity. Arabidopsis thaliana possesses three DRTS genes, called AtDRTS1, AtDRTS2 and AtDRTS3, that are located downstream of three members of the sec14-like SFH gene family. In this study, a characterization of the AtDRTS genes identified alternatively spliced transcripts coding for AtDRTS isoforms which may account for monofunctional DHFR enzymes supporting pathways unrelated to DNA synthesis. Moreover, we discovered a complex differential regulation of the AtDRTS genes that confirms the expected involvement of the AtDRTS genes in cell proliferation and endoreduplication, but indicates also functions related to other cellular activities. AtDRTS1 is widely expressed in both meristematic and differentiated tissues, whereas AtDRTS2 expression is almost exclusively limited to the apical meristems and AtDRTS3 is preferentially expressed in the shoot apex, in stipules and in root cap cells. The differential regulation of the AtDRTS genes is associated to distinctive promoter architectures and the expression of AtDRTS1 in the apical meristems is strictly dependent on the presence of an intragenic region that includes the second intron of the gene. Upon activation of cell proliferation in germinating seeds, the activity of the AtDRTS1 and AtDRTS2 promoters in meristematic cells appears to be maximal at the G1/S phase of the cell cycle. In addition, the promoters of AtDRTS2 and AtDRTS3 are negatively regulated through E2F cis-acting elements and both genes, but not AtDRTS1, are downregulated in plants overexpressing the AtE2Fa factor. Our study provides new information concerning the function and the regulation of plant DRTS genes and opens the way to further investigations addressing the importance of folate synthesis with respect to specific cellular activities.

Guilt by Association: A Phenotype-Based View of the Plant Phosphoinositide Network

Eukaryotic membranes contain small amounts of phospholipids that have regulatory effects on the physiological functions of cells, tissues, and organs. Phosphoinositides (PIs)-the phosphorylated derivatives of phosphatidylinositol-are one example of such regulatory lipids. Although PIs were described in plants decades ago, their contribution to the regulation of physiological processes in plants is not well understood. In the past few years, evidence has emerged that PIs are essential for plant function and development. Recently reported phenotypes associated with the perturbation of different PIs suggest that some subgroups of PIs influence specific processes. Although the molecular targets of PI-dependent regulation in plants are largely unknown, the effects of perturbed PI metabolism can be used to propose regulatory modules that involve particular downstream targets of PI regulation. This review summarizes phenotypes associated with the perturbation of the plant PI network to categorize functions and suggest possible downstream targets of plant PI regulation.

Genetic engineering of the biosynthesis of glycinebetaine leads to alleviate salt-induced potassium efflux and enhances salt tolerance in tomato plants

Tomato (Solanum lycopersicum cv. 'Moneymaker') was transformed with the choline oxidase gene codA from Arthrobacter globiformis, which was modified to allow for targeting to both chloroplasts and the cytosol. Glycine betaine (GB) was accumulated in transformed plants, while no detectable GB was found in wild-type (WT) plants. Compared to WT plants, transgenic lines showed significantly higher photosynthetic rates (Pn) and antioxidant enzyme activities and lower reactive oxygen species (ROS) accumulation in the leaves when exposed to salt stress. Furthermore, compared with WT plants, K⁺ efflux decreased and Na⁺ efflux increased in roots of transgenic plants under salt stress; resulted in lower Na⁺/K⁺ ratios in transgenic lines. The exogenous application of GB also significantly reduced NaCl-induced K⁺ efflux and increased Na⁺ efflux in WT plants. A qRT-PCR assay indicated that GB enhanced NaCl-induced expression of genes encoding the K⁺ transporter, Na⁺/H⁺ antiporter, and H⁺-ATPase. These results suggest that the enhanced salt tolerance conferred by codA in transgenic tomato plants might be due to the regulation of ion channel and transporters by GB, which would allow high potassium levels and low sodium levels to be maintained in transgenic plants under salt stress condition.

Arabidopsis PROTEIN S-ACYL TRANSFERASE4 mediates root hair growth

Polar growth of root hairs is critical for plant survival and requires fine-tuned Rho of plants (ROP) signaling. Multiple ROP regulators participate in root hair growth. However, protein S-acyl transferases (PATs), mediating the S-acylation and membrane partitioning of ROPs, are yet to be found. Using a reverse genetic approach, combining fluorescence probes, pharmacological drugs, site-directed mutagenesis and genetic analysis with related root-hair mutants, we have identified and characterized an Arabidopsis PAT, which may be responsible for ROP2 S-acylation in root hairs. Specifically, functional loss of PAT4 resulted in reduced root hair elongation, which was rescued by a wild-type but not an enzyme-inactive PAT4. Membrane-associated ROP2 was significantly reduced in pat4, similar to S-acylation-deficient ROP2 in the wild type. We further showed that PAT4 and SCN1, a ROP regulator, additively mediate the stability and targeting of ROP2. The results presented here indicate that PAT4-mediated S-acylation mediates the membrane association of ROP2 at the root hair apex and provide novel insights into dynamic ROP signaling during plant tip growth.

Isolation and functional characterization of a cold responsive phosphatidylinositol transfer-associated protein, ZmSEC14p, from maize (Zea may L.)

A Sec14-like protein, ZmSEC14p , from maize was structurally analyzed and functionally tested. Overexpression of ZmSEC14p in transgenic Arabidopsis conferred tolerance to cold stress. Sec14-like proteins are involved in essential biological processes, such as phospholipid metabolism, signal transduction, membrane trafficking, and stress response. Here, we reported a phosphatidylinositol transfer-associated protein, ZmSEC14p (accession no. KT932998), isolated from a cold-tolerant maize inbred line using the cDNA-AFLP approach and RACE-PCR method. Full-length cDNA that consisted of a single open reading frame (ORF) encoded a putative polypeptide of 295 amino acids. The ZmSEC14p protein was mainly localized in the nucleus, and its transcript was induced by cold, salt stresses, and abscisic acid (ABA) treatment in maize leaves and roots. Overexpression of ZmSEC14p in transgenic Arabidopsis conferred tolerance to cold stress. This tolerance was primarily displayed by the increased germination rate, root length, plant survival rate, accumulation of proline, activities of antioxidant enzymes, and the reduction of oxidative damage by reactive oxygen species (ROS). ZmSEC14p overexpression regulated the expression of phosphoinositide-specific phospholipase C, which cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) and generates second messengers (inositol 1,4,5-trisphosphate and 1,2-diacylglycerol) in the phosphoinositide signal transduction pathways. Moreover, up-regulation of some stress-responsive genes such as CBF3, COR6.6, and RD29B in transgenic plants under cold stress could be a possible mechanism for enhancing cold tolerance. Taken together, this study strongly suggests that ZmSEC14p plays an important role in plant tolerance to cold stress.

Identification of Phosphoinositide-Binding Protein PATELLIN2 as a Substrate of Arabidopsis MPK4 MAP Kinase during Septum Formation in Cytokinesis

The phosphorylation of proteins by protein kinases controls many cellular and physiological processes, which include intracellular signal transduction. However, the underlying molecular mechanisms of such controls and numerous substrates of protein kinases remain to be characterized. The mitogen-activated protein kinase (MAPK) cascade is of particular importance in a variety of extracellular and intracellular signaling processes. In plant cells, the progression of cytokinesis is an excellent example of an intracellular phenomenon that requires the MAPK cascade. However, the way in which MAPKs control downstream processes during cytokinesis in plant cells remains to be fully determined. We show here that comparisons, by two-dimensional difference gel electrophoresis, of phosphorylated proteins from wild-type Arabidopsis thaliana and mutant plants defective in a MAPK cascade allow identification of substrates of a specific MAPK. Using this method, we identified the PATELLIN2 (PATL2) protein, which has a SEC14 domain, as a substrate of MPK4 MAP kinase. PATL2 was concentrated at the cell division plane, as is MPK4, and had binding affinity for phosphoinositides. This binding affinity was altered after phosphorylation of PATL2 by MPK4, suggesting a role for the MAPK cascade in the formation of cell plates via regeneration of membranes during cytokinesis.

Male functions and malfunctions: the impact of phosphoinositides on pollen development and pollen tube growth

Phosphoinositides in pollen. In angiosperms, sexual reproduction is a series of complex biological events that facilitate the distribution of male generative cells for double fertilization. Angiosperms have no motile gametes, and the distribution units of generative cells are pollen grains, passively mobile desiccated structures, capable of delivering genetic material to compatible flowers over long distances and in an adverse environment. The development of pollen (male gametogenesis) and the formation of a pollen tube after a pollen grain has reached a compatible flower (pollen tube growth) are important aspects of plant developmental biology. In recent years, a wealth of information has been gathered about the molecular control of cell polarity, membrane trafficking and cytoskeletal dynamics underlying these developmental processes. In particular, it has been found that regulatory membrane phospholipids, such as phosphoinositides (PIs), are critical regulatory players, controlling key steps of trafficking and polarization. Characteristic features of PIs are the inositol phosphate headgroups of the lipids, which protrude from the cytosolic surfaces of membranes, enabling specific binding and recruitment of numerous protein partners containing specific PI-binding domains. Such recruitment is globally an early event in polarization processes of eukaryotic cells and also of key importance to pollen development and tube growth. Additionally, PIs serve as precursors of other signaling factors with importance to male gametogenesis. This review highlights the recent advances about the roles of PIs in pollen development and pollen function.

Two-ligand priming mechanism for potentiated phosphoinositide synthesis is an evolutionarily conserved feature of Sec14-like phosphatidylinositol and phosphatidylcholine exchange proteins

The two-ligand priming mechanism for stimulated phosphoinositide synthesis described for Saccharomyces Sec14 is also a conserved feature of Sec14-like phosphatidylinositol- and phosphatidylcholine-transfer proteins of the most evolutionarily advanced plants.

Lipid signaling, particularly phosphoinositide signaling, plays a key role in regulating the extreme polarized membrane growth that drives root hair development in plants. The Arabidopsis AtSFH1 gene encodes a two-domain protein with an amino-terminal Sec14-like phosphatidylinositol transfer protein (PITP) domain linked to a carboxy-terminal nodulin domain. AtSfh1 is critical for promoting the spatially highly organized phosphatidylinositol-4,5-bisphosphate signaling program required for establishment and maintenance of polarized root hair growth. Here we demonstrate that, like the yeast Sec14, the AtSfh1 PITP domain requires both its phosphatidylinositol (PtdIns)- and phosphatidylcholine (PtdCho)-binding properties to stimulate PtdIns-4-phosphate [PtdIns(4)P] synthesis. Moreover, we show that both phospholipid-binding activities are essential for AtSfh1 activity in supporting polarized root hair growth. Finally, we report genetic and biochemical evidence that the two-ligand mechanism for potentiation of PtdIns 4-OH kinase activity is a broadly conserved feature of plant Sec14-nodulin proteins, and that this strategy appeared only late in plant evolution. Taken together, the data indicate that the PtdIns/PtdCho-exchange mechanism for stimulated PtdIns(4)P synthesis either arose independently during evolution in yeast and in higher plants, or a suitable genetic module was introduced to higher plants from a fungal source and subsequently exploited by them.

Expression and characterization of a barley phosphatidylinositol transfer protein structurally homologous to the yeast Sec14p protein

Phosphatidylinositol transfer proteins (PITPs) include a large group of proteins implicated in the non-vesicular traffic of phosphatidylinositol (PI) between membranes. In yeast, the structure and function of the PITP Sec14-p protein have been well characterized. In contrast, the knowledge on plant PITP proteins is very scarce. In this work, we characterized a novel type of PITP protein in barley named HvSec14p and related to the yeast Sec14-p protein. Our data reveal that HvSec14p consists of only the Sec14p-domain structurally homologous to the yeast phosphoinositide binding domain. We show that HvSec14p expression is up-regulated at both transcript and protein levels at specific stages of development during seed formation and germination, and in leaves of a drought-tolerant barley genotype under osmotic constraints. Modeling analyses of the protein three-dimensional structure revealed its capacity to dock the phosphoinositides, PtdIns(3)P, PtdIns(4)P, PtdIns(5)P and PtdIns(3,5)P2. Consistently, the recombinant HvSec14p protein is able to bind in vitro most PIP types, the highest affinity being observed with PtdIns(3,5)P2. Based on the high gene expression at specific developmental stages and in drought-tolerant barley genotypes, we propose that HvSec14p plays essential roles in the biogenesis of membranes in expanding cells and in their preservation under osmotic stress conditions.