Different sensitivity to wortmannin of two vacuolar sorting signals indicates the presence of distinct sorting machineries in tobacco cells

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.

A century journey of organelles research in the plant endomembrane system

We are entering an exciting century in the study of the plant organelles in the endomembrane system. Over the past century, especially within the past 50 years, tremendous advancements have been made in the complex plant cell to generate a much clearer and informative picture of plant organelles, including the molecular/morphological features, dynamic/spatial behavior, and physiological functions. Importantly, all these discoveries and achievements in the identification and characterization of organelles in the endomembrane system would not have been possible without: (1) the innovations and timely applications of various state-of-art cell biology tools and technologies for organelle biology research; (2) the continuous efforts in developing and characterizing new organelle markers by the plant biology community; and (3) the landmark studies on the identification and characterization of the elusive organelles. While molecular aspects and results for individual organelles have been extensively reviewed, the development of the techniques for organelle research in plant cell biology is less appreciated. As one of the ASPB Centennial Reviews on "organelle biology," here we aim to take a journey across a century of organelle biology research in plants by highlighting the important tools (or landmark technologies) and key scientists that contributed to visualize organelles. We then highlight the landmark studies leading to the identification and characterization of individual organelles in the plant endomembrane systems.

Salicylic acid accelerates carbon starvation-induced leaf senescence in Arabidopsis thaliana by inhibiting autophagy through Nonexpressor of pathogenesis-related genes 1

In plants, leaf senescence is regulated by several factors, including age and carbon starvation. The molecular mechanism of age-regulated developmental leaf senescence differs from that of carbon starvation-induced senescence. Salicylic acid (SA) and Nonexpressor of pathogenesis-related genes 1 (NPR1) play important roles in promoting developmental leaf senescence. However, the relationship between SA signaling and carbon starvation-induced leaf senescence is not currently well understood. Here, we used Arabidopsis thaliana as material and found that carbon starvation-induced leaf senescence was accelerated in the SA dihydroxylase mutants s3hs5h compared to the Columbia ecotype (Col). Exogenous SA treatment significantly promoted carbon starvation-induced leaf senescence, especially in NPR1-GFP. Increasing the endogenous SA and overexpression of NPR1 inhibited carbon starvation-induced autophagy. However, mutation of NPR1 delayed carbon starvation-induced leaf senescence, increased autophagosome production and accelerated autophagic degradation of the Neighbor of BRCA1 gene 1 (NBR1). In conclusion, SA promotes carbon starvation-induced leaf senescence by inhibiting autophagy via NPR1.

Effects of Mycotoxin Fumagillin, Mevastatin, Radicicol, and Wortmannin on Photosynthesis of Chlamydomonas reinhardtii

Mycotoxins are one of the most important sources for the discovery of new pesticides and drugs because of their chemical structural diversity and fascinating bioactivity as well as unique novel targets. Here, the effects of four mycotoxins, fumagillin, mevastatin, radicicol, and wortmannin, on photosynthesis were investigated to identify their precise sites of action on the photosynthetic apparatus of Chlamydomonas reinhardtii. Our results showed that these four mycotoxins have multiple targets, acting mainly on photosystem II (PSII). Their mode of action is similar to that of diuron, inhibiting electron flow beyond the primary quinone electron acceptor (Q_A) by binding to the secondary quinone electron acceptor (Q_B) site of the D1 protein, thereby affecting photosynthesis. The results of PSII oxygen evolution rate and chlorophyll (Chl) a fluorescence imaging suggested that fumagillin strongly inhibited overall PSII activity; the other three toxins also exhibited a negative influence at the high concentration. Chl a fluorescence kinetics and the JIP test showed that the inhibition of electron transport beyond Q_A was the most significant feature of the four mycotoxins. Fumagillin decreased the rate of O₂ evolution by interrupting electron transfer on the PSII acceptor side, and had multiple negative effects on the primary photochemical reaction and PSII antenna size. Mevastatin caused a decrease in photosynthetic activity, mainly due to the inhibition of electron transport. Both radicicol and wortmannin decreased photosynthetic efficiency, mainly by inhibiting the electron transport efficiency of the PSII acceptor side and the activity of the PSII reaction centers. In addition, radicicol reduced the primary photochemical reaction efficiency and antenna size. The simulated molecular model of the four mycotoxins' binding to C. reinhardtii D1 protein indicated that the residue D1-Phe265 is their common site at the Q_B site. This is a novel target site different from those of commercial PSII herbicides. Thus, the interesting effects of the four mycotoxins on PSII suggested that they provide new ideas for the design of novel and efficient herbicide molecules.

Regulation of PIN-FORMED Protein Degradation

Auxin action largely depends on the establishment of auxin concentration gradient within plant organs, where PIN-formed (PIN) auxin transporter-mediated directional auxin movement plays an important role. Accumulating studies have revealed the need of polar plasma membrane (PM) localization of PIN proteins as well as regulation of PIN polarity in response to developmental cues and environmental stimuli, amongst which a typical example is regulation of PIN phosphorylation by AGCVIII protein kinases and type A regulatory subunits of PP2A phosphatases. Recent findings, however, highlight the importance of PIN degradation in reestablishing auxin gradient. Although the underlying mechanism is poorly understood, these findings provide a novel aspect to broaden the current knowledge on regulation of polar auxin transport. In this review, we summarize the current understanding on controlling PIN degradation by endosome-mediated vacuolar targeting, autophagy, ubiquitin modification and the related E3 ubiquitin ligases, cytoskeletons, plant hormones, environmental stimuli, and other regulators, and discuss the possible mechanisms according to recent studies.

Nanoparticle cellular internalization is not required for RNA delivery to mature plant leaves

Rapidly growing interest in the nanoparticle-mediated delivery of DNA and RNA to plants requires a better understanding of how nanoparticles and their cargoes translocate in plant tissues and into plant cells. However, little is known about how the size and shape of nanoparticles influence transport in plants and the delivery efficiency of their cargoes, limiting the development of nanotechnology in plant systems. In this study we employed non-biolistically delivered DNA-modified gold nanoparticles (AuNPs) of various sizes (5-20 nm) and shapes (spheres and rods) to systematically investigate their transport following infiltration into Nicotiana benthamiana leaves. Generally, smaller AuNPs demonstrated more rapid, higher and longer-lasting levels of association with plant cell walls compared with larger AuNPs. We observed internalization of rod-shaped but not spherical AuNPs into plant cells, yet, surprisingly, 10 nm spherical AuNPs functionalized with small-interfering RNA (siRNA) were the most efficient at siRNA delivery and inducing gene silencing in mature plant leaves. These results indicate the importance of nanoparticle size in efficient biomolecule delivery and, counterintuitively, demonstrate that efficient cargo delivery is possible and potentially optimal in the absence of nanoparticle cellular internalization. Overall, our results highlight nanoparticle features of importance for transport within plant tissues, providing a mechanistic overview of how nanoparticles can be designed to achieve efficacious biocargo delivery for future developments in plant nanobiotechnology.

Nanoparticle cellular internalization is not required for RNA delivery to mature plant leaves

Rapidly growing interest in the nanoparticle-mediated delivery of DNA and RNA to plants requires a better understanding of how nanoparticles and their cargoes translocate in plant tissues and into plant cells. However, little is known about how the size and shape of nanoparticles influence transport in plants and the delivery efficiency of their cargoes, limiting the development of nanotechnology in plant systems. In this study we employed non-biolistically delivered DNA-modified gold nanoparticles (AuNPs) of various sizes (5-20 nm) and shapes (spheres and rods) to systematically investigate their transport following infiltration into Nicotiana benthamiana leaves. Generally, smaller AuNPs demonstrated more rapid, higher and longer-lasting levels of association with plant cell walls compared with larger AuNPs. We observed internalization of rod-shaped but not spherical AuNPs into plant cells, yet, surprisingly, 10 nm spherical AuNPs functionalized with small-interfering RNA (siRNA) were the most efficient at siRNA delivery and inducing gene silencing in mature plant leaves. These results indicate the importance of nanoparticle size in efficient biomolecule delivery and, counterintuitively, demonstrate that efficient cargo delivery is possible and potentially optimal in the absence of nanoparticle cellular internalization. Overall, our results highlight nanoparticle features of importance for transport within plant tissues, providing a mechanistic overview of how nanoparticles can be designed to achieve efficacious biocargo delivery for future developments in plant nanobiotechnology.

Inhibition of Very Long Chain Fatty Acids Synthesis Mediates PI3P Homeostasis at Endosomal Compartments

A main characteristic of sphingolipids is the presence of a very long chain fatty acid (VLCFA) whose function in cellular processes is not yet fully understood. VLCFAs of sphingolipids are involved in the intracellular traffic to the vacuole and the maturation of early endosomes into late endosomes is one of the major pathways for vacuolar traffic. Additionally, the anionic phospholipid phosphatidylinositol-3-phosphate (PtdIns (3)P or PI3P) is involved in protein sorting and recruitment of small GTPase effectors at late endosomes/multivesicular bodies (MVBs) during vacuolar trafficking. In contrast to animal cells, PI3P mainly localizes to late endosomes in plant cells and to a minor extent to a discrete sub-domain of the plant's early endosome (EE)/trans-Golgi network (TGN) where the endosomal maturation occurs. However, the mechanisms that control the relative levels of PI3P between TGN and MVBs are unknown. Using metazachlor, an inhibitor of VLCFA synthesis, we found that VLCFAs are involved in the TGN/MVB distribution of PI3P. This effect is independent from either synthesis of PI3P by PI3-kinase or degradation of PI(3,5)P₂ into PI3P by the SUPPRESSOR OF ACTIN1 (SAC1) phosphatase. Using high-resolution live cell imaging microscopy, we detected transient associations between TGNs and MVBs but VLCFAs are not involved in those interactions. Nonetheless, our results suggest that PI3P might be transferable from TGN to MVBs and that VLCFAs act in this process.

Cellular internalization mechanism of novel Raman probes designed for plant cells

Diphenylacetylene derivatives containing different polymeric components, poly(l-lysine) (pLys) or tetra(ethylene glycol) (TEG) were designed as novel Raman imaging probes with high Raman sensitivity and low cytotoxicity in living plant cells. The pLys-conjugated probe is internalized via an endocytosis-dependent pathway, whereas TEG-conjugated probe most likely induces direct penetration into the plant cells.

Diphenyl acetylene derivatives containing various polymeric components have been designed as new Raman imaging probes. These are taken up by plant cells via different pathways, and the internalization of exogenous molecules can be visualized.

Subcellular Localization of PI3P in Arabidopsis

Phosphatidylinositol-3-phosphate (PI3P) is a signaling phospholipid enriched in the membranes of late endosomes (LE) and vacuoles. PI3P mediates vacuolar and endosomal trafficking through recruiting PI3P-binding effector proteins to the LE. PI3P is produced from phosphatidylinositol by the PI 3-kinase complex containing VACUOLAR PROTEIN SORTING 34 (VPS34). The role of PI3P has been elucidated by using genetically encoded PI3P biosensors. We previously showed that Arabidopsis VPS38, a component of the VPS34 complex, localized at the LE and that VPS38 is essential for proper PI3P distribution in endosomal and vacuolar trafficking routes. In this chapter, we describe methods for microscopic imaging of PI3P using the PI3P biosensor citrine-2 × FYVE and the PI 3-kinase inhibitors.

Specific Recruitment of Phosphoinositide Species to the Plant-Pathogen Interfacial Membrane Underlies Arabidopsis Susceptibility to Fungal Infection

Different phosphoinositides enriched at the membranes of specific subcellular compartments within plant cells contribute to organelle identity, ensuring appropriate cellular trafficking and function. During the infection of plant cells, biotrophic pathogens such as powdery mildews enter plant cells and differentiate into haustoria. Each haustorium is enveloped by an extrahaustorial membrane (EHM) derived from the host plasma membrane. Little is known about the EHM biogenesis and identity. Here, we demonstrate that among the two plasma membrane phosphoinositides in Arabidopsis (Arabidopsis thaliana), PI(4,5)P2 is dynamically up-regulated at powdery mildew infection sites and recruited to the EHM, whereas PI4P is absent in the EHM. Lateral transport of PI(4,5)P2 into the EHM occurs through a brefeldin A-insensitive but actin-dependent trafficking pathway. Furthermore, the lower levels of PI(4,5)P2 in pip5k1 pip5k2 mutants inhibit fungal pathogen development and cause disease resistance, independent of cell death-associated defenses and involving impaired host susceptibility. Our results reveal that plant biotrophic and hemibiotrophic pathogens modulate the subcellular distribution of host phosphoinositides and recruit PI(4,5)P2 as a susceptibility factor for plant disease.

MTV proteins unveil ER- and microtubule-associated compartments in the plant vacuolar trafficking pathway

The factors and mechanisms involved in vacuolar transport in plants, and in particular those directing vesicles to their target endomembrane compartment, remain largely unknown. To identify components of the vacuolar trafficking machinery, we searched for Arabidopsis modified transport to the vacuole (mtv) mutants that abnormally secrete the synthetic vacuolar cargo VAC2. We report here on the identification of 17 mtv mutations, corresponding to mutant alleles of MTV2/VSR4, MTV3/PTEN2A MTV7/EREL1, MTV8/ARFC1, MTV9/PUF2, MTV10/VPS3, MTV11/VPS15, MTV12/GRV2, MTV14/GFS10, MTV15/BET11, MTV16/VPS51, MTV17/VPS54, and MTV18/VSR1 Eight of the MTV proteins localize at the interface between the trans-Golgi network (TGN) and the multivesicular bodies (MVBs), supporting that the trafficking step between these compartments is essential for segregating vacuolar proteins from those destined for secretion. Importantly, the GARP tethering complex subunits MTV16/VPS51 and MTV17/VPS54 were found at endoplasmic reticulum (ER)- and microtubule-associated compartments (EMACs). Moreover, MTV16/VPS51 interacts with the motor domain of kinesins, suggesting that, in addition to tethering vesicles, the GARP complex may regulate the motors that transport them. Our findings unveil a previously uncharacterized compartment of the plant vacuolar trafficking pathway and support a role for microtubules and kinesins in GARP-dependent transport of soluble vacuolar cargo in plants.

The mysterious life of the plant trans-Golgi network: advances and tools to understand it better

By being at the interface of the exocytic and endocytic pathways, the plant trans-Golgi network (TGN) is a multitasking and highly diversified organelle. Despite governing vital cellular processes, the TGN remains one of the most uncharacterized organelle of plant cells. In this review, we highlight recent studies that have contributed new insights and to the generation of markers needed to answer several important questions on the plant TGN. Several drugs specifically affecting proteins critical for the TGN functions have been extremely useful for the identification of mutants of the TGN in the pursuit to understand how the morphology and the function of this organelle are controlled. In addition to these chemical tools, we review emerging microscopy techniques that help visualize the TGN at an unpreceded resolution and appreciate the heterogeneity and dynamics of this organelle in plant cells.

Structural and functional characterization of Solanum lycopersicum phosphatidylinositol 3-kinase C2 domain

Phosphatidylinositol 3-kinases (PI3Ks) are characterized by the presence of a C2 domain at the N-terminal end (class I, III); or at both the N-terminal and C-terminal ends (class II), sometimes including a Plextrin homology domain and/or a Ras domain. Plant PI3Ks are analogous to the class III mammalian PI3K. An N-terminal fragment (~170 aa) of the tomato PI3K regulatory domain including the C2 domain, was cloned and expressed in a bacterial system. This protein was purified to homogeneity and its physicochemical properties analyzed. The purified protein showed strong binding with monophosphorylated phosphatidylinositols, and the binding was dependent on calcium ion concentration and pH. In the overall tertiary structure of PI3K, C2 domain showed unique characteristics, having three antiparallel beta-sheets, hydrophobic regions, acidic as well as alkaline motifs, that can enable its membrane binding upon activation. To elucidate the functional significance of C2 domain, transgenic tobacco plants expressing the C2 domain of PI3K were generated. Transgenic plants showed defective pollen development and disrupted seed set. Flowers from the PI3K-C2 transgenic plants showed delayed wilting, and a decrease in ethylene production. It is likely that introduction of the PI3K-C2 segment may have interfered with the normal binding of PI3K to the membrane, delaying the onset of membrane lipid catabolism that lead to senescence.

Genetic Analyses of the Arabidopsis ATG1 Kinase Complex Reveal Both Kinase-Dependent and Independent Autophagic Routes during Fixed-Carbon Starvation

Under nutrient and energy-limiting conditions, plants up-regulate sophisticated catabolic pathways such as autophagy to remobilize nutrients and restore energy homeostasis. Autophagic flux is tightly regulated under these circumstances through the AuTophaGy-related1 (ATG1) kinase complex, which relays upstream nutrient and energy signals to the downstream components that drive autophagy. Here, we investigated the role(s) of the Arabidopsis (Arabidopsis thaliana) ATG1 kinase during autophagy through an analysis of a quadruple mutant deficient in all four ATG1 isoforms. These isoforms appear to act redundantly, including the plant-specific, truncated ATG1t variant, and like other well-characterized atg mutants, homozygous atg1abct quadruple mutants display early leaf senescence and hypersensitivity to nitrogen and fixed-carbon starvations. Although ATG1 kinase is essential for up-regulating autophagy under nitrogen deprivation and short-term carbon starvation, it did not stimulate autophagy under prolonged carbon starvation. Instead, an ATG1-independent response arose requiring phosphatidylinositol-3-phosphate kinase (PI3K) and SUCROSE NONFERMENTING1-RELATED PROTEIN KINASE1 (SnRK1), possibly through phosphorylation of the ATG6 subunit within the PI3K complex by the catalytic KIN10 subunit of SnRK1. Together, our data connect ATG1 kinase to autophagy and reveal that plants engage multiple pathways to activate autophagy during nutrient stress, which include the ATG1 route as well as an alternative route requiring SnRK1 and ATG6 signaling.plantcell;31/12/2973/FX1F1fx1.

PI4KIIIβ Activity Regulates Lateral Root Formation Driven by Endocytic Trafficking to the Vacuole

Lateral roots (LRs) increase the contact area of the root with the rhizosphere and thereby improve water and nutrient uptake from the soil. LRs are generated either via a developmentally controlled mechanism or through induction by external stimuli, such as water and nutrient availability. Auxin regulates LR organogenesis via transcriptional activation by an auxin complex receptor. Endocytic trafficking to the vacuole positively regulates LR organogenesis independently of the auxin complex receptor in Arabidopsis (Arabidopsis thaliana). Here, we demonstrate that phosphatidylinositol 4-phosphate (PI4P) biosynthesis regulated by the phosphatidylinositol 4-kinases PI4KIIIβ1 and PI4KIIIβ2 is essential for the LR organogenesis driven by endocytic trafficking to the vacuole. Stimulation with Sortin2, a biomodulator that promotes protein targeting to the vacuole, altered PI4P abundance at both the plasma membrane and endosomal compartments, a process dependent on PI4K activity. These findings suggest that endocytic trafficking to the vacuole regulated by the enzymatic activities of PI4KIIIβ1 and PI4KIIIβ2 participates in a mechanism independent of the auxin complex receptor that regulates LR organogenesis in Arabidopsis. Surprisingly, loss-of-function of PI4KIIIβ1 and PI4KIIIβ2 induced both LR primordium formation and endocytic trafficking toward the vacuole. This LR primordium induction was alleviated by exogenous PI4P, suggesting that PI4KIIIβ1 and PI4KIIIβ2 activity constitutively negatively regulates LR primordium formation. Overall, this research demonstrates a dual role of PI4KIIIβ1 and PI4KIIIβ2 in LR primordium formation in Arabidopsis.

How phosphoinositides shape autophagy in plant cells

Plant cells use autophagy to degrade their own cytoplasm in vacuoles, thereby not only recycling their breakdown products, but also ensuring the homeostasis of essential cytoplasmic constituents and organelles. Plants and other eukaryotes have a conserved set of core Autophagy-related (ATG) genes involved in the biogenesis of the autophagosome, the main autophagic compartment destined for the lytic vacuole. In the past decade, the core ATG genes were isolated from several plant species. The core ATG proteins include the components of the VACUOLAR PROTEIN SORTING 34 (VPS34) complex that is responsible for the local production of phosphatidylinositol 3-phosphate (PI3P) at the site of autophagosome formation. Dissecting the roles of PI3P and its effectors in autophagy is challenging, because of the multi-faceted links between autophagosomal and endosomal systems. This review highlights recent studies on putative plant PI3P effectors involved in autophagosome dynamics. Molecular mechanisms underlying the requirement of PI3P for autophagosome biogenesis and trafficking are also discussed.

Autophagy: The Master of Bulk and Selective Recycling

Plants have evolved sophisticated mechanisms to recycle intracellular constituents, which are essential for developmental and metabolic transitions; for efficient nutrient reuse; and for the proper disposal of proteins, protein complexes, and even entire organelles that become obsolete or dysfunctional. One major route is autophagy, which employs specialized vesicles to encapsulate and deliver cytoplasmic material to the vacuole for breakdown. In the past decade, the mechanics of autophagy and the scores of components involved in autophagic vesicle assembly have been documented. Now emerging is the importance of dedicated receptors that help recruit appropriate cargo, which in many cases exploit ubiquitylation as a signal. Although operating at a low constitutive level in all plant cells, autophagy is upregulated during senescence and various environmental challenges and is essential for proper nutrient allocation. Its importance to plant metabolism and energy balance in particular places autophagy at the nexus of robust crop performance, especially under suboptimal conditions.

Vacuole Integrity Maintained by DUF300 Proteins Is Required for Brassinosteroid Signaling Regulation

Brassinosteroid (BR) hormone signaling controls multiple processes during plant growth and development and is initiated at the plasma membrane through the receptor kinase BRASSINOSTEROID INSENSITIVE1 (BRI1) together with co-receptors such as BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1). BRI1 abundance is regulated by endosomal recycling and vacuolar targeting, but the role of vacuole-related proteins in BR receptor dynamics and BR responses remains elusive. Here, we show that the absence of two DUF300 domain-containing tonoplast proteins, LAZARUS1 (LAZ1) and LAZ1 HOMOLOG1 (LAZ1H1), causes vacuole morphology defects, growth inhibition, and constitutive activation of BR signaling. Intriguingly, tonoplast accumulation of BAK1 was substantially increased and appeared causally linked to enhanced BRI1 trafficking and degradation in laz1 laz1h1 plants. Since unrelated vacuole mutants exhibited normal BR responses, our findings indicate that DUF300 proteins play distinct roles in the regulation of BR signaling by maintaining vacuole integrity required to balance subcellular BAK1 pools and BR receptor distribution.

Vacuolar Trafficking Protein VPS38 Is Dispensable for Autophagy

Phosphatidylinositol 3-P (PI3P) is a signaling molecule that controls a variety of processes in endosomal, autophagic, and vacuolar/lysosomal trafficking in yeasts and mammals. Vacuolar protein sorting 34 (Vps34) is a conserved PI3K present in multiple complexes with specific functions and regulation. In yeast, the PI3K complex II consists of Vps34p, Vps15p, Vps30p/Atg6p, and Vps38p, and is essential for vacuolar protein sorting. Here, we describe the Arabidopsis (Arabidopsis thaliana) homolog of yeast Vps38p and human UV radiation resistance-associated gene protein. Arabidopsis VPS38 interacts with VPS30/ATG6 both in yeast and in planta. Although the level of PI3P in Arabidopsis vps38 mutants is similar to that in wild type, vps38 cells contain enlarged multivesicular endosomes and fewer organelles enriched in PI3P than the wild type. The vps38 mutants are defective in the trafficking of vacuolar cargo and its receptor VACUOLAR SORTING RECEPTOR2;1. The mutants also exhibit abnormal cytoplasmic distributions of endocytic cargo, such as auxin efflux carriers PINFORMED1 (PIN1) and PIN2. Constitutive autophagy is normal in the mutants but starvation-induced autophagy was slightly inhibited. We conclude that Arabidopsis VPS38 is dispensable for autophagy but essential for efficient targeting of biosynthetic and endocytic cargo to the vacuole.

The Use of Drugs in the Study of Vacuole Morphology and Trafficking to the Vacuole in Arabidopsis thaliana

Chemical compounds are useful to perturb biological functions in the same way as classical genetic approaches take advantage of mutations at the DNA level to perturb gene function. The use of bioactive chemicals currently called chemical genetic is especially valuable for cell biology. Chemical genetic approaches allow perturbations of cellular processes post-germination in a given time window controlling the severity of the effect by modifying or modulating the dose and/or the period of the treatment. Additionally, compounds can be applied directly to different mutants and translational fluorescent reporters/marker lines, expanding the repertoire of experimental setups addressing cell biology research. In this chapter, we describe standard protocols to visualize vacuole morphology and trafficking to the vacuole and the use of bioactive compounds as a proxy to study these biological processes.

Chemical suppressors of mlo-mediated powdery mildew resistance

Loss-of-function of barley mildew locus o (Mlo) confers durable broad-spectrum penetration resistance to the barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). Given the importance of mlo mutants in agriculture, surprisingly few molecular components have been identified to be required for this type of resistance in barley. With the aim to identify novel cellular factors contributing to mlo-based resistance, we devised a pharmacological inhibitor screen. Of the 41 rationally chosen compounds tested, five caused a partial suppression of mlo resistance in barley, indicated by increased levels of Bgh host cell entry. These chemicals comprise brefeldin A (BFA), 2',3'-dideoxyadenosine (DDA), 2-deoxy-d-glucose, spermidine, and 1-aminobenzotriazole. Further inhibitor analysis corroborated a key role for both anterograde and retrograde endomembrane trafficking in mlo resistance. In addition, all four ribonucleosides, some ribonucleoside derivatives, two of the five nucleobases (guanine and uracil), some guanine derivatives as well as various polyamines partially suppress mlo resistance in barley via yet unknown mechanisms. Most of the chemicals identified to be effective in partially relieving mlo resistance in barley also to some extent compromised powdery mildew resistance in an Arabidopsis mlo2 mlo6 double mutant. In summary, our study identified novel suppressors of mlo resistance that may serve as valuable probes to unravel further the molecular processes underlying this unusual type of disease resistance.

Chemical Genetic Dissection of Membrane Trafficking

The plant endomembrane system is an extensively connected functional unit for exchanging material between compartments. Secretory and endocytic pathways allow dynamic trafficking of proteins, lipids, and other molecules, regulating a myriad of biological processes. Chemical genetics-the use of compounds to perturb biological processes in a fast, tunable, and transient manner-provides elegant tools for investigating this system. Here, we review how chemical genetics has helped to elucidate different aspects of membrane trafficking. We discuss different strategies for uncovering the modes of action of such compounds and their use in unraveling membrane trafficking regulators. We also discuss how the bioactive chemicals that are currently used as probes to interrogate endomembrane trafficking were discovered and analyze the results regarding membrane trafficking and pathway crosstalk. The integration of different expertises and the rational implementation of chemical genetic strategies will improve the identification of molecular mechanisms that drive intracellular trafficking and our understanding of how trafficking interfaces with plant physiology and development.

Inhibitors of plant hormone transport

Here we present an overview of what is known about endogenous plant compounds that act as inhibitors of hormonal transport processes in plants, about their identity and mechanism of action. We have also summarized commonly and less commonly used compounds of non-plant origin and synthetic drugs that show at least partial 'specificity' to transport or transporters of particular phytohormones. Our main attention is focused on the inhibitors of auxin transport. The urgent need to understand precisely the molecular mechanism of action of these inhibitors is highlighted.

ENDOSOMAL RAB EFFECTOR WITH PX-DOMAIN, an Interacting Partner of RAB5 GTPases, Regulates Membrane Trafficking to Protein Storage Vacuoles in Arabidopsis

RAB5 GTPases act as molecular switches that regulate various endosomal functions in animal cells, including homotypic fusion of early endosomes, endosomal motility, endosomal signaling, and subcompartmentalization of the endosomal membrane. RAB5 proteins fulfill these diverse functions through interactions with downstream effector molecules. Two canonical RAB5 members, ARA7 and RAB HOMOLOG1 (RHA1), are encoded in the Arabidopsis thaliana genome. ARA7 and RHA1 play crucial roles in endocytic and vacuolar trafficking pathways. Plant RAB5 GTPases function via interactions with effector molecules, whose identities and functions are currently unclear. In this study, we searched for canonical RAB5 effector molecules of Arabidopsis and identified a candidate, which we called ENDOSOMAL RAB EFFECTOR WITH PX-DOMAIN (EREX). The intimate genetic interaction between EREX and RAB5 members, the results from subcellular colocalization experiments, and the direct interaction observed in an in vitro pull-down assay strongly suggest that EREX is a genuine effector of canonical RAB5s in Arabidopsis. We further found that close homologs of EREX play partially redundant functions with EREX in the transport of seed storage proteins. Our results indicate that canonical plant RAB5s acquired distinct effector molecules from those of non-plant systems to fulfill their functions.

Release of GTP Exchange Factor Mediated Down-Regulation of Abscisic Acid Signal Transduction through ABA-Induced Rapid Degradation of RopGEFs

The phytohormone abscisic acid (ABA) is critical to plant development and stress responses. Abiotic stress triggers an ABA signal transduction cascade, which is comprised of the core components PYL/RCAR ABA receptors, PP2C-type protein phosphatases, and protein kinases. Small GTPases of the ROP/RAC family act as negative regulators of ABA signal transduction. However, the mechanisms by which ABA controls the behavior of ROP/RACs have remained unclear. Here, we show that an Arabidopsis guanine nucleotide exchange factor protein RopGEF1 is rapidly sequestered to intracellular particles in response to ABA. GFP-RopGEF1 is sequestered via the endosome-prevacuolar compartment pathway and is degraded. RopGEF1 directly interacts with several clade A PP2C protein phosphatases, including ABI1. Interestingly, RopGEF1 undergoes constitutive degradation in pp2c quadruple abi1/abi2/hab1/pp2ca mutant plants, revealing that active PP2C protein phosphatases protect and stabilize RopGEF1 from ABA-mediated degradation. Interestingly, ABA-mediated degradation of RopGEF1 also plays an important role in ABA-mediated inhibition of lateral root growth. The presented findings point to a PP2C-RopGEF-ROP/RAC control loop model that is proposed to aid in shutting off ABA signal transduction, to counteract leaky ABA signal transduction caused by "monomeric" PYL/RCAR ABA receptors in the absence of stress, and facilitate signaling in response to ABA.

Is Wortmannin-Induced Reorganization of the trans-Golgi Network the Key to Explain Charasome Formation?

Wortmannin, a fungal metabolite and an inhibitor of phosphatidylinositol-3 (PI3) and phosphatidylinositol-4 (PI4) kinases, is widely used for the investigation and dissection of vacuolar trafficking routes and for the identification of proteins located at multivesicular bodies (MVBs). In this study, we applied wortmannin on internodal cells of the characean green alga Chara australis. Wortmannin was used at concentrations of 25 and 50 μM which, unlike in other cells, arrested neither constitutive, nor wounding-induced endocytosis via coated vesicles. Wortmannin caused the formation of "mixed compartments" consisting of MVBs and membranous tubules which were probably derived from the trans-Golgi network (TGN) and within these compartments MVBs fused into larger organelles. Most interestingly, wortmannin also caused pronounced changes in the morphology of the TGNs. After transient hypertrophy, the TGNs lost their coat and formed compact, three-dimensional meshworks of anastomosing tubules containing a central core. These meshworks had a size of up to 4 μm and a striking resemblance to charasomes, which are convoluted plasma membrane domains, and which serve to increase the area available for transporters. Our findings indicate that similar mechanisms are responsible for the formation of charasomes and the wortmannin-induced reorganization of the TGN. We hypothesize that both organelles grow because of a disturbance of clathrin-dependent membrane retrieval due to inhibition of PI3 and/or PI4 kinases. This leads to local inhibition of clathrin-mediated endocytosis during charasome formation in untreated cells and to inhibition of vesicle release from the TGN in wortmannin-treated cells, respectively. The morphological resemblance between charasomes and wortmannin-modified TGN compartments suggests that homologous proteins are involved in membrane curvature and organelle architecture.

NtGNL1a ARF-GEF acts in endocytosis in tobacco cells

BACKGROUND

Processes of anterograde and retrograde membrane trafficking play an important role in cellular homeostasis and dynamic rearrangements of the plasma membrane (PM) in all eukaryotes. These processes depend on the activity of adenosine ribosylation factors (ARFs), a family of GTP-binding proteins and their guanine exchange factors (GEFs). However, knowledge on the function and specificity of individual ARF-GEFs for individual steps of membrane trafficking pathways is still limited in plants.

RESULTS

In this work, treatments with various trafficking inhibitors showed that the endocytosis of FM 4-64 is largely dynamin-dependent and relies on proteins containing endocytic tyrosine-based internalization motif and intact cytoskeleton. Interestingly, brefeldin A (BFA), reported previously as an inhibitor of anterograde membrane trafficking in plants, appeared to be the most potent inhibitor of endocytosis in tobacco. In concert with this finding, we demonstrate that the point mutation in the Sec7 domain of the GNOM-LIKE protein1a (NtGNL1a) confers intracellular trafficking pathway-specific BFA resistance. The internalization of FM 4-64 and trafficking of PIN-FORMED1 (PIN1) auxin efflux carrier in BY-2 tobacco cells were studied to reveal the function of the ARF-GEF NtGNL1a in these.

CONCLUSIONS

Altogether, our observations uncovered the role of NtGNL1a in endocytosis, including endocytosis of PM proteins (as PIN1 auxin efflux carrier). Moreover these data emphasize the need of careful evaluation of mode of action of non-native inhibitors in various species. In addition, they demonstrate the potential of tobacco BY-2 cells for selective mapping of ARF-GEF-regulated endomembrane trafficking pathways.

A tetratricopeptide repeat domain-containing protein SSR1 located in mitochondria is involved in root development and auxin polar transport in Arabidopsis

Auxin polar transport mediated by a group of Pin-formed (PIN) transporters plays important roles in plant root development. However, the mechanism underlying the PIN expression and targeting in response to different developmental and environmental stimuli is still not fully understood. Here, we report a previously uncharacterized gene SSR1, which encodes a mitochondrial protein with tetratricopeptide repeat (TPR) domains, and show its function in root development in Arabidopsis thaliana. In ssr1-2, a SSR1 knock-out mutant, the primary root growth was dramatically inhibited due to severely impaired cell proliferation and cell elongation. Significantly lowered level of auxin was found in ssr1-2 roots by auxin measurement and was further supported by reduced expression of DR5-driven reporter gene. As a result, the maintenance of the root stem cell niche is compromised in ssr1-2. It is further revealed that the expression level of several PIN proteins, namely, PIN1, PIN2, PIN3, PIN4 and PIN7, were markedly reduced in ssr1-2 roots. In particular, we showed that the reduced protein level of PIN2 on cell membrane in ssr1-2 is due to impaired retrograde trafficking, possibly resulting from a defect in retromer sorting system, which destines PIN2 for degradation in vacuoles. In conclusion, our results indicated that SSR1 is functioning in root development in Arabidopsis, possibly by affecting PIN protein expression and subcellular targeting.

Phosphatidylinositol 4-phosphate negatively regulates chloroplast division in Arabidopsis

Chloroplast division is performed by the constriction of envelope membranes at the division site. Although constriction of a ring-like protein complex has been shown to be involved in chloroplast division, it remains unknown how membrane lipids participate in the process. Here, we show that phosphoinositides with unknown function in envelope membranes are involved in the regulation of chloroplast division in Arabidopsis thaliana. PLASTID DIVISION1 (PDV1) and PDV2 proteins interacted specifically with phosphatidylinositol 4-phosphate (PI4P). Inhibition of phosphatidylinositol 4-kinase (PI4K) decreased the level of PI4P in chloroplasts and accelerated chloroplast division. Knockout of PI4Kβ2 expression or downregulation of PI4Kα1 expression resulted in decreased levels of PI4P in chloroplasts and increased chloroplast numbers. PI4Kα1 is the main contributor to PI4P synthesis in chloroplasts, and the effect of PI4K inhibition was largely abolished in the pdv1 mutant. Overexpression of DYNAMIN-RELATED PROTEIN5B (DRP5B), another component of the chloroplast division machinery, which is recruited to chloroplasts by PDV1 and PDV2, enhanced the effect of PI4K inhibition, whereas overexpression of PDV1 and PDV2 had additive effects. The amount of DRP5B that associated with chloroplasts increased upon PI4K inhibition. These findings suggest that PI4P is a regulator of chloroplast division in a PDV1- and DRP5B-dependent manner.

Phosphatidylinositol 3-kinase and 4-kinase have distinct roles in intracellular trafficking of cellulose synthase complexes in Arabidopsis thaliana

The oriented deposition of cellulose microfibrils in the plant cell wall plays a crucial role in various plant functions such as cell growth, organ formation and defense responses. Cellulose is synthesized by cellulose synthase complexes (CSCs) embedded in the plasma membrane (PM), which comprise the cellulose synthases (CESAs). The abundance and localization of CSCs at the PM should be strictly controlled for precise regulation of cellulose deposition, which strongly depends on the membrane trafficking system. However, the mechanism of the intracellular transport of CSCs is still poorly understood. In this study, we explored requirements for phosphoinositides (PIs) in CESA trafficking by analyzing the effects of inhibitors of PI synthesis in Arabidopsis thaliana expressing green fluorescent protein-tagged CESA3 (GFP-CESA3). We found that a shift to a sucrose-free condition accelerated re-localization of PM-localized GFP-CESA3 into the periphery of the Golgi apparatus via the clathrin-enriched trans-Golgi network (TGN). Treatment with wortmannin (Wm), an inhibitor of phosphatidylinositol 3- (PI3K) and 4- (PI4K) kinases, and phenylarsine oxide (PAO), a more specific inhibitor for PI4K, inhibited internalization of GFP-CESA3 from the PM. In contrast, treatment with LY294002, which impairs the PI3K activity, did not exert such an inhibitory effect on the sequestration of GFP-CESA3, but caused a predominant accumulation of GFP-CESA3 at the ring-shaped periphery of the Golgi apparatus, resulting in the removal of GFP-CESA3 from the PM. These results indicate that PIs are essential elements for localization and intracellular transport of CESA3 and that PI4K and PI3K are required for distinct steps in secretory and/or endocytic trafficking of CESA3.

Knowing your friends and foes--plant receptor-like kinases as initiators of symbiosis or defence

The decision between defence and symbiosis signalling in plants involves alternative and modular plasma membrane-localized receptor complexes. A critical step in their activation is ligand-induced homo- or hetero-oligomerization of leucine-rich repeat (LRR)- and/or lysin motif (LysM) receptor-like kinases (RLKs). In defence signalling, receptor complexes form upon binding of pathogen-associated molecular patterns (PAMPs), including the bacterial flagellin-derived peptide flg22, or chitin. Similar mechanisms are likely to operate during the perception of microbial symbiont-derived (lipo)-chitooligosaccharides. The structurally related chitin-oligomer ligands chitooctaose and chitotetraose trigger defence and symbiosis signalling, respectively, and their discrimination involves closely related, if not identical, LysM-RLKs. This illustrates the demand for and the challenges imposed on decision mechanisms that ensure appropriate signal initiation. Appropriate signalling critically depends on abundance and localization of RLKs at the cell surface. This is regulated by internalization, which also provides a mechanism for the removal of activated signalling RLKs. Abundance of the malectin-like domain (MLD)-LRR-RLK Symbiosis Receptor-like Kinase (SYMRK) is additionally controlled by cleavage of its modular ectodomain, which generates a truncated and rapidly degraded RLK fragment. This review explores LRR- and LysM-mediated signalling, the involvement of MLD-LRR-RLKs in symbiosis and defence, and the role of endocytosis in RLK function.

How vacuolar sorting receptor proteins interact with their cargo proteins: crystal structures of apo and cargo-bound forms of the protease-associated domain from an Arabidopsis vacuolar sorting receptor

In plant cells, soluble proteins are directed to vacuoles because they contain vacuolar sorting determinants (VSDs) that are recognized by vacuolar sorting receptors (VSR). To understand how a VSR recognizes its cargo, we present the crystal structures of the protease-associated domain of VSR isoform 1 from Arabidopsis thaliana (VSR1PA) alone and complexed with a cognate peptide containing the barley (Hordeum vulgare) aleurain VSD sequence of 1ADSNPIRPVT10. The crystal structures show that VSR1PA binds the sequence, Ala-Asp-Ser, preceding the NPIR motif. A conserved cargo binding loop, with a consensus sequence of 95RGxCxF100, forms a cradle that accommodates the cargo-peptide. In particular, Arg-95 forms a hydrogen bond to the Ser-3 position of the VSD, and the essential role of Arg-95 and Ser-3 in receptor-cargo interaction was supported by a mutagenesis study. Cargo binding induces conformational changes that are propagated from the cargo binding loop to the C terminus via conserved residues in switch I-IV regions. The resulting 180° swivel motion of the C-terminal tail is stabilized by a hydrogen bond between Glu-24 and His-181. A mutagenesis study showed that these two residues are essential for cargo interaction and trafficking. Based on our structural and functional studies, we present a model of how VSRs recognize their cargos.

Vacuolar Sorting Receptor-Mediated Trafficking of Soluble Vacuolar Proteins in Plant Cells

Vacuoles are one of the most prominent organelles in plant cells, and they play various important roles, such as degradation of waste materials, storage of ions and metabolites, and maintaining turgor. During the past two decades, numerous advances have been made in understanding how proteins are specifically delivered to the vacuole. One of the most crucial steps in this process is specific sorting of soluble vacuolar proteins. Vacuolar sorting receptors (VSRs), which are type I membrane proteins, are involved in the sorting and packaging of soluble vacuolar proteins into transport vesicles with the help of various accessory proteins. To date, large amounts of data have led to the development of two different models describing VSR-mediated vacuolar trafficking that are radically different in multiple ways, particularly regarding the location of cargo binding to, and release from, the VSR and the types of carriers utilized. In this review, we summarize current literature aimed at elucidating VSR-mediated vacuolar trafficking and compare the two models with respect to the sorting signals of vacuolar proteins, as well as the molecular machinery involved in VSR-mediated vacuolar trafficking and its action mechanisms.

Tomato Pistil Factor STIG1 Promotes in Vivo Pollen Tube Growth by Binding to Phosphatidylinositol 3-Phosphate and the Extracellular Domain of the Pollen Receptor Kinase LePRK2

The speed of pollen tube growth is a major determinant of reproductive success in flowering plants. Tomato (Solanum lycopersicum) STIGMA-SPECIFIC PROTEIN1 (STIG1), a small Cys-rich protein from the pistil, was previously identified as a binding partner of the pollen receptor kinase LePRK2 and shown to promote pollen tube growth in vitro. However, the in vivo function of STIG1 and the underlying mechanism of its promotive effect were unknown. Here, we show that a 7-kD processed peptide of STIG1 is abundant in the stigmatic exudate and accumulates at the pollen tube surface, where it can bind LePRK2. Antisense LePRK2 pollen was less responsive than wild-type pollen to exogenous STIG1 in an in vitro pollen germination assay. Silencing of STIG1 reduced both the in vivo pollen tube elongation rate and seed production. Using partial deletion and point mutation analyses, two regions underlying the promotive activity of the STIG1 processed peptide were identified: amino acids 80 to 83, which interact with LePRK2; and amino acids 88 to 115, which bind specifically to phosphatidylinositol 3-phosphate [PI(3)P]. Furthermore, exogenous STIG1 elevated the overall redox potential of pollen tubes in both PI(3)P-dependent and LePRK2-dependent manners. Our results demonstrate that STIG1 conveys growth-promoting signals acting through the pollen receptor kinase LePRK2, a process that relies on the external phosphoinositide PI(3)P.

Homotypic vacuole fusion requires VTI11 and is regulated by phosphoinositides

Most plant cells contain a large central vacuole that is essential to maintain cellular turgor. We report a new mutant allele of VTI11 that implicates the SNARE protein VTI11 in homotypic fusion of protein storage and lytic vacuoles. Fusion of the multiple vacuoles present in vti11 mutants could be induced by treatment with Wortmannin and LY294002, which are inhibitors of Phosphatidylinositol 3-Kinase (PI3K). We provide evidence that Phosphatidylinositol 3-Phosphate (PtdIns(3)P) regulates vacuole fusion in vti11 mutants, and that fusion of these vacuoles requires intact microtubules and actin filaments. Finally, we show that Wortmannin also induced the fusion of guard cell vacuoles in fava beans, where vacuoles are naturally fragmented after ABA-induced stomata closure. These results suggest a ubiquitous role of phosphoinositides in vacuole fusion, both during the development of the large central vacuole and during the dynamic vacuole remodeling that occurs as part of stomata movements.

A revised assay for monitoring autophagic flux in Arabidopsis thaliana reveals involvement of AUTOPHAGY-RELATED9 in autophagy

Autophagy targets cytoplasmic cargo to a lytic compartment for degradation. Autophagy-related (Atg) proteins, including the transmembrane protein Atg9, are involved in different steps of autophagy in yeast and mammalian cells. Functional classification of core Atg proteins in plants has not been clearly confirmed, partly because of the limited availability of reliable assays for monitoring autophagic flux. By using proUBQ10-GFP-ATG8a as an autophagic marker, we showed that autophagic flux is reduced but not completely compromised in Arabidopsis thaliana atg9 mutants. In contrast, we confirmed full inhibition of autophagic flux in atg7 and that the difference in autophagy was consistent with the differences in mutant phenotypes such as hypersensitivity to nutrient stress and selective autophagy. Autophagic flux is also reduced by an inhibitor of phosphatidylinositol kinase. Our data indicated that atg9 is phenotypically distinct from atg7 and atg2 in Arabidopsis, and we proposed that ATG9 and phosphatidylinositol kinase activity contribute to efficient autophagy in Arabidopsis.

Profilin as a regulator of the membrane-actin cytoskeleton interface in plant cells

Membrane structures and cytoskeleton dynamics are intimately inter-connected in the eukaryotic cell. Recently, the molecular mechanisms operating at this interface have been progressively addressed. Many experiments have revealed that the actin cytoskeleton can interact with membranes through various discrete membrane domains. The actin-binding protein, profilin has been proven to inhibit actin polymerization and to promote F-actin elongation. This is dependent on many factors, such as the profilin/G-actin ratio and the ionic environment of the cell. Additionally, profilin has specific domains that interact with phosphoinositides and poly-L-proline rich proteins; theoretically, this gives profilin the opportunity to interact with membranes, and a large number of experiments have confirmed this possibility. In this article, we summarize recent findings in plant cells, and discuss the evidence of the connections among actin cytoskeleton, profilin and biomembranes through direct or indirect relationships.

The M3 phosphorylation motif has been functionally conserved for intracellular trafficking of long-looped PIN-FORMEDs in the Arabidopsis root hair cell

BACKGROUND

PIN-FORMED (PIN) efflux carriers contribute to polar auxin transport and plant development by exhibiting dynamic and diverse asymmetrical localization patterns in the plasma membrane (PM). Phosphorylation of the central hydrophilic loop (HL) of PINs has been implicated in the regulation of PIN trafficking. Recently, we reported that a phosphorylatable motif (M3) in the PIN3-HL is necessary for the polarity, intracellular trafficking, and biological functions of PIN3. In this study, using the root hair system for PIN activity assay, we investigated whether this motif has been functionally conserved among long-HL PINs.

RESULTS

Root hair-specific overexpression of wild-type PIN1, 2, or 7 greatly inhibited root hair growth by depleting auxin levels in the root hair cell, whereas overexpression of M3 phosphorylation-defective PIN mutants failed to inhibit root hair growth. Consistent with this root hair phenotype, the PM localization of M3 phosphorylation-defective PIN1 and PIN7 was partially disrupted, resulting in less auxin efflux and restoration of root hair growth. Partial formation of brefeldin A-compartments in these phosphorylation-mutant PIN lines also suggested that their PM targeting was partially disrupted. On the other hand, compared with the PIN1 and PIN7 mutant proteins, M3-phosphorylation-defective PIN2 proteins were almost undetectable. However, the mutant PIN2 protein levels were restored by wortmannin treatment almost to the wild-type PIN2 level, indicating that the M3 motif of PIN2, unlike that of other PINs, is implicated in PIN2 trafficking to the vacuolar lytic pathway.

CONCLUSIONS

These results suggest that the M3 phosphorylation motif has been functionally conserved to modulate the intracellular trafficking of long-HL PINs, but its specific function in trafficking has diverged among PIN members.

Apical F-actin-regulated exocytic targeting of NtPPME1 is essential for construction and rigidity of the pollen tube cell wall

In tip-confined growing pollen tubes, delivery of newly synthesized cell wall materials to the rapidly expanding apical surface requires spatial organization and temporal regulation of the apical F-actin filament and exocytosis. In this study, we demonstrate that apical F-actin is essential for the rigidity and construction of the pollen tube cell wall by regulating exocytosis of Nicotiana tabacum pectin methylesterase (NtPPME1). Wortmannin disrupts the spatial organization of apical F-actin in the pollen tube tip and inhibits polar targeting of NtPPME1, which subsequently alters the rigidity and pectic composition of the pollen tube cell wall, finally causing growth arrest of the pollen tube. In addition to mechanistically linking cell wall construction and apical F-actin, wortmannin can be used as a useful tool for studying endomembrane trafficking and cytoskeletal organization in pollen tubes.

Triacylglycerol mobilization is suppressed by brefeldin A in Chlamydomonas reinhardtii

Brefeldin A suppresses vesicle trafficking by inhibiting exchange of GDP for GTP in ADP-ribosylation factor. We report that brefeldin A suppresses mobilization of triacylglycerols in Chlamydomonas reinhardtii, a model organism of green microalgae. Analyses revealed that brefeldin A causes Chlamydomonas to form lipid droplets in which triacylglycerols accumulate in a dose-dependent manner. Pulse labeling experiment using fluorescent fatty acids suggested that brefeldin A inhibits the cells from degrading fatty acids. The experiment also revealed that the cells transiently form novel compartments that accumulate exogenously added fatty acids in the cytoplasm, designated fatty acid-induced microbodies (FAIMs). Brefeldin A up-regulates the formation of FAIMs, whereas nitrogen deprivation that up-regulates triacylglycerol synthesis in Chlamydomonas does not cause the cells to form FAIMs. These results underscore the role of the vesicle trafficking machinery in triacylglycerol metabolism in green microalgae.

Vesicular trafficking and stress response coupled to PI3K inhibition by LY294002 as revealed by proteomic and cell biological analysis

LY294002 is a synthetic quercetin-like compound, which, unlike wortmannin, is more specific inhibitor of phosphatidylinositol 3-kinase (PI3K). It inhibits endocytosis and vacuolar transport. We report here on the proteome-wide effects of LY294002 on Arabidopsis roots focusing on proteins involved in vesicular trafficking and stress response. At the subcellular level, LY294002 caused swelling and clustering of late endosomes leading to inhibition of vacuolar transport. At the proteome level, this compound caused changes in abundances of proteins categorized to 10 functional classes. Among proteins involved in vesicular trafficking, a small GTPase ARFA1f was more abundant, indicating its possible contribution to the aggregation and fusion of late endosomes triggered by LY294002. Our study provides new information on storage proteins and vacuolar hydrolases in vegetative tissues treated by LY294002. Vacuolar hydrolases were downregulated, while storage proteins were more abundant, suggesting that storage proteins were protected from degradation in swollen multivesicular bodies upon LY294002 treatment. Upregulation of 2S albumin was validated by immunoblotting and immunolabeling analyses. Our study also pointed to the control of antioxidant enzyme machinery by PI3K because LY294002 downregulated two isozymes of superoxide dismutase. This most likely occurred via PI3K-mediated downregulation of protein AtDJ1A. Finally, we discuss specificity differences of LY294002 and wortmannin against PI3K, which are reflected at the proteome level. Compared with wortmannin, LY294002 showed more narrow and perhaps also more specific effects on proteins, as suggested by gene ontology functional annotation.

An in vivo expression system for the identification of cargo proteins of vacuolar sorting receptors in Arabidopsis culture cells

Vacuolar sorting receptors (VSRs) are type I integral membrane family proteins that in plant cells are thought to recognize cargo proteins at the late Golgi or trans-Golgi network (TGN) for vacuolar transport via the pre-vacuolar compartment (PVC). However, little is known about VSR cargo proteins in plants. Here we developed and tested an in vivo expression system for the identification of VSR cargos which is based on the premise that the expressed N-terminus of VSRs will be secreted into the culture medium along with their corresponding cargo proteins. Indeed, transgenic Arabidopsis culture cell lines expressing VSR N-terminal binding domains (VSRNTs) were shown to secrete truncated VSRs (BP80NT, AtVSR1NT and AtVSR4NT) with attached cargo molecules into the culture medium. Putative cargo proteins were identified through mass spectrometry. Several identified cargo proteins were confirmed by localization studies and interaction analysis with VSRs. The screening strategy described here should be applicable to all VSRs and will help identify and study cargo proteins for individual VSR proteins. This method should be useful for both cargo identification and protein-protein interaction in vivo.

Trans-Golgi network localized ECHIDNA/Ypt interacting protein complex is required for the secretion of cell wall polysaccharides in Arabidopsis

The secretion of cell wall polysaccharides through the trans-Golgi network (TGN) is required for plant cell elongation. However, the components mediating the post-Golgi secretion of pectin and hemicellulose, the two major cell wall polysaccharides, are largely unknown. We identified evolutionarily conserved YPT/RAB GTPase Interacting Protein 4a (YIP4a) and YIP4b (formerly YIP2), which form a TGN-localized complex with ECHIDNA (ECH) in Arabidopsis thaliana. The localization of YIP4 and ECH proteins at the TGN is interdependent and influences the localization of VHA-a1 and SYP61, which are key components of the TGN. YIP4a and YIP4b act redundantly, and the yip4a yip4b double mutants have a cell elongation defect. Genetic, biochemical, and cell biological analyses demonstrate that the ECH/YIP4 complex plays a key role in TGN-mediated secretion of pectin and hemicellulose to the cell wall in dark-grown hypocotyls and in secretory cells of the seed coat. In keeping with these observations, Fourier transform infrared microspectroscopy analysis revealed that the ech and yip4a yip4b mutants exhibit changes in their cell wall composition. Overall, our results reveal a TGN subdomain defined by ECH/YIP4 that is required for the secretion of pectin and hemicellulose and distinguishes the role of the TGN in secretion from its roles in endocytic and vacuolar trafficking.

Successful transport to the vacuole of heterologously expressed mung bean 8S globulin occurs in seed but not in vegetative tissues

This study investigated the subcellular location of mung bean (Vigna radiata) 8S globulin in transient expression systems as well as in tobacco (Nicotiana tabacum) BY-2 cells and different tissues from a transgenic Arabidopsis (Arabidopsis thaliana) line stably expressing this storage globulin. When transiently expressed in protoplasts from both BY-2 cells and Arabidopsis suspension cultured cells, the 8S globulin located to structures that were neither Golgi nor pre-vacuolar compartments (PVCs). Immunogold electron microscopy of the transgenics reveals the 8S globulin-positive structures to be small, spherical, ribosome-covered endoplasmic reticulum (ER)-derived bodies. In BY-2 cells and all vegetative cells, the 8S globulin was present as a pro-form. However, in Arabidopsis embryos, with the onset of endogenous storage protein synthesis, the 8S globulin exited the ER and passed through the PVC to the protein storage vacuole where it was processed to its smaller mature form. These results clearly demonstrated that, when taken out of context and expressed in vegetative cells, the mung bean 8S storage globulin cannot exit the ER, and indicate that natural targeting of storage proteins to the vacuole should be better studied in the maturing seed.

Evidence that proliferation of golgi apparatus depends on both de novo generation from the endoplasmic reticulum and formation from pre-existing stacks during the growth of tobacco BY-2 cells

In higher plants, the numbers of cytoplasmic-distributed Golgi stacks differ based on function, age and cell type. It has not been clarified how the numbers are controlled, whether all the Golgi apparatus in a cell function equally and whether the increase in Golgi number is a result of the de novo formation from the endoplasmic reticulum (ER) or fission of pre-existing stacks. A tobacco prolyl 4-hydroxylase (NtP4H1.1), which is a cis-Golgi-localizing type II membrane protein, was tagged with a photoconvertible fluorescent protein, mKikGR (monomeric Kikume green red), and expressed in tobacco bright yellow 2 (BY-2) cells. Transformed cells were exposed to purple light to convert the fluorescence from green to red. A time-course analysis after the conversion revealed a progressive increase in green puncta and a decrease in the red puncta. From 3 to 6 h, we observed red, yellow and green fluorescent puncta corresponding to pre-existing Golgi; Golgi containing both pre-existing and newly synthesized protein; and newly synthesized Golgi. Analysis of the number and fluorescence of Golgi at different phases of the cell cycle suggested that an increase in Golgi number with both division and de novo synthesis occurred concomitantly with DNA replication. Investigation with different inhibitors suggested that the formation of new Golgi and the generation of Golgi containing both pre-existing and newly synthesized protein are mediated by different machineries. These results and modeling based on quantified results indicate that the Golgi apparatuses in tobacco BY-2 cells are not uniform and suggest that both de novo synthesis from the ER and Golgi division contribute almost equally to the increase in proliferating cells.

Structural basis for interactions of the Phytophthora sojae RxLR effector Avh5 with phosphatidylinositol 3-phosphate and for host cell entry

Oomycetes such as Phytophthora sojae employ effector proteins that enter plant cells to facilitate infection. Entry of some effector proteins is mediated by RxLR motifs in the effectors and phosphoinositides (PIP) resident in the host plasma membrane such as phosphatidylinositol 3-phosphate (PtdIns(3)P). Recent reports differ regarding the regions on RxLR effectors involved in PIP recognition. We have structurally and functionally characterized the P. sojae effector, avirulence homolog-5 (Avh5). Using nuclear magnetic resonance (NMR) spectroscopy, we demonstrate that Avh5 is helical in nature, with a long N-terminal disordered region. NMR titrations of Avh5 with the PtdIns(3)P head group, inositol 1,3-bisphosphate, directly identified the ligand-binding residues. A C-terminal lysine-rich helical region (helix 2) was the principal lipid-binding site, with the N-terminal RxLR (RFLR) motif playing a more minor role. Mutations in the RFLR motif affected PtdIns(3)P binding, while mutations in the basic helix almost abolished it. Mutations in the RFLR motif or in the basic region both significantly reduced protein entry into plant and human cells. Both regions independently mediated cell entry via a PtdIns(3)P-dependent mechanism. Based on these findings, we propose a model where Avh5 interacts with PtdIns(3)P through its C terminus, and by binding of the RFLR motif, which promotes host cell entry.

Trafficking of vacuolar proteins: the crucial role of Arabidopsis vacuolar protein sorting 29 in recycling vacuolar sorting receptor

The retromer is involved in recycling lysosomal sorting receptors in mammals. A component of the retromer complex in Arabidopsis thaliana, vacuolar protein sorting 29 (VPS29), plays a crucial role in trafficking storage proteins to protein storage vacuoles. However, it is not known whether or how vacuolar sorting receptors (VSRs) are recycled from the prevacuolar compartment (PVC) to the trans-Golgi network (TGN) during trafficking to the lytic vacuole (LV). Here, we report that VPS29 plays an essential role in the trafficking of soluble proteins to the LV from the TGN to the PVC. maigo1-1 (mag1-1) mutants, which harbor a knockdown mutation in VPS29, were defective in trafficking of two soluble proteins, Arabidopsis aleurain-like protein (AALP):green fluorescent protein (GFP) and sporamin:GFP, to the LV but not in trafficking membrane proteins to the LV or plasma membrane or via the secretory pathway. AALP:GFP and sporamin:GFP in mag1-1 protoplasts accumulated in the TGN but were also secreted into the medium. In mag1-1 mutants, VSR1 failed to recycle from the PVC to the TGN; rather, a significant proportion was transported to the LV; VSR1 overexpression rescued this defect. Moreover, endogenous VSRs were expressed at higher levels in mag1-1 plants. Based on these results, we propose that VPS29 plays a crucial role in recycling VSRs from the PVC to the TGN during the trafficking of soluble proteins to the LV.