Nanoscale glucan polymer network causes pathogen resistance

A Rapid Method for Screening Pathogen-Associated Molecular Pattern-Triggered Immunity-Intensifying Microbes

PAMP-triggered immunity (PTI) is the first layer of plant defense response that occurs on the plant plasma membrane. Recently, the application of a rhizobacterium, Bacillus amyloliquefaciens strain PMB05, has been demonstrated to enhance flg22Pst- or harpin-triggered PTI response such as callose deposition. This PTI intensification by PMB05 further contributes to plant disease resistance to different bacterial diseases. Under the demand for rapid and large-scale screening, it has become critical to establish a non-staining technology to identify microbial strains that can enhance PTI responses. Firstly, we confirmed that the expression of the GSL5 gene, which is required for callose synthesis, can be enhanced by PMB05 during PTI activation triggered by flg22 or PopW (a harpin from Ralstonia solanacearum). The promoter region of the GSL5 gene was further cloned and fused to the coding sequence of gfp. The constructed fragments were used to generate transgenic Arabidopsis plants through a plant transformation vector. The transgenic lines of AtGSL5-GFP were obtained. The analysis was performed by infiltrating flg22Pst or PopW in one homozygous line, and the results exhibited that the green fluorescent signals were observed until after 8 h. In addition, the PopW-induced fluorescent signal was significantly enhanced in the co-treatment with PMB05 at 4 h after inoculation. Furthermore, by using AtGSL5-GFP to analyze 13 Bacillus spp. strains, the regulation of PopW-induced fluorescent signal was observed. And, the regulation of these fluorescent signals was similar to that performed by callose staining. More importantly, the Bacillus strains that enhance PopW-induced fluorescent signals would be more effective in reducing the occurrence of bacterial wilt. Taken together, the technique by using AtGSL5-GFP would be a promising platform to screen plant immunity-intensifying microbes to control bacterial wilt.

Cellulose-Callose Hydrogels: Computational Exploration of Their Nanostructure and Mechanical Properties

Polysaccharides play a crucial role in virtually all living systems. They also represent the biocompatible and fully sustainable component of a variety of nanoparticles, which are of increasing interest in biomedicine, food processing, cosmetics, and structural reinforcement of polymeric materials. The computational modeling of complex polysaccharide phases will assist in understanding the properties and behavior of all these systems. In this paper, structural, bonding, and mechanical properties of 10 wt % cellulose-callose hydrogels (β-glucans coexisting in plant cell walls) were investigated by atomistic simulations. Systems of this kind have recently been introduced in experiments revealing unexpected interactions between the polysaccharides. Starting from initial configurations inspired by X-ray diffraction data, atomistic models made of ∼1.6 × 106 atoms provide a qualitatively consistent view of these hydrogels, displaying stability, homogeneity, connectivity, and elastic properties beyond those of a liquid suspension. The simulation shows that the relatively homogeneous distribution of saccharide nanofibers and chains in water is not due to the solubility of cellulose and callose, but to the formation of a number of cross-links among the various sample components. The broad distribution of strength and elasticity among the links implies a degree of anharmonicity and irreversible deformation already evident at low external load. Besides the qualitative agreement with experimental observations, the simulation results display also quantitative disagreements in the estimation of elastic coefficients, such as the Young's modulus, that require further investigation. Complementary simulations of dense cellulose-callose mixtures (no hydrogels) highlight the role of callose in smoothing the contact surface of different nanofibers forming larger bundles. Cellulose-callose structures in these systems displayed an enhanced water uptake and delayed dye release when compared to cellulose alone, highlighting potential new applications as drug delivery scaffolds. The simulation trajectories provide a tuning and testing ground for the development of coarse-grained models that are required for the large scale investigation of mechanical properties of cellulose and callose mixtures in a watery environment.

β-glucan-induced disease resistance in plants: A review

Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are caused by various factors, including both pathogenic and non-pathogenic ones. β-glucan primarily originates from bacteria and fungi, some species of these organisms work as biological agents in causing diseases. When β-glucan enters plants, it triggers the defense system, leading to various reactions such as the production of proteins related to pathogenicity and defense enzymes. By extracting β-glucan from disturbed microorganisms and using it as an inducing agent, plant diseases can be effectively controlled by activating the plant's defense system. β-glucan plays a crucial role during the interaction between plants and pathogens. Therefore, modeling the plant-pathogen relationship and using the molecules involved in this interaction can help in controlling plant diseases, as pathogens have genes related to resistance against pathogenicity. Thus, it is reasonable to identify and use biological induction agents at a large scale by extracting these compounds.

Balanced callose and cellulose biosynthesis in Arabidopsis quorum-sensing signaling and pattern-triggered immunity

The plant cell wall (CW) is one of the most important physical barriers that phytopathogens must conquer to invade their hosts. This barrier is a dynamic structure that responds to pathogen infection through a complex network of immune receptors, together with CW-synthesizing and CW-degrading enzymes. Callose deposition in the primary CW is a well-known physical response to pathogen infection. Notably, callose and cellulose biosynthesis share an initial substrate, UDP-glucose, which is the main load-bearing component of the CW. However, how these 2 critical biosynthetic processes are balanced during plant-pathogen interactions remains unclear. Here, using 2 different pathogen-derived molecules, bacterial flagellin (flg22) and the diffusible signal factor (DSF) produced by Xanthomonas campestris pv. campestris, we show a negative correlation between cellulose and callose biosynthesis in Arabidopsis (Arabidopsis thaliana). By quantifying the abundance of callose and cellulose under DSF or flg22 elicitation and characterizing the dynamics of the enzymes involved in the biosynthesis and degradation of these 2 polymers, we show that the balance of these 2 CW components is mediated by the activity of a β-1,3-glucanase (BG2). Our data demonstrate balanced cellulose and callose biosynthesis during plant immune responses.

A GH81-type β-glucan-binding protein enhances colonization by mutualistic fungi in barley

Cell walls are important interfaces of plant-fungal interactions, acting as robust physical and chemical barriers against invaders. Upon fungal colonization, plants deposit phenolics and callose at the sites of fungal penetration to prevent further fungal progression. Alterations in the composition of plant cell walls significantly impact host susceptibility. Furthermore, plants and fungi secrete glycan hydrolases acting on each other's cell walls. These enzymes release various sugar oligomers into the apoplast, some of which activate host immunity via surface receptors. Recent characterization of cell walls from plant-colonizing fungi has emphasized the abundance of β-glucans in different cell wall layers, which makes them suitable targets for recognition. To characterize host components involved in immunity against fungi, we performed a protein pull-down with the biotinylated β-glucan laminarin. Thereby, we identified a plant glycoside hydrolase family 81-type glucan-binding protein (GBP) as a β-glucan interactor. Mutation of GBP1 and its only paralog, GBP2, in barley led to decreased colonization by the beneficial root endophytes Serendipita indica and S. vermifera, as well as the arbuscular mycorrhizal fungus Rhizophagus irregularis. The reduction of colonization was accompanied by enhanced responses at the host cell wall, including an extension of callose-containing cell wall appositions. Moreover, GBP mutation in barley also reduced fungal biomass in roots by the hemibiotrophic pathogen Bipolaris sorokiniana and inhibited the penetration success of the obligate biotrophic leaf pathogen Blumeria hordei. These results indicate that GBP1 is involved in the establishment of symbiotic associations with beneficial fungi-a role that has potentially been appropriated by barley-adapted pathogens.

Callose synthesis at the center point of plant development-An evolutionary insight

Polar callose deposition into the extracellular matrix is tightly controlled in time and space. Its presence in the cell wall modifies the properties of the surrounding area, which is fundamental for the correct execution of numerous processes such as cell division, male gametophyte development, intercellular transport, or responses to biotic and abiotic stresses. Previous studies have been invaluable in characterizing specific callose synthases (CalSs) during individual cellular processes. However, the complex view of the relationships between a particular CalS and a specific process is still lacking. Here we review the recent proceedings on the role of callose and individual CalSs in cell wall remodelling from an evolutionary perspective and with a particular focus on cytokinesis. We provide a robust phylogenetic analysis of CalS across the plant kingdom, which implies a 3-subfamily distribution of CalS. We also discuss the possible linkage between the evolution of CalSs and their function in specific cell types and processes.

Genetic Regulation of Mitosis-Meiosis Fate Decision in Plants: Is Callose an Oversighted Polysaccharide in These Processes?

Timely progression of the meiotic cell cycle and synchronized establishment of male meiosis in anthers are key to ascertaining plant fertility. With the discovery of novel regulators of the plant cell cycle, the mechanisms underlying meiosis initiation and progression appear to be more complex than previously thought, requiring the conjunctive action of cyclins, cyclin-dependent kinases, transcription factors, protein-protein interactions, and several signaling components. Broadly, cell cycle regulators can be classified into two categories in plants based on the nature of their mutational effects: (1) those that completely arrest cell cycle progression; and (2) those that affect the timing (delay or accelerate) or synchrony of cell cycle progression but somehow complete the division process. Especially the latter effects reflect evasion or obstruction of major steps in the meiosis but have sometimes been overlooked due to their subtle phenotypes. In addition to meiotic regulators, very few signaling compounds have been discovered in plants to date. In this review, we discuss the current state of knowledge about genetic mechanisms to enter the meiotic processes, referred to as the mitosis-meiosis fate decision, as well as the importance of callose (β-1,3 glucan), which has been unsung for a long time in male meiosis in plants.

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

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

Plant-microbe interactions in the apoplast: Communication at the plant cell wall

The apoplast is a continuous plant compartment that connects cells between tissues and organs and is one of the first sites of interaction between plants and microbes. The plant cell wall occupies most of the apoplast and is composed of polysaccharides and associated proteins and ions. This dynamic part of the cell constitutes an essential physical barrier and a source of nutrients for the microbe. At the same time, the plant cell wall serves important functions in the interkingdom detection, recognition, and response to other organisms. Thus, both plant and microbe modify the plant cell wall and its environment in versatile ways to benefit from the interaction. We discuss here crucial processes occurring at the plant cell wall during the contact and communication between microbe and plant. Finally, we argue that these local and dynamic changes need to be considered to fully understand plant-microbe interactions.

The haustorium: The root of biotrophic fungal pathogens

Biotrophic plant pathogenic fungi are among the dreadful pathogens that continuously threaten the production of economically important crops. The interaction of biotrophic fungal pathogens with their hosts necessitates the development of unique infection mechanisms and involvement of various virulence-associated components. Biotrophic plant pathogenic fungi have an exceptional lifestyle that supports nutrient acquisition from cells of a living host and are fully dependent on the host for successful completion of their life cycle. The haustorium, a specialized infection structure, is the key organ for biotrophic fungal pathogens. The haustorium is not only essential in the uptake of nutrients without killing the host, but also in the secretion and delivery of effectors into the host cells to manipulate host immune system and defense responses and reprogram the metabolic flow of the host. Although there is a number of unanswered questions in this area yet, results from various studies indicate that the haustorium is the root of biotrophic fungal pathogens. This review provides an overview of current knowledge of the haustorium, its structure, composition, and functions, which includes the most recent haustorial transcriptome studies.

Cell wall characteristics during sexual reproduction of Mougeotia sp. (Zygnematophyceae) revealed by electron microscopy, glycan microarrays and RAMAN spectroscopy

Mougeotia spp. collected from field samples were investigated for their conjugation morphology by light-, fluorescence-, scanning- and transmission electron microscopy. During a scalarifom conjugation, the extragametangial zygospores were initially surrounded by a thin cell wall that developed into a multi-layered zygospore wall. Maturing zygospores turned dark brown and were filled with storage compounds such as lipids and starch. While M. parvula had a smooth surface, M. disjuncta had a punctated surface structure and a prominent suture. The zygospore wall consisted of a polysaccharide rich endospore, followed by a thin layer with a lipid-like appaerance, a massive electron dense mesospore and a very thin exospore composed of polysaccharides. Glycan microarray analysis of zygospores of different developmental stages revealed the occurrence of pectins and hemicelluloses, mostly composed of homogalacturonan (HG), xyloglucans, xylans, arabino-galactan proteins and extensins. In situ localization by the probe OG7-13^{AF 488} labelled HG in young zygospore walls, vegetative filaments and most prominently in conjugation tubes and cross walls. Raman imaging showed the distribution of proteins, lipids, carbohydrates and aromatic components of the mature zygospore with a spatial resolution of ~ 250 nm. The carbohydrate nature of the endo- and exospore was confirmed and in-between an enrichment of lipids and aromatic components, probably algaenan or a sporopollenin-like material. Taken together, these results indicate that during zygospore formation, reorganizations of the cell walls occured, leading to a resistant and protective structure.

Advances in Understanding Defense Mechanisms in Persea americana Against Phytophthora cinnamomi

Avocado (Persea americana) is an economically important fruit crop world-wide, the production of which is challenged by notable root pathogens such as Phytophthora cinnamomi and Rosellinia necatrix. Arguably the most prevalent, P. cinnamomi, is a hemibiotrophic oomycete which causes Phytophthora root rot, leading to reduced yields and eventual tree death. Despite its' importance, the development of molecular tools and resources have been historically limited, prohibiting significant progress toward understanding this important host-pathogen interaction. The development of a nested qPCR assay capable of quantifying P. cinnamomi during avocado infection has enabled us to distinguish avocado rootstocks as either resistant or tolerant - an important distinction when unraveling the defense response. This review will provide an overview of our current knowledge on the molecular defense pathways utilized in resistant avocado rootstock against P. cinnamomi. Notably, avocado demonstrates a biphasic phytohormone profile in response to P. cinnamomi infection which allows for the timely expression of pathogenesis-related genes via the NPR1 defense response pathway. Cell wall modification via callose deposition and lignification have also been implicated in the resistant response. Recent advances such as composite plant transformation, single nucleotide polymorphism (SNP) analyses as well as genomics and transcriptomics will complement existing molecular, histological, and biochemical assay studies and further elucidate avocado defense mechanisms.

Regulation and Function of Defense-Related Callose Deposition in Plants

Plants are constantly exposed to a wide range of potential pathogens and to protect themselves, have developed a variety of chemical and physical defense mechanisms. Callose is a β-(1,3)-D-glucan that is widely distributed in higher plants. In addition to its role in normal growth and development, callose plays an important role in plant defense. Callose is deposited between the plasma membrane and the cell wall at the site of pathogen attack, at the plasmodesmata, and on other plant tissues to slow pathogen invasion and spread. Since it was first reported more than a century ago, defense-related callose deposition has been extensively studied in a wide-spectrum of plant-pathogen systems. Over the past 20 years or so, a large number of studies have been published that address the dynamic nature of pathogen-induced callose deposition, the complex regulation of synthesis and transport of defense-related callose and associated callose synthases, and its important roles in plant defense responses. In this review, we summarize our current understanding of the regulation and function of defense-related callose deposition in plants and discuss both the progresses and future challenges in addressing this complex defense mechanism as a critical component of a plant immune system.

ShNPSN11, a vesicle-transport-related gene, confers disease resistance in tomato to Oidium neolycopersici

Tomato powdery mildew, caused by Oidium neolycopersici, is a fungal disease that results in severe yield loss in infected plants. Herein, we describe the function of a class of proteins, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which play a role in vesicle transport during defense signaling. To date, there have been no reports describing the function of tomato SNAREs during resistance signaling to powdery mildew. Using a combination of classical plant pathology-, genetics-, and cell biology-based approaches, we evaluate the role of ShNPSN11 in resistance to the powdery mildew pathogen O. neolycopersici. Quantitative RT-PCR analysis of tomato SNAREs revealed that ShNPSN11 mRNA accumulation in disease-resistant varieties was significantly increased following pathogen, compared with susceptible varieties, suggesting a role during induced defense signaling. Using in planta subcellular localization, we demonstrate that ShNPSN11 was primarily localized at the plasma membrane, consistent with the localization of SNARE proteins and their role in defense signaling and trafficking. Silencing of ShNPSN11 resulted in increased susceptibility to O. neolycopersici, with pathogen-induced levels of H2O2 and cell death elicitation in ShNPSN11-silenced lines showing a marked reduction. Transient expression of ShNPSN11 did not result in the induction of a hypersensitive cell death response or suppress cell death induced by BAX. Taken together, these data demonstrate that ShNPSNl11 plays an important role in defense activation and host resistance to O. neolycopersici in tomato LA1777.

Multicolor 3D-dSTORM Reveals Native-State Ultrastructure of Polysaccharides' Network during Plant Cell Wall Assembly

The plant cell wall, a form of the extracellular matrix, is a complex and dynamic network of polymers mediating a plethora of physiological functions. How polysaccharides assemble into a coherent and heterogeneous matrix remains mostly undefined. Further progress requires improved molecular-level visualization methods that would gain a deeper understanding of the cell wall nanoarchitecture. dSTORM, a type of super-resolution microscopy, permits quantitative nanoimaging of the cell wall. However, due to the lack of single-cell model systems and the requirement of tissue-level imaging, its use in plant science is almost absent. Here we overcome these limitations; we compare two methods to achieve three-dimensional dSTORM and identify optimal photoswitching dyes for tissue-level multicolor nanoscopy. Combining dSTORM with spatial statistics, we reveal and characterize the ultrastructure of three major polysaccharides, callose, mannan, and cellulose, in the plant cell wall precursor and provide evidence for cellulose structural re-organization related to callose content.

Influence of cell wall polymers and their modifying enzymes during plant-aphid interactions

Aphids are a major issue for commercial crops. These pests drain phloem nutrients and transmit ~50% of the known insect-borne viral diseases. During aphid feeding, trophic structures called stylets advance toward the phloem intercellularly, disrupting cell wall polymers. It is thought that cell wall-modifying enzymes (CWMEs) present in aphid saliva facilitate stylet penetration through this intercellular polymer network. Additionally, different studies have demonstrated that host settling preference, feeding behavior, and colony performance of aphids are influenced by modulating the CWME expression levels in host plants. CWMEs have been described as critical defensive elements for plants, but also as a key virulence factor for plant pathogens. However, whether CWMEs are elements of the plant defense mechanisms or the aphid infestation process remains unclear. Therefore, in order to better consider the function of CWMEs and cell wall-derived damage-associated molecular patterns (DAMPs) during plant-aphid interactions, the present review integrates different hypotheses, perspectives, and experimental evidence in the field of plant-aphid interactions and discusses similarities to other well-characterized models such as the fungi-plant pathosystems from the host and the attacker perspectives.

CRISPR/Cas9-targeted mutagenesis of the tomato susceptibility gene PMR4 for resistance against powdery mildew

BACKGROUND

The development of CRISPR/Cas9 technology has facilitated targeted mutagenesis in an efficient and precise way. Previously, RNAi silencing of the susceptibility (S) gene PowderyMildewResistance 4 (PMR4) in tomato has been shown to enhance resistance against the powdery mildew pathogen Oidium neolycopersici (On).

RESULTS

To study whether full knock-out of the tomato PMR4 gene would result in a higher level of resistance than in the RNAi-silenced transgenic plants we generated tomato PMR4 CRISPR mutants. We used a CRISPR/Cas9 construct containing four single-guide RNAs (sgRNAs) targeting the tomato PMR4 gene to increase the possibility of large deletions in the mutants. After PCR-based selection and sequencing of transformants, we identified five different mutation events, including deletions from 4 to 900-bp, a 1-bp insertion and a 892-bp inversion. These mutants all showed reduced susceptibility to On based on visual scoring of disease symptoms and quantification of relative fungal biomass. Histological observations revealed a significantly higher occurrence of hypersensitive response-like cell death at sites of fungal infection in the pmr4 mutants compared to wild-type plants. Both haustorial formation and hyphal growth were diminished but not completely inhibited in the mutants.

CONCLUSION

CRISPR/Cas-9 targeted mutagenesis of the tomato PMR4 gene resulted in mutants with reduced but not complete loss of susceptibility to the PM pathogen On. Our study demonstrates the efficiency and versatility of the CRISPR/Cas9 system as a powerful tool to study and characterize S-genes by generating different types of mutations.

Identification of lncRNAs and their regulatory relationships with target genes and corresponding miRNAs in melon response to powdery mildew fungi

Melon (Cucumis melo L.), an economically beneficial crop widely cultivated around the world, is vulnerable to powdery mildew (PM). However, the studies on molecular mechanism of melon response to PM fungi is still limited. Long non coding RNAs (lncRNAs) have emerged as new regulators in plants response to biotic stresses. We predicted and identified the intricate regulatory roles of lncRNAs in melon response to PM fungi. A total of 539 lncRNAs were identified from PM-resistant (MR-1) and susceptible melon (Top Mark), in which 254 were significantly altered after PM fungi infection. Multiple target genes of lncRNAs were found to be involved in the hydrolysis of chitin, callose deposition and cell wall thickening, plant-pathogen interaction and plant hormone signal transduction pathway. Additionally, a total of 42 lncRNAs possess the various functions with microRNAs (miRNAs), including lncRNAs that are targeted by miRNAs and function as miRNA precursors or miRNA sponges. These findings provide a comprehensive view of potentially functional lncRNAs, corresponding target genes and related lncRNA-miRNA pairs, which will greatly increase our knowledge of the mechanism underlying susceptibility and resistance to PM in melon.

Imaging and Analysis of the Content of Callose, Pectin, and Cellulose in the Cell Wall of Arabidopsis Pollen Tubes Grown In Vitro

To achieve fertilization, pollen tubes have to protect and properly deliver sperm cells through the pistil to the ovules. Pollen tube growth is a representative example of polarized growth where new components of the cell wall and plasma membrane are continuously deposited at the tip of the growing cell. The integrity of the cell wall is of fundamental importance to maintain apical growth. For this reason, pollen tube growth has become an excellent model to study the role of polysaccharides and structural cell wall proteins involved in polar cell expansion. However, quantification of structural polysaccharides at the pollen tube cell wall has been challenging due to technical complexity and the difficulty of finding specific dyes. Here, we propose simple methods for imaging and quantification of callose, pectin , and cellulose using specific dyes such as Aniline Blue, Propidium Iodide, and Pontamine Fast Scarlet 4B.

Synergistic chemo-photodynamic therapy mediated by light-activated ROS-degradable nanocarriers

The combination of chemotherapy and photodynamic therapy (PDT) using polymeric nanocarriers is effective for improving therapeutic efficiency against cancer. Yet, in most reported cases, due to the lack of synergistic mechanisms, chemotherapy and PDT work independently rather than synergistically-the functions of chemotherapeutic drugs and photosensitizers in nanocarriers are independent when they are delivered to cancer cells. Here, we demonstrate the construction of reactive oxygen species (ROS)-degradable nanoparticles (NPs) based on phenylboronic pinacol ester-conjugated dextran (PPE-Dex) through a membrane-extrusion emulsification approach for the co-delivery of anticancer drug (e.g., doxorubicin, Dox) and photosensitizer (e.g., chlorin e6, Ce6). When exposed to 655 nm laser irradiation, ROS generated by encapsulated Ce6 not only induced a significant PDT effect in cancer cells, but also triggered the rapid oxidization and degradation of PPE-Dex, resulting in the quick release and enhanced intra-nuclei accumulation of Dox. In vitro cytotoxicity and combination index (CI) assay indicated that the PPE-Dex NPs offered remarkable synergistic therapeutic effects of Dox and Ce6 against cancer cells under irradiation. Furthermore, the drug release profiles can be well regulated by changing the irradiation time to satisfy different demands in various treatment programs. Our results demonstrated that such ROS-degradable polymeric NPs with light-activated disassembly capability are promising carriers for synergistic photodynamic-chemo therapy in cancer therapy.

Interactions between callose and cellulose revealed through the analysis of biopolymer mixtures

The properties of (1,3)-β-glucans (i.e., callose) remain largely unknown despite their importance in plant development and defence. Here we use mixtures of (1,3)-β-glucan and cellulose, in ionic liquid solution and hydrogels, as proxies to understand the physico-mechanical properties of callose. We show that after callose addition the stiffness of cellulose hydrogels is reduced at a greater extent than predicted from the ideal mixing rule (i.e., the weighted average of the individual components’ properties). In contrast, yield behaviour after the elastic limit is more ductile in cellulose-callose hydrogels compared with sudden failure in 100% cellulose hydrogels. The viscoelastic behaviour and the diffusion of the ions in mixed ionic liquid solutions strongly indicate interactions between the polymers. Fourier-transform infrared analysis suggests that these interactions impact cellulose organisation in hydrogels and cell walls. We conclude that polymer interactions alter the properties of callose-cellulose mixtures beyond what it is expected by ideal mixing.

Despite their importance in plant development and defence the properties of (1,3)-β-glucan remain largely unknown. Here, the authors find that addition of (1,3)-β-glucans increases the flexibility of cellulose and its resilience to high strain, an effect originating in molecular level interactions.

OXI1 kinase plays a key role in resistance of Arabidopsis towards aphids (Myzus persicae)

Plants have co-evolved with a diverse array of pathogens and insect herbivores and so have evolved an extensive repertoire of constitutive and induced defence mechanisms activated through complex signalling pathways. OXI1 kinase is required for activation of mitogen-activated protein kinases (MAPKs) and is an essential part of the signal transduction pathway linking oxidative burst signals to diverse downstream responses. Furthermore, changes in the levels of OXI1 appear to be crucial for appropriate signalling. Callose deposition also plays a key role in defence. Here we demonstrate, for the first time, that OXI1 plays an important role in defence against aphids. The Arabidopsis mutant, oxi1-2, showed significant resistance both in terms of population build-up (p < 0.001) and the rate of build-up (p < 0.001). Arabidopsis mutants for β-1,3-glucanase, gns2 and gns3, showed partial aphid resistance, significantly delaying developmental rate, taking two-fold longer to reach adulthood. Whilst β-1,3-glucanase genes GNS1, GNS2, GNS3 and GNS5 were not induced in oxi1-2 in response to aphid feeding, GNS2 was expressed to high levels in the corresponding WT (Col-0) in response to aphid feeding. Callose synthase GSL5 was up-regulated in oxi1-2 in response to aphids. The results suggest that resistance in oxi1-2 mutants is through induction of callose deposition via MAPKs resulting in ROS induction as an early response. Furthermore, the results suggest that the β-1,3-glucanase genes, especially GNS2, play an important role in host plant susceptibility to aphids. Better understanding of signalling cascades underpinning tolerance to biotic stress will help inform future breeding programmes for enhancing crop resilience.

Plant-microbe interactions: organelles and the cytoskeleton in action

Contents Summary 1012 I. Introduction 1012 II. The endomembrane system in plant-microbe interactions 1013 III. The cytoskeleton in plant-microbe interactions 1017 IV. Organelles in plant-microbe interactions 1019 V. Inter-organellar communication in plant-microbe interactions 1022 VI. Conclusions and prospects 1023 Acknowledgements 1024 References 1024 SUMMARY: Plants have evolved a multilayered immune system with well-orchestrated defense strategies against pathogen attack. Multiple immune signaling pathways, coordinated by several subcellular compartments and interactions between these compartments, play important roles in a successful immune response. Pathogens use various strategies to either directly attack the plant's immune system or to indirectly manipulate the physiological status of the plant to inhibit an immune response. Microscopy-based approaches have allowed the direct visualization of membrane trafficking events, cytoskeleton reorganization, subcellular dynamics and inter-organellar communication during the immune response. Here, we discuss the contributions of organelles and the cytoskeleton to the plant's defense response against microbial pathogens, as well as the mechanisms used by pathogens to target these compartments to overcome the plant's defense barrier.

The Ceratopteris (fern) developing motile gamete walls contain diverse polysaccharides, but not pectin

Unlike most plant cell walls, the five consecutive walls laid down during spermatogenesis in the model fern Ceratopteris contain sparse cellulose, lack pectin and are enriched with callose and hemicelluloses. Seed-free plants like bryophytes and pteridophytes produce swimming male gametes for sexual reproduction. During spermatogenesis, unique walls are formed that are essential to the appropriate development and maturation of the motile gametes. Other than the detection of callose and general wall polysaccharides in scattered groups, little is known about the sequence of wall formation and the composition of these walls during sperm cell differentiation in plants that produce swimming sperm. Using histochemistry and immunogold localizations, we examined the distribution of callose, cellulose, mannan and xylan-containing hemicelluloses, and homogalacturonan (HG) pectins in the special walls deposited during spermatogenesis in Ceratopteris. Five walls are produced in sequence and each has a unique fate. The first wall (W1) contains callose and sparse xylan-containing hemicelluloses. Wall two (W2) is thin and composed of cellulose crosslinked by xylan-containing hemicelluloses. The third wall (W3) is thick and composed entirely of callose, and the fourth wall (W4) is built of cellulose heavily crosslinked by galactoxyloglucan hemicelluloses. Wall five (W5) is an arabinogalactan protein (AGP)-rich matrix in which the gamete changes shape and multiple flagella elongate. We detected no esterified or unesterified HG pectins in any of the walls laid down during spermatogenesis. To consider evolutionary modifications in cell walls associated with motile gametes, comparisons are presented with male gametophyte and spermatogenous cell walls across plant groups.

Soft mechanical stimulation induces a defense response against Botrytis cinerea in strawberry

Genes associated with plant mechanical stimulation were found in strawberry genome. A soft mechanical stimulation (SMS) induces molecular and biochemical changes in strawberry plants, conferring protection against Botrytis cinerea. Plants have the capacity to induce a defense response after exposure to abiotic stresses acquiring resistance towards pathogens. It was reported that when leaves of Arabidopsis thaliana were wounded or treated with a soft mechanical stimulation (SMS), they could resist much better the attack of the fungal pathogen Botrytis cinerea, and this effect was accompanied by an oxidative burst and the expression of touch-inducible genes (TCH). However, no further work was carried out to better characterize the induced defense response. In this paper, we report that TCH genes were identified for first time in the genomes of the strawberry species Fragaria ananassa (e.g. FaTCH2, FaTCH3, FaTCH4 and FaCML39) and Fragaria vesca (e.g. FvTCH2, FvTCH3, FvTCH4 and FvCML39). Phylogenetic studies revealed that F. ananassa TCH genes exhibited high similarity with the orthologous of F. vesca and lower with A. thaliana ones. We also present evidence that after SMS treatment on strawberry leaves, plants activate a rapid oxidative burst, callose deposition, and the up-regulation of TCH genes as well as plant defense genes such as FaPR1, FaCHI2-2, FaCAT, FaACS1 and FaOGBG-5. The latter represents the first report showing that TCH- and defense-induced genes participate in SMS-induced resistance in plants, bringing a rational explanation why plants exposed to a SMS treatment acquired an enhance resistance toward B. cinerea.

Holaphyllamine, a steroid, is able to induce defense responses in Arabidopsis thaliana and increases resistance against bacterial infection

MAIN CONCLUSION

A chemical screen of plant-derived compounds identified holaphyllamine, a steroid, able to trigger defense responses in Arabidopsis thaliana and improve resistance against the pathogenic bacterium Pseudomonas syringae pv tomato DC3000. A chemical screen of 1600 plant-derived compounds was conducted and allowed the identification of a steroid able to activate defense responses in A. thaliana at a concentration of 1 µM without altering growth. The identified compound is holaphyllamine (HPA) whose chemical structure is similar to steroid pregnanes of mammals. Our data show that HPA, which is not constitutively present in A. thaliana, is able to trigger the formation of reactive oxygen species, deposition of callose and expression of several pathogenesis-related genes of the salicylic and jasmonic acid pathways. In addition, the results show that pre-treatment of A. thaliana seedlings with HPA before infection with the pathogenic bacterium Pseudomonas syringae pv tomato DC3000 results in a significant reduction of symptoms (i.e., reduction of bacterial colonies). Using A. thaliana mutants, we have found that the activation of defense responses by HPA does not depend on BRI1/BAK1 receptor kinases. Finally, a structure/function study reveals that the minimal structure required for activity is a 5-pregnen-20-one steroid with an equatorial nucleophilic group in C-3. Together, these findings demonstrate that HPA can activate defense responses that lead to improved resistance against bacterial infection in A. thaliana.

Histochemical Analyses Reveal That Stronger Intrinsic Defenses in Gossypium barbadense Than in G. hirsutum Are Associated With Resistance to Verticillium dahliae

Verticillium wilt, caused by Verticillium dahliae Kleb., is a serious threat to cotton (Gossypium spp.) crop production. To enhance our understanding of the plant's complex defensive mechanism, we examined colonization patterns and interactions between V. dahliae and two cotton species, the resistant G. barbadense and the susceptible G. hirsutum. Microscopic examinations and grafting experiments showed that the progression of infection was restricted within G. barbadense. At all pre- and postinoculation sampling times, levels of salicylic acid (SA) were also higher in that species than in G. hirsutum. Comparative RNA-Seq analyses indicated that infection induced dramatic changes in the expression of thousands of genes in G. hirsutum, whereas those changes were fewer and weaker in G. barbadense. Investigations of the morphological and biochemical nature of cell-wall barriers demonstrated that depositions of lignin, phenolic compounds, and callose were significantly higher in G. barbadense. To determine the contribution of a known resistance gene to these processes, we silenced GbEDS1 and found that the transformed plants had decreased SA production, which led to the upregulation of PLASMODESMATA-LOCATED PROTEIN (PDLP) 1 and PDLP6. This was followed by a decline in callose deposition in the plasmodesmata, which then led to increased pathogen susceptibility. This comparison between resistant and susceptible species indicated that both physical and chemical mechanisms play important roles in the defenses of cotton against V. dahliae.

Regulation of cellulose synthesis in response to stress

The cell wall is a complex polysaccharide network that provides stability and protection to the plant and is one of the first layers of biotic and abiotic stimuli perception. A controlled remodeling of the primary cell wall is essential for the plant to adapt its growth to environmental stresses. Cellulose, the main component of plant cell walls is synthesized by plasma membrane-localized cellulose synthases moving along cortical microtubule tracks. Recent advancements demonstrate a tight regulation of cellulose synthesis at the primary cell wall by phytohormone networks. Stress-induced perturbations at the cell wall that modify cellulose synthesis and microtubule arrangement activate similar phytohormone-based stress response pathways. The integration of stress perception at the primary cell wall and downstream responses are likely to be tightly regulated by phytohormone signaling pathways in the context of cellulose synthesis and microtubule arrangement.

Components of the SNARE-containing regulon are co-regulated in root cells undergoing defense

The term regulon has been coined in the genetic model plant Arabidopsis thaliana, denoting a structural and physiological defense apparatus defined genetically through the identification of the penetration (pen) mutants. The regulon is composed partially by the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) syntaxin PEN1. PEN1 has homology to a Saccharomyces cerevisae gene that regulates a Secretion (Sec) protein, Suppressor of Sec 1 (Sso1p). The regulon is also composed of the β-glucosidase (PEN2) and an ATP binding cassette (ABC) transporter (PEN3). While important in inhibiting pathogen infection, limited observations have been made regarding the transcriptional regulation of regulon genes until now. Experiments made using the model agricultural Glycine max (soybean) have identified co-regulated gene expression of regulon components. The results explain the observation of hundreds of genes expressed specifically in the root cells undergoing the natural process of defense. Data regarding additional G. max genes functioning within the context of the regulon are presented here, including Sec 14, Sec 4 and Sec 23. Other examined G. max homologs of membrane fusion genes include an endosomal bromo domain-containing protein1 (Bro1), syntaxin6 (SYP6), SYP131, SYP71, SYP8, Bet1, coatomer epsilon (ϵ-COP), a coatomer zeta (ζ-COP) paralog and an ER to Golgi component (ERGIC) protein. Furthermore, the effectiveness of biochemical pathways that would function within the context of the regulon ave been examined, including xyloglucan xylosyltransferase (XXT), reticuline oxidase (RO) and galactinol synthase (GS). The experiments have unveiled the importance of the regulon during defense in the root and show how the deposition of callose relates to the process.

Measuring Callose Deposition, an Indicator of Cell Wall Reinforcement, During Bacterial Infection in Arabidopsis

The plant cell wall responds dynamically during interaction with various pathogens. Upon recognition of "nonself" components, plant cells deploy a variety of immune responses including cell wall fortification. Callose, a β-(1, 3)-D-glucan polymer, is a component of the material deposited at the site of infection between the plasma membrane and the preexisting cell wall that is hypothesized to serve as a physical barrier and platform for directed antimicrobial compound deposition. The defense-associated function of callose deposition is supported by its induction during pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI) and its inhibition by defense suppressing virulence effectors. Thus, callose deposition is a commonly monitored read-out in plant defense. This protocol describes the use of aniline blue staining and fluorescent microscopy to measure callose deposition in bacteria-infected or elicitor-challenged Arabidopsis leaf tissues.

Cellulose and callose synthesis and organization in focus, what's new?

Plant growth and development are supported by plastic but strong cell walls. These walls consist largely of polysaccharides that vary in content and structure. Most of the polysaccharides are produced in the Golgi apparatus and are then secreted to the apoplast and built into the growing walls. However, the two glucan polymers cellulose and callose are synthesized at the plasma membrane by cellulose or callose synthase complexes, respectively. Cellulose is the most common cell wall polymer in land plants and provides strength to the walls to support directed cell expansion. In contrast, callose is integral to specialized cell walls, such as the cell plate that separates dividing cells and growing pollen tube walls, and maintains important functions during abiotic and biotic stress responses. The last years have seen a dramatic increase in our understanding of how cellulose and callose are manufactured, and new factors that regulate the synthases have been identified. Much of this knowledge has been amassed via various microscopy-based techniques, including various confocal techniques and super-resolution imaging. Here, we summarize and synthesize recent findings in the fields of cellulose and callose synthesis in plant biology.

Neutrophil Attack Triggers Extracellular Trap-Dependent Candida Cell Wall Remodeling and Altered Immune Recognition

Pathogens hide immunogenic epitopes from the host to evade immunity, persist and cause infection. The opportunistic human fungal pathogen Candida albicans, which can cause fatal disease in immunocompromised patient populations, offers a good example as it masks the inflammatory epitope β-glucan in its cell wall from host recognition. It has been demonstrated previously that β-glucan becomes exposed during infection in vivo but the mechanism behind this exposure was unknown. Here, we show that this unmasking involves neutrophil extracellular trap (NET) mediated attack, which triggers changes in fungal cell wall architecture that enhance immune recognition by the Dectin-1 β-glucan receptor in vitro. Furthermore, using a mouse model of disseminated candidiasis, we demonstrate the requirement for neutrophils in triggering these fungal cell wall changes in vivo. Importantly, we found that fungal epitope unmasking requires an active fungal response in addition to the stimulus provided by neutrophil attack. NET-mediated damage initiates fungal MAP kinase-driven responses, particularly by Hog1, that dynamically relocalize cell wall remodeling machinery including Chs3, Phr1 and Sur7. Neutrophil-initiated cell wall disruptions augment some macrophage cytokine responses to attacked fungi. This work provides insight into host-pathogen interactions during disseminated candidiasis, including valuable information about how the C. albicans cell wall responds to the biotic stress of immune attack. Our results highlight the important but underappreciated concept that pattern recognition during infection is dynamic and depends on the host-pathogen dialog.

Opportunistic fungal infections, including those caused by C. albicans, have emerged as a significant global health burden and the disseminated form of these infections still have unacceptably high mortality rates despite modern antifungal treatments. The fungal cell wall controls its interaction with the host environment and immune recognition, although cell wall dynamics during infection are poorly understood. C. albicans organizes its cell wall to mask the inflammatory β-glucan as a form of immune evasion and it is known that during infection this β-glucan becomes exposed. Here, we investigated how β-glucan becomes exposed and discovered a dynamic interaction where host NETs provoke an active fungal response that disrupts cell wall architecture and unmasks β-glucan. We revealed an unexpected level of local fungal cell wall dynamics in response to immune mediated stress, suggesting this may represent a model that can be leveraged to identify novel drug targets. Our results highlight the understudied concept that the cell wall is a dynamic landscape during infection and can be influenced by the host.

Friends or foes? Emerging insights from fungal interactions with plants

Fungi interact with plants in various ways, with each interaction giving rise to different alterations in both partners. While fungal pathogens have detrimental effects on plant physiology, mutualistic fungi augment host defence responses to pathogens and/or improve plant nutrient uptake. Tropic growth towards plant roots or stomata, mediated by chemical and topographical signals, has been described for several fungi, with evidence of species-specific signals and sensing mechanisms. Fungal partners secrete bioactive molecules such as small peptide effectors, enzymes and secondary metabolites which facilitate colonization and contribute to both symbiotic and pathogenic relationships. There has been tremendous advancement in fungal molecular biology, omics sciences and microscopy in recent years, opening up new possibilities for the identification of key molecular mechanisms in plant-fungal interactions, the power of which is often borne out in their combination. Our fragmentary knowledge on the interactions between plants and fungi must be made whole to understand the potential of fungi in preventing plant diseases, improving plant productivity and understanding ecosystem stability. Here, we review innovative methods and the associated new insights into plant-fungal interactions.

Biotrophy at Its Best: Novel Findings and Unsolved Mysteries of the Arabidopsis-Powdery Mildew Pathosystem

It is generally accepted in plant-microbe interactions research that disease is the exception rather than a common outcome of pathogen attack. However, in nature, plants with symptoms that signify colonization by obligate biotrophic powdery mildew fungi are omnipresent. The pervasiveness of the disease and the fact that many economically important plants are prone to infection by powdery mildew fungi drives research on this interaction. The competence of powdery mildew fungi to establish and maintain true biotrophic relationships renders the interaction a paramount example of a pathogenic plant-microbe biotrophy. However, molecular details underlying the interaction are in many respects still a mystery. Since its introduction in 1990, the Arabidopsis-powdery mildew pathosystem has become a popular model to study molecular processes governing powdery mildew infection. Due to the many advantages that the host Arabidopsis offers in terms of molecular and genetic tools this pathosystem has great capacity to answer some of the questions of how biotrophic pathogens overcome plant defense and establish a persistent interaction that nourishes the invader while in parallel maintaining viability of the plant host.

Localization and Quantification of Callose in the Streptophyte Green Algae Zygnema and Klebsormidium: Correlation with Desiccation Tolerance

Freshwater green algae started to colonize terrestrial habitats about 460 million years ago, giving rise to the evolution of land plants. Today, several streptophyte green algae occur in aero-terrestrial habitats with unpredictable fluctuations in water availability, serving as ideal models for investigating desiccation tolerance. We tested the hypothesis that callose, a β-d-1,3-glucan, is incorporated specifically in strained areas of the cell wall due to cellular water loss, implicating a contribution to desiccation tolerance. In the early diverging genus Klebsormidium, callose was drastically increased already after 30 min of desiccation stress. Localization studies demonstrated an increase in callose in the undulating cross cell walls during cellular water loss, allowing a regulated shrinkage and expansion after rehydration. This correlates with a high desiccation tolerance demonstrated by a full recovery of the photosynthetic yield visualized at the subcellular level by Imaging-PAM. Furthermore, abundant callose in terminal cell walls might facilitate cell detachment to release dispersal units. In contrast, in the late diverging Zygnema, the callose content did not change upon desiccation for up to 3.5 h and was primarily localized in the corners between individual cells and at terminal cells. While these callose deposits still imply reduction of mechanical damage, the photosynthetic yield did not recover fully in the investigated young cultures of Zygnema upon rehydration. The abundance and specific localization of callose correlates with the higher desiccation tolerance in Klebsormidium when compared with Zygnema.

Glucanocellulosic ethanol: the undiscovered biofuel potential in energy crops and marine biomass

Converting biomass to biofuels is a key strategy in substituting fossil fuels to mitigate climate change. Conventional strategies to convert lignocellulosic biomass to ethanol address the fermentation of cellulose-derived glucose. Here we used super-resolution fluorescence microscopy to uncover the nanoscale structure of cell walls in the energy crops maize and Miscanthus where the typical polymer cellulose forms an unconventional layered architecture with the atypical (1, 3)-β-glucan polymer callose. This raised the question about an unused potential of (1, 3)-β-glucan in the fermentation of lignocellulosic biomass. Engineering biomass conversion for optimized (1, 3)-β-glucan utilization, we increased the ethanol yield from both energy crops. The generation of transgenic Miscanthus lines with an elevated (1, 3)-β-glucan content further increased ethanol yield providing a new strategy in energy crop breeding. Applying the (1, 3)-β-glucan-optimized conversion method on marine biomass from brown macroalgae with a naturally high (1, 3)-β-glucan content, we not only substantially increased ethanol yield but also demonstrated an effective co-fermentation of plant and marine biomass. This opens new perspectives in combining different kinds of feedstock for sustainable and efficient biofuel production, especially in coastal regions.

Interplays between the cell wall and phytohormones in interaction between plants and necrotrophic pathogens

The plant cell wall surrounds every cell in plants. During microbial infection, the cell wall provides a dynamic interface for interaction with necrotrophic phytopathogens as a rich source of carbohydrates for the growth of pathogens, as a physical barrier restricting the progression of the pathogens, and as an integrity sensory system that can activate intracellular signaling cascades and ultimately lead to a multitude of inducible host defense responses. Studies over the last decade have provided evidence of interplays between the cell wall and phytohormone signaling. This review summarizes the current state of knowledge about the cell wall-phytohormone interplays, with the focus on auxin, cytokinin, brassinosteroids, and abscisic acid, and discuss how they impact the outcome of plant-necrotrophic pathogen interaction.

Experimental approaches to study plant cell walls during plant-microbe interactions

Plant cell walls provide physical strength, regulate the passage of bio-molecules, and act as the first barrier of defense against biotic and abiotic stress. In addition to providing structural integrity, plant cell walls serve an important function in connecting cells to their extracellular environment by sensing and transducing signals to activate cellular responses, such as those that occur during pathogen infection. This mini review will summarize current experimental approaches used to study cell wall functions during plant-pathogen interactions. Focus will be paid to cell imaging, spectroscopic analyses, and metabolic profiling techniques.

Callose biosynthesis in Arabidopsis with a focus on pathogen response: what we have learned within the last decade

BACKGROUND

(1,3)-β-Glucan callose is a cell wall polymer that is involved in several fundamental biological processes, ranging from plant development to the response to abiotic and biotic stresses. Despite its importance in maintaining plant integrity and plant defence, knowledge about the regulation of callose biosynthesis at its diverse sites of action within the plant is still limited. The moderately sized family of GSL (GLUCAN SYNTHASE-LIKE) genes is predicted to encode callose synthases with a specific biological function and subcellular localization. Phosphorylation and directed translocation of callose synthases seem to be key post-translational mechanisms of enzymatic regulation, whereas transcriptional control of GSL genes might only have a minor function in response to biotic or abiotic stresses.

SCOPE AND CONCLUSIONS

Among the different sites of callose biosynthesis within the plant, particular attention has been focused on the formation of callose in response to pathogen attack. Here, callose is deposited between the plasma membrane and the cell wall to act as a physical barrier to stop or slow invading pathogens. Arabidopsis (Arabidopsis thaliana) is one of the best-studied models not only for general plant defence responses but also for the regulation of pathogen-induced callose biosynthesis. Callose synthase GSL5 (GLUCAN SYNTHASE-LIKE5) has been shown to be responsible for stress-induced callose deposition. Within the last decade of research into stress-induced callose, growing evidence has been found that the timing of callose deposition in the multilayered system of plant defence responses could be the key parameter for optimal effectiveness. This timing seems to be achieved through co-ordinated transport and formation of the callose synthase complex.

The Podosphaera xanthii haustorium, the fungal Trojan horse of cucurbit-powdery mildew interactions

The powdery mildew fungi are obligate biotrophic plant pathogens that develop a specialized structure for parasitism termed haustorium, which is responsible for nutrient uptake and factor exchange with the plant. In this work, we present a detailed microscopy analysis of the haustoria of the cucurbit powdery mildew fungus Podosphaera xanthii, a major limiting factor for cucurbit production worldwide. Despite being located inside plant epidermal cells, transmission electron microscopy (TEM) analysis showed the characteristic highly irregular outline of the extrahaustorial membrane that separates the extrahaustorial matrix of haustoria from the cytoplasm of the plant cell. TEM analysis also revealed the presence of some vesicles and electron-dense plaques of material surrounding the haustoria. In confocal microscopy analysis and aniline blue staining we found a positive correlation between haustorial development and deposition of callose, which is distributed as plaques around haustorial complex. In this study, a method for the isolation of P. xanthii haustoria was also adapted, which permitted the analysis of the formation of haustorial lobes and the visualization of vacuoles and the pool of vesicles inside the haustorial complex. Our findings suggested that the haustorial lobes were responsible for vesicular trafficking and most likely act as the main mediators of the fungus-plant dialogue. All of these findings were integrated into a model of the P. xanthii-host cellular interactions.

Visualization and analysis of hepatitis C virus structural proteins at lipid droplets by super-resolution microscopy

Cytosolic lipid droplets are central organelles in the Hepatitis C Virus (HCV) life cycle. The viral capsid protein core localizes to lipid droplets and initiates the production of viral particles at lipid droplet–associated ER membranes. Core is thought to encapsidate newly synthesized viral RNA and, through interaction with the two envelope proteins E1 and E2, bud into the ER lumen. Here, we visualized the spatial distribution of HCV structural proteins core and E2 in vicinity of small lipid droplets by three-color 3D super-resolution microscopy. We observed and analyzed small areas of colocalization between the two structural proteins in HCV-infected cells with a diameter of approximately 100 nm that might represent putative viral assembly sites.

Interaction of the Arabidopsis GTPase RabA4c with its effector PMR4 results in complete penetration resistance to powdery mildew

The (1,3)-β-glucan callose is a major component of cell wall thickenings in response to pathogen attack in plants. GTPases have been suggested to regulate pathogen-induced callose biosynthesis. To elucidate the regulation of callose biosynthesis in Arabidopsis thaliana, we screened microarray data and identified transcriptional upregulation of the GTPase RabA4c after biotic stress. We studied the function of RabA4c in its native and dominant negative (dn) isoform in RabA4c overexpression lines. RabA4c overexpression caused complete penetration resistance to the virulent powdery mildew Golovinomyces cichoracearum due to enhanced callose deposition at early time points of infection, which prevented fungal ingress into epidermal cells. By contrast, RabA4c(dn) overexpression did not increase callose deposition or penetration resistance. A cross of the resistant line with the pmr4 disruption mutant lacking the stress-induced callose synthase PMR4 revealed that enhanced callose deposition and penetration resistance were PMR4-dependent. In live-cell imaging, tagged RabA4c was shown to localize at the plasma membrane prior to infection, which was broken in the pmr4 disruption mutant background, with callose deposits at the site of attempted fungal penetration. Together with our interactions studies including yeast two-hybrid, pull-down, and in planta fluorescence resonance energy transfer assays, we concluded that RabA4c directly interacts with PMR4, which can be seen as an effector of this GTPase.

The use of nanoscale fluorescence microscopic to decipher cell wall modifications during fungal penetration

Plant diseases are one of the most studied subjects in the field of plant science due to their impact on crop yield and food security. Our increased understanding of plant-pathogen interactions was mainly driven by the development of new techniques that facilitated analyses on a subcellular and molecular level. The development of labeling technologies, which allowed the visualization and localization of cellular structures and proteins in live cell imaging, promoted the use of fluorescence and laser-scanning microscopy in the field of plant-pathogen interactions. Recent advances in new microscopic technologies opened their application in plant science and in the investigation of plant diseases. In this regard, in planta Förster/Fluorescence resonance energy transfer has demonstrated to facilitate the measurement of protein-protein interactions within the living tissue, supporting the analysis of regulatory pathways involved in plant immunity and putative host-pathogen interactions on a nanoscale level. Localization microscopy, an emerging, non-invasive microscopic technology, will allow investigations with a nanoscale resolution leading to new possibilities in the understanding of molecular processes.

Callose-mediated resistance to pathogenic intruders in plant defense-related papillae

Plants are exposed to a wide range of potential pathogens, which derive from diverse phyla. Therefore, plants have developed successful defense mechanisms during co-evolution with different pathogens. Besides many specialized defense mechanisms, the plant cell wall represents a first line of defense. It is actively reinforced through the deposition of cell wall appositions, so-called papillae, at sites of interaction with intruding microbial pathogens. The papilla is a complex structure that is formed between the plasma membrane and the inside of the plant cell wall. Even though the specific biochemical composition of papillae can vary between different plant species, some classes of compounds are commonly found which include phenolics, reactive oxygen species, cell wall proteins, and cell wall polymers. Among these polymers, the (1,3)-β-glucan callose is one of the most abundant and ubiquitous components. Whereas the function of most compounds could be directly linked with cell wall reinforcement or an anti-microbial effect, the role of callose has remained unclear. An evaluation of recent studies revealed that the timing of the different papilla-forming transport processes is a key factor for successful plant defense.