A barley ROP GTPase ACTIVATING PROTEIN associates with microtubules and regulates entry of the barley powdery mildew fungus into leaf epidermal cells

Three Rho of Plant (ROP)-GTPase regulatory proteins control ROP-mediated alkaloid biosynthesis in Catharanthus roseus

Rho of plant (ROP)-GTPase regulatory proteins (RGRPs) have been shown to control plant morphogenesis, development, and immunity; however, their role in specialized metabolism remains unknown. Here, we demonstrate that specific RGRPs control monoterpene indole alkaloid (MIA) biosynthesis by interacting with distinct ROPs in Madagascar periwinkle (Catharanthus roseus). Among the 5 guanine nucleotide exchange factors (GEFs), 4 GTPase-activating proteins (GAPs), and 2 GDP dissociation inhibitors (GDIs) identified in the C. roseus genome, only CrGEF1, CrGAP1, and CrGDI2 specifically interacted with CrROP3 and CrROP5. These RGRPs displayed distinct cytosolic and/or membrane localization patterns, with their transcripts predominantly expressed in aerial tissues. Functional studies revealed that CrGEF1 acts as a positive regulator of MIA biosynthesis, as silencing its gene led to a reduction in MIA production, while overexpression enhanced MIA levels. Conversely, CrGAP1 and CrGDI2 function as negative regulators, with silencing resulting in increased MIA production and overexpression causing reduced MIA levels. Notably, terminal truncated forms of these RGRPs showed interaction with CrROP3 or CrROP5 but failed to influence MIA biosynthesis, underscoring the importance of these domains in their regulatory functions. Overall, our findings uncover a mechanism by which distinct RGRPs coordinate with specific ROPs to regulate transcription factors and fine-tune MIA biosynthesis in C. roseus.

Microtubule-mediated defence reaction of grapevine to Neofusicoccum parvum via the transcription factor VrWRKY22 promoting the kinesin-like protein VrKIN10C

Grapevine Trunk Diseases (GTDs) are among the most destructive diseases in viticulture due to global climate change. Some causal agents like Neofusicoccum parvum can be latent endophytic and become pathogenic under abiotic stress. Microtubules (MTs) have been found to play a role in mediating the pathogen-related signaling in grapevine. In this study, a novel transcription factor VrWRKY22 was identified and cloned from the native American grapevine Vitis rupestris. Leaves of the table grape variety 'Kyoho' (V. vinifera × V. labrusca L.) overexpressing VrWRKY22 showed less necroses after N. parvum Bt-67 inoculation and activated signaling pathways. VrWRKY22 interacted with VrMPK3 and then bounded to the TTGACC motif in the promoter of VrKIN10C, which was confirmed by Y2H and Y1H assays. Since VrKIN10C is one of the important kinesin-like proteins associated with MTs, a grapevine MT marker line overexpressing VrWRKY22 was generated to test the responses of grapevine cells to N. parvum Bt-67. An increased number of prompt movement proteins can be traced within the peri-nuclear MTs and along the cortical MTs. The skewness and thickness of both central and cortical MTs were significantly increased. Moreover, a prominent (resulting from both the number and the rate) accumulation of speckles appeared in the nucleus and cortical MTs. A significant reduction in cell mortality and a stronger antioxidant capacity were detected. This study demonstrates that VrWRKY22 plays positive roles during N. parvum Bt-67 invasion by rapidly increasing the concentration and dynamics of MTs in the peri-nuclear and cortical regions via VrKIN10, and will facilitate the interpretation of the results of further GTD mitigation studies.

A Ralstonia solanacearum type III effector alters the actin and microtubule cytoskeleton to promote bacterial virulence in plants

Cellular responses to biotic stress frequently involve signaling pathways that are conserved across eukaryotes. These pathways include the cytoskeleton, a proteinaceous network that senses external cues at the cell surface and signals to interior cellular components. During biotic stress, dynamic cytoskeletal rearrangements serve as a platform from which early immune-associated processes are organized and activated. Bacterial pathogens of plants and animals use proteins called type III effectors (T3Es) to interfere with host immune signaling, thereby promoting virulence. We previously found that RipU, a T3E from the soilborne phytobacterial pathogen Ralstonia solanacearum, co-localizes with the plant cytoskeleton. Here, we show that RipU from R. solanacearum K60 (RipUK60) associated with and altered the organization of both the actin and microtubule cytoskeleton. We found that pharmacological disruption of the tomato (Solanum lycopersicum) cytoskeleton promoted R. solanacearum K60 colonization. Importantly, tomato plants inoculated with R. solanacearum K60 lacking RipUK60 (ΔripUK60) had reduced wilting symptoms and significantly reduced root colonization when compared to plants inoculated with wild-type R. solanacearum K60. Collectively, our data suggest that R. solanacearum K60 uses the type III effector RipUK60 to remodel cytoskeletal organization, thereby promoting pathogen virulence.

Vesicle trafficking pathways in defence-related cell wall modifications: papillae and encasements

Filamentous pathogens that cause plant diseases such as powdery mildew, rust, anthracnose, and late blight continue to represent an enormous challenge for farmers worldwide. Interestingly, these pathogens, although phylogenetically distant, initiate pathogenesis in a very similar way by penetrating the cell wall and establishing a feeding structure inside the plant host cell. To prevent pathogen ingress, the host cell responds by forming defence structures known as papillae and encasements that are thought to mediate pre- and post-invasive immunity, respectively. This form of defence is evolutionarily conserved in land plants and is highly effective and durable against a broad selection of non-adapted filamentous pathogens. As most pathogens have evolved strategies to overcome the defences of only a limited range of host plants, the papilla/encasement response could hold the potential to become an optimal transfer of resistance from one plant species to another. In this review I lay out current knowledge of the involvement of membrane trafficking that forms these important defence structures and highlight some of the questions that still need to be resolved.

Cloning and Functional Analysis of CsROP5 and CsROP10 Genes Involved in Cucumber Resistance to Corynespora cassiicola

The cloning of resistance-related genes CsROP5/CsROP10 and the analysis of their mechanism of action provide a theoretical basis for the development of molecular breeding of disease-resistant cucumbers. The structure domains of two Rho-related guanosine triphosphatases from plant (ROP) genes were systematically analyzed using the bioinformatics method in cucumber plants, and the genes CsROP5 (Cucsa.322750) and CsROP10 (Cucsa.197080) were cloned. The functions of the two genes were analyzed using reverse-transcription quantitative PCR (RT-qPCR), virus-induced gene silencing (VIGS), transient overexpression, cucumber genetic transformation, and histochemical staining technology. The conserved elements of the CsROP5/CsROP10 proteins include five sequence motifs (G1-G5), a recognition site for serine/threonine kinases, and a hypervariable region (HVR). The knockdown of CsROP10 through VIGS affected the transcript levels of ABA-signaling-pathway-related genes (CsPYL, CsPP2Cs, CsSnRK2s, and CsABI5), ROS-signaling-pathway-related genes (CsRBOHD and CsRBOHF), and defense-related genes (CsPR2 and CsPR3), thereby improving cucumber resistance to Corynespora cassiicola. Meanwhile, inhibiting the expression of CsROP5 regulated the expression levels of ROS-signaling-pathway-related genes (CsRBOHD and CsRBOHF) and defense-related genes (CsPR2 and CsPR3), thereby enhancing the resistance of cucumber to C. cassiicola. Overall, CsROP5 and CsROP10 may participate in cucumber resistance to C. cassiicola through the ROS and ABA signaling pathways.

Rho of Plants patterning: linking mathematical models and molecular diversity

ROPs (Rho of Plants) are plant specific small GTPases involved in many membrane patterning processes and play important roles in the establishment and communication of cell polarity. These small GTPases can produce a wide variety of patterns, ranging from a single cluster in tip-growing root hairs and pollen tubes to an oriented stripe pattern controlling protoxylem cell wall deposition. For an understanding of what controls these various patterns, models are indispensable. Consequently, many modelling studies on small GTPase patterning exist, often focusing on yeast or animal cells. Multiple patterns occurring in plants, however, require the stable co-existence of multiple active ROP clusters, which does not occur with the most common yeast/animal models. The possibility of such patterns critically depends on the precise model formulation. Additionally, different small GTPases are usually treated interchangeably in models, even though plants possess two types of ROPs with distinct molecular properties, one of which is unique to plants. Furthermore, the shape and even the type of ROP patterns may be affected by the cortical cytoskeleton, and cortex composition and anisotropy differ dramatically between plants and animals. Here, we review insights into ROP patterning from modelling efforts across kingdoms, as well as some outstanding questions arising from these models and recent experimental findings.

Two subtypes of GTPase-activating proteins coordinate tip growth and cell size regulation in Physcomitrium patens

The establishment of cell polarity is a prerequisite for many developmental processes. However, how it is achieved during tip growth in plants remains elusive. Here, we show that the RHO OF PLANTs (ROPs), ROP GUANINE NUCLEOTIDE EXCHANGE FACTORs (RopGEFs), and ROP GTPASE-ACTIVATING PROTEINs (RopGAPs) assemble into membrane domains in tip-growing cells of the moss Physcomitrium patens. The confinement of membrane domains requires redundant global inactivation of ROPs by PpRopGAPs and the PLECKSTRIN HOMOLOGY (PH) domain-containing RenGAP PpREN. Unexpectedly, PpRopGAPs and PpREN exert opposing effects on domain size and cell width upon overexpression. Biochemical and functional analyses indicate that PpRopGAPs are recruited to the membrane by active ROPs to restrict domain size through clustering, whereas PpREN rapidly inactivates ROPs and inhibits PpRopGAP-induced clustering. We propose that the activity- and clustering-based domain organization by RopGAPs and RenGAPs is a general mechanism for coordinating polarized cell growth and cell size regulation in plants.

Emerging role of small GTPases and their interactome in plants to combat abiotic and biotic stress

Plants are frequently subjected to abiotic and biotic stress which causes major impediments in their growth and development. It is emerging that small guanosine triphosphatases (small GTPases), also known as monomeric GTP-binding proteins, assist plants in managing environmental stress. Small GTPases function as tightly regulated molecular switches that get activated with the aid of guanosine triphosphate (GTP) and deactivated by the subsequent hydrolysis of GTP to guanosine diphosphate (GDP). All small GTPases except Rat sarcoma (Ras) are found in plants, including Ras-like in brain (Rab), Rho of plant (Rop), ADP-ribosylation factor (Arf) and Ras-like nuclear (Ran). The members of small GTPases in plants interact with several downstream effectors to counteract the negative effects of environmental stress and disease-causing pathogens. In this review, we describe processes of stress alleviation by developing pathways involving several small GTPases and their associated proteins which are important for neutralizing fungal infections, stomatal regulation, and activation of abiotic stress-tolerant genes in plants. Previous reviews on small GTPases in plants were primarily focused on Rab GTPases, abiotic stress, and membrane trafficking, whereas this review seeks to improve our understanding of the role of all small GTPases in plants as well as their interactome in regulating mechanisms to combat abiotic and biotic stress. This review brings to the attention of scientists recent research on small GTPases so that they can employ genome editing tools to precisely engineer economically important plants through the overexpression/knock-out/knock-in of stress-related small GTPase genes.

Barley RIC157, a potential RACB scaffold protein, is involved in susceptibility to powdery mildew

CRIB motif-containing barley RIC157 is a novel ROP scaffold protein that interacts directly with barley RACB, promotes susceptibility to fungal penetration, and colocalizes with RACB at the haustorial neck. Successful obligate pathogens benefit from host cellular processes. For the biotrophic ascomycete fungus Blumeria hordei (Bh) it has been shown that barley RACB, a small monomeric G-protein (ROP, Rho of plants), is required for full susceptibility to fungal penetration. The susceptibility function of RACB probably lies in its role in cell polarity, which may be co-opted by the pathogen for invasive ingrowth of its haustorium. However, how RACB supports fungal penetration success and which other host proteins coordinate this process is incompletely understood. RIC (ROP-Interactive and CRIB-(Cdc42/Rac Interactive Binding) motif-containing) proteins are considered scaffold proteins which can interact directly with ROPs via a conserved CRIB motif. Here we describe a previously uncharacterized barley RIC protein, RIC157, which can interact directly with RACB in planta. We show that, in the presence of constitutively activated RACB, RIC157 shows a localization at the cell periphery/plasma membrane, whereas it otherwise localizes to the cytoplasm. RIC157 appears to mutually stabilize the plasma membrane localization of the activated ROP. During fungal infection, RIC157 and RACB colocalize at the penetration site, particularly at the haustorial neck. Additionally, transiently overexpressed RIC157 renders barley epidermal cells more susceptible to fungal penetration. We discuss that RIC157 may promote fungal penetration into barley epidermal cells by operating probably downstream of activated RACB.

The Cytoskeleton in Plant Immunity: Dynamics, Regulation, and Function

The plant cytoskeleton, consisting of actin filaments and microtubules, is a highly dynamic filamentous framework involved in plant growth, development, and stress responses. Recently, research has demonstrated that the plant cytoskeleton undergoes rapid remodeling upon sensing pathogen attacks, coordinating the formation of microdomain immune complexes, the dynamic and turnover of pattern-recognizing receptors (PRRs), the movement and aggregation of organelles, and the transportation of defense compounds, thus serving as an important platform for responding to pathogen infections. Meanwhile, pathogens produce effectors targeting the cytoskeleton to achieve pathogenicity. Recent findings have uncovered several cytoskeleton-associated proteins mediating cytoskeletal remodeling and defense signaling. Furthermore, the reorganization of the actin cytoskeleton is revealed to further feedback-regulate reactive oxygen species (ROS) production and trigger salicylic acid (SA) signaling, suggesting an extremely complex role of the cytoskeleton in plant immunity. Here, we describe recent advances in understanding the host cytoskeleton dynamics upon sensing pathogens and summarize the effectors that target the cytoskeleton. We highlight advances in the regulation of cytoskeletal remodeling associated with the defense response and assess the important function of the rearrangement of the cytoskeleton in the immune response. Finally, we propose suggestions for future research in this area.

Barley guanine nucleotide exchange factor HvGEF14 is an activator of the susceptibility factor HvRACB and supports host cell entry by Blumeria graminis f. sp. hordei

In barley (Hordeum vulgare), signalling rat sarcoma homolog (RHO) of plants guanosine triphosphate hydrolases (ROP GTPases) support the penetration success of Blumeria graminis f. sp. hordei but little is known about ROP activation. Guanine nucleotide exchange factors (GEFs) facilitate the exchange of ROP-bound GDP for GTP and thereby turn ROPs into a signalling-activated ROP-GTP state. Plants possess a unique class of GEFs harbouring a plant-specific ROP nucleotide exchanger domain (PRONE). Here, we performed phylogenetic analyses and annotated barley PRONE-GEFs. The leaf epidermal-expressed PRONE-GEF HvGEF14 undergoes a transcriptional down-regulation on inoculation with B. graminis f. sp. hordei and directly interacts with the ROP GTPase and susceptibility factor HvRACB in yeast and in planta. Overexpression of activated HvRACB or of HvGEF14 led to the recruitment of ROP downstream interactor HvRIC171 to the cell periphery. HvGEF14 further supported direct interaction of HvRACB with a HvRACB-GTP-binding CRIB (Cdc42/Rac Interactive Binding motif) domain-containing HvRIC171 truncation. Finally, the overexpression of HvGEF14 caused enhanced susceptibility to fungal entry, while HvGEF14 RNAi provoked a trend to more penetration resistance. HvGEF14 might therefore play a role in the activation of HvRACB in barley epidermal cells during fungal penetration.

Host nuclear repositioning and actin polarization towards the site of penetration precedes fungal ingress during compatible pea-powdery mildew interactions

MAIN CONCLUSION

Actin polarization and actin-driven host nuclear movement towards the fungal penetration site facilitates successful host colonization during compatible pea-Erysiphe pisi interactions. Proper nuclear positioning in plant cells is crucial for developmental processes and response to (a)biotic stimuli. During plant-fungal interactions, the host nucleus moves toward the infection site, a process regulated by the plant cytoskeleton. Notably, rearrangement of the plant cytoskeleton is one of the earliest cellular responses to pathogen invasion and is known to impact penetration efficiency. Yet, the connection between host nuclear movement and fungal ingress is still elusive, particularly in legumes. Here, we investigated the host nuclear dynamics during compatible interactions between Pisum sativum (pea) and the adapted powdery mildew (PM) fungus Erysiphe pisi to gain insights into the functional relevance of PM-induced nuclear movement in legumes. We show that the host nucleus moves towards the fungal appressorium before penetration and becomes associated with the primary haustorium. However, the nucleus migrates away from the primary infection site as the infection progresses toward colony expansion and sporulation. Treatment of pea leaves with the actin-polymerization inhibitor, cytochalasin D, abolished host nuclear movement towards the fungal penetration site and restricted PM growth. In contrast, treatment with oryzalin, a microtubule-polymerization inhibitor, had no effect. In addition to nuclear movement, strong polarization of host actin filaments towards the site of appressorial contact was evident at early infection stages. Our results suggest that actin focusing mediates host nuclear movement to the fungal penetration site and facilitates successful colonization during compatible pea-PM interactions.

Research on the Molecular Interaction Mechanism between Plants and Pathogenic Fungi

Plant diseases caused by fungi are one of the major threats to global food security and understanding the interactions between fungi and plants is of great significance for plant disease control. The interaction between pathogenic fungi and plants is a complex process. From the perspective of pathogenic fungi, pathogenic fungi are involved in the regulation of pathogenicity by surface signal recognition proteins, MAPK signaling pathways, transcription factors, and pathogenic factors in the process of infecting plants. From the perspective of plant immunity, the signal pathway of immune response, the signal transduction pathway that induces plant immunity, and the function of plant cytoskeleton are the keys to studying plant resistance. In this review, we summarize the current research progress of fungi–plant interactions from multiple aspects and discuss the prospects and challenges of phytopathogenic fungi and their host interactions.

Posttranslational modification of the RHO of plants protein RACB by phosphorylation and cross-kingdom conserved ubiquitination

Small RHO-type G-proteins act as signaling hubs and master regulators of polarity in eukaryotic cells. Their activity is tightly controlled, as defective RHO signaling leads to aberrant growth and developmental defects. Two major processes regulate G-protein activity: canonical shuttling between different nucleotide bound states and posttranslational modification (PTM), of which the latter can support or suppress RHO signaling, depending on the individual PTM. In plants, regulation of Rho of plants (ROPs) signaling activity has been shown to act through nucleotide exchange and GTP hydrolysis, as well as through lipid modification, but there is little data available on phosphorylation or ubiquitination of ROPs. Hence, we applied proteomic analyses to identify PTMs of the barley ROP RACB. We observed in vitro phosphorylation by barley ROP binding kinase 1 and in vivo ubiquitination of RACB. Comparative analyses of the newly identified RACB phosphosites and human RHO protein phosphosites revealed conservation of modified amino acid residues, but no overlap of actual phosphorylation patterns. However, the identified RACB ubiquitination site is conserved in all ROPs from Hordeum vulgare, Arabidopsis thaliana and Oryza sativa and in mammalian Rac1 and Rac3. Point mutation of this ubiquitination site leads to stabilization of RACB. Hence, this highly conserved lysine residue may regulate protein stability across different kingdoms.

Arabidopsis pavement cell shape formation involves spatially confined ROPGAP regulators

In many plant species, pavement cell development relies on the coordinated formation of interdigitating lobes and indentations. Polarity signaling via the activity of antagonistic Rho-related GTPases from plants (ROPs) was implicated in pavement cell development, but the spatiotemporal regulation remained unclear. Here, we report on the role of the PLECKSTRIN HOMOLOGY GTPase ACTIVATING PROTEINS (PHGAPS) during multipolar growth in pavement cell shape establishment. Loss of function in phgap1phgap2 double mutants severely affected the shape of Arabidopsis leaf epidermal pavement cells. Predominantly, PHGAPs interacted with ROP2 and displayed a distinct and microtubule-dependent enrichment along the anticlinal cell face and transfacial boundary of pavement cell indentation regions. This localization was established upon undulation initiation and was maintained throughout the expansion of the cell. Our data suggest that PHGAP1/REN2 and PHGAP2/REN3 are key players in the establishment of ROP2 activity gradients and underscore the importance of locally controlled ROP activity for the orchestrated establishment of multipolarity in epidermal cells.

The small GTP-binding protein TaRop10 interacts with TaTrxh9 and functions as a negative regulator of wheat resistance against the stripe rust

Small GTP-binding proteins, also known as ROPs (Rho of Plants), are a subfamily of the Ras superfamily of signaling G-proteins and are required for numerous signaling processes, ranging from growth and development to biotic and abiotic signaling. In this study, we cloned and characterized wheat TaRop10, a homolog of Arabidopsis ROP10 and member of the class II ROP, and uncovered a role for TaRop10 in wheat response to Puccinia striiformis f. sp. tritici (Pst). TaRop10 was downregulated by actin depolymerization and was observed to be differentially induced by abiotic stress and the perception of plant hormones. A combination of yeast two-hybrid and bimolecular fluorescence complementation assays revealed that TaRop10 interacted with a h-type thioredoxin (TaTrxh9). Knocking-down of TaRop10 and TaTrxh9 was performed using the BSMV-VIGS (barley stripe mosaic virus-based virus-induced gene silencing) technique and revealed that TaRop10 and TaTrxh9 play a role in the negative regulation of defense signaling in response to Pst infection. In total, the data presented herein further illuminate our understanding of how intact plant cells accommodate fungal infection structures, and furthermore, support the function of TaRop10 and TaTrxh9 in negative modulation of defense signaling in response to stripe rust infection.

Recognition and defence of plant-infecting fungal pathogens

Attempted infections of plants with fungi result in diverse outcomes ranging from symptom-less resistance to severe disease and even death of infected plants. The deleterious effect on crop yield have led to intense focus on the cellular and molecular mechanisms that explain the difference between resistance and susceptibility. This research has uncovered plant resistance or susceptibility genes that explain either dominant or recessive inheritance of plant resistance with many of them coding for receptors that recognize pathogen invasion. Approaches based on cell biology and phytochemistry have contributed to identifying factors that halt an invading fungal pathogen from further invasion into or between plant cells. Plant chemical defence compounds, antifungal proteins and structural reinforcement of cell walls appear to slow down fungal growth or even prevent fungal penetration in resistant plants. Additionally, the hypersensitive response, in which a few cells undergo a strong local immune reaction, including programmed cell death at the site of infection, stops in particular biotrophic fungi from spreading into surrounding tissue. In this review, we give a general overview of plant recognition and defence of fungal parasites tracing back to the early 20th century with a special focus on Triticeae and on the progress that was made in the last 30 years.

A holistic view on plant effector-triggered immunity presented as an iceberg model

The immune system of plants is highly complex. It involves pattern-triggered immunity (PTI), which is signaled and manifested through branched multi-step pathways. To counteract this, pathogen effectors target and inhibit individual PTI steps. This in turn can cause specific plant cytosolic nucleotide-binding leucine-rich repeat (NLR) receptors to activate effector-triggered immunity (ETI). Plants and pathogens have many genes encoding NLRs and effectors, respectively. Yet, only a few segregate genetically as resistance (R) genes and avirulence (Avr) effector genes in wild-type populations. In an attempt to explain this contradiction, a model is proposed where far most of the NLRs, the effectors and the effector targets keep one another in a silent state. In this so-called "iceberg model", a few NLR-effector combinations are genetically visible above the surface, while the vast majority is hidden below. Besides, addressing the existence of many NLRs and effectors, the model also helps to explain why individual downregulation of many effectors causes reduced virulence and why many lesion-mimic mutants are found. Finally, the iceberg model accommodates genuine plant susceptibility factors as potential effector targets.

ROP INTERACTIVE PARTNER b Interacts with RACB and Supports Fungal Penetration into Barley Epidermal Cells

Rho of Plants (ROP) G-proteins are key components of cell polarization processes in plant development. The barley (Hordeum vulgare) ROP protein RACB is a susceptibility factor in the interaction of barley with the barley powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh). RACB also drives polar cell development, and this function might be coopted during the formation of fungal haustoria in barley epidermal cells. To understand RACB signaling during the interaction of barley with Bgh, we searched for potential downstream interactors of RACB. Here, we show that ROP INTERACTIVE PARTNER b (RIPb; synonym: INTERACTOR OF CONSTITUTIVE ACTIVE ROP b) directly interacts with RACB in yeast and in planta. Overexpression of RIPb supports the susceptibility of barley to Bgh RIPb further interacts with itself at microtubules. However, the interaction with activated RACB largely takes place at the plasma membrane. Both RIPb and RACB are recruited to the site of fungal attack around the neck of developing haustoria, suggesting locally enhanced ROP activity. We further assigned different functions to different domains of the RIPb protein. The N-terminal coiled-coil CC1 domain is required for microtubule localization, while the C-terminal coiled-coil CC2 domain is sufficient to interact with RACB and to fulfill a function in susceptibility at the plasma membrane. Hence, RIPb appears to be localized at microtubules and is then recruited by activated RACB for a function at the plasma membrane during formation of the haustorial complex.

Regulation and Functions of ROP GTPases in Plant-Microbe Interactions

Rho proteins of plants (ROPs) form a specific clade of Rho GTPases, which are involved in either plant immunity or susceptibility to diseases. They are intensively studied in grass host plants, in which ROPs are signaling hubs downstream of both cell surface immune receptor kinases and intracellular nucleotide-binding leucine-rich repeat receptors, which activate major branches of plant immune signaling. Additionally, invasive fungal pathogens may co-opt the function of ROPs for manipulation of the cytoskeleton, cell invasion and host cell developmental reprogramming, which promote pathogenic colonization. Strikingly, mammalian bacterial pathogens also initiate both effector-triggered susceptibility for cell invasion and effector-triggered immunity via Rho GTPases. In this review, we summarize central concepts of Rho signaling in disease and immunity of plants and briefly compare them to important findings in the mammalian research field. We focus on Rho activation, downstream signaling and cellular reorganization under control of Rho proteins involved in disease progression and pathogen resistance.

Auxin-induced signaling protein nanoclustering contributes to cell polarity formation

Cell polarity is fundamental to the development of both eukaryotes and prokaryotes, yet the mechanisms behind its formation are not well understood. Here we found that, phytohormone auxin-induced, sterol-dependent nanoclustering of cell surface transmembrane receptor kinase 1 (TMK1) is critical for the formation of polarized domains at the plasma membrane (PM) during the morphogenesis of cotyledon pavement cells (PC) in Arabidopsis. Auxin-induced TMK1 nanoclustering stabilizes flotillin1-associated ordered nanodomains, which in turn promote the nanoclustering of ROP6 GTPase that acts downstream of TMK1 to regulate cortical microtubule organization. In turn, cortical microtubules further stabilize TMK1- and flotillin1-containing nanoclusters at the PM. Hence, we propose a new paradigm for polarity formation: A diffusive signal triggers cell polarization by promoting cell surface receptor-mediated nanoclustering of signaling components and cytoskeleton-mediated positive feedback that reinforces these nanodomains into polarized domains.

TaRac6 Is a Potential Susceptibility Factor by Regulating the ROS Burst Negatively in the Wheat-Puccinia striiformis f. sp. tritici Interaction

Rac/Rop proteins play important roles in the regulation of cell growth and plant defense responses. However, the function of Rac/Rop proteins in wheat remains largely unknown. In this study, a small G protein gene, designated as TaRac6, was characterized from wheat (Triticum aestivum) in response to Puccinia striiformis f. sp. tritici (Pst) and was found to be highly homologous to the Rac proteins identified in other plant species. Transient expression analyses of the TaRac6-GFP fusion protein in Nicotiana benthamiana leaves showed that TaRac6 was localized in the whole cell. Furthermore, transient expression of TaRac6 inhibited Bax-triggered plant cell death (PCD) in N. benthamiana. Transcript accumulation of TaRac6 was increased at 24 h post-inoculation (hpi) in the compatible interaction between wheat and Pst, while it was not induced in an incompatible interaction. More importantly, silencing of TaRac6 by virus induced gene silencing (VIGS) enhanced the resistance of wheat (Suwon 11) to Pst (CYR31) by producing fewer uredinia. Histological observations revealed that the hypha growth of Pst was markedly inhibited along with more H₂O₂ generated in the TaRac6-silenced leaves in response to Pst. Moreover, transcript levels of TaCAT were significantly down-regulated, while those of TaSOD and TaNOX were significantly up-regulated. These results suggest that TaRac6 functions as a potential susceptibility factor, which negatively regulate the reactive oxygen species (ROS) burst in the wheat-Pst interaction.

Hunting modulators of plant defence: the grapevine trunk disease fungus Eutypa lata secretes an amplifier for plant basal immunity

Grapevine trunk diseases (GTDs) are progressively affecting vineyard longevity and productivity worldwide. To be able to understand and combat these diseases, we need a different concept of the signals exchanged between the grapevine and fungi than the well-studied pathogen-associated molecular pattern and effector concepts. We screened extracts from fungi associated with GTDs for their association with basal defence responses in suspension cells of grapevine. By activity-guided fractionation of the two selected extracts, O-methylmellein was identified as a candidate modulator of grapevine immunity. O-Methylmellein could not induce immune responses by itself (i.e. does not act as an elicitor), but could amplify some of the defence responses triggered by the bacterial elicitor flg22, such as the induction level of defence genes and actin remodelling. These findings show that Eutypa lata, exemplarily selected as an endophytic fungus linked with GTDs, can secrete compounds that act as amplifiers of basal immunity. Thus, in addition to elicitors that can trigger basal immunity, and effectors that down-modulate antibacterial basal immunity, once it had been activated, E. lata seems to secrete a third type of chemical signal that amplifies basal immunity and may play a role in the context of consortia of mutually competing microorganisms.

Barley ROP-Interactive Partner-a organizes into RAC1- and MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1-dependent membrane domains

BACKGROUND

Small ROP (also called RAC) GTPases are key factors in polar cell development and in interaction with the environment. ROP-Interactive Partner (RIP) proteins are predicted scaffold or ROP-effector proteins, which function downstream of activated GTP-loaded ROP proteins in establishing membrane heterogeneity and cellular organization. Grass ROP proteins function in cell polarity, resistance and susceptibility to fungal pathogens but grass RIP proteins are little understood.

RESULTS

We found that the barley (Hordeum vulgare L.) RIPa protein can interact with barley ROPs in yeast. Fluorescent-tagged RIPa, when co-expressed with the constitutively activated ROP protein CA RAC1, accumulates at the cell periphery or plasma membrane. Additionally, RIPa, locates into membrane domains, which are laterally restricted by microtubules when co-expressed with RAC1 and MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1. Both structural integrity of MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1 and microtubule stability are key to maintenance of RIPa-labeled membrane domains. In this context, RIPa also accumulates at the interface of barley and invading hyphae of the powdery mildew fungus Blumeria graminis f.sp. hordei.

CONCLUSIONS

Data suggest that barley RIPa interacts with barley ROPs and specifies RAC1 activity-associated membrane domains with potential signaling capacity. Lateral diffusion of this RAC1 signaling capacity is spatially restricted and the resulting membrane heterogeneity requires intact microtubules and MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1. Focal accumulation of RIPa at sites of fungal attack may indicate locally restricted ROP activity at sites of fungal invasion.

Shotgun proteomics provides an insight into pathogenesis-related proteins using anamorphic stage of the biotroph, Erysiphe pisi pathogen of garden pea

E. pisi is an ascomycete member causing powdery mildew disease of garden pea. It is a biotrophic pathogen, requiring a living host for its survival. Our understanding of molecular mechanisms underlying its pathogenesis is limited. The identification of proteins expressed in the pathogen is required to gain an insight into the functional mechanisms of an obligate biotrophic fungal pathogen. In this study, the proteome of the anamorphic stage of E. pisi pathogen has been elucidated through the nano LC-MS/MS approach. A total of 328 distinct proteins were detected from Erysiphe isolates infecting the susceptible pea cultivar, Arkel. The proteome is available via ProteomeXchange with identifier PXD010238. The functional classification of protein accessions based on Gene Ontology revealed proteins related to signal transduction, secondary metabolite formation and stress which might be involved in virulence and pathogenesis. The functional validation carried through differential expression of genes encoding G-protein beta subunit, a Cyclophilin (Peptidyl prolyl cis-transisomerase) and ABC transporter in a time course study confirmed their putative role in pathogenesis between resistant and susceptible genotypes, JI2480 and Arkel. The garden pea-powdery mildew pathosystem is largely unexplored, therefore, the identified proteome provides a first-hand information and will form a basis to analyze mechanisms involving pathogen survival, pathogenesis and virulence.

Overexpression of a Novel ROP Gene from the Banana (MaROP5g) Confers Increased Salt Stress Tolerance

Rho-like GTPases from plants (ROPs) are plant-specific molecular switches that are crucial for plant survival when subjected to abiotic stress. We identified and characterized 17 novel ROP proteins from Musa acuminata (MaROPs) using genomic techniques. The identified MaROPs fell into three of the four previously described ROP groups (Groups II⁻IV), with MaROPs in each group having similar genetic structures and conserved motifs. Our transcriptomic analysis showed that the two banana genotypes tested, Fen Jiao and BaXi Jiao, had similar responses to abiotic stress: Six genes (MaROP-3b, -5a, -5c, -5f, -5g, and -6) were highly expressed in response to cold, salt, and drought stress conditions in both genotypes. Of these, MaROP5g was most highly expressed in response to salt stress. Co-localization experiments showed that the MaROP5g protein was localized at the plasma membrane. When subjected to salt stress, transgenic Arabidopsis thaliana overexpressing MaROP5g had longer primary roots and increased survival rates compared to wild-type A. thaliana. The increased salt tolerance conferred by MaROP5g might be related to reduced membrane injury and the increased cytosolic K⁺/Na⁺ ratio and Ca²⁺ concentration in the transgenic plants as compared to wild-type. The increased expression of salt overly sensitive (SOS)-pathway genes and calcium-signaling pathway genes in MaROP5g-overexpressing A. thaliana reflected the enhanced tolerance to salt stress by the transgenic lines in comparison to wild-type. Collectively, our results suggested that abiotic stress tolerance in banana plants might be regulated by multiple MaROPs, and that MaROP5g might enhance salt tolerance by increasing root length, improving membrane injury and ion distribution.

Evolutionarily conserved partial gene duplication in the Triticeae tribe of grasses confers pathogen resistance

Background

The large and highly repetitive genomes of the cultivated species Hordeum vulgare (barley), Triticum aestivum (wheat), and Secale cereale (rye) belonging to the Triticeae tribe of grasses appear to be particularly rich in gene-like sequences including partial duplicates. Most of them have been classified as putative pseudogenes. In this study we employ transient and stable gene silencing- and over-expression systems in barley to study the function of HvARM1 (for H. vulgare Armadillo 1), a partial gene duplicate of the U-box/armadillo-repeat E3 ligase HvPUB15 (for H. vulgare Plant U-Box 15).

Results

The partial ARM1 gene is derived from a gene-duplication event in a common ancestor of the Triticeae and contributes to quantitative host as well as nonhost resistance to the biotrophic powdery mildew fungus Blumeria graminis. In barley, allelic variants of HvARM1 but not of HvPUB15 are significantly associated with levels of powdery mildew infection. Both HvPUB15 and HvARM1 proteins interact in yeast and plant cells with the susceptibility-related, plastid-localized barley homologs of THF1 (for Thylakoid formation 1) and of ClpS1 (for Clp-protease adaptor S1) of Arabidopsis thaliana. A genome-wide scan for partial gene duplicates reveals further events in barley resulting in stress-regulated, potentially neo-functionalized, genes.

Conclusion

The results suggest neo-functionalization of the partial gene copy HvARM1 increases resistance against powdery mildew infection. It further links plastid function with susceptibility to biotrophic pathogen attack. These findings shed new light on a novel mechanism to employ partial duplication of protein-protein interaction domains to facilitate the expansion of immune signaling networks.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1472-7) contains supplementary material, which is available to authorized users.

A barley powdery mildew fungus non-autonomous retrotransposon encodes a peptide that supports penetration success on barley

Pathogens overcome plant immunity by means of secreted effectors. Host effector targets often act in pathogen defense, but might also support fungal accommodation or nutrition. The barley ROP GTPase HvRACB is involved in accommodation of fungal haustoria of the powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh) in barley epidermal cells. We found that HvRACB interacts with the ROP-interactive peptide 1 (ROPIP1) that is encoded on the active non-long terminal repeat retroelement Eg-R1 of Bgh. Overexpression of ROPIP1 in barley epidermal cells and host-induced post-transcriptional gene silencing (HIGS) of ROPIP1 suggested that ROPIP1 is involved in virulence of Bgh. Bimolecular fluorescence complementation and co-localization supported that ROPIP1 can interact with activated HvRACB in planta. We show that ROPIP1 is expressed by Bgh on barley and translocated into the cytoplasm of infected barley cells. ROPIP1 is recruited to microtubules upon co-expression of MICROTUBULE ASSOCIATED ROP GTPase ACTIVATING PROTEIN (HvMAGAP1) and can destabilize cortical microtubules. The data suggest that Bgh ROPIP targets HvRACB and manipulates host cell microtubule organization for facilitated host cell entry. This points to a possible neo-functionalization of retroelement-derived transcripts for the evolution of a pathogen virulence effector.

Abscisic acid-induced degradation of Arabidopsis guanine nucleotide exchange factor requires calcium-dependent protein kinases

Abscisic acid (ABA) plays essential roles in plant development and responses to environmental stress. ABA induces subcellular translocation and degradation of the guanine nucleotide exchange factor RopGEF1, thus facilitating ABA core signal transduction. However, the underlying mechanisms for ABA-triggered RopGEF1 trafficking/degradation remain unknown. Studies have revealed that RopGEFs associate with receptor-like kinases to convey developmental signals to small ROP GTPases. However, how the activities of RopGEFs are modulated is not well understood. Type 2C protein phosphatases stabilize the RopGEF1 protein, indicating that phosphorylation may trigger RopGEF1 trafficking and degradation. We have screened inhibitors followed by several protein kinase mutants and find that quadruple-mutant plants in the Arabidopsis calcium-dependent protein kinases (CPKs) cpk3/4/6/11 disrupt ABA-induced trafficking and degradation of RopGEF1. Moreover, cpk3/4/6/11 partially impairs ABA inhibition of cotyledon emergence. Several CPKs interact with RopGEF1. CPK4 binds to and phosphorylates RopGEF1 and promotes the degradation of RopGEF1. CPK-mediated phosphorylation of RopGEF1 at specific N-terminal serine residues causes the degradation of RopGEF1 and mutation of these sites also compromises the RopGEF1 overexpression phenotype in root hair development in Arabidopsis Our findings establish the physiological and molecular functions and relevance of CPKs in regulation of RopGEF1 and illuminate physiological roles of a CPK-GEF-ROP module in ABA signaling and plant development. We further discuss that CPK-dependent RopGEF degradation during abiotic stress could provide a mechanism for down-regulation of RopGEF-dependent growth responses.

Commonalities and Differences in Controlling Multipartite Intracellular Infections of Legume Roots by Symbiotic Microbes

Legumes have the almost unique ability to establish symbiotic associations with rhizobia and arbuscular mycorrhizal fungi. Forward and reverse genetics have identified a large number of genes that are required for either or both interactions. However, and in sharp contrast to natural soils, these interactions have been almost exclusively investigated under laboratory conditions by using separate inoculation systems, whereas both symbionts are simultaneously present in the field. Considering our recent understanding of the individual symbioses, the community is now promisingly positioned to co-inoculate plants with two or more microbes in order to understand mechanistically how legumes efficiently balance, regulate and potentially separate these symbioses and other endophytic microbes within the same root. Here, we discuss a number of key control layers that should be considered when assessing tri- or multipartite beneficial interactions and that may contribute to colonization patterns in legume roots.

In silico identification and experimental validation of amino acid motifs required for the Rho-of-plants GTPase-mediated activation of receptor-like cytoplasmic kinases

Several amino acid motifs required for Rop-dependent activity were found to form a common surface on RLCKVI_A kinases. This indicates a unique mechanism for Rho-type GTPase-mediated kinase activation in plants. Rho-of-plants (Rop) G-proteins are implicated in the regulation of various cellular processes, including cell growth, cell polarity, hormonal and pathogen responses. Our knowledge about the signalling pathways downstream of Rops is continuously increasing. However, there are still substantial gaps in this knowledge. One reason for this is that these pathways are considerably different from those described for yeast and/or animal Rho-type GTPases. Among others, plants lack all Rho/Rac/Cdc42-activated kinase families. Only a small group of plant-specific receptor-like cytoplasmic kinases (RLCK VI_A) has been shown to exhibit Rop-binding-dependent in vitro activity. These kinases do not carry any known GTPase-binding motifs. Based on the sequence comparison of the Rop-activated RLCK VI_A and the closely related but constitutively active RLCK VI_B kinases, several distinguishing amino acid residues/motifs were identified. All but one of these were found to be required for the Rop-mediated regulation of the in vitro activity of two RLCK VI_A kinases. Structural modelling indicated that these motifs might form a common Rop-binding surface. Based on in silico data mining, kinases that have the identified Rop-binding motifs are present in Embryophyta but not in unicellular green algae. It can, therefore, be supposed that Rops recruited these plant-specific kinases for signalling at an early stage of land plant evolution.

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.

Barley susceptibility factor RACB modulates transcript levels of signalling protein genes in compatible interaction with Blumeria graminis f.sp. hordei

RHO (rat sarcoma homologue) GTPases (guanosine triphosphatases) are regulators of downstream transcriptional responses of eukaryotes to intracellular and extracellular stimuli. For plants, little is known about the function of Rho-like GTPases [called RACs (rat sarcoma-related C botulinum substrate) or ROPs (RHO of plants)] in transcriptional reprogramming of cells. However, in plant hormone response and innate immunity, RAC/ROP proteins influence gene expression patterns. The barley RAC/ROP RACB is required for full susceptibility of barley to the powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh). We compared the transcriptomes of barley plants either silenced for RACB or over-expressing constitutively activated RACB with and without inoculation with Bgh. This revealed a large overlap of the barley transcriptome during the early response to Bgh and during the over-expression of constitutively activated RACB. Global pathway analyses and stringent analyses of differentially expressed genes suggested that RACB influences, amongst others, the expression of signalling receptor kinases. Transient induced gene silencing of RACB-regulated signalling genes (a leucine-rich repeat protein, a leucine-rich repeat receptor-like kinase and an S-domain SD1-receptor-like kinase) suggested that they might be involved in RACB-modulated susceptibility to powdery mildew. We discuss the function of RACB in regulating the transcriptional responses of susceptible barley to Bgh.

Barley ADH-1 modulates susceptibility to Bgh and is involved in chitin-induced systemic resistance

The plant primary energy metabolism is profoundly reorganized under biotic stress conditions and there is increasing evidence for a role of the fermentative pathway in biotic interactions. Previously we showed via transient gene silencing or overexpression a function of barley alcohol dehydrogenase 1 (HvADH-1) in the interaction of barley with the parasitic fungus Blumeria graminis f.sp. hordei (Bgh). Here we extend our studies on stable transgenic barley events over- or under-expressing HvADH-1 to analyse ADH-1 functions at the level of whole plants. Knock-down (KD) of HvADH-1 by dsRNA interference resulted in reduced and overexpression of HvADH-1 in strongly increased HvADH-1 enzyme activity in leaves of stable transgenic barley plants. The KD of HvADH-1 coincided with a reduced susceptibility to Bgh of both excised leaves and leaves of intact plants. Overexpression (OE) of HvADH-1 results in increased susceptibility to Bgh when excised leaves but not when whole seedlings were inoculated. When first leaves of 10-day-old barley plants were treated with a chitin elicitor, we observed a reduced enzyme activity of ADH-1/-1 homodimers at 48 h after treatment in the second, systemic leaf for empty vector controls and HvADH-1 KD events, but not for the HvADH-1 OE events. Reduced ADH-1 activity in the systemic leaf of empty vector controls and HvADH-1 KD events coincided with chitin-induced resistance to Bgh. Taken together, stable HvADH-1 (KD) or systemic down-regulation of ADH-1/-1 activity by chitin treatment modulated the pathogen response of barley to the biotrophic fungal parasite Bgh and resulted in less successful infections by Bgh.

ROP GTPases Structure-Function and Signaling Pathways

Interactions between receptor like kinases and guanyl nucleotide exchange factors together with identification of effector proteins reveal putative ROP GTPases signaling cascades.

Subcellular Localization Screening of Colletotrichum higginsianum Effector Candidates Identifies Fungal Proteins Targeted to Plant Peroxisomes, Golgi Bodies, and Microtubules

The genome of the hemibiotrophic anthracnose fungus, Colletotrichum higginsianum, encodes a large inventory of putative secreted effector proteins that are sequentially expressed at different stages of plant infection, namely appressorium-mediated penetration, biotrophy and necrotrophy. However, the destinations to which these proteins are addressed inside plant cells are unknown. In the present study, we selected 61 putative effector genes that are highly induced in appressoria and/or biotrophic hyphae. We then used Agrobacterium-mediated transformation to transiently express them as N-terminal fusions with fluorescent proteins in cells of Nicotiana benthamiana for imaging by confocal microscopy. Plant compartments labeled by the fusion proteins in N. benthamiana were validated by co-localization with specific organelle markers, by transient expression of the proteins in the true host plant, Arabidopsis thaliana, and by transmission electron microscopy-immunogold labeling. Among those proteins for which specific subcellular localizations could be verified, nine were imported into plant nuclei, three were imported into the matrix of peroxisomes, three decorated cortical microtubule arrays and one labeled Golgi stacks. Two peroxisome-targeted proteins harbored canonical C-terminal tripeptide signals for peroxisome import via the PTS1 (peroxisomal targeting signal 1) pathway, and we showed that these signals are essential for their peroxisome localization. Our findings provide valuable information about which host processes are potentially manipulated by this pathogen, and also reveal plant peroxisomes, microtubules, and Golgi as novel targets for fungal effectors.

Diversification of defensins and NLRs in Arabidopsis species by different evolutionary mechanisms

BACKGROUND

Genes encoding proteins underlying host-pathogen co-evolution and which are selected for new resistance specificities frequently are under positive selection, a process that maintains diversity. Here, we tested the contribution of natural selection, recombination and transcriptional divergence to the evolutionary diversification of the plant defensins superfamily in three Arabidopsis species. The intracellular NOD-like receptor (NLR) family was used for comparison because positive selection has been well documented in its members. Similar to defensins, NLRs are encoded by a large and polymorphic gene family and many of their members are involved in the immune response.

RESULTS

Gene trees of Arabidopsis defensins (DEFLs) show a high prevalence of clades containing orthologs. This indicates that their diversity dates back to a common ancestor and species-specific duplications did not significantly contribute to gene family expansion. DEFLs are characterized by a pervasive pattern of neutral evolution with infrequent positive and negative selection as well as recombination. In comparison, most NLR alignment groups are characterized by frequent occurrence of positive selection and recombination in their leucine-rich repeat (LRR) domain as well negative selection in their nucleotide-binding (NB-ARC) domain. While major NLR subgroups are expressed in pistils and leaves both in presence or absence of pathogen infection, the members of DEFL alignment groups are predominantly transcribed in pistils. Furthermore, conserved groups of NLRs and DEFLs are differentially expressed in response to Fusarium graminearum regardless of whether these genes are under positive selection or not.

CONCLUSIONS

The present analyses of NLRs expands previous studies in Arabidopsis thaliana and highlights contrasting patterns of purifying and diversifying selection affecting different gene regions. DEFL genes show a different evolutionary trend, with fewer recombination events and significantly fewer instances of natural selection. Their heterogeneous expression pattern suggests that transcriptional divergence probably made the major contribution to functional diversification. In comparison to smaller families encoding pathogenesis-related (PR) proteins under positive selection, DEFLs are involved in a wide variety of processes that altogether might pose structural and functional trade-offs to their family-wide pattern of evolution.

Membrane nanodomains and microdomains in plant-microbe interactions

During plant-microbe interactions, host cells need to keep stringent control over the approaching pathogens and symbionts. This requires specific spatio-temporal assemblies of pattern recognition receptors and other complex constituents and a strict physical separation of genetically overlapping pathways. Increasing evidence suggests that this is, at least partially, achieved by the formation of nanometer scale membrane platforms that might act as signaling hubs. These and other larger-scale sub-compartments have been termed 'membrane rafts', 'nanodomains' and 'microdomains'. This review focuses on recent advances in understanding these nano-scale signaling platforms during plant-microbe interactions and proposes a common definition meant to facilitate the precise discrimination between different types of membrane domains in the future.

A ROP2-RIC1 pathway fine-tunes microtubule reorganization for salt tolerance in Arabidopsis

The reorganization of microtubules induced by salt stress is required for Arabidopsis survival under high salinity conditions. RIC1 is an effector of Rho-related GTPase from plants (ROPs) and a known microtubule-associated protein. In this study, we demonstrated that RIC1 expression decreased with long-term NaCl treatment, and ric1-1 seedlings exhibited a higher survival rate under salt stress. We found that RIC1 reduced the frequency of microtubule transition from shortening to growing status and knockout of RIC1 improved the reassembly of depolymerized microtubules caused by either oryzalin treatment or salt stress. Further investigation showed that constitutively active ROP2 promoted the reassembly of microtubules and the survival of seedlings under salt stress. A rop2-1 ric1-1 double mutant rescued the salt-sensitive phenotype of rop2-1, indicating that ROP2 functions in salt tolerance through RIC1. Although ROP2 did not regulate RIC1 expression upon salt stress, a quick but mild increase of ROP2 activity was induced, led to reduction of RIC1 on microtubules. Collectively, our study reveals an ROP2-RIC1 pathway that fine-tunes microtubule dynamics in response to salt stress in Arabidopsis. This finding not only reveals a new regulatory mechanism for microtubule reorganization under salt stress but also the importance of ROP signalling for salinity tolerance.

Similarities between Reproductive and Immune Pistil Transcriptomes of Arabidopsis Species

Independent lines of evidence suggest that members from ancient and polymorphic gene families such as defensins and receptor-like kinases mediate intercellular communication during both the immune response and reproduction. Here, we report a large-scale analysis to investigate the extent of overlap between these processes by comparing differentially expressed genes (DEGs) in the pistil transcriptomes of Arabidopsis thaliana and Arabidopsis halleri during self-pollination and interspecific pollination and during infection with Fusarium graminearum In both Arabidopsis species, the largest number of DEGs was identified in infected pistils, where genes encoding regulators of cell division and development were most frequently down-regulated. Comparison of DEGs between infection and various pollination conditions showed that up to 79% of down-regulated genes are shared between conditions and include especially defensin-like genes. Interspecific pollination of A.thaliana significantly up-regulated thionins and defensins. The significant overrepresentation of similar groups of DEGs in the transcriptomes of reproductive and immune responses of the pistil makes it a prime system in which to study the consequences of plant-pathogen interactions on fertility and the evolution of intercellular communication in pollination.

Small GTPases in plant biotic interactions

The superfamily of small monomeric GTPases originated in a common ancestor of eukaryotic multicellular organisms and, since then, it has evolved independently in each lineage to cope with the environmental challenges imposed by their different life styles. Members of the small GTPase family function in the control of vesicle trafficking, cytoskeleton rearrangements and signaling during crucial biological processes, such as cell growth and responses to environmental cues. In this review, we discuss the emerging roles of these small GTPases in the pathogenic and symbiotic interactions established by plants with microorganisms present in their nearest environment, in which membrane trafficking is crucial along the different steps of the interaction, from recognition and signal transduction to nutrient exchange.

RING-H2-type E3 gene VpRH2 from Vitis pseudoreticulata improves resistance to powdery mildew by interacting with VpGRP2A

Grapevine is one of the world's most important fruit crops. European cultivated grape species have the best fruit quality but show almost no resistance to powdery mildew (PM). PM caused by Uncinula necator is a harmful disease that has a significant impact on the economic value of the grape crop. In this study, we examined a RING-H2-type ubiquitin ligase gene VpRH2 that is associated with significant PM-resistance of Chinese wild-growing grape Vitis pseudoreticulata accession Baihe-35-1. The expression of VpRH2 was clearly induced by U. necator inoculation compared with its homologous gene VvRH2 in a PM-susceptible grapevine V. vinifera cv. Thompson Seedless. Using a yeast two-hybrid assay we confirmed that VpRH2 interacted with VpGRP2A, a glycine-rich RNA-binding protein. The degradation of VpGRP2A was inhibited by treatment with the proteasome inhibitor MG132 while VpRH2 did not promote the degradation of VpGRP2A. Instead, the transcripts of VpRH2 were increased by over-expressing VpGRP2A while VpRH2 suppressed the expression of VpGRP2A. Furthermore, VpGRP2A was down-regulated in both Baihe-35-1 and Thompson Seedless after U. necator inoculation. Specifically, we generated VpRH2 overexpression transgenic lines in Thompson Seedless and found that the transgenic plants showed enhanced resistance to powdery mildew compared with the wild-type. In summary, our results indicate that VpRH2 interacts with VpGRP2A and plays a positive role in resistance to powdery mildew.

Plant ER geometry and dynamics: biophysical and cytoskeletal control during growth and biotic response

The endoplasmic reticulum (ER) is an intricate and dynamic network of membrane tubules and cisternae. In plant cells, the ER 'web' pervades the cortex and endoplasm and is continuous with adjacent cells as it passes through plasmodesmata. It is therefore the largest membranous organelle in plant cells. It performs essential functions including protein and lipid synthesis, and its morphology and movement are linked to cellular function. An emerging trend is that organelles can no longer be seen as discrete membrane-bound compartments, since they can physically interact and 'communicate' with one another. The ER may form a connecting central role in this process. This review tackles our current understanding and quantification of ER dynamics and how these change under a variety of biotic and developmental cues.

Barley disease susceptibility factor RACB acts in epidermal cell polarity and positioning of the nucleus

RHO GTPases are regulators of cell polarity and immunity in eukaryotes. In plants, RHO-like RAC/ROP GTPases are regulators of cell shaping, hormone responses, and responses to microbial pathogens. The barley (Hordeum vulgare L.) RAC/ROP protein RACB is required for full susceptibility to penetration by Blumeria graminis f.sp. hordei (Bgh), the barley powdery mildew fungus. Disease susceptibility factors often control host immune responses. Here we show that RACB does not interfere with early microbe-associated molecular pattern-triggered immune responses such as the oxidative burst or activation of mitogen-activated protein kinases. RACB also supports rather than restricts expression of defence-related genes in barley. Instead, silencing of RACB expression by RNAi leads to defects in cell polarity. In particular, initiation and maintenance of root hair growth and development of stomatal subsidiary cells by asymmetric cell division is affected by silencing expression of RACB. Nucleus migration is a common factor of developmental cell polarity and cell-autonomous interaction with Bgh RACB is required for positioning of the nucleus near the site of attack from Bgh We therefore suggest that Bgh profits from RACB's function in cell polarity rather than from immunity-regulating functions of RACB.

Polarized Defense Against Fungal Pathogens Is Mediated by the Jacalin-Related Lectin Domain of Modular Poaceae-Specific Proteins

Modular proteins are an evolutionary answer to optimize performance of proteins that physically interact with each other for functionality. Using a combination of genetic and biochemical experiments, we characterized the rice protein OsJAC1, which consists of a jacalin-related lectin (JRL) domain predicted to bind mannose-containing oligosaccharides, and a dirigent domain which might function in stereoselective coupling of monolignols. Transgenic overexpression of OsJAC1 in rice resulted in quantitative broad-spectrum resistance against different pathogens including bacteria, oomycetes, and fungi. Overexpression of this gene or its wheat ortholog TAJA1 in barley enhanced resistance against the powdery mildew fungus. Both protein domains of OsJAC1 are required to establish resistance as indicated by single or combined transient expression of individual domains. Expression of artificially separated and fluorescence-tagged protein domains showed that the JRL domain is sufficient for targeting the powdery mildew penetration site. Nevertheless, co-localization of the lectin and the dirigent domain occurred. Phylogenetic analyses revealed orthologs of OsJAC1 exclusively within the Poaceae plant family. Dicots, by contrast, only contain proteins with either JRL or dirigent domain(s). Altogether, our results identify OsJAC1 as a representative of a novel type of resistance protein derived from a plant lineage-specific gene fusion event for better function in local pathogen defense.

Coexpression network analysis of the genes regulated by two types of resistance responses to powdery mildew in wheat

Powdery mildew disease caused by Blumeria graminis f. sp. tritici (Bgt) inflicts severe economic losses in wheat crops. A systematic understanding of the molecular mechanisms involved in wheat resistance to Bgt is essential for effectively controlling the disease. Here, using the diploid wheat Triticum urartu as a host, the genes regulated by immune (IM) and hypersensitive reaction (HR) resistance responses to Bgt were investigated through transcriptome sequencing. Four gene coexpression networks (GCNs) were developed using transcriptomic data generated for 20 T. urartu accessions showing IM, HR or susceptible responses. The powdery mildew resistance regulated (PMRR) genes whose expression was significantly correlated with Bgt resistance were identified, and they tended to be hubs and enriched in six major modules. A wide occurrence of negative regulation of PMRR genes was observed. Three new candidate immune receptor genes (TRIUR3_13045, TRIUR3_01037 and TRIUR3_06195) positively associated with Bgt resistance were discovered. Finally, the involvement of TRIUR3_01037 in Bgt resistance was tentatively verified through cosegregation analysis in a F2 population and functional expression assay in Bgt susceptible leaf cells. This research provides insights into the global network properties of PMRR genes. Potential molecular differences between IM and HR resistance responses to Bgt are discussed.