The ubiquitin ligase PUB22 targets a subunit of the exocyst complex required for PAMP-triggered responses in Arabidopsis

Solanaceous exocyst subunits are involved in immunity to diverse plant pathogens

The exocyst, a multiprotein complex consisting of eight subunits, plays an essential role in many biological processes by mediating secretion of post-Golgi-derived vesicles towards the plasma membrane. In recent years, roles for plant exocyst subunits in pathogen defence have been uncovered, largely based on studies in the model plant Arabidopsis. Only a few studies have been undertaken to assign the role of exocyst subunits in plant defence in other plants species, including crops. In this study, predicted protein sequences from exocyst subunits were retrieved by mining databases from the Solanaceous plants Nicotiana benthamiana, tomato, and potato. Subsequently, their evolutionary relationship with Arabidopsis exocyst subunits was analysed. Gene silencing in N. benthamiana showed that several exocyst subunits are required for proper plant defence against the (hemi-)biotrophic plant pathogens Phytophthora infestans and Pseudomonas syringae. In contrast, some exocyst subunits seem to act as susceptibility factors for the necrotrophic pathogen Botrytis cinerea. Furthermore, the majority of the exocyst subunits were found to be involved in callose deposition, suggesting that they play a role in basal plant defence. This study provides insight into the evolution of exocyst subunits in Solanaceous plants and is the first to show their role in immunity against multiple unrelated pathogens.

News from the PUB: plant U-box type E3 ubiquitin ligases

Plant U-box type E3 ubiquitin ligases (PUBs) are well known for their functions in a variety of stress responses, including immune responses and the adaptation to abiotic stresses. First linked to pollen self-incompatibility, their repertoire of roles has grown to encompass also the regulation of developmental processes. Notably, new studies provide clues to their mode of action, underline the existence of conserved PUB-kinase modules, and suggest new links to G-protein signalling, placing PUBs at the crossroads of major signalling hubs. The frequent association with membranes, by interacting and/or targeting membrane proteins, as well as through a recently reported direct interaction with phospholipids, indicates a general function in the control of vesicle transport and their cargoes. This review aims to give an overview of the most significant advances in the field, while also trying to identify common themes of PUB function.

Real-time detection of N-end rule-mediated ubiquitination via fluorescently labeled substrate probes

The N-end rule pathway has emerged as a major system for regulating protein functions by controlling their turnover in medical, animal and plant sciences as well as agriculture. Although novel functions and enzymes of the pathway have been discovered, the ubiquitination mechanism and substrate specificity of N-end rule pathway E3 ubiquitin ligases have remained elusive. Taking the first discovered bona fide plant N-end rule E3 ligase PROTEOLYSIS1 (PRT1) as a model, we used a novel tool to molecularly characterize polyubiquitination live, in real time. We gained mechanistic insights into PRT1 substrate preference and activation by monitoring live ubiquitination using a fluorescent chemical probe coupled to artificial substrate reporters. Ubiquitination was measured by rapid in-gel fluorescence scanning as well as in real time by fluorescence polarization. The enzymatic activity, substrate specificity, mechanisms and reaction optimization of PRT1-mediated ubiquitination were investigated ad hoc instantaneously and with significantly reduced reagent consumption. We demonstrated that PRT1 is indeed an E3 ligase, which has been hypothesized for over two decades. These results demonstrate that PRT1 has the potential to be involved in polyubiquitination of various substrates and therefore pave the way to understanding recently discovered phenotypes of prt1 mutants.

The Novel Secreted Meloidogyne incognita Effector MiISE6 Targets the Host Nucleus and Facilitates Parasitism in Arabidopsis

Meloidogyne incognita is highly specialized parasite that interacts with host plants using a range of strategies. The effectors are synthesized in the esophageal glands and secreted into plant cells through a needle-like stylet during parasitism. In this study, based on RNA-seq and bioinformatics analysis, we predicted 110 putative Meloidogyne incognita effectors that contain nuclear localization signals (NLSs). Combining the Burkholderia glumae-pEDV based screening system with subcellular localization, from 20 randomly selected NLS effector candidates, we identified an effector MiISE6 that can effectively suppress B. glumae-induced cell death in Nicotiana benthamiana, targets to the nuclei of plant cells, and is highly expressed in early parasitic J2 stage. Sequence analysis showed that MiISE6 is a 157-amino acid peptide, with an OGFr_N domain and two NLS motifs. Hybridization in situ verified that MiISE6 is expressed in the subventral esophageal glands. Yeast invertase secretion assay validated the function of the signal peptide harbored in MiISE6. Transgenic Arabidopsis thaliana plants expressing MiISE6 become more susceptible to M. incognita. Inversely, the host-derived RNAi of MiISE6 of the nematode can decrease its parasitism on host. Based on transcriptome analysis of the MiISE6 transgenic Arabidopsis samples and the wild-type samples, we obtained 852 differentially expressed genes (DEGs). Integrating Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses, we found that expression of MiISE6 in Arabidopsis can suppress jasmonate signaling pathway. In addition, the expression of genes related to cell wall modification and the ubiquitination proteasome pathway also have detectable changes in the transgenic plants. Results from the present study suggest that MiISE6 is involved in interaction between nematode-plant, and plays an important role during the early stages of parasitism by interfering multiple signaling pathways of plant. Moreover, we found homologs of MiISE6 in other sedentary nematodes, Meloidogyne hapla and Globodera pallida. Our experimental results provide evidence to decipher the molecular mechanisms underlying the manipulation of host immune defense responses by plant parasitic nematodes, and transcriptome data also provide useful information for further study nematode-plant interactions.

Exocyst and autophagy-related membrane trafficking in plants

Endomembrane traffic in eukaryotic cells functions partially as a means of communication; delivery of membrane in one direction has to be balanced with a reduction at the other end. This effect is typically the case during the defence against pathogens. To combat pathogens, cellular growth and differentiation are suppressed, while endomembrane traffic is poised towards limiting the pathogen attack. The octameric exocyst vesicle-tethering complex was originally discovered as a factor facilitating vesicle-targeting and vesicle-plasma membrane (PM) fusion during exocytosis prior to and possibly during SNARE complex formation. Interestingly, it was recently implicated both in animals and plants in autophagy membrane traffic. In animal cells, the exocyst is integrated into the mTOR-regulated energy metabolism stress/starvation pathway, participating in the formation and especially initiation of an autophagosome. In plants, the first functional link was to autophagy-related anthocyanin import to the vacuole and to starvation. In this concise review, we summarize the current knowledge of exocyst functions in autophagy and defence in plants that might involve unconventional secretion and compare it with animal conditions. Formation of different exocyst complexes during undisturbed cell growth, as opposed to periods of cellular stress reactions involving autophagy, might contribute to the coordination of endomembrane trafficking pathways.

Conventional and unconventional ubiquitination in plant immunity

Ubiquitination is one of the most abundant types of protein post-translational modification (PTM) in plant cells. The importance of ubiquitination in the regulation of many aspects of plant immunity has been increasingly appreciated in recent years. Most of the studies linking ubiquitination to the plant immune system, however, have been focused on the E3 ubiquitin ligases and the conventional ubiquitination that leads to the degradation of the substrate proteins by the 26S proteasome. By contrast, our knowledge about the role of unconventional ubiquitination that often serves as non-degradative, regulatory signal remains a significant gap. We discuss, in this review, the recent advances in our understanding of ubiquitination in the modulation of plant immunity, with a particular focus on the E3 ubiquitin ligases. We approach the topic from a perspective of two broadly defined types of ubiquitination in an attempt to highlight the importance, yet current scarcity, in our knowledge about the regulation of plant immunity by unconventional ubiquitination.

Multisubunit tethering complexes in higher plants

Tethering complexes mediate the initial, specific contact between donor and acceptor membranes. This review focuses on the modularity and function of multisubunit tethering complexes (MTCs) in higher plants. One emphasis is on molecular interactions of plant MTCs. Here, a number of insights have been gained concerning interactions between different tethering complexes, and between tethers and microtubule-associated proteins. The roles of tethering complexes in abiotic stress responses appear indirect, but in the context of biotic stress responses it has been suggested that some tethers are direct targets of pathogen effectors or virulence factors. In light of the central roles tethering complexes play in plant development, an emerging concept is that tethers may be co-opted for plant adaptive responses.

The Arabidopsis U-box E3 ubiquitin ligase PUB30 negatively regulates salt tolerance by facilitating BRI1 kinase inhibitor 1 (BKI1) degradation

The Arabidopsis U-box E3 ubiquitin ligases play an important role in the ubiquitin/26S proteasome-mediated protein degradation pathway. Recently, PUB30 has been reported to participate in the salt stress response during seed germination stage in abscisic acid (ABA)-independent manner, but the molecular mechanism remains to be elucidated. Here, we displayed that the pub30 mutant was more tolerant to salt stress during seed germination, whereas the mutant of its closest homologue PUB31 showed mild sensitivity to salt stress. PUB30 exhibited E3 ubiquitin ligase activity in vitro. PUB30 specifically interacted with BRI1 kinase inhibitor 1 (BKI1), a regulator playing dual roles in brassinosteroids signaling, in vitro and in vivo. We found that BKI1 protein was ubiquitinated and degraded by the 26S proteasome. The degradation of BKI1 was slowed down in the pub30-1 mutant compared with that in the wild type. The bki1 mutant was sensitive to salt, whereas the transgenic plants overexpressing BKI1 showed salt tolerant phenotype. All these results indicate that PUB30 negatively regulates salt tolerance probably through regulating the degradation of BKI1 and brassinosteroids signaling in Arabidopsis.

The RING-Type E3 Ligase XBAT35.2 Is Involved in Cell Death Induction and Pathogen Response

XBAT35 belongs to a subfamily of Arabidopsis (Arabidopsis thaliana) RING-type E3s that are similar in domain architecture to the rice (Oryza sativa) XA21 Binding Protein3, a defense protein. The XBAT35 transcript undergoes alternative splicing to produce two protein isoforms, XBAT35.1 and XBAT35.2. Here, we demonstrate that XBAT35.2 localizes predominantly to the Golgi and is involved in cell death induction and pathogen response. XBAT35.2, but not XBAT35.1, was found to trigger cell death when overexpressed in tobacco (Nicotiana benthamiana) leaves and does so in a manner that requires its RING domain. Loss of XBAT35 gene function disrupts the plant's ability to defend against pathogen attack, whereas overexpression of XBAT35.2 enhances resistance to pathogens. XBAT35.2 was found to be unstable and promotes its own degradation, suggesting self-regulation. Inoculation with virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae pv tomato DC3000 results in a drastic reduction in the levels of ubiquitinated XBAT35.2 and an increase in the abundance of the E3. This implies that pathogen infection prohibits XBAT35.2 self-regulation and stabilizes the E3. In agreement with a role in defending against pathogens, XBAT35.2 interacts with defense-related Accelerated Cell Death11 (ACD11) in planta and promotes the proteasome-dependent turnover of ACD11 in cell-free degradation assays. In accordance with regulation by a stabilized XBAT35.2, the levels of ubiquitinated ACD11 increased considerably, and the abundance of ACD11 was reduced following pathogen infection. In addition, treatment of transgenic seedlings with a proteasome inhibitor results in the accumulation of ACD11, confirming proteasome-dependent degradation. Collectively, these results highlight a novel role for XBAT35.2 in cell death induction and defense against pathogens.

E3 Ubiquitin Ligases: Ubiquitous Actors in Plant Development and Abiotic Stress Responses

Understanding the precise regulatory mechanisms of plant development and stress responses at the post-translational level is currently a topic of intensive research. Protein ubiquitination, including the sequential performances of ubiquitin-activating (E1), ubiquitin-conjugating (E2) and ubiquitin ligase (E3) enzymes, is a refined post-translational modification ubiquitous in all eukaryotes. Plants are an integral part of our ecosystem and, as sessile organisms, the ability to perceive internal and external signals and to adapt well to various environmental challenges is crucial for their survival. Over recent decades, extensive studies have demonstrated that protein ubiquitination plays key roles in multiple plant developmental stages (e.g. seed dormancy and germination, root growth, flowering time control, self-incompatibility and chloroplast development) and several abiotic stress responses (e.g. drought and high salinity), by regulating the abundance, activities or subcellular localizations of a variety of regulatory polypeptides and enzymes. Importantly, diverse E3 ligases are involved in these regulatory pathways by mediating phytohormone and light signaling or other pathways. In this updated review, we mainly summarize recent advances in our understanding of the regulatory roles of protein ubiquitination in plant development and plant-environment interactions, and primarily focus on different types of E3 ligases because they play critical roles in determining substrate specificity.

E3 ligase SAUL1 serves as a positive regulator of PAMP-triggered immunity and its homeostasis is monitored by immune receptor SOC3

In both plants and animals, intracellular nucleotide-binding leucine-rich repeat proteins (NLRs; or Nod-like receptors) serve as immune receptors to recognize pathogen-derived molecules and mount effective immune responses against microbial infections. Plant NLRs often guard the presence or activity of other host proteins, which are the direct virulence targets of pathogen effectors. These guardees are sometimes immune-promoting components such as those in a mitogen-activated protein kinase cascade. Plant E3 ligases serve many roles in immune regulation, but it is unclear whether they can also be guarded by NLRs. Here, we report on an immune-regulating E3 ligase SAUL1, whose homeostasis is monitored by a Toll interleukin 1 receptor (TIR)-type NLR (TNL), SOC3. SOC3 can associate with SAUL1, and either loss or overexpression of SAUL1 triggers autoimmunity mediated by SOC3. By contrast, SAUL1 functions redundantly with its close homolog PUB43 to promote PAMP-triggered immunity (PTI). Taken together, the E3 ligase SAUL1 serves as a positive regulator of PTI and its homeostasis is monitored by the TNL SOC3.

Early Arabidopsis root hair growth stimulation by pathogenic strains of Pseudomonas syringae

BACKGROUND AND AIMS

Selected beneficial Pseudomonas spp. strains have the ability to influence root architecture in Arabidopsis thaliana by inhibiting primary root elongation and promoting lateral root and root hair formation. A crucial role for auxin in this long-term (1week), long-distance plant-microbe interaction has been demonstrated.

METHODS

Arabidopsis seedlings were cultivated in vitro on vertical plates and inoculated with pathogenic strains Pseudomonas syringae pv. maculicola (Psm) and P. syringae pv. tomato DC3000 (Pst), as well as Agrobacterium tumefaciens (Atu) and Escherichia coli (Eco). Root hair lengths were measured after 24 and 48h of direct exposure to each bacterial strain. Several Arabidopsis mutants with impaired responses to pathogens, impaired ethylene perception and defects in the exocyst vesicle tethering complex that is involved in secretion were also analysed.

KEY RESULTS

Arabidopsis seedling roots infected with Psm or Pst responded similarly to when infected with plant growth-promoting rhizobacteria; root hair growth was stimulated and primary root growth was inhibited. Other plant- and soil-adapted bacteria induced similar root hair responses. The most compromised root hair growth stimulation response was found for the knockout mutants exo70A1 and ein2. The single immune pathways dependent on salicylic acid, jasmonic acid and PAD4 are not directly involved in root hair growth stimulation; however, in the mutual cross-talk with ethylene, they indirectly modify the extent of the stimulation of root hair growth. The Flg22 peptide does not initiate root hair stimulation as intact bacteria do, but pretreatment with Flg22 prior to Psm inoculation abolished root hair growth stimulation in an FLS2 receptor kinase-dependent manner. These early response phenomena are not associated with changes in auxin levels, as monitored with the pDR5::GUS auxin reporter.

CONCLUSIONS

Early stimulation of root hair growth is an effect of an unidentified component of living plant pathogenic bacteria. The root hair growth response is triggered in the range of hours after bacterial contact with roots and can be modulated by FLS2 signalling. Bacterial stimulation of root hair growth requires functional ethylene signalling and an efficient exocyst-dependent secretory machinery.

Arabidopsis E3 Ubiquitin Ligases PUB22 and PUB23 Negatively Regulate Drought Tolerance by Targeting ABA Receptor PYL9 for Degradation

Drought causes osmotic stress and rapidly triggers abscisic acid (ABA) accumulation in plants. The roles of various ABA receptors in drought tolerance and molecular mechanisms regulating ABA receptor stability needs to be elucidated. Here, we report that Arabidopsis plants overexpressing PYL9, one of the 14 pyrabactin resistance (PYR)/pyrabactin resistance-like (PYL)/regulatory component of ABA receptors (RCAR) family ABA receptors, gained drought tolerance trait. Osmotic stress induced accumulation of the PYL9 protein, which was regulated by the 26S proteasome. PYL9 interacted with two highly homologous plant U-box E3 ubiquitin ligases PUB22 and PUB23. In the cell-free degradation assay, the degradation of GST-PYL9 was accelerated in protein extract from plants overexpressing PUB22 but slowed down in protein extract from the pub22 pub23 double mutant. The in vivo decay of Myc-PYL9 was significantly reduced in the pub22 pub23 double mutant as compared with the wild-type. Additionally, PUB22 also interacted with other ABA receptors such as PYL5, PYL7 and PYL8. Considering the improved drought tolerance in the pub22 pub23 double mutant in previous studies, our results suggest that PUB22 and PUB23 negatively regulate drought tolerance in part by facilitating ABA receptors degradation.

Membrane Trafficking in Plant Immunity

Plants employ sophisticated mechanisms to interact with pathogenic as well as beneficial microbes. Of those, membrane trafficking is key in establishing a rapid and precise response. Upon interaction with pathogenic microbes, surface-localized immune receptors undergo endocytosis for signal transduction and activity regulation while cell wall components, antimicrobial compounds, and defense proteins are delivered to pathogen invasion sites through polarized secretion. To sustain mutualistic associations, host cells also reprogram the membrane trafficking system to accommodate invasive structures of symbiotic microbes. Here, we provide an analysis of recent advances in understanding the roles of secretory and endocytic membrane trafficking pathways in plant immune activation. We also discuss strategies deployed by adapted microbes to manipulate these pathways to subvert or inhibit plant defense.

A look at plant immunity through the window of the multitasking coreceptor BAK1

Recognition of microbe- and danger-associated molecular patterns (MAMPs and DAMPs, respectively) by pattern recognition receptors (PRRs) is central to innate immunity in both plants and animals. The plant PRRs described to date are all cell surface-localized receptors. According to their ligand-binding ectodomains, each PRR engages a specific coreceptor or adaptor kinase in its signaling complexes to regulate defense signaling. With a focus on the coreceptor RLK BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) and related SOMATIC EMBRYOGENESIS RECEPTOR KINASEs (SERKs), here we review the increasing inventory of BAK1 partners and their functions in plant immunity. We also discuss the significance of autoimmunity triggered by BAK1/SERK4 disintegration in shaping the strategies for attenuation of PRR signaling by infectious microbes and host plants.

RIN4 recruits the exocyst subunit EXO70B1 to the plasma membrane

The exocyst is a conserved vesicle-tethering complex with principal roles in cell polarity and morphogenesis. Several studies point to its involvement in polarized secretion during microbial pathogen defense. In this context, we have found an interaction between the Arabidopsis EXO70B1 exocyst subunit, a protein which was previously associated with both the defense response and autophagy, and RPM1 INTERACTING PROTEIN 4 (RIN4), the best studied member of the NOI protein family and a known regulator of plant defense pathways. Interestingly, fragments of RIN4 mimicking the cleavage caused by the Pseudomonas syringae effector protease, AvrRpt2, fail to interact strongly with EXO70B1. We observed that transiently expressed RIN4, but not the plasma membrane (PM) protein aquaporin PIP2, recruits EXO70B1 to the PM. Unlike EXO70B1, RIN4 does not recruit the core exocyst subunit SEC6 to the PM under these conditions. Furthermore, the AvrRpt2 effector protease delivered by P. syringae is able to release both RIN4 and EXO70B1 to the cytoplasm. We present a model for how RIN4 might regulate the localization and putative function of EXO70B1 and speculate on the role the AvrRpt2 protease might have in the regulation of this defense response.

Arabidopsis E3 ubiquitin ligase PLANT U-BOX13 (PUB13) regulates chitin receptor LYSIN MOTIF RECEPTOR KINASE5 (LYK5) protein abundance

Long-chain chitooligosaccharides are fungal microbe-associated molecular patterns (MAMPs) that are recognized by LYSIN MOTIF RECEPTOR KINASE5 (LYK5), inducing the formation of a complex with CHITIN ELICITOR RECEPTOR KINASE1 (CERK1). Formation of this complex leads to activation of the CERK1 intracellular kinase domain and induction of plant innate immunity in Arabidopsis. We found that addition of chitooctaose induced LYK5 protein accumulation as a result of de novo gene expression and the inhibition of LYK5 protein degradation. Screening the putative E3 ligases for interaction with LYK5 identified PLANT U-BOX13 (PUB13), which complexed with LYK5, but this complex dissociated upon addition of chitooctaose. Consistent with these results, LYK5 protein abundance was higher in pub13 mutants compared with the wild type without chitooctaose treatment, while similar abundance was detected with the addition of chitooctaose. The pub13 mutants showed hypersensitivity to chitooctaose-induced rapid responses, such as the production of reactive oxygen species (ROS) and mitogen-activated protein (MAP) kinase phosphorylation, but exhibited normal responses to subsequent long-term chitooctaose treatment, such as gene expression and callose deposition. In addition, PUB13 could ubiquitinate the LYK5 kinase domain in vitro. Taken together, our results suggest an important regulatory function for the turnover of LYK5 mediated by the E3 ligase PUB13.

VpPUB24, a novel gene from Chinese grapevine, Vitis pseudoreticulata, targets VpICE1 to enhance cold tolerance

The ubiquitination system plays important roles in the degradation and modification of substrate proteins. In this study, we characterize a putative U-box type E3 ubiquitin ligase gene, VpPUB24 (plant U-box protein 24), from Chinese wild grapevine, Vitis pseudoreticulata accession Baihe-35-1. We show that VpPUB24 is induced by a number of stresses, especially cold treatment. Real-time PCR analysis indicated that the PUB24 transcripts were increased after cold stress in different grapevine species, although the relative expression level was different. In grapevine protoplasts, we found that VpPUB24 was expressed at a low level at 22 °C but accumulated rapidly following cold treatment. A yeast two-hybrid assay revealed that VpPUB24 interacted physically with VpICE1. Further experiments indicated that VpICE1 is targeted for degradation via the 26S proteasome and that the degradation is accelerated by VpHOS1, and not by VpPUB24. Immunoblot analyses indicated that VpPUB24 promotes the accumulation of VpICE1 and suppresses the expression of VpHOS1 to regulate the abundance of VpICE1. Furthermore, VpICE1 promotes transcription of VpPUB24 at low temperatures. We also found that VpPUB24 interacts with VpHOS1 in a yeast two-hybrid assay. Additionally, over-expression of VpPUB24 in Arabidopsis thaliana enhanced cold tolerance. Collectively, our results suggest that VpPUB24 interacts with VpICE1 to play a role in cold stress.

EXO70C2 Is a Key Regulatory Factor for Optimal Tip Growth of Pollen

The exocyst, a eukaryotic tethering complex, coregulates targeted exocytosis as an effector of small GTPases in polarized cell growth. In land plants, several exocyst subunits are encoded by double or triple paralogs, culminating in tens of EXO70 paralogs. Out of 23 Arabidopsis thaliana EXO70 isoforms, we analyzed seven isoforms expressed in pollen. Genetic and microscopic analyses of single mutants in EXO70A2, EXO70C1, EXO70C2, EXO70F1, EXO70H3, EXO70H5, and EXO70H6 genes revealed that only a loss-of-function EXO70C2 allele resulted in a significant male-specific transmission defect (segregation 40%:51%:9%) due to aberrant pollen tube growth. Mutant pollen tubes grown in vitro exhibited an enhanced growth rate and a decreased thickness of the tip cell wall, causing tip bursts. However, exo70C2 pollen tubes could frequently recover and restart their speedy elongation, resulting in a repetitive stop-and-go growth dynamics. A pollen-specific depletion of the closest paralog, EXO70C1, using artificial microRNA in the exo70C2 mutant background, resulted in a complete pollen-specific transmission defect, suggesting redundant functions of EXO70C1 and EXO70C2. Both EXO70C1 and EXO70C2, GFP tagged and expressed under the control of their native promoters, localized in the cytoplasm of pollen grains, pollen tubes, and also root trichoblast cells. The expression of EXO70C2-GFP complemented the aberrant growth of exo70C2 pollen tubes. The absent EXO70C2 interactions with core exocyst subunits in the yeast two-hybrid assay, cytoplasmic localization, and genetic effect suggest an unconventional EXO70 function possibly as a regulator of exocytosis outside the exocyst complex. In conclusion, EXO70C2 is a novel factor contributing to the regulation of optimal tip growth of Arabidopsis pollen tubes.

CALCIUM-DEPENDENT PROTEIN KINASE5 Associates with the Truncated NLR Protein TIR-NBS2 to Contribute to exo70B1-Mediated Immunity

Calcium-dependent protein kinases (CPKs) function as calcium sensors and play important roles in plant immunity. Loss of function of the exocyst complex subunit EXO70B1 leads to autoimmunity caused by activation of TN2, a truncated Toll/interleukin-1 receptor-nucleotide binding sequence protein. Here we show, based on a screen for suppressors of exo70B1, that exo70B1-activated autoimmune responses require CPK5 However, the CPK5 homologs CPK4, CPK6, and CPK11, which were previously reported to function redundantly with CPK5 in effector-triggered immunity, did not contribute to exo70B1-associated phenotypes, indicating that CPK5 plays a unique role in plant immunity. Overexpressing CPK5 results in TN2-dependent autoimmunity and enhanced disease resistance, reminiscent of the exo70B1 phenotypes. Ectopic expression of CPK5 in the exo70B1 mutant led to constitutive CPK5 protein kinase activity, which was not detectable in tn2 mutants. Furthermore, TN2 interacts with the CPK5 N-terminal variable and kinase domains, stabilizing CPK5 kinase activity in vitro. This work uncovers a direct functional link between an atypical immune receptor and a crucial component of early immune signaling: increased immunity in exo70B1 depends on TN2 and CPK5 and, in a positive feedback loop, TN2 keeps CPK5 enzymatically active beyond the initiating stimulus.

Changes in PUB22 Ubiquitination Modes Triggered by MITOGEN-ACTIVATED PROTEIN KINASE3 Dampen the Immune Response

Crosstalk between posttranslational modifications, such as ubiquitination and phosphorylation, play key roles in controlling the duration and intensity of signaling events to ensure cellular homeostasis. However, the molecular mechanisms underlying the regulation of negative feedback loops remain poorly understood. Here, we uncover a pathway in Arabidopsis thaliana by which a negative feedback loop involving the E3 ubiquitin ligase PUB22 that dampens the immune response is triggered by MITOGEN-ACTIVATED PROTEIN KINASE3 (MPK3), best known for its function in the activation of signaling. PUB22's stability is controlled by MPK3-mediated phosphorylation of residues localized in and adjacent to the E2 docking domain. We show that phosphorylation is critical for stabilization by inhibiting PUB22 oligomerization and, thus, autoubiquitination. The activity switch allows PUB22 to dampen the immune response. This regulatory mechanism also suggests that autoubiquitination, which is inherent to most single unit E3s in vitro, can function as a self-regulatory mechanism in vivo.

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.

Analysis of Exocyst Subunit EXO70 Family Reveals Distinct Membrane Polar Domains in Tobacco Pollen Tubes

The vesicle-tethering complex exocyst is one of the crucial cell polarity regulators. The EXO70 subunit is required for the targeting of the complex and is represented by many isoforms in angiosperm plant cells. This diversity could be partly responsible for the establishment and maintenance of membrane domains with different composition. To address this hypothesis, we employed the growing pollen tube, a well-established cell polarity model system, and performed large-scale expression, localization, and functional analysis of tobacco (Nicotiana tabacum) EXO70 isoforms. Various isoforms localized to different regions of the pollen tube plasma membrane, apical vesicle-rich inverted cone region, nucleus, and cytoplasm. The overexpression of major pollen-expressed EXO70 isoforms resulted in growth arrest and characteristic phenotypic deviations of tip swelling and apical invaginations. NtEXO70A1a and NtEXO70B1 occupied two distinct and mutually exclusive plasma membrane domains. Both isoforms partly colocalized with the exocyst subunit NtSEC3a at the plasma membrane, possibly forming different exocyst complex subpopulations. NtEXO70A1a localized to the small area previously characterized as the site of exocytosis in the tobacco pollen tube, while NtEXO70B1 surprisingly colocalized with the zone of clathrin-mediated endocytosis. Both NtEXO70A1a and NtEXO70B1 colocalized to different degrees with markers for the anionic signaling phospholipids phosphatidylinositol 4,5-bisphosphate and phosphatidic acid. In contrast, members of the EXO70 C class, which are specifically expressed in tip-growing cells, exhibited exocytosis-related functional effects in pollen tubes despite the absence of apparent plasma membrane localization. Taken together, our data support the existence of multiple membrane-trafficking domains regulated by different EXO70-containing exocyst complexes within a single cell.

Homologous U-box E3 Ubiquitin Ligases OsPUB2 and OsPUB3 Are Involved in the Positive Regulation of Low Temperature Stress Response in Rice (Oryza sativa L.)

Rice U-box E3 Ub ligases (OsPUBs) are implicated in biotic stress responses. However, their cellular roles in response to abiotic stress are poorly understood. In this study, we performed functional analyses of two homologous OsPUB2 and OsPUB3 in response to cold stress (4°C). OsPUB2 was up-regulated by high salinity, drought, and cold, whereas OsPUB3 was constitutively expressed. A subcellular localization assay revealed that OsPUB2 and OsPUB3 were localized to the exocyst positive organelle (EXPO)-like punctate structures. OsPUB2 was also localized to the nuclei. OsPUB2 and OsPUB3 formed a hetero-dimeric complex as well as homo-dimers in yeast cells and in vitro. OsPUB2/OsPUB3 exhibited self-ubiquitination activities in vitro and were rapidly degraded in the cell-free extracts with apparent half-lives of 150-160 min. This rapid degradation of OsPUB2/OsPUB3 was delayed in the presence of the crude extracts of cold-treated seedlings (apparent half-lives of 200-280 min). Moreover, a hetero-dimeric form of OsPUB2/OsPUB3 was more stable than the homo-dimers. These results suggested that OsPUB2 and OsPUB3 function coordinately in response to cold stress. OsPUB2- and OsPUB3-overexpressing transgenic rice plants showed markedly better tolerance to cold stress than did the wild-type plants in terms of survival rates, chlorophyll content, ion leakage, and expression levels of cold stress-inducible marker genes. Taken together, these results suggested that the two homologous rice U-box E3 Ub ligases OsPUB2 and OsPUB3 are positive regulators of the response to cold stress.

The N-Terminal UND Motif of the Arabidopsis U-Box E3 Ligase PUB18 Is Critical for the Negative Regulation of ABA-Mediated Stomatal Movement and Determines Its Ubiquitination Specificity for Exocyst Subunit Exo70B1

The Arabidopsis thaliana U-box E3 ligases PUB18/PUB19 and PUB22/PUB23 are negative regulators of drought stress responses. PUB18/PUB19 regulate the drought stress response in an abscisic acid (ABA)-dependent manner, whereas PUB22/PUB23 regulate this response in an ABA-independent manner. A major structural difference between PUB18/PUB19 and PUB22/PUB23 is the presence of the UND (U-box N-terminal domain). Here, we focused on elucidating the molecular mechanism that mediates the functional difference between PUB18 and PUB22 and found that the UND^PUB18 was critically involved in the negative regulation of ABA-mediated stomatal movements. Exo70B1, a subunit of the exocyst complex, was identified as a target of PUB18, whereas Exo70B2 was a substrate of PUB22. However, the ∆UND-PUB18 derivative failed to ubiquitinate Exo70B1, but ubiquitinated Exo70B2. By contrast, the UND^PUB18-PUB22 chimeric protein ubiquitinated Exo70B1 instead of Exo70B2, suggesting that the ubiquitination specificities of PUB18 and PUB22 to Exo70B1 and Exo70B2, respectively, are dependent on the presence or absence of the UND^PUB18 motif. The ABA-insensitive phenotypes of the pub18 pub19 exo70b1 triple mutant were reminiscent of those of exo70b1 rather than pub18 pub19, indicating that Exo70B1 functions downstream of PUB18. Overall, our results suggest that the UND^PUB18 motif is crucial for the negative regulation of ABA-dependent stomatal movement and for determination of its ubiquitination specificity to Exo70B1.

The Proteasome Acts as a Hub for Plant Immunity and Is Targeted by Pseudomonas Type III Effectors

Recent evidence suggests that the ubiquitin-proteasome system is involved in several aspects of plant immunity and that a range of plant pathogens subvert the ubiquitin-proteasome system to enhance their virulence. Here, we show that proteasome activity is strongly induced during basal defense in Arabidopsis (Arabidopsis thaliana). Mutant lines of the proteasome subunits RPT2a and RPN12a support increased bacterial growth of virulent Pseudomonas syringae pv tomato DC3000 (Pst) and Pseudomonas syringae pv maculicola ES4326. Both proteasome subunits are required for pathogen-associated molecular pattern-triggered immunity responses. Analysis of bacterial growth after a secondary infection of systemic leaves revealed that the establishment of systemic acquired resistance (SAR) is impaired in proteasome mutants, suggesting that the proteasome also plays an important role in defense priming and SAR In addition, we show that Pst inhibits proteasome activity in a type III secretion-dependent manner. A screen for type III effector proteins from Pst for their ability to interfere with proteasome activity revealed HopM1, HopAO1, HopA1, and HopG1 as putative proteasome inhibitors. Biochemical characterization of HopM1 by mass spectrometry indicates that HopM1 interacts with several E3 ubiquitin ligases and proteasome subunits. This supports the hypothesis that HopM1 associates with the proteasome, leading to its inhibition. Thus, the proteasome is an essential component of pathogen-associated molecular pattern-triggered immunity and SAR, which is targeted by multiple bacterial effectors.

The ubiquitin conjugating enzyme, TaU4 regulates wheat defence against the phytopathogen Zymoseptoria tritici

Mycosphaerella graminicola (Zymoseptoria tritici commonly known as Septoria), the causal agent of Septoria Leaf Blotch (STB), is considered one of the major threats to European wheat production. Previous studies have shown the importance of ubiquitination in plant defence against a multitude of pathogens. However the ubiquitination machinery in wheat is under studied, particularly E2 enzymes that have the ability to control the ubiquitination and thereby the fate of many different target proteins. In this study we identify an E2 enzyme, Triticum aestivum Ubiquitin conjugating enzyme 4 (TaU4) that functions in wheat defence against Septoria. We demonstrate TaU4 to be a bona fide E2 enzyme through an E2 charging assay. TaU4 localises in both the cytoplasm and nucleus, therefore potentially interacting with E3 ligases and substrate proteins in multiple compartments. Virus Induced Gene Silencing of TaU4 in wheat leaves resulted in delayed development of disease symptoms, reduced Septoria growth and reproduction. We conclude that TaU4 is a novel negative regulator of defence against Septoria.

Mutations in exocyst complex subunit SEC6 gene impaired polar auxin transport and PIN protein recycling in Arabidopsis primary root

Polar auxin transport, which is critical for land plant pattern formation and directional growth, is largely depended on asymmetric distribution of PIN proteins at the plasma membrane (PM). Endocytosis and recycling processes play important roles in regulating PIN protein distribution and abundance at the PM. Two subunits (SEC8, EXO70A1) of exocyst, an octameric vesicle-tethering complex, have been reported to be involved in PIN protein recycling in Arabidopsis. However, the function of exocyst complex in PIN protein recycling and polar auxin transport remains incompletely understood. In this study, we utilized two SEC6 down-regulation mutants (PRsec6-1 and PRsec6-2) to investigate the role of exocyst subunit SEC6 in the primary root development, polar auxin transport and PIN proteins recycling. We found that in PRsec6 mutants: 1. Primary root growth was retarded, and lateral root initiation were compromised. 2. Primary roots were sensitive to exogenous auxin 1-napthalene acetic acid (NAA) but not 2,4-dichlorophenoxy (2.4-D). 3. Recycling of PIN1 and PIN2 proteins from the Brefeldin A (BFA) compartment to the PM was delayed. 4. Vesicles accumulated in the primary root tip cells, especially accumulated in the cytosol closed to the PM. These results further demonstrated that the exocyst complex plays an important role in PIN protein recycling and polar auxin transport in Arabidopsis primary root.

PPP1, a plant-specific regulator of transcription controls Arabidopsis development and PIN expression

Directional transport of auxin is essential for plant development, with PIN auxin transport proteins representing an integral part of the machinery that controls hormone distribution. However, unlike the rapidly emerging framework of molecular determinants regulating PIN protein abundance and subcellular localization, insights into mechanisms controlling PIN transcription are still limited. Here we describe PIN2 PROMOTER BINDING PROTEIN 1 (PPP1), an evolutionary conserved plant-specific DNA binding protein that acts on transcription of PIN genes. Consistent with PPP1 DNA-binding activity, PPP1 reporter proteins are nuclear localized and analysis of PPP1 null alleles and knockdown lines indicated a function as a positive regulator of PIN expression. Furthermore, we show that ppp1 pleiotropic mutant phenotypes are partially reverted by PIN overexpression, and results are presented that underline a role of PPP1-PIN promoter interaction in PIN expression control. Collectively, our findings identify an elementary, thus far unknown, plant-specific DNA-binding protein required for post-embryonic plant development, in general, and correct expression of PIN genes, in particular.

Arabidopsis Proline Dehydrogenase Contributes to Flagellin-Mediated PAMP-Triggered Immunity by Affecting RBOHD

Plants activate different defense systems to counteract the attack of microbial pathogens. Among them, the recognition of conserved microbial- or pathogen-associated molecular patterns (MAMPs or PAMPs) by pattern-recognition receptors stimulates MAMP- or PAMP-triggered immunity (PTI). In recent years, the elicitors, receptors, and signaling pathways leading to PTI have been extensively studied. However, the contribution of organelles to this program deserves further characterization. Here, we studied how processes altering the mitochondrial electron transport chain (mETC) influence PTI establishment. With particular emphasis, we evaluated the effect of proline dehydrogenase (ProDH), an enzyme that can load electrons into the mETC and regulate the cellular redox state. We found that mETC uncouplers (antimycin or rotenone) and manganese superoxide dismutase deficiency impair flg22-induced responses such as accumulation of reactive oxygen species (ROS) and bacterial growth limitation. ProDH mutants also reduce these defenses, decreasing callose deposition as well. Using ProDH inhibitors and ProDH inducers (exogenous Pro treatment), we showed that this enzyme modulates the generation of ROS by the plasma membrane respiratory burst NADPH oxidase homolog D. In this way, we contribute to the understanding of mitochondrial activities influencing early and late PTI responses and the coordination of the redox-associated mitochondrial enzyme ProDH with defense events initiated at the plasma membrane.

Tethering Complexes in the Arabidopsis Endomembrane System

Targeting of endomembrane transport containers is of the utmost importance for proper land plant growth and development. Given the immobility of plant cells, localized membrane vesicle secretion and recycling are amongst the main processes guiding proper cell, tissue and whole plant morphogenesis. Cell wall biogenesis and modification are dependent on vectorial membrane traffic, not only during normal development, but also in stress responses and in plant defense against pathogens and/or symbiosis. It is surprising how little we know about these processes in plants, from small GTPase regulation to the tethering complexes that act as their effectors. Tethering factors are single proteins or protein complexes mediating first contact between the target membrane and arriving membrane vesicles. In this review we focus on the tethering complexes of the best-studied plant model—Arabidopsis thaliana. Genome-based predictions indicate the presence of all major tethering complexes in plants that are known from a hypothetical last eukaryotic common ancestor (LECA). The evolutionary multiplication of paralogs of plant tethering complex subunits has produced the massively expanded EXO70 family, indicating a subfunctionalization of the terminal exocytosis machinery in land plants. Interpretation of loss of function (LOF) mutant phenotypes has to consider that related, yet clearly functionally-specific complexes often share some common core subunits. It is therefore impossible to conclude with clarity which version of the complex is responsible for the phenotypic deviations observed. Experimental interest in the analysis of plant tethering complexes is growing and we hope to contribute with this review by attracting even more attention to this fascinating field of plant cell biology.

Potato NPH3/RPT2-Like Protein StNRL1, Targeted by a Phytophthora infestans RXLR Effector, Is a Susceptibility Factor

Plant pathogens deliver effectors to manipulate host processes. We know little about how fungal and oomycete effectors target host proteins to promote susceptibility, yet such knowledge is vital to understand crop disease. We show that either transient expression in Nicotiana benthamiana, or stable transgenic expression in potato (Solanum tuberosum), of the Phytophthora infestans RXLR effector Pi02860 enhances leaf colonization by the pathogen. Expression of Pi02860 also attenuates cell death triggered by the P. infestans microbe-associated molecular pattern INF1, indicating that the effector suppresses pattern-triggered immunity. However, the effector does not attenuate cell death triggered by Cf4/Avr4 coexpression, showing that it does not suppress all cell death activated by cell surface receptors. Pi02860 interacts in yeast two-hybrid assays with potato NPH3/RPT2-LIKE1 (NRL1), a predicted CULLIN3-associated ubiquitin E3 ligase. Interaction of Pi02860 in planta was confirmed by coimmunoprecipitation and bimolecular fluorescence complementation assays. Virus-induced gene silencing of NRL1 in N. benthamiana resulted in reduced P. infestans colonization and accelerated INF1-mediated cell death, indicating that this host protein acts as a negative regulator of immunity. Moreover, whereas NRL1 virus-induced gene silencing had no effect on the ability of the P. infestans effector Avr3a to suppress INF1-mediated cell death, such suppression by Pi02860 was significantly attenuated, indicating that this activity of Pi02860 is mediated by NRL1. Transient overexpression of NRL1 resulted in the suppression of INF1-mediated cell death and enhanced P. infestans leaf colonization, demonstrating that NRL1 acts as a susceptibility factor to promote late blight disease.

The Exocyst Complex in Health and Disease

Exocytosis involves the fusion of intracellular secretory vesicles with the plasma membrane, thereby delivering integral membrane proteins to the cell surface and releasing material into the extracellular space. Importantly, exocytosis also provides a source of lipid moieties for membrane extension. The tethering of the secretory vesicle before docking and fusion with the plasma membrane is mediated by the exocyst complex, an evolutionary conserved octameric complex of proteins. Recent findings indicate that the exocyst complex also takes part in other intra-cellular processes besides secretion. These various functions seem to converge toward defining a direction of membrane growth in a range of systems from fungi to plants and from neurons to cilia. In this review we summarize the current knowledge of exocyst function in cell polarity, signaling and cell-cell communication and discuss implications for plant and animal health and disease.

Molecular Mechanisms of Floral Boundary Formation in Arabidopsis

Boundary formation is a crucial developmental process in plant organogenesis. Boundaries separate cells with distinct identities and act as organizing centers to control the development of adjacent organs. In flower development, initiation of floral primordia requires the formation of the meristem-to-organ (M-O) boundaries and floral organ development depends on the establishment of organ-to-organ (O-O) boundaries. Studies in this field have revealed a suite of genes and regulatory pathways controlling floral boundary formation. Many of these genes are transcription factors that interact with phytohormone pathways. This review will focus on the functions and interactions of the genes that play important roles in the floral boundaries and discuss the molecular mechanisms that integrate these regulatory pathways to control the floral boundary formation.

CaPUB1, a Hot Pepper U-box E3 Ubiquitin Ligase, Confers Enhanced Cold Stress Tolerance and Decreased Drought Stress Tolerance in Transgenic Rice (Oryza sativa L.)

Abiotic stresses such as drought and low temperature critically restrict plant growth, reproduction, and productivity. Higher plants have developed various defense strategies against these unfavorable conditions. CaPUB1 (Capsicum annuum Putative U-box protein 1) is a hot pepper U-box E3 Ub ligase. Transgenic Arabidopsis plants that constitutively expressed CaPUB1 exhibited drought-sensitive phenotypes, suggesting that it functions as a negative regulator of the drought stress response. In this study, CaPUB1 was over-expressed in rice (Oryza sativa L.), and the phenotypic properties of transgenic rice plants were examined in terms of their drought and cold stress tolerance. Ubi:CaPUB1 T3 transgenic rice plants displayed phenotypes hypersensitive to dehydration, suggesting that its role in the negative regulation of drought stress response is conserved in dicot Arabidopsis and monocot rice plants. In contrast, Ubi:CaPUB1 progeny exhibited phenotypes markedly tolerant to prolonged low temperature (4°C) treatment, compared to those of wild-type plants, as determined by survival rates, electrolyte leakage, and total chlorophyll content. Cold stress-induced marker genes, including DREB1A, DREB1B, DREB1C, and Cytochrome P450, were more up-regulated by cold treatment in Ubi:CaPUB1 plants than in wild-type plants. These results suggest that CaPUB1 serves as both a negative regulator of the drought stress response and a positive regulator of the cold stress response in transgenic rice plants. This raises the possibility that CaPUB1 participates in the cross-talk between drought and low-temperature signaling pathways.

Membrane-Associated Ubiquitin Ligase SAUL1 Suppresses Temperature- and Humidity-Dependent Autoimmunity in Arabidopsis

Plants have evolved elaborate mechanisms to regulate pathogen defense. Imbalances in this regulation may result in autoimmune responses that are affecting plant growth and development. In Arabidopsis, SAUL1 encodes a plant U-box ubiquitin ligase and regulates senescence and cell death. Here, we show that saul1-1 plants exhibit characteristics of an autoimmune mutant. A decrease in relative humidity or temperature resulted in reduced growth and systemic lesioning of saul1-1 rosettes. These physiological changes are associated with increased expression of salicylic acid-dependent and pathogenesis-related (PR) genes. Consistently, resistance of saul1-1 plants against Pseudomonas syringae pv. maculicola ES4326, P. syringae pv. tomato DC3000, or Hyaloperonospora arabidopsidis Noco2 was enhanced. Transmission electron microscopy revealed alterations in saul1-1 chloroplast ultrastructure and cell-wall depositions. Confocal analysis on aniline blue-stained leaf sections and cellular universal micro spectrophotometry further showed that these cell-wall depositions contain callose and lignin. To analyze signaling downstream of SAUL1, we performed epistasis analyses between saul1-1 and mutants in the EDS1/PAD4/SAG101 hub. All phenotypes observed in saul1-1 plants at low temperature were dependent on EDS1 and PAD4 but not SAG101. Taken together, SAUL1 negatively regulates immunity upstream of EDS1/PAD4, likely through the degradation of an unknown activator of the pathway.

QTL Mapping by SLAF-seq and Expression Analysis of Candidate Genes for Aphid Resistance in Cucumber

Cucumber, a very important vegetable crop worldwide, is easily damaged by pests. Aphid is one of the most serious cucumber pests and frequently cause severe damage to commercially produced crops. Understanding the genetic mechanisms underlying pest resistance is important for aphid-resistant cucumber varieties breeding. In this study, two parental cucumber lines, JY30 (aphid susceptible) and EP6392 (aphid resistant), and pools of resistant and susceptible (n = 50 each) plants from 1000 F2 individuals derived from crossing JY30 with EP6392, were used to detect genomic regions associated with aphid resistance in cucumbers. The analysis was performed using specific length amplified fragment sequencing (SLAF-seq), bulked segregant analysis (BSA), and single nucleotide polymorphism index (SNP-index) methods. A main effect QTL (quantitative trait locus) of 0.31 Mb on Chr5, including 43 genes, was identified by association analysis. Sixteen of the 43 genes were identified as potentially associated with aphid resistance through gene annotation analysis. The effect of aphid infestation on the expression of these candidate genes screened by SLAF-seq was investigated in EP6392 plants by qRT-PCR. The results indicated that seven genes including encoding transcription factor MYB59-like (Csa5M641610.1), auxin transport protein BIG-like (Csa5M642140.1), F-box/kelch-repeat protein At5g15710-like (Csa5M642160.1), transcription factor HBP-1a-like (Csa5M642710.1), beta-glucan-binding protein (Csa5M643380.1), endo-1,3(4)-beta-glucanase 1-like (Csa5M643880.1), and proline-rich receptor-like protein kinase PERK10-like (Csa5M643900.1), out of the 16 genes were down regulated after aphid infestation, whereas 5 genes including encoding probable leucine-rich repeat (LRR) receptor-like serine/threonine-protein kinase At5g15730-like (Csa5M642150.1), Stress-induced protein KIN2 (Csa5M643240.1 and Csa5M643260.1), F-box family protein (Csa5M643280.1), F-box/kelch-repeat protein (Csa5M643290.1), were up-regulated after aphid infestation. The gene Csa5M642150.1, encoding probable LRR receptor-like serine/threonine-protein kinase At5g15730-like, was most likely a key candidate gene in cucumber plants in response to infestation. This study provides a certain theoretical basis of molecular biology for genetic improvement of cucumber aphid resistance and aphid resistant variety breeding.

Role of Ubiquitin-Mediated Degradation System in Plant Biology

Ubiquitin-mediated proteasomal degradation is an important mechanism to control protein load in the cells. Ubiquitin binds to a protein on lysine residue and usually promotes its degradation through 26S proteasome system. Abnormal proteins and regulators of many processes, are targeted for degradation by the ubiquitin-proteasome system. It allows cells to maintain the response to cellular level signals and altered environmental conditions. The ubiquitin-mediated proteasomal degradation system plays a key role in the plant biology, including abiotic stress, immunity, and hormonal signaling by interfering with key components of these pathways. The involvement of the ubiquitin system in many vital processes led scientists to explore more about the ubiquitin machinery and most importantly its targets. In this review, we have summarized recent discoveries of the plant ubiquitin system and its involvement in critical processes of plant biology.

The ROP2-RIC7 pathway negatively regulates light-induced stomatal opening by inhibiting exocyst subunit Exo70B1 in Arabidopsis

Stomata are the tiny valves on the plant surface that mediate gas exchange between the plant and its environment. Stomatal opening needs to be tightly regulated to facilitate CO2 uptake and prevent excess water loss. Plant Rho-type (ROP) GTPase 2 (ROP2) is a molecular component of the system that negatively regulates light-induced stomatal opening. Previously, ROP-interactive Cdc42- and Rac-interactive binding motif-containing protein 7 (RIC7) was suggested to function downstream of ROP2. However, the underlying molecular mechanism remains unknown. To understand the mechanism by which RIC7 regulates light-induced stomatal opening, we analyzed the stomatal responses of ric7 mutant Arabidopsis plants and identified the target protein of RIC7 using a yeast two-hybrid screen. Light-induced stomatal opening was promoted by ric7 knockout, whereas it was inhibited by RIC7 overexpression, indicating that RIC7 negatively regulates stomatal opening in Arabidopsis. RIC7 interacted with exocyst subunit Exo70 family protein B1 (Exo70B1), a component of the vesicle trafficking machinery. RIC7 and Exo70B1 localized to the plasma membrane region under light or constitutively active ROP2 conditions. The knockout mutant of Exo70B1 and ric7/exo70b1 exhibited retarded light-induced stomatal opening. Our results suggest that ROP2 and RIC7 suppress excess stomatal opening by inhibiting Exo70B1, which most likely participates in the vesicle trafficking required for light-induced stomatal opening.

Disruption of OsEXO70A1 Causes Irregular Vascular Bundles and Perturbs Mineral Nutrient Assimilation in Rice

Normal uptake, transportation, and assimilation of primary nutrients are essential to plant growth. Tracheary elements (TEs) are tissues responsible for the transport of water and minerals and characterized by patterned secondary cell wall (SCW) thickening. Exocysts are involved in the regulation of SCW deposition by mediating the targeted transport of materials and enzymes to specific membrane areas. EXO70s are highly duplicated in plants and provide exocysts with functional specificity. In this study, we report the isolation of a rice mutant rapid leaf senescence2 (rls2) that exhibits dwarfism, ferruginous spotted necrotic leaves, decreased hydraulic transport, and disordered primary nutrient assimilation. Histological analysis of rls2-1 mutants has indicated impaired cell expansion, collapsed vascular tissues, and irregular SCW deposition. Map-based cloning has revealed that RLS2 encodes OsEXO70A1, which is one of the 47 members of EXO70s in rice. RLS2 was widely expressed and spatially restricted in vascular bundles. Subcellular localization analysis demonstrated that RLS2 was present on both membrane and nuclear regions. Expression analysis revealed that mutations in rls2 triggers transcriptional fluctuation of orthologous EXO70 genes and affects genes involved in primary nutrient absorption and transport. In brief, our study revealed that RLS2 is required for normal vascular bundle differentiation and primary nutrient assimilation.

RNA Silencing of Exocyst Genes in the Stigma Impairs the Acceptance of Compatible Pollen in Arabidopsis

Initial pollen-pistil interactions in the Brassicaceae are regulated by rapid communication between pollen grains and stigmatic papillae and are fundamentally important, as they are the first step toward successful fertilization. The goal of this study was to examine the requirement of exocyst subunits, which function in docking secretory vesicles to sites of polarized secretion, in the context of pollen-pistil interactions. One of the exocyst subunit genes, EXO70A1, was previously identified as an essential factor in the stigma for the acceptance of compatible pollen in Arabidopsis (Arabidopsis thaliana) and Brassica napus. We hypothesized that EXO70A1, along with other exocyst subunits, functions in the Brassicaceae dry stigma to deliver cargo-bearing secretory vesicles to the stigmatic papillar plasma membrane, under the pollen attachment site, for pollen hydration and pollen tube entry. Here, we investigated the functions of exocyst complex genes encoding the remaining seven subunits, SECRETORY3 (SEC3), SEC5, SEC6, SEC8, SEC10, SEC15, and EXO84, in Arabidopsis stigmas following compatible pollinations. Stigma-specific RNA-silencing constructs were used to suppress the expression of each exocyst subunit individually. The early postpollination stages of pollen grain adhesion, pollen hydration, pollen tube penetration, seed set, and overall fertility were analyzed in the transgenic lines to evaluate the requirement of each exocyst subunit. Our findings provide comprehensive evidence that all eight exocyst subunits are necessary in the stigma for the acceptance of compatible pollen. Thus, this work implicates a fully functional exocyst complex as a component of the compatible pollen response pathway to promote pollen acceptance.

An Arabidopsis WDR protein coordinates cellular networks involved in light, stress response and hormone signals

The WD-40 repeat (WDR) protein acts as a scaffold for protein interactions in various cellular events. An Arabidopsis WDR protein exhibited sequence similarity with human WDR26, a scaffolding protein implicated in H2O2-induced cell death in neural cells. The AtWDR26 transcript was induced by auxin, abscisic acid (ABA), ethylene (ET), osmostic stress and salinity. The expression of AtWDR26 was regulated by light, and seed germination of the AtWDR26 overexpression (OE) and seedling growth of the T-DNA knock-out (KO) exhibited altered sensitivity to light. Root growth of the OE seedlings increased tolerance to ZnSO4 and NaCl stresses and were hypersensitive to inhibition of osmotic stress. Seedlings of OE and KO altered sensitivities to multiple hormones. Transcriptome analysis of the transgenic plants overexpressing AtWDR26 showed that genes involved in the chloroplast-related metabolism constituted the largest group of the up-regulated genes. AtWDR26 overexpression up-regulated a large number of genes related to defense cellular events including biotic and abiotic stress response. Furthermore, several members of genes functioning in the regulation of Zn homeostasis, and hormone synthesis and perception of auxin and JA were strongly up-regulated in the transgenic plants. Our data provide physiological and transcriptional evidence for AtWDR26 role in hormone, light and abiotic stress cellular events.

Phytophthora infestans RXLR Effector AVR1 Interacts with Exocyst Component Sec5 to Manipulate Plant Immunity

Phytophthora infestans secretes numerous RXLR effectors that modulate host defense and thereby pave the way for successful invasion. Here, we show that the RXLR effector AVR1 is a virulence factor that promotes colonization and suppresses callose deposition, a hallmark of basal defense. To identify host targets of AVR1, we performed yeast two-hybrid screens and selected Sec5 as a candidate. Sec5 is a subunit of the exocyst, a protein complex that is involved in vesicle trafficking. AVR1-like (A-L), a close homolog of AVR1, also acts as a virulence factor, but unlike AVR1, A-L does not suppress CRINKLER2 (CRN2)-induced cell death or interact with Sec5. Compared with AVR1, A-L is shorter and lacks the carboxyl-terminal tail, the T-region that is crucial for CRN2-induced cell death suppression and Sec5 interaction. In planta analyses revealed that AVR1 and Sec5 are in close proximity, and coimmunoprecipitation confirmed the interaction. Sec5 is required for secretion of the pathogenesis-related protein PR-1 and callose deposition and also plays a role in CRN2-induced cell death. Our findings show that P. infestans manipulates an exocyst subunit and thereby potentially disturbs vesicle trafficking, a cellular process that is important for basal defense. This is a novel strategy that oomycete pathogens exploit to modulate host defense.

Dynamic transcription profiles of "Qinguan" apple (Malus × domestica) leaves in response to Marssonina coronaria inoculation

Marssonina apple blotch, caused by the fungus Marssonina coronaria, is one of the most destructive apple diseases in China and East Asia. A better understanding of the plant's response to fungi during pathogenesis is urgently needed to improve plant resistance and to breed resistant cultivars. To address this, the transcriptomes of "Qinguan" (a cultivar with high resistance to M. coronaria) apple leaves were sequenced at 12, 24, 48, and 72 h post-inoculation (hpi) with Marssonina coronaria. The comparative results showed that a total of 1956 genes were differentially expressed between the inoculated and control samples at the 4 time points. Gene ontology (GO) term enrichment analysis of differentially expressed genes (DEGs) revealed changes in cellular component, secondary metabolism including chalcone isomerase activity, phytoalexin biosynthetic process, anthocyanin-containing compound biosynthetic process, lignin biosynthetic process, positive regulation of flavonoid biosynthetic process; and molecular functions or biological processes related to the defense response, biotic stimulus response, wounding response and fungus response. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that DEGs were significantly enriched in flavonoid biosynthesis, vitamin B6 metabolism, phenylpropanoid biosynthesis, and the stilbenoid, diarylheptanoid and gingerol biosynthesis pathways. Furthermore, the importance of changes in cellular components and partial polyphenol compounds when encountering M. coronaria are discussed.

E3 ubiquitin ligase gene CMPG1-V from Haynaldia villosa L. contributes to powdery mildew resistance in common wheat (Triticum aestivum L.)

Powdery mildew is one of the most devastating wheat fungal diseases. A diploid wheat relative, Haynaldia villosa L., is highly resistant to powdery mildew, and its genetic resource of resistances, such as the Pm21 locus, is now widely used in wheat breeding. Here we report the cloning of a resistance gene from H. villosa, designated CMPG1-V, that encodes a U-box E3 ubiquitin ligase. Expression of the CMPG1-V gene was induced in the leaf and stem of H. villosa upon inoculation with Blumeria graminis f. sp. tritici (Bgt) fungus, and the presence of Pm21 is essential for its rapid induction of expression. CMPG1-V has conserved key residues for E3 ligase, and possesses E3 ligase activity in vitro and in vivo. CMPG1-V is localized in the nucleus, endoplasmic reticulum, plasma membrane and partially in trans-Golgi network/early endosome vesicles. Transgenic wheat over-expressing CMPG1-V showed improved broad-spectrum powdery mildew resistance at seedling and adult stages, associated with an increase in expression of salicylic acid-responsive genes, H2 O2 accumulation, and cell-wall protein cross-linking at the Bgt infection sites, and the expression of CMPG1-V in H. villosa was increased when treated with salicylic acid, abscisic acid and H2 O2 . These results indicate the involvement of E3 ligase in defense responses to Bgt fungus in wheat, particularly in broad-spectrum disease resistance, and suggest association of reactive oxidative species and the phytohormone pathway with CMPG1-V-mediated powdery mildew resistance.

Rice Exo70 interacts with a fungal effector, AVR-Pii, and is required for AVR-Pii-triggered immunity

Vesicle trafficking including the exocytosis pathway is intimately associated with host immunity against pathogens. However, we still have insufficient knowledge about how it contributes to immunity, and how pathogen factors affect it. In this study, we explore host factors that interact with the Magnaporthe oryzae effector AVR-Pii. Gel filtration chromatography and co-immunoprecipitation assays identified a 150 kDa complex of proteins in the soluble fraction comprising AVR-Pii and OsExo70-F2 and OsExo70-F3, two rice Exo70 proteins presumably involved in exocytosis. Simultaneous knockdown of OsExo70-F2 and F3 totally abrogated Pii immune receptor-dependent resistance, but had no effect on Pia- and Pik-dependent resistance. Knockdown levels of OsExo70-F3 but not OsExo70-F2 correlated with reduction of Pii function, suggesting that OsExo70-F3 is specifically involved in Pii-dependent resistance. Under our current experimental conditions, over-expression of AVR-Pii or knockdown of OsExo70-F2 and -F3 genes in rice did not affect the virulence of compatible isolates of M. oryzae. AVR-Pii interaction with OsExo70-F3 appears to play a crucial role in immunity triggered by Pii, suggesting a role for OsExo70 as a decoy or helper in Pii/AVR-Pii interactions.

Quo vadis, Pep? Plant elicitor peptides at the crossroads of immunity, stress, and development

The first line of inducible plant defence, pattern-triggered immunity (PTI), is activated by the recognition of exogenous as well as endogenous elicitors. Exogenous elicitors, also called microbe-associated molecular patterns, signal the presence of microbes. In contrast, endogenous elicitors seem to be generated and recognized under more diverse circumstances, making the evaluation of their biological relevance much more complex. Plant elicitor peptides (Peps) are one class of such endogenous elicitors, which contribute to immunity against attack by bacteria, fungi, as well as herbivores. Recent studies indicate that the Pep-triggered signalling pathways also operate during the response to a more diverse set of stresses including starvation stress. In addition, in silico data point to an involvement in the regulation of plant development, and a study on Pep-mediated inhibition of root growth supports this indication. Importantly, Peps are neither limited to the model plant Arabidopsis nor to a specific plant family like the previously intensively studied systemin peptides. On the contrary, they are present and active in angiosperms all across the phylogenetic tree, including many important crop plants. Here we summarize the progress made in research on Peps from their discovery in 2006 until now. We discuss the two main models which describe their likely function in plant immunity, highlight the studies supporting additional roles of Pep-triggered signalling and identify urgent research tasks to further uncover their biological relevance.

EXO70I Is Required for Development of a Sub-domain of the Periarbuscular Membrane during Arbuscular Mycorrhizal Symbiosis

In eukaryotic cells, polarized secretion mediated by exocytotic fusion of membrane vesicles with the plasma membrane is essential for spatially restricted expansion of the plasma membrane and for the delivery of molecules to specific locations at the membrane and/or cell surface. The EXOCYST complex is central to this process, and in yeast, regulation of the EXO70 subunit influences exocytosis and cargo specificity. In contrast to yeast and mammalian cells, plants have upwards of 23 EXO70 genes with largely unknown roles. During arbuscular mycorrhizal (AM) symbiosis, deposition of the plant periarbuscular membrane (PAM) around the fungal arbuscule creates an intracellular membrane interface between the symbionts. The PAM has two major membrane sub-domains, and symbiosis-specific transporter proteins are localized in the branch domain. Currently, the mechanisms and cellular machinery involved in biogenesis of the PAM are largely unknown. Here, we identify an EXO70I protein present exclusively in plants forming AM symbiosis. Medicago truncatula exo70i mutants are unable to support normal arbuscule development, and incorporation of two PAM-resident ABC transporters, STR and STR2, is limited. During arbuscule branching, EXO70I is located in spatially restricted zones adjacent to the PAM around the arbuscule hyphal tips where it interacts with Vapyrin, a plant-specific protein required for arbuscule development. We conclude that EXO70I provides a specific exocytotic capacity necessary for development of the main functional sub-domain of the PAM. Furthermore, in contrast to other eukaryotes, plant EXO70s have evolved distinct specificities and interaction partners to fulfill their specialized secretory requirements.

PLANT U-BOX PROTEIN10 Regulates MYC2 Stability in Arabidopsis

MYC2 is an important regulator for jasmonic acid (JA) signaling, but little is known about its posttranslational regulation. Here, we show that the MYC2 C-terminal region interacted with the PLANT U-BOX PROTEIN10 (PUB10) armadillo repeats in vitro. MYC2 was efficiently polyubiquitinated by PUB10 with UBC8 as an E2 enzyme and the conserved C249 in PUB10 was required for activity. The inactive PUB10(C249A) mutant protein retained its ability to heterodimerize with PUB10, thus blocking PUB10 E3 activity as a dominant-negative mutant. Both MYC2 and PUB10 were nucleus localized and coimmunoprecipitation experiments confirmed their interaction in vivo. Although unstable in the wild type, MYC2 stability was enhanced in pub10, suggesting destabilization by PUB10. Moreover, MYC2 half-life was shortened or prolonged by induced expression of PUB10 or the dominant-negative PUB10(C249A) mutant, respectively. Root growth of pub10 seedlings phenocopied 35S:MYC2 seedlings and was hypersensitive to methyl jasmonate, whereas 35S:PUB10 and jin1-9 (myc2) seedlings were hyposensitive. In addition, the root phenotype conferred by MYC2 overexpression in double transgenic plants was reversed or enhanced by induced expression of PUB10 or PUB10(C249A), respectively. Similar results were obtained with three other JA-regulated genes, TAT, JR2, and PDF1.2. Collectively, our results show that MYC2 is targeted by PUB10 for degradation during JA responses.