BTB-BACK Domain Protein POB1 Suppresses Immune Cell Death by Targeting Ubiquitin E3 ligase PUB17 for Degradation

Potato E3 ubiquitin ligase StRFP1 positively regulates late blight resistance by degrading sugar transporters StSWEET10c and StSWEET11

Potato (Solanum tuberosum) is the fourth largest food crop in the world. Late blight, caused by oomycete Phytophthora infestans, is the most devastating disease threatening potato production. Previous research has shown that StRFP1, a potato Arabidopsis Tóxicos en Levadura (ATL) family protein, positively regulates late blight resistance via its E3 ligase activity. However, the underlying mechanism is unknown. Here, we reveal that StRFP1 is associated with the plasma membrane (PM) and undergoes constitutive endocytic trafficking. Its PM localization is essential for inhibiting P. infestans colonization. Through in vivo and in vitro assays, we investigated that StRFP1 interacts with two sugar transporters StSWEET10c and StSWEET11 at the PM. Overexpression (OE) of StSWEET10c or StSWEET11 enhances P. infestans colonization. Both StSWEET10c and StSWEET11 exhibit sucrose transport ability in yeast, and OE of StSWEET10c leads to an increased sucrose content in the apoplastic fluid of potato leaves. StRFP1 ubiquitinates StSWEET10c and StSWEET11 to promote their degradation. We illustrate a novel mechanism by which a potato ATL protein enhances disease resistance by degrading susceptibility (S) factors, such as Sugars Will Eventually be Exported Transporters (SWEETs). This offers a potential strategy for improving disease resistance by utilizing host positive immune regulators to neutralize S factors.

A systematic analysis of ARM genes revealed that GhARM144 regulates the resistance against Verticillium dahliae via interaction with GhOSM34

Proteins of the armadillo repeat gene family play important roles in plant pathogen response. Here, 169 armadillo (ARM) genes were identified in upland cotton (Gossypium hirsutum). Phylogenetic analysis grouped these into 11 subfamilies, with conserved protein structures within each subfamily. The results signify that the expansion of the gene family occurred via whole genome duplication and dispersed duplication. Expression profiling and network analysis suggest that GhARM144 may regulate cotton resistance to Verticillium dahliae. GhARM144 was upregulated in roots by V. dahliae infection or salicylic acid treatment. This upregulation indicates a negative regulatory role of GhARM144' in the cotton immune responses, potentially by manipulating salicylic acid biosynthesis. Protein interaction studies found that GhARM144 associates with an osmotin-like protein, GhOSM34, at the plasma membrane. Silencing GhOSM34 reduced the resistance to V. dahliae, suggesting it may play a positive regulatory role. The results demonstrate that GhARM144 modulates cotton immunity through interaction with GhOSM34 and salicylic acid signalling. Further study of these proteins may yield insights into disease resistance mechanisms in cotton and other plants.

E3 Ubiquitin Ligase PUB23 in Kiwifruit Interacts with Trihelix Transcription Factor GT1 and Negatively Regulates Immune Responses against Pseudomonas syringae pv. actinidiae

Kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) is the most serious disease threatening kiwifruit production. Our previous study found genes encoding the U-box containing proteins were significantly regulated by Psa infection. Here, we report a U-box type E3 ubiquitin ligase PUB23 in kiwifruit which acts as a negative regulator of immune responses against Psa. PUB23 was found to physically interact with GT1, a trihelix transcription factor, in vitro and in vivo. The expression of GT1 was up-regulated in PUB23-silenced plants, indicating that interacting with PUB23 may directly or indirectly suppress GT1 expression. The silencing of PUB23 led to enhanced immune responses of PAMP-triggered immunity (PTI), including a higher expression level of defense marker genes PR1 and RIN4, and increased accumulation of hydrogen peroxide and superoxide anion. Our results reveal a negative role PUB23 plays in kiwifruit immune responses against Psa and may regulate gene expression by interacting with GT1.

The Plant Ubiquitin-Proteasome System as a Target for Microbial Manipulation

The plant immune system perceives pathogens to trigger defense responses. In turn, pathogens secrete effector molecules to subvert these defense responses. The initiation and maintenance of defense responses involve not only de novo synthesis of regulatory proteins and enzymes but also their regulated degradation. The latter is achieved through protein degradation pathways such as the ubiquitin-proteasome system (UPS). The UPS regulates all stages of immunity, from the perception of the pathogen to the execution of the response, and, therefore, constitutes an ideal candidate for microbial manipulation of the host. Pathogen effector molecules interfere with the plant UPS through several mechanisms. This includes hijacking general UPS functions or perturbing its ability to degrade specific targets. In this review, we describe how the UPS regulates different immunity-related processes and how pathogens subvert this to promote disease.

Genetic Mapping of the Root Mycobiota in Rice and its Role in Drought Tolerance

BACKGROUND

Rice is the second most produced crop worldwide, but is highly susceptible to drought. Micro-organisms can potentially alleviate the effects of drought. The aim of the present study was to unravel the genetic factors involved in the rice-microbe interaction, and whether genetics play a role in rice drought tolerance. For this purpose, the composition of the root mycobiota was characterized in 296 rice accessions (Oryza sativa L. subsp. indica) under control and drought conditions. Genome wide association mapping (GWAS) resulted in the identification of ten significant (LOD > 4) single nucleotide polymorphisms (SNPs) associated with six root-associated fungi: Ceratosphaeria spp., Cladosporium spp., Boudiera spp., Chaetomium spp., and with a few fungi from the Rhizophydiales order. Four SNPs associated with fungi-mediated drought tolerance were also found. Genes located around those SNPs, such as a DEFENSIN-LIKE (DEFL) protein, EXOCYST TETHERING COMPLEX (EXO70), RAPID ALKALINIZATION FACTOR-LIKE (RALFL) protein, peroxidase and xylosyltransferase, have been shown to be involved in pathogen defense, abiotic stress responses and cell wall remodeling processes. Our study shows that rice genetics affects the recruitment of fungi, and that some fungi affect yield under drought. We identified candidate target genes for breeding to improve rice-fungal interactions and hence drought tolerance.

An activated form of NB-ARC protein RLS1 functions with cysteine-rich receptor-like protein RMC to trigger cell death in rice

A key event that follows pathogen recognition by a resistance (R) protein containing an NB-ARC (nucleotide-binding adaptor shared by Apaf-1, R proteins, and Ced-4) domain is hypersensitive response (HR)-type cell death accompanied by accumulation of reactive oxygen species and nitric oxide. However, the integral mechanisms that underlie this process remain relatively opaque. Here, we show that a gain-of-function mutation in the NB-ARC protein RLS1 (Rapid Leaf Senescence 1) triggers high-light-dependent HR-like cell death in rice. The RLS1-mediated defense response is largely independent of salicylic acid accumulation, NPR1 (Nonexpressor of Pathogenesis-Related Gene 1) activity, and RAR1 (Required for Mla12 Resistance 1) function. A screen for suppressors of RLS1 activation identified RMC (Root Meander Curling) as essential for the RLS1-activated defense response. RMC encodes a cysteine-rich receptor-like secreted protein (CRRSP) and functions as an RLS1-binding partner. Intriguingly, their co-expression resulted in a change in the pattern of subcellular localization and was sufficient to trigger cell death accompanied by a decrease in the activity of the antioxidant enzyme APX1. Collectively, our findings reveal an NB-ARC-CRRSP signaling module that modulates oxidative state, the cell death process, and associated immunity responses in rice.

Mining of the CULLIN E3 ubiquitin ligase genes in the whole genome of Salvia miltiorrhiza

CULLIN (CUL) proteins are E3 ubiquitin ligases that are involved in a wide variety of biological processes as well as in response to stress in plants. In Salvia miltiorrhiza, CUL genes have not been characterized and its role in plant development, stress response and secondary metabolite synthesis have not been studied. In this study, genome-wide analyses were performed to identify and to predict the structure and function of CUL of S. miltiorrhiza. Eight CUL genes were identified from the genome of S. miltiorrhiza. The CUL genes were clustered into four subgroups according to phylogenetic relationships. The CUL domain was highly conserved across the family of CUL genes. Analysis of cis-acting elements suggested that CUL genes might play important roles in a variety of biological processes, including abscission reaction acid (ABA) processing. To investigate this hypothesis, we treated hairy roots of S. miltiorrhiza with ABA. The expression of CUL genes varied obviously after ABA treatment. Co-expression network results indicated that three CUL genes might be involved in the biosynthesis of phenolic acid or tanshinone. In summary, the mining of the CUL genes in the whole genome of S. miltiorrhiza contribute novel information to the understanding of the CUL genes and its functional roles in plant secondary metabolites, growth and development.

Emerging roles of the ubiquitin-proteasome pathway in enhancing crop yield by optimizing seed agronomic traits

Ubiquitin-proteasome pathway has the potential to modulate crop productivity by influencing agronomic traits. Being sessile, the plant often uses the ubiquitin-proteasome pathway to maintain the stability of different regulatory proteins to survive in an ever-changing environment. The ubiquitin system influences plant reproduction, growth, development, responses to the environment, and processes that control critical agronomic traits. E3 ligases are the major players in this pathway, and they are responsible for recognizing and tagging the targets/substrates. Plants have a variety of E3 ubiquitin ligases, whose functions have been studied extensively, ranging from plant growth to defense strategies. Here we summarize three agronomic traits influenced by ubiquitination: seed size and weight, seed germination, and accessory plant agronomic traits particularly panicle architecture, tillering in rice, and tassels branch number in maize. This review article highlights some recent progress on how the ubiquitin system influences the stability/modification of proteins that determine seed agronomic properties like size, weight, germination and filling, and ultimately agricultural productivity and quality. Further research into the molecular basis of the aforementioned processes might lead to the identification of genes that could be modified or selected for crop development. Likewise, we also propose advances and future perspectives in this regard.

The E3 ubiquitin ligase RING1 interacts with COP9 Signalosome Subunit 4 to positively regulate resistance to root-knot nematodes in Solanum lycopersicum L

Globally, root-knot nematodes (RKNs) cause massive production losses in all major crops. E3 ubiquitin ligases are involved in plant growth, development and immune response. But their roles in plant defense against RKNs are largely unclear. Here, we show that tomato E3 ubiquitin ligase RING1 interacts with COP9 Signalosome Subunit 4 (CSN4) which is essential for jasmonic acid (JA)-dependent basal defense against RKNs. Tissue-specific expression analysis showed that RING1 expression was the highest in tomato roots and the expression was significantly increased with RKN (Meloidogyne incognita) infection. Compared with the wild-type plants, the number of egg masses in roots significantly increased in the ring1 mutants, while RING1 overexpression conferred resistance against RKNs. Furthermore, RKN infection increased the accumulation of CSN4 protein in the roots of wild-type plants, which was largely compromised in the ring1 mutants but was enhanced in the RING1 overexpressing plants. The RKN-induced transcripts of JA biosynthetic and signaling genes as well as the accumulation of JA and JA-isoleucine were compromised in ring1 mutants but were increased in RING1 overexpressing plants. These results suggest that RING1 positively regulates JA-dependent basal defense against RKNs by interacting with CSN4 proteins.

A BTB/POZ domain-containing protein negatively regulates plant immunity in Nicotiana benthamiana

Plants have evolved immune systems to fight against pathogens. However, it is still largely unknown how the plant immunity is finely regulated. Here we identified a BTB/POZ domain-containing protein, namely NbBTB, which is predicted to be a member of the ubiquitin E3 ligase complex. The NbBTB expression is downregulated upon the oomycete pathogen Phytophthora parasitica infection. Overexpression of NbBTB in Nicotiana benthamiana promoted plant susceptibility to P. parasitica infection, and silencing NbBTB increased plant resistance to P. parasitica, indicating that NbBTB negatively modulates plant basal defense. Interestingly, overexpressing or silencing NbBTB did not affect plant resistance to two bacterial pathogens Ralstonia solanacearum and Pseudomonas syringae, suggesting that NbBTB is specifically involved in basal defense against oomycete pathogen. Expression of NbBTB suppressed hypersensitive response (HR) triggered by avirulence proteins from both R. sonanacearum and P. infestans, and silencing NbBTB showed the opposite effect, indicating that NbBTB negatively regulates effector-triggered immunity (ETI). Protein accumulation of avirulence effectors in NbBTB-silenced plants was significantly enhanced, suggesting that NbBTB is likely to negatively modulate ETI by affecting effector protein accumulation. Together, our results demonstrated that NbBTB is a negative regulator in both plant basal defense and ETI.

Nitrate-inducible MdBT2 acts as a restriction factor to limit apple necrotic mosaic virus genome replication in Malus domestica

Apple necrotic mosaic virus (ApNMV) is highly associated with the occurrence of apple mosaic disease in China. However, ApNMV-host interactions and defence mechanisms of host plants against this virus are poorly studied. Here, we report that nitrate treatment restrains ApNMV genomic RNA accumulation by destabilizing viral replication protein 1a through the MdBT2-mediated ubiquitin-proteasome pathway. MdBT2, a nitrate-responsive BTB/TAZ domain-containing protein, was identified in a yeast two-hybrid screen of an apple cDNA library using viral protein 1a as bait, and 1a was further confirmed to interact with MdBT2 both in vivo and in vitro. It was further verified that MdBT2 promoted the ubiquitination and degradation of viral protein 1a through the ubiquitin-proteasome pathway in an MdCUL3A-independent manner. Viral genomic RNA accumulation was reduced in MdBT2-overexpressing transgenic apple leaves but enhanced in MdBT2-antisense leaves compared to the wild type. Moreover, MdBT2 was found to interfere with the interaction between viral replication proteins 1a and 2a^pol by competitively interacting with 1a. Taken together, our results demonstrate that nitrate-inducible MdBT2 functions as a limiting factor in ApNMV viral RNA accumulation by promoting the ubiquitination and degradation of viral protein 1a and interfering with interactions between viral replication proteins.

How Many Faces Does the Plant U-Box E3 Ligase Have?

Ubiquitination is a major type of post-translational modification of proteins in eukaryotes. The plant U-Box (PUB) E3 ligase is the smallest family in the E3 ligase superfamily, but plays a variety of essential roles in plant growth, development and response to diverse environmental stresses. Hence, PUBs are potential gene resources for developing climate-resilient crops. However, there is a lack of review of the latest advances to fully understand the powerful gene family. To bridge the gap and facilitate its use in future crop breeding, we comprehensively summarize the recent progress of the PUB family, including gene evolution, classification, biological functions, and multifarious regulatory mechanisms in plants.

Genome-wide identification and functional analysis of U-box E3 ubiquitin ligases gene family related to drought stress response in Chinese white pear (Pyrus bretschneideri)

BACKGROUND

The plant U-box (PUB) proteins are a family of ubiquitin ligases (E3) enzymes that involved in diverse biological processes, as well as in responses to plant stress response. However, the characteristics and functional divergence of the PUB gene family have not yet been previously studied in the Chinese white pear (Pyrus bretschneideri).

RESULTS

In the present study, we identified 62 PbrPUBs in Chinese white pear genome. Based on the phylogenetic relationship, 62 PUB genes were clustered into five groups. The results of conserved motif and gene structure analysis supported the classification phylogenetic tree. The PbrPUB genes were unevenly distribution on 17 pear chromosomes, chromosome 15 housed most member of PUB family, with eight PUB genes. Cis-acting element analysis indicated that PUB genes might participate in diverse biological processes, especially in the response to abiotic stresses. Based on RNA-data from 'Dangshansuli' at seven tissues, we found that PUB genes exhibited diverse of expression level in seven tissues, and qRT-PCR experiment further supported the reliable of RNA-Seq data. To identify candidate genes associated with resistance, we conducted qRT-PCR experiment the expression level of pear seed plant under four abiotic stresses, including: ABA, dehydration, salt and cold treatment. One candidate PUB gene associated with dehydration stress was selected to conduct further functional experiment. Subcellular localization revealed PbrPUB18 protein was located on cell nucleus. Furthermore, heterologous over-expression of PbrPUB18 in Arabidopsis indicated that the over-expression of PbrPUB18 could enhance resistance in drought treatment. In conclusions, we systematically identified the PUB genes in pear, and provided useful knowledge for functional identification of PUB genes in pear.

CUL3 E3 ligases in plant development and environmental response

Thirty years of research have revealed the fundamental role of the ubiquitin-proteasome system in diverse aspects of cellular regulation in eukaryotes. The ubiquitin-protein ligases or E3s are central to the ubiquitin-proteasome system since they determine the specificity of ubiquitylation. The cullin-RING ligases (CRLs) constitute one large class of E3s that can be subdivided based on the cullin isoform and the substrate adapter. SCF complexes, composed of CUL1 and the SKP1/F-box protein substrate adapter, are perhaps the best characterized in plants. More recently, accumulating evidence has demonstrated the essential roles of CRL3 E3s, consisting of a CUL3 protein and a BTB/POZ substrate adaptor. In this Review, we describe the variety of CRL3s functioning in plants and the wide range of processes that they regulate. Furthermore, we illustrate how different classes of E3s may cooperate to regulate specific pathways or processes.

The Ubiquitin E3 Ligase PUB17 Positively Regulates Immunity by Targeting a Negative Regulator, KH17, for Degradation

Ubiquitination is a post-translational modification that regulates many processes in plants. Several ubiquitin E3 ligases act as either positive or negative regulators of immunity by promoting the degradation of different substrates. StPUB17 is an E3 ligase that has previously been shown to positively regulate immunity to bacteria, fungi and oomycetes, including the late blight pathogen Phytophthora infestans. Silencing of StPUB17 promotes pathogen colonization and attenuates Cf4/avr4 cell death. Using yeast-2-hybrid and co-immunoprecipitation we identified the putative K-homology (KH) RNA-binding protein (RBP), StKH17, as a candidate substrate for degradation by StPUB17. StKH17 acts as a negative regulator of immunity that promotes P. infestans infection and suppresses specific immune pathways. A KH RBP domain mutant of StKH17 (StKH17GDDG) is no longer able to negatively regulate immunity, indicating that RNA binding is likely required for StKH17 function. As StPUB17 is a known target of the ubiquitin E3 ligase, StPOB1, we reveal an additional step in an E3 ligase regulatory cascade that controls plant defense.

A Comprehensive Gene Expression Profile of Pectin Degradation Enzymes Reveals the Molecular Events during Cell Wall Degradation and Pathogenesis of Rice Sheath Blight Pathogen Rhizoctonia solani AG1-IA

Sheath blight disease of rice caused by Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris) remains a global challenge due to the absence of reliable resistance genes and poor understanding of pathogen biology. Pectin, one of the most vital constituents of the plant cell wall, is targeted by pectin methylesterases, polygalacturonases, and few other enzymes of fungal pathogens. In this study, we catalogued the expressed genes of the fungal genome from RNAseq of R. solani infected four rice genotypes. Analysis of RNAseq revealed 3325 pathogen genes commonly expressed in all rice genotypes, in which 49, 490, and 83 genes were specific to BPT5204, Tetep, and Pankaj genotypes, respectively. To identify the early and late responding genes of R. solani during plant cell wall degradation, a real-time PCR analysis of 30 pectinolytic enzymes was done at six different time points after inoculation. The majority of these genes showed maximum induction at the 72 h time point, suggesting that it is the most crucial stage of infection. Pankaj showed lesser induction of these genes as compared to other genotypes. Leaf-blade tissue and 45 days old-growth stage are more favorable for the expression of pectin degradation genes of R. solani. Additionally, the expression analysis of these genes from four different strains of R. solani suggested differential regulation of genes but no distinct expression pattern between highly virulent and mild strains. The implications of the differential regulation of these genes in disease development have been discussed. This study provides the first such comprehensive analysis of R. solani genes encoding pectin degrading enzymes, which would help to decipher the pathogen biology and sheath blight disease development.

SHR4z, a novel decoy effector from the haustorium of the parasitic weed Striga gesnerioides, suppresses host plant immunity

Cowpea (Vigna unguiculata) cultivar B301 is resistant to races SG4 and SG3 of the root parasitic weed Striga gesnerioides, developing a hypersensitive response (HR) at the site of parasite attachment. By contrast, race SG4z overcomes B301 resistance and successfully parasitises the plant. Comparative transcriptomics and in silico analysis identified a small secreted effector protein dubbed Suppressor of Host Resistance 4z (SHR4z) in the SG4z haustorium that upon transfer to the host roots causes a loss of host immunity (i.e. decreased HR and increased parasite growth). SHR4z has significant homology to the short leucine-rich repeat (LRR) domain of SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) family proteins and functions by binding to VuPOB1, a host BTB-BACK domain-containing ubiquitin E3 ligase homologue, leading to its rapid turnover. VuPOB1 is shown to be a positive regulator of HR since silencing of VuPOB1 expression in transgenic B301 roots lowers the frequency of HR and increases the levels of successful SG4 parasitism and overexpression decreases parasitism by SG4z. These findings provide new insights into how parasitic weeds overcome host defences and could potentially contribute to the development of novel strategies for controlling Striga and other parasitic weeds thereby enhancing crop productivity and food security globally.

BTB-BACK Domain E3 Ligase MdPOB1 Suppresses Plant Pathogen Defense against Botryosphaeria dothidea by Ubiquitinating and Degrading MdPUB29 Protein in Apple

Apple ring rot is a severe disease that affects the yield and quality of apple fruits worldwide. However, the underlying molecular mechanism that involved in this process still remains largely unexplored. Here, we report that apple POZ/BTB CONTAINING-PROTEIN 1 (MdPOB1), a BTB-BACK domain E3 ligase protein, functions to suppress apple pathogen defense against Botryosphaeria dothidea (B. dothidea). Both in vitro and in vivo assays indicated that MdPOB1 interacted directly with and degraded apple U-box E3 ligase MdPUB29, a well-established positive regulator of plant innate immunity, through the ubiquitin/26S proteasome pathway. A series of transgenic analyses in apple fruits demonstrated that MdPOB1 affected apple pathogen defense against B. dothidea at least partially, if not completely, via regulating MdPUB29. Additionally, it was found that the apple pathogen defense against B. dothidea was correlated with the H2O2 contents and the relative expression of salicylic acid (SA) synthesis- and SA signaling-related genes, which might be regulated via degradation of MdPUB29 by MdPOB1. Overall, our findings provide new insights into the mechanism of the MdPOB1 modulation of apple ring rot resistance, which occur by directly regulating potential downstream target protein MdPUB29 for proteasomal degradation in apple.

New Insights of Salicylic Acid Into Stamen Abortion of Female Flowers in Tung Tree (Vernicia fordii)

Tung tree (Vernicia fordii), an economically important woody oil plant, is a monoecious and diclinous species with male and female flowers on the same inflorescence. The extremely low proportion of female flowers leads to low fruit yield in tung orchards. The female flower normally develops along with stamen abortion; otherwise sterile ovules will be produced. However, little knowledge is known about the molecular basis of the female flower development in tung tree. In this study, integrated analyses of morphological and cytological observations, endogenous phytohormone assay and RNA-seq were conducted to understand the molecular mechanism of the female flower development in tung tree. Cytological observation suggested that the abortion of stamens in female flowers (SFFs) belongs to the type of programmed cell death (PCD), which was caused by tapetum degeneration at microspore mother cell stage. A total of 1,366 differentially expressed genes (DEGs) were identified in female flowers by RNA-seq analysis, of which 279 (20.42%) DEGs were significantly enriched in phenylpropanoid biosynthesis, phenylalanine metabolism, flavonoid biosynthesis, starch and sucrose metabolism, and plant hormone signal transduction. Stage-specific transcript identification detected dynamically expressed genes of important transcription regulators in female flowers that may be involved in PCD and floral organ development. Gene expression patterns revealed that 17 anther and pollen development genes and 37 PCD-related genes might be involved in the abortion of SFF. Further analyses of phytohormone levels and co-expression networks suggested that salicylic acid (SA) accumulation could trigger PCD and inhibit the development of SFF in tung tree. This study provides new insights into the role of SA in regulating the abortion of SFF to develop normal female flowers.

Phytophthora infestans effector SFI3 targets potato UBK to suppress early immune transcriptional responses

The potato blight agent Phytophthora infestans secretes a range of RXLR effectors to promote disease. Recent evidence indicates that some effectors suppress early pattern-triggered immunity (PTI) following perception of microbe-associated molecular patterns (MAMPs). Phytophthora infestans effector PiSFI3/Pi06087/PexRD16 has been previously shown to suppress MAMP-triggered pFRK1-Luciferase reporter gene activity. How PiSFI3 suppresses immunity is unknown. We employed yeast-two-hybrid (Y2H) assays, co-immunoprecipitation, transcriptional silencing by RNA interference and virus-induced gene silencing (VIGS), and X-ray crystallography for structure-guided mutagenesis, to investigate the function of PiSFI3 in targeting a plant U-box-kinase protein (StUBK) to suppress immunity. We discovered that PiSFI3 is active in the host nucleus and interacts in yeast and in planta with StUBK. UBK is a positive regulator of specific PTI pathways in both potato and Nicotiana benthamiana. Importantly, it contributes to early transcriptional responses that are suppressed by PiSFI3. PiSFI3 forms an unusual trans-homodimer. Mutation to disrupt dimerization prevents nucleolar localisation of PiSFI3 and attenuates both its interaction with StUBK and its ability to enhance P. infestans leaf colonisation. PiSFI3 is a 'WY-domain' RXLR effector that forms a novel trans-homodimer which is required for its ability to suppress PTI via interaction with the U-box-kinase protein StUBK.

GhCyP3 improves the resistance of cotton to Verticillium dahliae by inhibiting the E3 ubiquitin ligase activity of GhPUB17

A U-box E3 ubiquitin ligase GhPUB17 is inhibited by GhCyP3 with antifungal activity and acts as a negative regulator involved in cotton resistance to Verticillium dahliae. E3 ubiquitin ligases, the key component enzymes of the ubiquitin-proteasome system, which contains the most diverse structural and functional members involved in the determination of target specificity and the regulation of metabolism, have been well documented in previous studies. Here, we identify GhPUB17, a U-box E3 ligase in cotton that has ubiquitination activity and is involved in the cotton immune response to Verticillium dahliae. The expression level of GhPUB17 is downregulated in the ssn mutant with a constitutively activated immune response (Sun et al., Nat Commun 5:5372, 2014). Infection with V. dahliae or exogenous hormone treatment, including jasmonic acid and salicylic acid, significantly upregulated GhPUB17 in cotton roots, which suggested a possible role for this E3 ligase in the plant immune response to pathogens. Moreover, GhPUB17-knockdown cotton plants are more resistant to V. dahliae, whereas GhPUB17-overexpressing plants are more susceptible to the pathogen, which indicated that GhPUB17 is a negative regulator of cotton resistance to V. dahliae. A yeast two-hybrid (Y2H) assay identified GhCyP3 as a protein that interacts with GhPUB17, and this finding was confirmed by further protein interaction assays. The downregulation of GhCyP3 in cotton seedlings attenuated the plants' resistance to V. dahliae. In addition, GhCyP3 showed antifungal activity against V. dahliae, and the E3 ligase activity of GhPUB17 was repressed by GhCyP3 in vitro. These results suggest that GhPUB17 negatively regulates cotton immunity to V. dahliae and that the antifungal protein GhCyP3 likely interacts with and inhibits the ligase activity of GhPUB17 and plays an important role in the cotton-Verticillium interaction.

AtPUB48 E3 ligase plays a crucial role in the thermotolerance of Arabidopsis

As the global temperature gradually increases, thermotolerance is vital to the growth and survival for plants. Ubiquitin-mediated protein degradation is a central regulator of many key cellular and physiological processes, including responses to biotic and abiotic stresses. E3 Ubiquitin-ligases, as the major components in the ubiquitination pathway, confer specificity of substrate recognition. Herein, we report that AtPUB48 expression was induced by heat stress, including basal and acquired thermotolerance. AtPUB48-overexpressing lines (OEs) of plants were generated to detect the functions of AtPUB48 in the heat response signaling pathway in Arabidopsis. Seeds of Atpub48-2 mutant had a lower germination rate than those of wild-type (WT) and OE plants when suffered from high temperatures. On the contrary, overexpression of AtPUB48 in Arabidopsis enhanced basal and acquired thermotolerance in seed germination and seedling growth. Moreover, the transcript expression levels of several heat-related downstream genes were highly improved in the OE lines under heat stress, although there were lower levels in the Atpub48-2 mutant compared with that of WT. An in vitro ubiquitination assay confirmed that AtPUB48 with U-box and ARM-repeats functioned as an E3 ubiquitin ligase. The subcellular localization showed that AtPUB48 localized to the nucleus. Collectively, these data imply that AtPUB48 acts as a novel regulator in the heat response signaling pathway. AtPUB48 may target the unknown substrate receptor to 26S proteasome proteolysis.

The Nitrate-Responsive Protein MdBT2 Regulates Anthocyanin Biosynthesis by Interacting with the MdMYB1 Transcription Factor

In addition to scavenging reactive oxygen species, anthocyanins are pigments that give organs their color. In apple (Malus domestica), R2R3-MYB transcription factor MdMYB1 is a master regulator of anthocyanin biosynthesis and fruit coloration. In this study, we found that MdMYB1 was degraded via a ubiquitin-dependent pathway in response to nitrate, an inhibitor of anthocyanin synthesis. Using a yeast two-hybrid (Y2H) approach, we found that the BTB-TAZ protein encoded by the nitrate-responsive gene MdBT2 interacts with MdMYB1. Pull-down and coimmunoprecipitation assays supported this conclusion. In vivo and in vitro experiments revealed that MdBT2 promoted the ubiquitination and degradation of MdMYB1 through a cullin protein MdCUL3-independent pathway. Expression analysis demonstrated that MdBT2 and MdMYB1 were inversely regulated by nitrate and other environmental signals. Furthermore, we used transgenic approaches in apple and Arabidopsis (Arabidopsis thaliana) to characterize the function of MdBT2 in regulating anthocyanin biosynthesis in response to nitrate. Our findings provide insight into a mechanism involving the MdBT2-MdMYB1 pathway that regulates anthocyanin accumulation in apple and possibly in other plant species.

Plant pathogen effector utilizes host susceptibility factor NRL1 to degrade the immune regulator SWAP70

Plant pathogens deliver effectors into plant cells to suppress immunity. Whereas many effectors inactivate positive immune regulators, other effectors associate with negative regulators of immunity: so-called susceptibility (S) factors. Little is known about how pathogens exploit S factors to suppress immunity. Phytophthora infestans RXLR effector Pi02860 interacts with host protein NRL1, which is an S factor whose activity suppresses INF1-triggered cell death (ICD) and is required for late blight disease. We show that NRL1 interacts in yeast and in planta with a guanine nucleotide exchange factor called SWAP70. SWAP70 associates with endosomes and is a positive regulator of immunity. Virus-induced gene silencing of SWAP70 in Nicotiana benthamiana enhances P. infestans colonization and compromises ICD. In contrast, transient overexpression of SWAP70 reduces P. infestans infection and accelerates ICD. Expression of Pi02860 and NRL1, singly or in combination, results in proteasome-mediated degradation of SWAP70. Degradation of SWAP70 is prevented by silencing NRL1, or by mutation of Pi02860 to abolish its interaction with NRL1. NRL1 is a BTB-domain protein predicted to form the substrate adaptor component of a CULLIN3 ubiquitin E3 ligase. A dimerization-deficient mutant, NRL1NQ, fails to interact with SWAP70 but maintains its interaction with Pi02860. NRL1NQ acts as a dominant-negative mutant, preventing SWAP70 degradation in the presence of effector Pi02860, and reducing P. infestans infection. Critically, Pi02860 enhances the association between NRL1 and SWAP70 to promote proteasome-mediated degradation of the latter and, thus, suppress immunity. Preventing degradation of SWAP70 represents a strategy to combat late blight disease.

PAMP-responsive ATL gene StRFP1 and its orthologue NbATL60 positively regulate Phytophthora infestans resistance in potato and Nicotiana benthamiana

Ubiquitination is a post-translational modification that plays a crucial role during the regulation of plant immune signalling. The plant ATL family consists of a large number of putative RING type ubiquitin ligases. We show that potato ATL family gene StRFP1 and its orthologue NbATL60 from N. benthamiana both respond to Phytophthora infestans culture filtrate (CF) and flg22 induction. StRFP1 positively regulates immunity against P. infestans in potato. Ectopic transient expression of StRFP1 or expression of NbATL60 in N. benthamiana also enhances late blight resistance. By contrast, silencing NbATL60 in N. benthamiana reduces late blight resistance and leads to plant growth inhibition. Both StRFP1 and NbATL60 localize to the plasma membrane and intracellular puncta and possess E3 Ligase activity in vitro. Furthermore we demonstrate that the RING finger domain mutants of StRFP1 and NbATL60 lost E3 ligase activity and fail to suppress P. infestans colonization in N. benthamiana, indicating that E3 ligase activity is critical for StRFP1 and NbATL60 to regulate immunity. Overexpression or RNA interference of StRFP1 in transgenic potato led to increased or decreased expression of PTI maker genes (WRKY7, WRKY8, ACRE31 and Pti5) respectively. Similarly silencing of NbATL60 in N. benthamiana decreases expression of these PTI marker genes. Moreover, VIGS of NbATL60 in N. benthamiana did not compromise P. infestans PAMP INF1 or R2/Avr2, R3a/AVR3a, Rx/Cp and Pto/AvrPto triggered cell death. These results indicate that ATL genes StRFP1 and NbATL60 contribute to basal immunity (PTI) in Solanaceous plants.

Roles of E3 Ubiquitin-Ligases in Nuclear Protein Homeostasis during Plant Stress Responses

Ubiquitination, the reversible protein conjugation with ubiquitin (Ub), is a post-translational modification that enables rapid and specific cellular responses to stimuli without requirement of de novo protein synthesis. Although ubiquitination also displays non-proteolytic functions, it often acts as a signal for selective protein degradation through the ubiquitin-proteasome system (UPS). In plants, it has become increasingly apparent that the UPS is a central regulator of many key cellular and physiological processes, including responses to biotic and abiotic stresses. In the nucleus, protein regulation via the UPS orchestrates gene expression, genome maintenance, and signal transduction. Here, we focus on E3 Ub-ligase proteins as major components of the ubiquitination cascade that confer specificity of substrate recognition. We provide an overview on how they contribute to nuclear proteome plasticity during plant responses to environmental stress signals.

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.