Assaying proteasomal degradation in a cell-free system in plants

Brassica napus Plants Gain Improved Salt-Stress Tolerance and Increased Storage Oil Biosynthesis by Interfering with CRL3BPM Activities

Generating new strategies to improve plant performance and yield in crop plants becomes increasingly relevant with ongoing and predicted global climate changes. E3 ligases that function as key regulators within the ubiquitin proteasome pathway often are involved in abiotic stress responses, development, and metabolism in plants. The aim of this research was to transiently downregulate an E3 ligase that uses BTB/POZ-MATH proteins as substrate adaptors in a tissue-specific manner. Interfering with the E3 ligase at the seedling stage and in developing seeds results in increased salt-stress tolerance and elevated fatty acid levels, respectively. This novel approach can help to improve specific traits in crop plants to maintain sustainable agriculture.

E3 ligase, the Oryza sativa salt-induced RING finger protein 4 (OsSIRP4), negatively regulates salt stress responses via degradation of the OsPEX11-1 protein

OsSIRP4 is an E3 ligase that acts as a negative regulator in the plant response to salt stress via the 26S proteasomal system regulation of substrate proteins, OsPEX11-1, which it provides important information for adaptation and regulation in rice. Plants are sessile organisms that can be exposed to environmental stress. Plants alter their cellular processes to survive under potentially unfavorable conditions. Protein ubiquitination is an important post-translational modification that has a crucial role in various cellular signaling processes in abiotic stress response. In this study, we characterized Oryza sativa salt-induced RING finger protein 4, OsSIRP4, a membrane and cytosol-localized RING E3 ligase in rice. OsSIRP4 transcripts were highly induced under salt stress in rice. We found that OsSIRP4 possesses E3 ligase activity; however, no E3 ligase activity was observed with a single amino acid substitution (OsSIRP4^C269A). The results of the yeast two hybrid system, in vitro pull-down assay, BiFC analysis, in vitro ubiquitination assay, and in vitro degradation assay indicate that OsSIRP4 regulates degradation of a substrate protein, OsPEX11-1 (Oryza sativa peroxisomal biogenesis factor 11-1) via the 26S proteasomal system. Phenotypic analysis of OsSIRP4-overexpressing plants demonstrated hypersensitivity to salt response compared to that of the wild type and mutated OsSIRP4^C269A plants. In addition, OsSIRP4-overexpressing plants exhibited significant low enzyme activities of superoxide dismutase, catalase, and peroxidase, and accumulation of proline and soluble sugar, but a high level of H₂O₂. Furthermore, qRT data on transgenic plants suggest that OsSIRP4 acted as a negative regulator of salt response by diminishing the expression of genes related to Na⁺/K⁺ homeostasis (AtSOS1, AtAKT1, AtNHX1, and AtHKT1;1) in transgenic plants under salt stress. These results suggest that OsSIRP4 plays a negative regulatory role in response to salt stress by modulating the target protein levels.

Molecular characterization of a RING E3 ligase SbHCI1 in sorghum under heat and abscisic acid stress

Molecular function ofRING E3 ligase SbHCI1is involved in ABA-mediated basal heat stress tolerancein sorghum. Global warming generally reduces plant survival, owing to the negative effects of high temperatures on plant development. However, little is known about the role of Really Interesting New Gene (RING) E3 ligase in the heat stress responses of plants. As such, the aim of the present study was to characterize the molecular functions of the Sorghum bicolor ortholog of the Oryza sativa gene for Heat- and Cold-Induced RING finger protein 1 (SbHCI1). Subcellular localization revealed that SbHCI1 was mainly associated with the cytosol and that it moved to the Golgi apparatus under heat stress conditions. The fluorescent signals of SbHCI1 substrate proteins were observed to migrate to the cytoplasm under heat stress conditions. Bimolecular fluorescence complementation (BiFC) and yeast two-hybrid (Y2H) assays revealed that SbHCI1 physically interacted with OsHCI1 ortholog partner proteins in the cytoplasm. Moreover, an in vitro ubiquitination assay revealed that SbHCI1 polyubiquitinated each of the three interacting proteins. The ectopic overexpression of SbHCI1 in Arabidopsis revealed that the protein was capable of inducing abscisic acid (ABA)-hypersensitivity and basal heat stress tolerance. Therefore, SbHCI1 possesses E3 ligase activity and may function as a positive regulator of heat stress responses through the modulation of interacting proteins.

MaXB3 Modulates MaNAC2, MaACS1, and MaACO1 Stability to Repress Ethylene Biosynthesis during Banana Fruit Ripening

Ethylene plays a critical regulatory role in climacteric fruit ripening, and its biosynthesis is fine-tuned at the transcriptional and posttranslational levels. Nevertheless, the mechanistic link between transcriptional and posttranslational regulation of ethylene biosynthesis during fruit ripening is largely unknown. This study uncovers a coordinated transcriptional and posttranslational mechanism of controlling ethylene biosynthesis during banana (Musa acuminata) fruit ripening. NAC (NAM, ATAF, and CUC) proteins MaNAC1 and MaNAC2 repress the expression of MaERF11, a protein previously known to negatively regulate ethylene biosynthesis genes MaACS1 and MaACO1 A RING E3 ligase MaXB3 interacts with MaNAC2 to promote its ubiquitination and degradation, leading to the inhibition of MaNAC2-mediated transcriptional repression. In addition, MaXB3 also targets MaACS1 and MaACO1 for proteasome degradation. Further evidence supporting the role of MaXB3 is provided by its transient and ectopic overexpression in banana fruit and tomato (Solanum lycopersicum), respectively, which delays fruit ripening via repressing ethylene biosynthesis and thus ethylene response. Strikingly, MaNAC1 and MaNAC2 directly repress MaXB3 expression, suggesting a feedback regulatory mechanism that maintains a balance of MaNAC2, MaACS1, and MaACO1 levels. Collectively, our findings establish a multilayered regulatory cascade involving MaXB3, MaNACs, MaERF11, and MaACS1/MaACO1 that controls ethylene biosynthesis during climacteric ripening.

Oryza sativa drought-, heat-, and salt-induced RING finger protein 1 (OsDHSRP1) negatively regulates abiotic stress-responsive gene expression

Plants are sessile and unable to avoid environmental stresses, such as drought, high temperature, and high salinity, which often limit the overall plant growth. Plants have evolved many complex mechanisms to survive these abiotic stresses via post-translational modifications. Recent evidence suggests that ubiquitination plays a crucial role in regulating abiotic stress responses in plants by regulating their substrate proteins. Here, we reported the molecular function of a RING finger E3 ligase, Oryza sativa Drought, Heat and Salt-induced RING finger protein 1 (OsDHSRP1), involved in regulating plant abiotic stress tolerance via the Ub/26S proteasome system. The OsDHSRP1 gene transcripts were highly expressed under various abiotic stresses such as NaCl, drought, and heat and the phytohormone abscisic acid (ABA). In addition, in vitro ubiquitination assays demonstrated that the OsDHSRP1 protein possesses a RING-H2 type domain that confers ligase functionality. The results of yeast two-hybrid (Y2H), in vitro pull-down, and bimolecular fluorescence complementation assays support that OsDHSRP1 is able to regulate two substrates, O. sativa glyoxalase (OsGLYI-11.2) and O. sativa abiotic stress-induced cysteine proteinase 1 (OsACP1). We further confirmed that these two substrate proteins were ubiquitinated by OsDHSRP1 E3 ligase and caused protein degradation via the Ub/26S proteasome system. The Arabidopsis plants overexpressing OsDHSRP1 exhibited hypersensitivity to drought, heat, and NaCl stress and a decrease in their germination rates and root lengths compared to the control plants because the degradation of the OsGLYI-11.2 protein maintained lower glyoxalase levels, which increased the methylglyoxal amount in transgenic Arabidopsis plants. However, the OsDHSRP1-overexpressing plants showed no significant difference when treated with ABA. Our finding supports the hypothesis that the OsDHSRP1 E3 ligase acts as a negative regulator, and the degradation of its substrate proteins via ubiquitination plays important roles in regulating various abiotic stress responses via an ABA-independent pathway.

Oryza sativa heat-induced RING finger protein 1 (OsHIRP1) positively regulates plant response to heat stress

OsHIRP1 is an E3 ligase that acts as a positive regulator in the plant response to heat stress, thus providing important information relating to adaptation and regulation under heat stress in plant. Extreme temperature adversely affects plant growth, development, and productivity. Here, we report the molecular functions of Oryza sativa heat-induced RING finger protein 1 (OsHIRP1), which might play an important role in the response to heat. Transcription of the OsHIRP1 was upregulated in response to heat and drought treatment. We found that the OsHIRP1-EYFP fusion protein was localized to the nucleus after heat treatment (45 °C). Two interacting partners, OsARK4 and OsHRK1, were identified via yeast-two-hybrid screening, which were mainly targeted to the nucleus (OsARK4) and cytosol (OsHRK1), and their interactions with OsHIRP1 were confirmed by biomolecular fluorescence complementation (BiFC). An in vitro ubiquitination assay showed that OsHIRP1 E3 ligase directly ubiquitinates its interacting proteins, OsAKR4 and OsHRK1, as substrates. Using an in vitro cell-free degradation assay, we observed a clear reduction in the levels of the two proteins under high temperature (45 °C), but not under low temperature conditions (4 °C and 30 °C). Seeds of OsHIRP1-overexpressing plants exhibited high germination rates compared with the control under heat stress. The OsHIRP1-overexpressing plants presented high survival rates of approximately 62-68%, whereas control plants displayed a low recovery rate of 34% under condition of acquired thermo-tolerance. Some heat stress-inducible genes (HsfA3, HSP17.3, HSP18.2 and HSP20) were up-regulated in OsHIRP1-overexpressing Arabidopsis than control plants under heat stress conditions. Collectively, these results suggest that OsHIRP1, an E3 ligase, positively regulates plant response to heat stress.

The Agrobacterium F-Box Protein Effector VirF Destabilizes the Arabidopsis GLABROUS1 Enhancer/Binding Protein-Like Transcription Factor VFP4, a Transcriptional Activator of Defense Response Genes

Agrobacterium-mediated genetic transformation not only represents a technology of choice to genetically manipulate plants, but it also serves as a model system to study mechanisms employed by invading pathogens to counter the myriad defenses mounted against them by the host cell. Here, we uncover a new layer of plant defenses that is targeted by A. tumefaciens to facilitate infection. We show that the Agrobacterium F-box effector VirF, which is exported into the host cell, recognizes an Arabidopsis transcription factor VFP4 and targets it for proteasomal degradation. We hypothesize that VFP4 resists Agrobacterium infection and that the bacterium utilizes its VirF effector to degrade VFP4 and thereby mitigate the VFP4-based defense. Indeed, loss-of-function mutations in VFP4 resulted in differential expression of numerous biotic stress-response genes, suggesting that one of the functions of VFP4 is to control a spectrum of plant defenses, including those against Agrobacterium tumefaciens. We identified one such gene, ATL31, known to mediate resistance to bacterial pathogens. ATL31 was transcriptionally repressed in VFP4 loss-of-function plants and activated in VFP4 gain-of-function plants. Gain-of-function lines of VFP4 and ATL31 exhibited recalcitrance to Agrobacterium tumorigenicity, suggesting that A. tumefaciens may utilize the host ubiquitin/proteasome system to destabilize transcriptional regulators of the host disease response machinery.

Real-time monitoring of PtaHMGB activity in poplar transactivation assays

BACKGROUND

Precise control of gene expression is essential to synchronize plant development with the environment. In perennial plants, transcriptional regulation remains poorly understood, mainly due to the long time required to perform functional studies. Transcriptional reporters based on luciferase have been useful to study circadian and diurnal regulation of gene expression, both by transcription factors and chromatin remodelers. The high mobility group proteins are considered transcriptional chaperones that also modify the chromatin architecture. They have been found in several species, presenting in some cases a circadian expression of their mRNA or protein.

RESULTS

Transactivation experiments have been shown as a powerful and fast method to obtain information about the potential role of transcription factors upon a certain reporter. We designed and validated a luciferase transcriptional reporter using the 5' sequence upstream ATG of Populus tremula × alba LHY2 gene. We showed the robustness of this reporter line under long day and continuous light conditions. Moreover, we confirmed that pPtaLHY2::LUC activity reproduces the accumulation of PtaLHY2 mRNA. We performed transactivation studies by transient expression, using the reporter line as a genetic background, unraveling a new function of a high mobility group protein in poplar, which can activate the PtaLHY2 promoter in a gate-dependent manner. We also showed PtaHMGB2/3 needs darkness to produce that activation and exhibits an active degradation after dawn, mediated by the 26S proteasome.

CONCLUSIONS

We generated a stable luciferase reporter poplar line based on the circadian clock gene PtaLHY2, which can be used to investigate transcriptional regulation and signal transduction pathway. Using this reporter line as a genetic background, we established a methodology to rapidly assess potential regulators of diurnal and circadian rhythms. This tool allowed us to demonstrate that PtaHMGB2/3 promotes the transcriptional activation of our reporter in a gate-dependent manner. Moreover, we added new information about the PtaHMGB2/3 protein regulation along the day. This methodology can be easily adapted to other transcription factors and reporters.

The Banana Fruit SINA Ubiquitin Ligase MaSINA1 Regulates the Stability of MaICE1 to be Negatively Involved in Cold Stress Response

The regulation of ICE1 protein stability is important to ensure effective cold stress response, and is extensively studied in Arabidopsis. Currently, how ICE1 stability in fruits under cold stress is controlled remains largely unknown. Here, we reported the possible involvement of a SEVEN IN ABSENTIA (SINA) ubiquitin ligase MaSINA1 from banana fruit in affecting MaICE1 stability. MaSINA1 was identified based on a yeast two-hybrid screening using MaICE1 as bait. Further yeast two-hybrid, pull-down, bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (CoIP) assays confirmed that MaSINA1 interacted with MaICE1. The expression of MaSINA1 was repressed by cold stress. Subcellular localization analysis in tobacco leaves showed that MaSINA1 was localized predominantly in the nucleus. In vitro ubiquitination assay showed that MaSINA1 possessed E3 ubiquitin ligase activity. More importantly, in vitro and semi-in vivo experiments indicated that MaSINA1 can ubiquitinate MaICE1 for the 26S proteasome-dependent degradation, and therefore suppressed the transcriptional activation of MaICE1 to MaNAC1, an important regulator of cold stress response of banana fruit. Collectively, our data reveal a mechanism in banana fruit for control of the stability of ICE1 and for the negative regulation of cold stress response by a SINA E3 ligase via the ubiquitin proteasome system.

A disulphide isomerase gene (PDI-V) from Haynaldia villosa contributes to powdery mildew resistance in common wheat

In this study, we report the contribution of a PDI-like gene from wheat wild relative Haynaldia villosa in combating powdery mildew. PDI-V protein contains two conserved thioredoxin (TRX) active domains (a and a′) and an inactive domain (b). PDI-V interacted with E3 ligase CMPG1-V protein, which is a positive regulator of powdery mildew response. PDI-V was mono-ubiquitinated by CMPG1-V without degradation being detected. PDI-V was located on H. villosa chromosome 5V and encoded for a protein located in the endoplasmic reticulum. Bgt infection in leaves of H. villosa induced PDI-V expression. Virus induced gene silencing of PDIs in a T. durum-H. villosa amphiploid compromised the resistance. Single cell transient over-expression of PDI-V or a truncated version containing the active TXR domain a decreased the haustorial index in moderately susceptible wheat cultivar Yangmai 158. Stable transgenic lines over-expressing PDI-V in Yangmai 158 displayed improved powdery mildew resistance at both the seedling and adult stages. By contrast over-expression of point-mutated PDI-V^C57A did not increase the level of resistance in Yangmai 158. The above results indicate a pivotal role of PDI-V in powdery mildew resistance and showed that conserved TRX domain a is critical for its function.

Association analysis for disease resistance to Fusarium oxysporum in cape gooseberry (Physalis peruviana L)

BACKGROUND

Vascular wilt caused by Fusarium oxysporum is the most important disease in cape gooseberry (Physalis peruviana L.) in Colombia. The development of resistant cultivars is considered one of the most cost-effective means to reduce the impact of this disease. In order to do so, it is necessary to provide breeders with molecular markers and promising germplasm for introgression of different resistance loci as part of breeding schemes. Here we described an association mapping study in cape gooseberry with the goal to: (i) select promising materials for use in plant breeding and (ii) identify SNPs associated with the cape gooseberry resistance response to the F. oxysporum pathogen under greenhouse conditions, as potential markers for cape gooseberry breeding.

RESULTS

We found a total of 21 accessions with different resistance responses within a diversity panel of 100 cape gooseberry accessions. A total of 60,663 SNPs were also identified within the same panel by means of GBS (Genotyping By Sequencing). Model-based population structure and neighbor-joining analyses showed three populations comprising the cape gooseberry panel. After correction for population structure and kinship, we identified SNPs markers associated with the resistance response against F. oxysporum. The identification of markers was based on common tags using the reference genomes of tomato and potato as well as the root/stem transcriptome of cape gooseberry. By comparing their location with the tomato genome, 16 SNPs were found in genes involved in defense/resistance response to pathogens, likewise when compared with the genome of potato, 12 markers were related.

CONCLUSIONS

The work presented herein provides the first association mapping study in cape gooseberry showing both the identification of promising accessions with resistance response phenotypes and the identification of a set of SNP markers mapped to defense/resistance response genes of reference genomes. Thus, the work also provides new knowledge on candidate genes involved in the P. peruviana - F. oxysporum pathosystem as a foundation for further validation in marker-assisted selection. The results have important implications for conservation and breeding strategies in cape gooseberry.

Nopaline-type Ti plasmid of Agrobacterium encodes a VirF-like functional F-box protein

During Agrobacterium-mediated genetic transformation of plants, several bacterial virulence (Vir) proteins are translocated into the host cell to facilitate infection. One of the most important of such translocated factors is VirF, an F-box protein produced by octopine strains of Agrobacterium, which presumably facilitates proteasomal uncoating of the invading T-DNA from its associated proteins. The presence of VirF also is thought to be involved in differences in host specificity between octopine and nopaline strains of Agrobacterium, with the current dogma being that no functional VirF is encoded by nopaline strains. Here, we show that a protein with homology to octopine VirF is encoded by the Ti plasmid of the nopaline C58 strain of Agrobacterium. This protein, C58VirF, possesses the hallmarks of functional F-box proteins: it contains an active F-box domain and specifically interacts, via its F-box domain, with SKP1-like (ASK) protein components of the plant ubiquitin/proteasome system. Thus, our data suggest that nopaline strains of Agrobacterium have evolved to encode a functional F-box protein VirF.

Interaction of Arabidopsis Trihelix-Domain Transcription Factors VFP3 and VFP5 with Agrobacterium Virulence Protein VirF

Agrobacterium is a natural genetic engineer of plants that exports several virulence proteins into host cells in order to take advantage of the cell machinery to facilitate transformation and support bacterial growth. One of these effectors is the F-box protein VirF, which presumably uses the host ubiquitin/proteasome system (UPS) to uncoat the packaging proteins from the invading bacterial T-DNA. By analogy to several other bacterial effectors, VirF most likely has several functions in the host cell and, therefore, several interacting partners among host proteins. Here we identify one such interactor, an Arabidopsis trihelix-domain transcription factor VFP3, and further show that its very close homolog VFP5 also interacted with VirF. Interestingly, interactions of VirF with either VFP3 or VFP5 did not activate the host UPS, suggesting that VirF might play other UPS-independent roles in bacterial infection. To better understand the potential scope of VFP3 function, we used RNAi to reduce expression of the VFP3 gene. Transcriptome profiling of these VFP3-silenced plants using high-throughput cDNA sequencing (RNA-seq) revealed that VFP3 substantially affected plant gene expression; specifically, 1,118 genes representing approximately 5% of all expressed genes were significantly either up- or down-regulated in the VFP3 RNAi line compared to wild-type Col-0 plants. Among the 507 up-regulated genes were genes implicated in the regulation of transcription, protein degradation, calcium signaling, and hormone metabolism, whereas the 611 down-regulated genes included those involved in redox regulation, light reactions of photosynthesis, and metabolism of lipids, amino acids, and cell wall. Overall, this pattern of changes in gene expression is characteristic of plants under stress. Thus, VFP3 likely plays an important role in controlling plant homeostasis.