Exploring the pleiotropy of hos1

HOS1 ubiquitinates SPL9 for degradation to modulate salinity-delayed flowering

Soil salinity is a serious environmental threat to plant growth and flowering. Flowering in the right place, at the right time, ensures maximal reproductive success for plants. Salinity-delayed flowering is considered a stress coping/survival strategy and the molecular mechanisms underlying this process require further studies to enhance the crop's salt tolerance ability. A nuclear pore complex (NPC) component, HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 1 (HOS1), has been recognized as a negative regulator of plant cold responses and flowering. Here, we challenged the role of HOS1 in regulating flowering in response to salinity stress. Interestingly, we discovered that HOS1 can directly interact with and ubiquitinate transcription factor SPL9 (SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 9) to promote its protein degradation in response to salinity stress. Moreover, we demonstrated that HOS1 and SPL9 antagonistically regulate plant flowering under both normal and salt stress conditions. HOS1 was further shown to negatively regulate the expression of SPLs and several key flowering genes in response to salinity stress. These results jointly revealed that HOS1 is an important integrator in the process of modulating salinity-delayed flowering, thus offering new perspectives on a salinity stress coping strategy of plants.

Involvement of epigenetic factors in flavonoid accumulation during plant cold adaptation

Plant responses to cold stress include either induction of flavonoid biosynthesis as part of defense responses or initially elevated levels of these substances to mitigate sudden temperature fluctuations. The role of chromatin modifying factors and, in general, epigenetic variability in these processes is not entirely clear. In this work, we review the literature to establish the relationship between flavonoids, cold and chromatin modifications. We demonstrate the relationship between cold acclimation and flavonoid accumulation, and then describe the cold adaptation signaling pathways and their relationship with chromatin modifying factors. Particular attention was paid to the cold signaling module OST1-HOS1-ICE1 and the novel function of the E3 ubiquitin protein ligase HOS1 (a protein involved in chromatin modification during cold stress) in flavonoid regulation.

The ubiquitin codes in cellular stress responses

Ubiquitination/ubiquitylation, one of the most fundamental post-translational modifications, regulates almost every critical cellular process in eukaryotes. Emerging evidence has shown that essential components of numerous biological processes undergo ubiquitination in mammalian cells upon exposure to diverse stresses, from exogenous factors to cellular reactions, causing a dazzling variety of functional consequences. Various forms of ubiquitin signals generated by ubiquitylation events in specific milieus, known as ubiquitin codes, constitute an intrinsic part of myriad cellular stress responses. These ubiquitination events, leading to proteolytic turnover of the substrates or just switch in functionality, initiate, regulate, or supervise multiple cellular stress-associated responses, supporting adaptation, homeostasis recovery, and survival of the stressed cells. In this review, we attempted to summarize the crucial roles of ubiquitination in response to different environmental and intracellular stresses, while discussing how stresses modulate the ubiquitin system. This review also updates the most recent advances in understanding ubiquitination machinery as well as different stress responses and discusses some important questions that may warrant future investigation.

The nuclear pore Y-complex functions as a platform for transcriptional regulation of FLOWERING LOCUS C in Arabidopsis

The nuclear pore complex (NPC) has multiple functions beyond the nucleo-cytoplasmic transport of large molecules. Subnuclear compartmentalization of chromatin is critical for gene expression in animals and yeast. However, the mechanism by which the NPC regulates gene expression is poorly understood in plants. Here we report that the Y-complex (Nup107-160 complex, a subcomplex of the NPC) self-maintains its nucleoporin homeostasis and modulates FLOWERING LOCUS C (FLC) transcription via changing histone modifications at this locus. We show that Y-complex nucleoporins are intimately associated with FLC chromatin through their interactions with histone H2A at the nuclear membrane. Fluorescence in situ hybridization assays revealed that Nup96, a Y-complex nucleoporin, enhances FLC positioning at the nuclear periphery. Nup96 interacted with HISTONE DEACETYLASE 6 (HDA6), a key repressor of FLC expression via histone modification, at the nuclear membrane to attenuate HDA6-catalyzed deposition at the FLC locus and change histone modifications. Moreover, we demonstrate that Y-complex nucleoporins interact with RNA polymerase II to increase its occupancy at the FLC locus, facilitating transcription. Collectively, our findings identify an attractive mechanism for the Y-complex in regulating FLC expression via tethering the locus at the nuclear periphery and altering its histone modification.

Suppression of the HOS1 Gene Affects the Level of ROS Depending on Light and Cold

The E3 ubiquitin-protein ligase HOS1 is an important integrator of temperature information and developmental processes. HOS1 is a negative regulator of plant cold tolerance, and silencing HOS1 leads to increased cold tolerance. In the present work, we studied ROS levels in hos1^Cas9Arabidopsis thaliana plants, in which the HOS1 gene was silenced by disruption of the open reading frame via CRISPR/Cas9 technology. Confocal imaging of intracellular reactive oxygen species (ROS) showed that the hos1 mutation moderately increased levels of ROS under both low and high light (HL) conditions, but wild-type (WT) and hos1^Cas9 plants exhibited similar ROS levels in the dark. Visualization of single cells did not reveal differences in the intracellular distribution of ROS between WT and hos1^Cas9 plants. The hos1^Cas9 plants contained a high basal level of ascorbic acid, maintained a normal balance between reduced and oxidized glutathione (GSH and GSSG), and generated a strong antioxidant defense response against paraquat under HL conditions. Under cold exposure, the hos1 mutation decreased the ROS level and substantially increased the expression of the ascorbate peroxidase genes Apx1 and Apx2. When plants were pre-exposed to cold and further exposed to HL, the expression of the NADPH oxidase genes RbohD and RbohF was increased in the hos1^Cas9 plants but not in WT plants. hos1-mediated changes in the level of ROS are cold-dependent and cold-independent, which implies different levels of regulation. Our data indicate that HOS1 is required to maintain ROS homeostasis not only under cold conditions, but also under conditions of both low and high light intensity. It is likely that HOS1 prevents the overinduction of defense mechanisms to balance growth.

CRISPR/Cas9-Mediated Knockout of HOS1 Reveals Its Role in the Regulation of Secondary Metabolism in Arabidopsis thaliana

In Arabidopsis, the RING finger-containing E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1 (HOS1) functions as a main regulator of the cold signaling. In this study, CRISPR/Cas9-mediated targeted mutagenesis of the HOS1 gene in the first exon was performed. DNA sequencing showed that frameshift indels introduced by genome editing of HOS1 resulted in the appearance of premature stop codons, disrupting the open reading frame. Obtained hos1^Cas9 mutant plants were compared with the SALK T-DNA insertion mutant, line hos1-3, in terms of their tolerance to abiotic stresses, accumulation of secondary metabolites and expression levels of genes participating in these processes. Upon exposure to cold stress, enhanced tolerance and expression of cold-responsive genes were observed in both hos1-3 and hos1^Cas9 plants. The hos1 mutation caused changes in the synthesis of phytoalexins in transformed cells. The content of glucosinolates (GSLs) was down-regulated by 1.5-times, while flavonol glycosides were up-regulated by 1.2 to 4.2 times in transgenic plants. The transcript abundance of the corresponding MYB and bHLH transcription factors, which are responsible for the regulation of secondary metabolism in Arabidopsis, were also altered. Our data suggest a relationship between HOS1-regulated downstream signaling and phytoalexin biosynthesis.

HOS1 activates DNA repair systems to enhance plant thermotolerance

Plants possess an astonishing capability of effectively adapting to a wide range of temperatures, ranging from freezing to near-boiling temperatures^1,2. Yet, heat is a critical obstacle to plant survival. The deleterious effects of heat shock on cell function include misfolding of cellular proteins, disruption of cytoskeletons and membranes, and disordering of RNA metabolism and genome integrity^3-5. Plants stimulate diverse heat shock response pathways in response to abrupt temperature increases. While it is known that stressful high temperatures disturb genome integrity by causing nucleotide modifications and strand breakages or impeding DNA repair⁶, it is largely unexplored how plants cope with heat-induced DNA damages. Here, we demonstrated that high expression of osmotically reponsive genes 1 (HOS1) induces thermotolerance by activating DNA repair components. Thermotolerance and DNA repair capacity were substantially reduced in HOS1-deficient mutants, in which thermal induction of genes encoding DNA repair systems, such as the DNA helicase RECQ2, was markedly decreased. Notably, HOS1 proteins were thermostabilized in a heat shock factor A1/heat shock protein 90 (HSP90)-dependent manner. Our data indicate that the thermoresponsive HSP90-HOS1-RECQ2 module contributes to sustaining genome integrity during the acquisition of thermotolerance, providing a distinct molecular link between DNA repair and thermotolerance.

Nucleoporin 160 Regulates Flowering through Anchoring HOS1 for Destabilizing CO in Arabidopsis

Nuclear pore complexes (NPCs), which comprise multiple copies of nucleoporins (Nups), are large protein assemblies embedded in the nuclear envelope connecting the nucleus and cytoplasm. Although it has been known that Nups affect flowering in Arabidopsis, the underlying mechanisms are poorly understood. Here, we show that loss of function of Nucleoporin 160 (Nup160) leads to increased abundance of CONSTANS (CO) protein and the resulting upregulation of FLOWERING LOCUS T (FT) specifically in the morning. We demonstrate that Nup160 regulates CO protein stability through affecting NPC localization of an E3-ubiquitin ligase, HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES1 (HOS1), which destabilizes CO protein in the morning period. Taken together, these results provide a mechanistic understanding of Nup function in the transition from vegetative to reproductive growth, suggesting that deposition of HOS1 at NPCs by Nup160 is essential for preventing precocious flowering in response to photoperiod in Arabidopsis.

Nucleocytosolic mRNA transport in plants: export factors and their influence on growth and development

In eukaryotes, the regulated transport of mRNAs from the cell nucleus to the cytosol is a critical step in the expression of protein-coding genes, as it links nuclear mRNA synthesis with cytosolic translation. The pre-mRNAs that are synthesised by RNA polymerase II are processed by 5´-capping, splicing, and 3´-polyadenylation. The multi-subunit THO/TREX complex integrates mRNA biogenesis with their nucleocytosolic transport. Various export factors are recruited to the mRNAs during their maturation, which occurs essentially co-transcriptionally. These RNA-bound export factors ensure efficient transport of the export-competent mRNAs through nuclear pore complexes. In recent years, several factors involved in plant mRNA export have been functionally characterised. Analysis of mutant plants has demonstrated that impaired mRNA export causes defects in growth and development. Moreover, there is accumulating evidence that mRNA export can influence processes such as plant immunity, circadian regulation, and stress responses. Therefore, it is important to learn more details about the mechanism of nucleocytosolic mRNA transport in plants and its physiological significance.

croFGD: Catharanthus roseus Functional Genomics Database

Catharanthus roseus is a medicinal plant, which can produce monoterpene indole alkaloid (MIA) metabolites with biological activity and is rich in vinblastine and vincristine. With release of the scaffolded genome sequence of C. roseus, it is necessary to annotate gene functions on the whole-genome level. Recently, 53 RNA-seq datasets are available in public with different tissues (flower, root, leaf, seedling, and shoot) and different treatments (MeJA, PnWB infection and yeast elicitor). We used in-house data process pipeline with the combination of PCC and MR algorithms to construct a co-expression network exploring multi-dimensional gene expression (global, tissue preferential, and treat response) through multi-layered approaches. In the meanwhile, we added miRNA-target pairs, predicted PPI pairs into the network and provided several tools such as gene set enrichment analysis, functional module enrichment analysis, and motif analysis for functional prediction of the co-expression genes. Finally, we have constructed an online croFGD database (http://bioinformatics.cau.edu.cn/croFGD/). We hope croFGD can help the communities to study the C. roseus functional genomics and make novel discoveries about key genes involved in some important biological processes.

Cytokinin at the Crossroads of Abiotic Stress Signalling Pathways

Cytokinin is a multifaceted plant hormone that plays major roles not only in diverse plant growth and development processes, but also stress responses. We summarize knowledge of the roles of its metabolism, transport, and signalling in responses to changes in levels of both macronutrients (nitrogen, phosphorus, potassium, sulphur) and micronutrients (boron, iron, silicon, selenium). We comment on cytokinin's effects on plants' xenobiotic resistance, and its interactions with light, temperature, drought, and salinity signals. Further, we have compiled a list of abiotic stress-related genes and demonstrate that their expression patterns overlap with those of cytokinin metabolism and signalling genes.

An Arabidopsis Nucleoporin NUP85 modulates plant responses to ABA and salt stress

Several nucleoporins in the nuclear pore complex (NPC) have been reported to be involved in abiotic stress responses in plants. However, the molecular mechanism of how NPC regulates abiotic stress responses, especially the expression of stress responsive genes remains poorly understood. From a forward genetics screen using an abiotic stress-responsive luciferase reporter (RD29A-LUC) in the sickle-1 (sic-1) mutant background, we identified a suppressor caused by a mutation in NUCLEOPORIN 85 (NUP85), which exhibited reduced expression of RD29A-LUC in response to ABA and salt stress. Consistently, the ABA and salinity induced expression of several stress responsive genes such as RD29A, COR15A and COR47 was significantly compromised in nup85 mutants and other nucleoporin mutants such as nup160 and hos1. Subsequently, Immunoprecipitation and mass spectrometry analysis revealed that NUP85 is potentially associated with HOS1 and other nucleoporins within the nup107-160 complex, along with several mediator subunits. We further showed that there is a direct physical interaction between MED18 and NUP85. Similar to NUP85 mutations, MED18 mutation was also found to attenuate expression of stress responsive genes. Taken together, we not only revealed the involvement of NUP85 and other nucleoporins in regulating ABA and salt stress responses, but also uncovered a potential relation between NPC and mediator complex in modulating the gene expression in plants.

Nuclear pore complex (NPC) mediates the traffic between nucleus and cytoplasm. This work identified NUCLEOPORIN 85 (NUP85) as an important factor for the expression of stress-responsive luciferase reporter gene RD29A-LUC in response to ABA and salt stress from a forward genetics screen. Mutation in NUP85 and other NPC components such as NUP160 and HOS1 resulted in decreased expression of several stress responsive genes such as RD29A, COR15A and COR47. Proteomics data uncovered a list of putative NUP85 associated proteins. Furthermore, NUP85 was demonstrated to interact with MED18, a master transcriptional regulator, to control the expression of stress responsive genes. The study has added a new layer of knowledge about the diverse functions of NPC in abiotic stress responses.

Cell Biology of the Plant Nucleus

The eukaryotic nucleus is enclosed by the nuclear envelope, which is perforated by the nuclear pores, the gateways of macromolecular exchange between the nucleoplasm and cytoplasm. The nucleoplasm is organized in a complex three-dimensional fashion that changes over time and in response to stimuli. Within the cell, the nucleus must be viewed as an organelle (albeit a gigantic one) that is a recipient of cytoplasmic forces and capable of morphological and positional dynamics. The most dramatic reorganization of this organelle occurs during mitosis and meiosis. Although many of these aspects are less well understood for the nuclei of plants than for those of animals or fungi, several recent discoveries have begun to place our understanding of plant nuclei firmly into this broader cell-biological context.

Nup82 functions redundantly with Nup136 in a salicylic acid-dependent defense response of Arabidopsis thaliana

The nuclear pore complex (NPC) comprises more than 30 nucleoporins (Nups). NPC mediates macromolecular trafficking between the nucleoplasm and the cytoplasm, but specific roles of individual Nups are poorly understood in higher plants. Here, we show that the novel nucleoporin unique to angiosperm plants (designated as Nup82) functions in a salicylic acid-dependent defense in a redundant manner with Nup136, which is a component of the nuclear basket in the NPC. Arabidopsis thaliana Nup82 had a similar amino acid sequence to the N-terminal half of Nup136 and a Nup82-GFP fusion was localized on the nuclear envelope. Immunoprecipitation and bimolecular fluorescence complementation analyses revealed that Nup82 interacts with the NPC components Nup136 and RAE1. The double knockout mutant nup82 nup136 showed severe growth defects, while the single knockout mutant nup82 did not, suggesting that Nup82 functions redundantly with Nup136. nup82 nup136 impaired benzothiadiazole (an analog of salicylic acid)-induced resistance to the virulent bacteria Pseudomonas syringae pv. tomato DC3000. Furthermore, transcriptome analysis of nup82 nup136 indicates that deficiency of Nup82 and Nup136 causes noticeable downregulation of immune-related genes. These results suggest that Nup82 and Nup136 are redundantly involved in transcriptional regulation of salicylic acid-responsive genes through nuclear transport of signaling molecules.

Rice root curling, a response to mechanosensing, is modulated by the rice E3-ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE1 (OsHOS1)

Plant development depends on the perception of external cues, such as light, gravity, touch, wind or nutrients, among others. Nevertheless, little is known regarding signal transduction pathways integrating these stimuli. Recently, we have reported the involvement of a rice E3-ubiquitin ligase (OsHOS1, HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE1), previously associated with abiotic stress response, in root responses to mechanical stimuli. We showed that OsHOS1 is involved in the regulation of root curling after mechanosensing and that RNAi::OsHOS1 plants failed to exhibit the root curling phenotype observed in WT. Interestingly, the straight root phenotype of these transgenics correlated with the up-regulation of rice ROOT MEANDER CURLING (OsRMC, a negative regulator of rice root curling) and was reverted by the exogenous application of jasmonic acid. Altogether, our results highlight the role of the proteasome modulating plant responses to mechanical stimuli and suggest that OsHOS1 is a hub integrating environmental and hormonal signaling into plant growth and development.

Red Light-Mediated Degradation of CONSTANS by the E3 Ubiquitin Ligase HOS1 Regulates Photoperiodic Flowering in Arabidopsis

The regulation of CONSTANS (CO) gene expression is crucial to accurately measure changes in daylength, which influences flowering time in Arabidopsis thaliana. CO expression is under both transcriptional and posttranslational control mechanisms. We previously showed that the E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES1 (HOS1) physically interacts with CO in Arabidopsis. This interaction is required to precisely modulate the timing of CO accumulation and, consequently, to maintain low levels of FLOWERING LOCUS T expression during the first part of the day. The data presented here demonstrate that HOS1 is involved in the red light-mediated degradation of CO that takes place in the early stages of the daylight period. Our results show that phytochrome B (phyB) is able to regulate flowering time, acting in the phloem companion cells, as previously described for CO and HOS1. Moreover, we reveal that phyB physically interacts with HOS1 and CO, indicating that the three proteins may be present in a complex in planta that is required to coordinate a correct photoperiodic response in Arabidopsis.