A comprehensive analysis of interaction and localization of Arabidopsis SKP1-like (ASK) and F-box (FBX) proteins

Effects of the skp1 gene of the SCF complex on lipid metabolism and response to abiotic stress in Chlamydomonas reinhardtii

SKP1 (S-phase kinase-associated protein 1) is a key member of the SCF (SKP1-Cullin1-F-box) E3 ligase complex. The SCF complex is involved in regulating various levels of plant physiology, including regulation of cellular signaling and response to abiotic stresses. While the function of SKP1 in plants is well known, its function in algae remains poorly understood. In this study, we investigated the role of the Chlamydomonas reinhardtii skp1 gene using RNAi interference and overexpression approaches. Subcellular localization of SKP1 was performed by transient expression in onion epidermal cells. For abiotic stress assays, the growth of skp1 RNAi and overexpression recombinant strains was examined under conditions of high osmolality (sorbitol), high salinity (NaCl) and high temperature (37°C). Our results showed that skp1 silencing significantly reduced oil accumulation by 38%, whereas skp1 overexpressing led to a 37% increase in oil content, suggesting that skp1 plays a crucial role in regulating oil synthesis and may influence lipid accumulation by regulating photosynthetic carbon flux partitioning. Subcellular localization analysis revealed that skp1 was predominantly localized within the nucleus. Furthermore, our results showed that SKP1 responds to abiotic stresses. Under sorbol and NaCl stress conditions, RNAi interference strains exhibited better growth than controls; however, their growth was comparatively impaired under 37°C stress compared to controls. On the other hand, overexpression strains showed weaker growth under sorbol and NaCl stress but were more tolerant to 37°C heat stress. These results illustrate the functional diversity of SKP1 in Chlamydomonas. This study provides an important complement for lipid metabolism and abiotic stress regulation in microalgae.

Combined Bulked Segregant Analysis-Sequencing and Transcriptome Analysis to Identify Candidate Genes Associated with Cold Stress in Brassica napus L

Cold tolerance in rapeseed is closely related to its growth, yield, and geographical distribution. However, the mechanisms underlying cold resistance in rapeseed remain unclear. This study aimed to explore cold resistance genes and provide new insights into the molecular mechanisms of cold resistance in rapeseed. Rapeseed M98 (cold-sensitive line) and D1 (cold-tolerant line) were used as parental lines. In their F2 population, 30 seedlings with the lowest cold damage levels and 30 with the highest cold damage levels were selected to construct cold-tolerant and cold-sensitive pools, respectively. The two pools and parental lines were analyzed using bulk segregant sequencing (BSA-seq). The G'-value analysis indicated a single peak on Chromosome C09 as the candidate interval, which had a 2.59 Mb segment with 69 candidate genes. Combined time-course and weighted gene co-expression network analyses were performed at seven time points to reveal the genetic basis of the two-parent response to low temperatures. Twelve differentially expressed genes primarily involved in plant cold resistance were identified. Combined BSA-seq and transcriptome analysis revealed BnaC09G0354200ZS, BnaC09G0353200ZS, and BnaC09G0356600ZS as the candidate genes. Quantitative real-time PCR validation of the candidate genes was consistent with RNA-seq. This study facilitates the exploration of cold tolerance mechanisms in rapeseed.

Genome-Wide Identification and Functional Characterization of SKP1-like Gene Family Reveal Its Involvement in Response to Stress in Cotton

SKP1 constitutes the Skp1-Cullin-F-box ubiquitin E3 ligase (SCF), which plays a role in plant growth and development and biotic and abiotic stress in ubiquitination. However, the response of the SKP1-like gene family to abiotic and biotic stresses in cotton has not been well characterized. In this study, a total of 72 SKP1-like genes with the conserved domain of SKP1 were identified in four Gossypium species. Synteny and collinearity analyses revealed that segmental duplication played a major role in the expansion of the cotton SKP1-like gene family. All SKP1-like proteins were classified into three different subfamilies via phylogenetic analysis. Furthermore, we focused on a comprehensive analysis of SKP1-like genes in G. hirsutum. The cis-acting elements in the promoter site of the GhSKP1-like genes predict their involvement in multiple hormonal and defense stress responses. The expression patterns results indicated that 16 GhSKP1-like genes were expressed in response to biotic or abiotic stresses. To further validate the role of the GhSKP1-like genes in salt stress, four GhSKP1-like genes were randomly selected for gene silencing via VIGS. The results showed that the silencing of GhSKP1-like_7A resulted in the inhibition of plant growth under salt stress, suggesting that GhSKP1-like_7A was involved in the response to salt stress. In addition, yeast two-hybrid results revealed that GhSKP1-like proteins have different abilities to interact with F-box proteins. These results provide valuable information for elucidating the evolutionary relationships of the SKP1-like gene family and aiding further studies on the function of SKP1-like genes in cotton.

An F-box Kelch repeat protein, PmFBK2, from Persicaria minor interacts with GID1b to modulate gibberellin signalling

The F-box protein (FBP) family plays diverse functions in the plant kingdom, with the function of many members still unrevealed. In this study, a specific FBP called PmFBK2, containing Kelch repeats from Persicaria minor, was functionally investigated. Employing the yeast two-hybrid (Y2H) assay, PmFBK2 was found to interact with Skp1-like proteins from P. minor, suggesting its potential to form an E3 ubiquitin ligase, known as the SCF complex. Y2H and co-immunoprecipitation tests revealed that PmFBK2 interacts with full-length PmGID1b. The interaction marks the first documented binding between these two protein types, which have never been reported in other plants before, and they exhibited a negative effect on gibberellin (GA) signal transduction. The overexpression of PmFBK2 in the kmd3 mutant, a homolog from Arabidopsis, demonstrated the ability of PmFBK2 to restore the function of the mutated KMD3 gene. The function restoration was supported by morphophysiological and gene expression analyses, which exhibited patterns similar to the wild type (WT) compared to the kmd3 mutant. Interestingly, the overexpression of PmFBK2 or PmGID1b in Arabidopsis had opposite effects on rosette diameter, seed weight, and plant height. This study provides new insights into the complex GA signalling. It highlights the crucial roles of the interaction between FBP and the GA receptor (GID1b) in regulating GA responses. These findings have implications for developing strategies to enhance plant growth and yield by modulating GA signalling in crops.

The de-ubiquitinase UBQUITIN SPECIFIC PROTEASE 15 (UBP15) interacts with the SCF E3 complex adaptor ARABIDOPSIS SKP1 HOMOLOGUE 1 (ASK1) to regulate petal size and fertility in Arabidopsis thaliana

Ubiquitination is a pivotal type of post-translational modification, which plays a far-reaching role in plant growth and development, as well as in the response of plants to stress. Just like the two sides of a coin, de-ubiquitination also plays an important role in plant life, which has been gradually discovered in recent years. Here, we demonstrate that the UBQUITIN SPECIFIC PROTEASE 15 (UBP15), which is a UBP-type de-ubiquitinase, interacts with the SCF E3 complex adaptor ARABIDOPSIS SKP1 HOMOLOGUE 1 (ASK1) and influences its protein stability to regulate plant fertility and petal size. The UBP15 is associated with the ASK1 physically, as verified by yeast-two-hybrid (Y2H) and protein pull-down in vitro assays. Disruption of ASK1 by a T-DNA insertion generates some abnormal phenotypes, such as low fertility and small petals. Genetic analysis shows that the UBP15 mutation enhances the low-fertility and small-petal phenotypes of ask1 mutant plants. By proteomic analysis, many types of proteins were identified as potential candidate downstream genes associated with the phenotypes of ubp15 ask1 double mutant plants. Taken together, these findings reveal a molecular relationship between ASK1 and UBP15 and their interaction in the regulation of petal size and fertility, which would benefit in-depth research about the ubiquitin-related pathway in plant physiological processes in the future.

Two imprinted genes primed by DEMETER in the central cell and activated by WRKY10 in the endosperm

The early development of the endosperm is crucial for balancing the allocation of maternal nutrients to offspring. This process is believed to be evolutionarily associated with genomic imprinting, resulting in parentally biased allelic gene expression. Beyond FertilizationIndependentSeed (FIS) genes, the number of imprinted genes involved in early endosperm development and seed size determination remains limited. This study introduces early endosperm-expressed HAIKU (IKU) downstream Candidate F-box 1 (ICF1) and ICF2 as maternally expressed imprinted genes (MEGs) in Arabidopsis thaliana. Although these genes are also demethylated by DEMETER (DME) in the central cell, their activation differs from the direct DME-mediated activation seen in classical MEGs such as the FIS genes. Instead, ICF maternal alleles carry pre-established hypomethylation in their promoters, priming them for activation by the WRKY10 transcription factor in the endosperm. On the contrary, paternal alleles are predominantly suppressed by CG methylation. Furthermore, we find that ICF genes partially contribute to the small seed size observed in iku mutants. Our discovery reveals a two-step regulatory mechanism that highlights the important role of conventional transcription factors in the activation of imprinted genes, which was previously not fully recognized. Therefore, the mechanism provides a new dimension to understand the transcriptional regulation of imprinting in plant reproduction and development.

Dynamic interactions between SPX proteins, the ubiquitination machinery, and signalling molecules for stress adaptation at a whole-plant level

The plant macronutrient phosphorus is a scarce resource and plant-available phosphate is limiting in most soil types. Generally, a gene regulatory module called the phosphate starvation response (PSR) enables efficient phosphate acquisition by roots and translocation to other organs. Plants growing on moderate to nutrient-rich soils need to co-ordinate availability of different nutrients and repress the highly efficient PSR to adjust phosphate acquisition to the availability of other macro- and micronutrients, and in particular nitrogen. PSR repression is mediated by a small family of single SYG1/Pho81/XPR1 (SPX) domain proteins. The SPX domain binds higher order inositol pyrophosphates that signal cellular phosphorus status and modulate SPX protein interaction with PHOSPHATE STARVATION RESPONSE1 (PHR1), the central transcriptional regulator of PSR. Sequestration by SPX repressors restricts PHR1 access to PSR gene promoters. Here we focus on SPX4 that primarily acts in shoots and sequesters many transcription factors other than PHR1 in the cytosol to control processes beyond the classical PSR, such as nitrate, auxin, and jasmonic acid signalling. Unlike SPX1 and SPX2, SPX4 is subject to proteasomal degradation not only by singular E3 ligases, but also by SCF-CRL complexes. Emerging models for these different layers of control and their consequences for plant acclimation to the environment will be discussed.

A Sorghum F-Box Protein Induces an Oxidative Burst in the Defense Against Colletotrichum sublineola

The hemibiotrophic fungal pathogen Colletotrichum sublineola is the causal agent of anthracnose in sorghum (Sorghum bicolor), resulting in leaf blight, stalk rot, and head blight in susceptible genotypes, with yield losses of up to 50%. The development of anthracnose-resistant cultivars can reduce reliance on fungicides and provide a more sustainable and economical means for disease management. A previous genome-wide association study of the sorghum association panel identified the candidate resistance gene Sobic.005G172300 encoding an F-box protein. To better understand the role of this gene in the defense against C. sublineola, gene expression following infection with C. sublineola was monitored by RNA sequencing in seedlings of sorghum accession SC110, which harbored the resistance allele, and three accessions that harbored a susceptible allele. Only in SC110 did the expression of Sobic.005G172300 increase during the biotrophic phase of infection. Subsequent transcriptome analysis, gene co-expression networks, and gene regulatory networks of inoculated and mock-inoculated seedlings of resistant and susceptible accessions suggest that the increase in expression of Sobic.005G172300 induces an oxidative burst by lowering the concentration of ascorbic acid during the biotrophic phase of infection. Based on gene regulatory network analysis, the protein encoded by Sobic.005G172300 is proposed to target proteins involved in the biosynthesis of ascorbic acid for polyubiquitination through the SCF E3 ubiquitin ligase, causing their degradation via the proteasome.

Genome-Wide Network Analysis of Above- and Below-Ground Co-growth in Populus euphratica

Tree growth is the consequence of developmental interactions between above- and below-ground compartments. However, a comprehensive view of the genetic architecture of growth as a cohesive whole is poorly understood. We propose a systems biology approach for mapping growth trajectories in genome-wide association studies viewing growth as a complex (phenotypic) system in which above- and below-ground components (or traits) interact with each other to mediate systems behavior. We further assume that trait-trait interactions are controlled by a genetic system composed of many different interactive genes and integrate the Lotka-Volterra predator-prey model to dissect phenotypic and genetic systems into pleiotropic and epistatic interaction components by which the detailed genetic mechanism of above- and below-ground co-growth can be charted. We apply the approach to analyze linkage mapping data of Populus euphratica, which is the only tree species that can grow in the desert, and characterize several loci that govern how above- and below-ground growth is cooperated or competed over development. We reconstruct multilayer and multiplex genetic interactome networks for the developmental trajectories of each trait and their developmental covariation. Many significant loci and epistatic effects detected can be annotated to candidate genes for growth and developmental processes. The results from our model may potentially be useful for marker-assisted selection and genetic editing in applied tree breeding programs. The model provides a general tool to characterize a complete picture of pleiotropic and epistatic genetic architecture in growth traits in forest trees and any other organisms.

Exploring the role of FBXL fbxl gene family in Soybean: Implications for plant height and seed size regulation

F-box proteins constitute a significant family in eukaryotes and, as a component of the Skp1p-cullin-F-box complex, are considered critical for cellular protein degradation and other biological processes in plants. Despite their importance, the functions of F-box proteins, particularly those with C-terminal leucine-rich repeat (LRR) domains, remain largely unknown in plants. Therefore, the present study conducted genome-wide identification and in silico characterization of F-BOX proteins with C-terminal LRR domains in soybean (Glycine max L.) (GmFBXLs). A total of 45 GmFBXLs were identified. The phylogenetic analysis showed that GmFBXLs could be subdivided into ten subgroups and exhibited a close relationship with those from Arabidopsis thaliana, Cicer aretineum, and Medicago trunculata. It was observed that most cis-regulatory elements in the promoter regions of GmFBXLs are involved in hormone signalling, stress responses, and developmental stages. In silico transcriptome data illustrated diverse expression patterns of the identified GmFBXLs across various tissues, such as shoot apical meristem, flower, green pods, leaves, nodules, and roots. Overexpressing (OE) GmFBXL12 in Tianlong No.1 cultivar resulted in a significant difference in seed size, number of pods, and number of seeds per plant, indicated a potential increase in yield compared to wild type. This study offers valuable perspectives into the role of FBXLs in soybean, serving as a foundation for future research. Additionally, the identified OE lines represent valuable genetic resources for enhancing seed-related traits in soybean.

Identification and Expression Analysis of the SKP1-Like Gene Family under Phytohormone and Abiotic Stresses in Apple (Malus domestica)

Ubiquitination participates in plant hormone signaling and stress response to adversity. SKP1-Like, a core component of the SCF (Skp1-Cullin-F-box) complex, is the final step in catalyzing the ubiquitin-mediated protein degradation pathway. However, the SKP1-Like gene family has not been well characterized in response to apple abiotic stresses and hormonal treatments. This study revealed that 17 MdSKP1-Like gene family members with the conserved domain of SKP1 were identified in apples and were unevenly distributed on eight chromosomes. The MdSKP1-Like genes located on chromosomes 1, 10, and 15 were highly homologous. The MdSKP1-like genes were divided into three subfamilies according to the evolutionary affinities of monocotyledons and dicotyledons. MdSKP1-like members of the same group or subfamily show some similarity in gene structure and conserved motifs. The predicted results of protein interactions showed that members of the MdSKP1-like family have strong interactions with members of the F-Box family of proteins. A selection pressure analysis showed that MdSKP1-Like genes were in purifying selection. A chip data analysis showed that MdSKP1-like14 and MdSKP1-like15 were higher in flowers, whereas MdSKP1-like3 was higher in fruits. The upstream cis-elements of MdSKP1-Like genes contained a variety of elements related to light regulation, drought, low temperature, and many hormone response elements, etc. Meanwhile, qRT-PCR also confirmed that the MdSKP1-Like gene is indeed involved in the response of the apple to hormonal and abiotic stress treatments. This research provides evidence for regulating MdSKP1-Like gene expression in response to hormonal and abiotic stresses to improve apple stress resistance.

Genome-wide association study of multiyear dynamic growth traits in hybrid Liriodendron identifies robust genetic loci associated with growth trajectories

The genetic factors underlying growth traits differ over time points or stages. However, most current studies of phenotypes at single time points do not capture all loci or explain the genetic differences underlying growth trajectories. Hybrid Liriodendron exhibits obvious heterosis and is widely cultivated, although its complex genetic mechanism underlying growth traits remains unknown. A genome-wide association study (GWAS) is an effective method for elucidating the genetic architecture by identifying genetic loci underlying complex quantitative traits. In the present study, using a GWAS, we identified robust loci associated with growth trajectories in hybrid Liriodendron populations. We selected 233 hybrid progenies derived from 25 crosses for resequencing, and measured their tree height (H) and diameter at breast height (DBH) for 11 consecutive years; 192 972 high-quality single nucleotide polymorphisms (SNPs) were obtained. The dynamics of the multiyear single-trait GWAS showed that year-specific SNPs predominated, and only five robust SNPs for DBH were identified in at least three different years. Multitrait GWAS analysis with model parameters as latent variables also revealed 62 SNPs for H and 52 for DBH associated with the growth trajectory, displaying different biomass accumulation patterns, among which four SNPs exerted pleiotropic effects. All identified SNPs also exhibited temporal variations in effect sizes and inheritance patterns potentially related to different growth and developmental stages. The haplotypes resulting from these significant SNPs might pyramid favorable loci, benefitting the selection of superior genotypes. The present study provides insights into the genetic architecture of dynamic growth traits and lays a basis for future molecular-assisted breeding.

The N-coil and the globular N-terminal domain of plant ARGONAUTE1 are interaction hubs for regulatory factors

The effector complex of RNA interference (RNAi) contains at its core an ARGONAUTE (AGO) protein bound to a small guide RNA. AGO proteins adopt a two-lobed structure in which the N-terminal (N) and Piwi-Argonaute-Zwille (PAZ) domains make up one lobe, while the middle (MID) and Piwi domains make up the other. Specific biochemical functions of PAZ, MID and Piwi domains of eukaryotic AGO proteins have been described, but the functions of the N domain remain less clear. Here, we use yeast two-hybrid screening with the N domain of the founding member of the AGO protein family, Arabidopsis AGO1, to reveal that it interacts with many factors involved in regulated proteolysis. Interaction with a large group of proteins, including the autophagy cargo receptors ATI1 and ATI2, requires residues in a short, linear region, the N-coil, that joins the MID-Piwi lobe in the three-dimensional structure of AGO. In contrast, the F-box protein AUF1 interacts with AGO1 independently of the N-coil and requires distinct residues in the globular N domain itself. Mutation of AGO1 residues necessary for interaction with protein degradation factors in yeast stabilizes reporters fused to the AGO1 N domain in plants, supporting their in vivo relevance. Our results define distinct regions of the N domain implicated in protein-protein interaction, and point to a particular importance of the AGO1 N-coil as a site of interaction with regulatory factors.

Overexpression of a rice Tubby-like protein-encoding gene, OsFBT4, confers tolerance to abiotic stresses

The OsFBT4 belongs to a small sub-class of rice F-box proteins called TLPs (Tubby-like proteins) containing the conserved N-terminal F-box domain and a C-terminal Tubby domain. These proteins have largely been implicated in both abiotic and biotic stress responses, besides developmental roles in plants. Here, we investigated the role of OsFBT4 in abiotic stress signalling. The OsFBT4 transcript was strongly upregulated in response to different abiotic stresses in rice, including exogenous ABA. When ectopically expressed, in Arabidopsis, under a constitutive CaMV 35S promoter, the overexpression (OE) caused hypersensitivity to most abiotic stresses, including ABA, during seed germination and early seedling growth. At the 5-day-old seedling growth stage, the OE conferred tolerance to all abiotic stresses. The OE lines displayed significant tolerance to salinity and water deficit at the mature growth stage. The stomatal size and density were seen to be altered in the OE lines, accompanied by hypersensitivity to ABA and hydrogen peroxide (H₂O₂) and a reduced water loss rate. Overexpression of OsFBT4 caused upregulation of several ABA-regulated/independent stress-responsive genes at more advanced stages of growth, showing wide and intricate roles played by OsFBT4 in stress signalling. The OsFBT4 showed interaction with several OSKs (Oryza SKP1 proteins) and localized to the plasma membrane (PM). The protein translocates to the nucleus, in response to oxidative and osmotic stresses, but failed to show transactivation activity in the yeast system. The OE lines also displayed morphological deviations from the wild-type (WT) plants, suggesting a role of the gene also in plant development.

AtTLP2, a Tubby-like protein, plays intricate roles in abiotic stress signalling

The Arabidopsis Tubby-like protein (TLP) encoding gene, AtTLP2, plays intricate roles during ABA-dependent abiotic stress signalling, particularly salt and dehydration stress responses. TLPs (Tubby-like proteins) are a small group of eukaryotic proteins characterized by the presence of a Tubby domain. The plant TLPs have been widely shown to play important roles during abiotic stress signaling. In this study, we investigated the role of an Arabidopsis TLP, AtTLP2, in mediating abiotic stress responses. Both attlp2 null mutant and overexpression (OE) lines, in Arabidopsis, were studied which indicated the role of the gene also in development. The attlp2 mutant showed an overall dwarfism, while its overexpression caused enhanced growth. AtTLP2 localized to the plasma membrane (PM) and showed nuclear translocation in response to dehydration stress. The protein interacted with ASK1 and ASK2, but failed to show transactivation activity in yeast. AtTLP2 was transcriptionally induced by stress, caused by salt, dehydration and ABA. The attlp2 mutant was insensitive to ABA, but hypersensitive to oxidative stress at all stages of growth. ABA insensitivity conferred tolerance to salt and osmotic stresses at the germination and early seedling growth stages, but caused hypersensitivity to salt and drought stresses at advanced stages of growth. The OE lines were more sensitive to ABA, causing increased sensitivity to most stresses at the seed germination stage, but conferring tolerance to salt and osmotic stresses at more advanced stages of development. The stomata of the attlp2 mutant were less responsive to ABA and H₂O_2, while that of the OE lines exhibited greater sensitivity. Several ABA-regulated stress responsive marker genes were found to be downregulated in the mutant, but upregulated in the OE lines. The study establishes that AtTLP2 plays intricate roles in abiotic stress signaling, and the response may be largely ABA dependent.

Identification and function of miRNA-mRNA interaction pairs during lateral root development of hemi-parasitic Santalum album L. seedlings

Sandalwood (Santalum album L.) is a hemi-parasitic tree species famous for its santalol and santalene, which are extracted from its heartwood and roots. The ability to understand root functionality within its branched root system would benefit the regulation of sandalwood growth and enhance the commercial value of sandalwood. Phenotypic and anatomical evidence in this study indicated that seed germination stage 4 (SG4) seemed pivotal for lateral root (LR) morphogenesis. Small RNA (sRNA) high-throughput sequencing of root tissues at three sub-stages of SG4 (lateral root primordia initiation (LRPI), lateral root primordia development (LRPD), and lateral root primordia emergence (LRPE)) was performed to identify microRNAs (miRNAs) associated with LR development. A total of 135 miRNAs, including 70 differentially expressed miRNAs (DEMs), were screened. Ten DEMs were selected to investigate transcript abundance in different organs or developmental stages. Among 100 negative DEM-mRNA interaction pairs, four targets (Sa-miR166m_2, 408d, 858a, and novel_Sa-miR8) were selected for studying cleavage sites by 5' RLM-RACE validation. The expression mode of the four miRNA-mRNA pairs was investigated after indole-3-acetic acid (IAA) treatment. IAA enhanced the abundance of homeobox-leucine-zipper protein 32 (HOX32), laccase 12 (LAC12), myeloblastosis86 (MYB86), and pectin methylesterase inhibitor6 (PMEI6) target transcripts by reducing the expression of Sa-miR166m_2, 408d, 858a, and novel_Sa-miR8 in the first 10 min. A schematic model of miRNA-regulated LR development is proposed for this hemi-parasitic species. This novel genetic information for improving sandalwood root growth and development may allow for the cultivation of fast-growing and high-yielding plantations.

F-box receptor mediated control of substrate stability and subcellular location organizes cellular development of Aspergillus nidulans

Fungal growth and development are coordinated with specific secondary metabolism. This coordination requires 8 of 74 F-box proteins of the filamentous fungus Aspergillus nidulans. F-box proteins recognize primed substrates for ubiquitination by Skp1-Cul1-Fbx (SCF) E3 ubiquitin RING ligases and degradation by the 26S proteasome. 24 F-box proteins are found in the nuclear fraction as part of SCFs during vegetative growth. 43 F-box proteins interact with SCF proteins during growth, development or stress. 45 F-box proteins are associated with more than 700 proteins that have mainly regulatory roles. This corroborates that accurate surveillance of protein stability is prerequisite for organizing multicellular fungal development. Fbx23 combines subcellular location and protein stability control, illustrating the complexity of F-box mediated regulation during fungal development. Fbx23 interacts with epigenetic methyltransferase VipC which interacts with fungal NF-κB-like velvet domain regulator VeA that coordinates fungal development with secondary metabolism. Fbx23 prevents nuclear accumulation of methyltransferase VipC during early development. These results suggest that in addition to their role in protein degradation, F-box proteins also control subcellular accumulations of key regulatory proteins for fungal development.

Genetic Determinants of Fiber-Associated Traits in Flax Identified by Omics Data Integration

In this paper, we explore potential genetic factors in control of flax phenotypes associated with fiber by mining a collection of 306 flax accessions from the Federal Research Centre of the Bast Fiber Crops, Torzhok, Russia. In total, 11 traits were assessed in the course of 3 successive years. A genome-wide association study was performed for each phenotype independently using six different single-locus models implemented in the GAPIT3 R package. Moreover, we applied a multivariate linear mixed model implemented in the GEMMA package to account for trait correlations and potential pleiotropic effects of polymorphisms. The analyses revealed a number of genomic variants associated with different fiber traits, implying the complex and polygenic control. All stable variants demonstrate a statistically significant allelic effect across all 3 years of the experiment. We tested the validity of the predicted variants using gene expression data available for the flax fiber studies. The results shed new light on the processes and pathways associated with the complex fiber traits, while the pinpointed candidate genes may be further used for marker-assisted selection.

A small chromosomal inversion mediated by MITE transposons confers cleistogamy in Brassica napus

Cleistogamy, self-pollination within closed flowers, can help maintain seed purity, accelerate breeding speed, and aid in the development of ornamental flowers. However, the mechanism underlying petal closing/opening behavior remains elusive. Here, we found that a Brassica napus petal closing/opening behavior was inherited in a Mendelian manner. Fine mapping and positional cloning experiments revealed that the Mendelian factor originated from a short (29.8 kb) inversion mediated by BnDTH9 miniature inverted-repeat transposable elements (MITEs) on chromosome C03. This inversion led to tissue-specific gene promoter exchange between BnaC03.FBA (BnaC03G0156800ZS encoding an F-Box-associated domain-containing protein) and BnaC03.EFO1 (BnaC03G0157400ZS encoding an EARLY FLOWERING BY OVEREXPRESSION 1 protein) positioned near the respective inversion breakpoints. Our genetic transformation work demonstrated that the cleistogamy originated from high tissue-specific expression of the BnaC03.FBA gene caused by promoter changes due to the MITE-mediated inversion. BnaC03.FBA is involved in the formation of an SCF (Skp1-Cullin-F-box) complex, which participates in ubiquitin-mediated protein targeting for degradation through the ubiquitin 26S-proteasome system. Our results shed light on a molecular model of petal-closing behavior.

Constitutive expression of a pea apyrase, psNTP9, increases seed yield in field-grown soybean

To address the demand for food by a rapidly growing human population, agricultural scientists have carried out both plant breeding and genetic engineering research. Previously, we reported that the constitutive expression of a pea apyrase (Nucleoside triphosphate, diphosphohydrolase) gene, psNTP9, under the control of the CaMV35S promoter, resulted in soybean plants with an expanded root system architecture, enhanced drought resistance and increased seed yield when they are grown in greenhouses under controlled conditions. Here, we report that psNTP9-expressing soybean lines also show significantly enhanced seed yields when grown in multiple different field conditions at multiple field sites, including when the gene is introgressed into elite germplasm. The transgenic lines have higher leaf chlorophyll and soluble protein contents and decreased stomatal density and cuticle permeability, traits that increase water use efficiency and likely contribute to the increased seed yields of field-grown plants. These altered properties are explained, in part, by genome-wide gene expression changes induced by the transgene.

Construction of Chromosome Segment Substitution Lines and Inheritance of Seed-Pod Characteristics in Wild Soybean

Genetic populations provide the basis for genetic and genomic research, and chromosome segment substitution lines (CSSLs) are a powerful tool for the fine mapping of quantitative traits, new gene mining, and marker-assisted breeding. In this study, 213 CSSLs were obtained by self-crossing, backcrossing, and marker-assisted selection between cultivated soybean (Glycine max [L.] Merr.) variety Suinong14 (SN14) and wild soybean (Glycine soja Sieb. et Zucc.) ZYD00006. The genomes of these 213 CSSLs were resequenced and 580,524 single-nucleotide polymorphism markers were obtained, which were divided into 3,780 bin markers. The seed-pod-related traits were analyzed by quantitative trait locus (QTL) mapping using CSSLs. A total of 170 QTLs were detected, and 32 QTLs were detected stably for more than 2 years. Through epistasis analysis, 955 pairs of epistasis QTLs related to seed-pod traits were obtained. Furthermore, the hundred-seed weight QTL was finely mapped to the region of 64.4 Kb on chromosome 12, and Glyma.12G088900 was identified as a candidate gene. Taken together, a set of wild soybean CSSLs was constructed and upgraded by a resequencing technique. The seed-pod-related traits were studied by bin markers, and a candidate gene for the hundred-seed weight was finely mapped. Our results have revealed the CSSLs can be an effective tool for QTL mapping, epistatic effect analysis, and gene cloning.

Genome-wide identification and characterization of F-box family proteins in sweet potato and its expression analysis under abiotic stress

In this study, genome-wide characterization of F-box proteins in sweet potato yielded 243 IbFBX genes, unevenly distributed on the 15 chromosomes of sweet potato. Gene duplication analysis suggested segmental duplication as the principal factor influencing the expansive evolution of IbFBX genes in sweet potato. Phylogenetic analysis clustered F-box proteins in sweet potato, Arabidopsis, and rice into six clades (I-VI). Gene structure analysis of the IbFBX genes revealed that most of the genes within the same clade were highly conserved. Expression profiles of IbFBX family genes in 9 different tissues and under stress conditions revealed that the IbFBXs were highly upregulated or downregulated in response to salt and drought stress, suggesting their significant roles in abiotic stress response and adaptation. Knowledge of the diverse functions and expression patterns of IbFBXs presents a solid theoretical basis for annotating the functions of IbFBXs and further facilitate the molecular breeding of sweet potato.

Iduronate-2-sulfatase interactome: Validation by Yeast Two-Hybrid Assay

Mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome, is a rare X-linked recessive disease caused by a deficiency of the lysosomal enzyme iduronate-2-sulfatase (IDS), which activates intracellular accumulation of nonmetabolized glycosaminoglycans such as heparan sulfate and dermatan sulfate. This accumulation causes severe damage to several tissues, principally the central nervous system. Previously, we identified 187 IDS-protein interactions in the mouse brain. To validate a subset of these interactions, we selected and cloned the coding regions of 10 candidate genes to perform a targeted yeast two-hybrid assay. The results allowed the identification of the physical interaction of IDS with LSAMP and SYT1. Although the physiological relevance of these complexes is unknown, recent advances allow us to point out that these interactions could be involved in vesicular trafficking of IDS through the interaction with SYT1, as well as to the ability to form a transcytosis module between the cellular components of the blood-brain-barrier (BBB) through its interaction with LSAMP. These results may shed light on the role of IDS on cellular homeostasis and may also contribute to the understanding of MPS II physiopathology and the development of novel therapeutic strategies to transport recombinant IDS through the brain endothelial cells toward the brain parenchyma.

A common F-box gene regulates the leucine homeostasis of Medicago truncatula and Arabidopsis thaliana

The F-box domain is a conserved structural protein motif that most frequently interacts with the SKP1 protein, the core of the SCFs (SKP1-CULLIN-F-box protein ligase) E3 ubiquitin protein ligases. As part of the SCF complexes, the various F-box proteins recruit substrates for degradation through ubiquitination. In this study, we functionally characterized an F-box gene (MtF-box) identified earlier in a population of Tnt1 retrotransposon-tagged mutants of Medicago truncatula and its Arabidopsis thaliana homolog (AtF-box) using gain- and loss-of-function plants. We highlighted the importance of MtF-box in leaf development of M. truncatula. Protein-protein interaction analyses revealed the 2-isopropylmalate synthase (IPMS) protein as a common interactor partner of MtF-box and AtF-box, being a key enzyme in the biosynthesis pathway of the branched-chain amino acid leucine. For further detailed analysis, we focused on AtF-box and its role during the cell division cycle. Based on this work, we suggest a mechanism for the role of the studied F-box gene in regulation of leucine homeostasis, which is important for growth.

Interactome of Arabidopsis Thaliana

More than 95,000 protein–protein interactions of Arabidopsis thaliana have been published and deposited in databases. This dataset was supplemented by approximately 900 additional interactions, which were identified in the literature from the years 2002–2021. These protein–protein interactions were used as the basis for a Cytoscape network and were supplemented with data on subcellular localization, gene ontologies, biochemical properties and co-expression. The resulting network has been exemplarily applied in unraveling the PPI-network of the plant vacuolar proton-translocating ATPase (V-ATPase), which was selected due to its central importance for the plant cell. In particular, it is involved in cellular pH homeostasis, providing proton motive force necessary for transport processes, trafficking of proteins and, thereby, cell wall synthesis. The data points to regulation taking place on multiple levels: (a) a phosphorylation-dependent regulation by 14-3-3 proteins and by kinases such as WNK8 and NDPK1a, (b) an energy-dependent regulation via HXK1 and the glucose receptor RGS1 and (c) a Ca2+-dependent regulation by SOS2 and IDQ6. The known importance of V-ATPase for cell wall synthesis is supported by its interactions with several proteins involved in cell wall synthesis. The resulting network was further analyzed for (experimental) biases and was found to be enriched in nuclear, cytosolic and plasma membrane proteins but depleted in extracellular and mitochondrial proteins, in comparison to the entity of protein-coding genes. Among the processes and functions, proteins involved in transcription were highly abundant in the network. Subnetworks were extracted for organelles, processes and protein families. The degree of representation of organelles and processes reveals limitations and advantages in the current knowledge of protein–protein interactions, which have been mainly caused by a high number of database entries being contributed by only a few publications with highly specific motivations and methodologies that favor, for instance, interactions in the cytosol and the nucleus.

The reference genome and full-length transcriptome of pakchoi provide insights into cuticle formation and heat adaption

Brassica rapa includes various vegetables with high economic value. Among them, green petiole type pakchoi (B. rapa ssp. chinensis) is one of the major vegetables grown in southern China. Compared with other B. rapa varieties, green petiole type pakchoi shows a higher level of heat resistance, which is partially derived from the rich epicuticular wax. Here we sequence a high-quality genome of green petiole type pakchoi, which has been widely used as the parent in breeding. Our results reveal that long terminal repeat retrotransposon insertion plays critical roles in promoting the genome expansion and transcriptional diversity of pakchoi genes through preferential insertions, particularly in cuticle biosynthetic genes. After whole-genome triplication, over-retained pakchoi genes escape stringent selection pressure, and among them a set of cuticle-related genes are retained. Using bulked-segregant analysis of a heat-resistant pakchoi cultivar, we identify a frame-shift deletion across the third exon and the subsequent intron of BrcCER1 in candidate regions. Using Nanopore long-read sequencing, we analyze the full-length transcriptome of two pakchoi cultivars with opposite sensitivity to high temperature. We find that the heat-resistant pakchoi cultivar can mitigate heat-caused leaf damage by activating an unfolded protein response, as well as by inhibiting chloroplast development and energy metabolism, which are presumably mediated by both transcriptional regulation and splicing factors. Our study provides valuable resources for Brassica functional genomics and breeding research, and deepens our understanding of plant stress resistance.

Unique N-Terminal Interactions Connect F-BOX STRESS INDUCED (FBS) Proteins to a WD40 Repeat-like Protein Pathway in Arabidopsis

SCF-type E3 ubiquitin ligases provide specificity to numerous selective protein degradation events in plants, including those that enable survival under environmental stress. SCF complexes use F-box (FBX) proteins as interchangeable substrate adaptors to recruit protein targets for ubiquitylation. FBX proteins almost universally have structure with two domains: A conserved N-terminal F-box domain interacts with a SKP protein and connects the FBX protein to the core SCF complex, while a C-terminal domain interacts with the protein target and facilitates recruitment. The F-BOX STRESS INDUCED (FBS) subfamily of plant FBX proteins has an atypical structure, however, with a centrally located F-box domain and additional conserved regions at both the N- and C-termini. FBS proteins have been linked to environmental stress networks, but no ubiquitylation target(s) or biological function has been established for this subfamily. We have identified two WD40 repeat-like proteins in Arabidopsis that are highly conserved in plants and interact with FBS proteins, which we have named FBS INTERACTING PROTEINs (FBIPs). FBIPs interact exclusively with the N-terminus of FBS proteins, and this interaction occurs in the nucleus. FBS1 destabilizes FBIP1, consistent with FBIPs being ubiquitylation targets SCFFBS1 complexes. This work indicates that FBS proteins may function in stress-responsive nuclear events, and it identifies two WD40 repeat-like proteins as new tools with which to probe how an atypical SCF complex, SCFFBS, functions via FBX protein N-terminal interaction events.

SDC mediates DNA methylation-controlled clock pace by interacting with ZTL in Arabidopsis

Molecular bases of eukaryotic circadian clocks mainly rely on transcriptional-translational feedback loops (TTFLs), while epigenetic codes also play critical roles in fine-tuning circadian rhythms. However, unlike histone modification codes that play extensive and well-known roles in the regulation of circadian clocks, whether DNA methylation (5mC) can affect the circadian clock, and the associated underlying molecular mechanisms, remains largely unexplored in many organisms. Here we demonstrate that global genome DNA hypomethylation can significantly lengthen the circadian period of Arabidopsis. Transcriptomic and genetic evidence demonstrate that SUPPRESSOR OF drm1 drm2 cmt3 (SDC), encoding an F-box containing protein, is required for the DNA hypomethylation-tuned circadian clock. Moreover, SDC can physically interact with another F-box containing protein ZEITLUPE (ZTL) to diminish its accumulation. Genetic analysis further revealed that ZTL and its substrate TIMING OF CAB EXPRESSION 1 (TOC1) likely act downstream of DNA methyltransferases to control circadian rhythm. Together, our findings support the notion that DNA methylation is important to maintain proper circadian pace in Arabidopsis, and further established that SDC links DNA hypomethylation with a proteolytic cascade to assist in tuning the circadian clock.

Physiological, biochemical, and transcriptional regulation in a leguminous forage Trifolium pratense L. responding to silver ions

Trifolium pratense L. (red clover) is an important leguminous crop with great potential for Ag-contaminated environment remediation. Whereas, the molecular mechanisms of Ag tolerance in red clover are largely unknown. Red clover seedlings were used for physiological and transcriptomic investigation under 0, 20, 50, and 100 mg/L Ag⁺ stress in our research to reveal potential molecular resistance mechanism. Research showed that red clover possessed fairly strong Ag absorbance capacity, the Ag level reached 0.14 and 2.35 mg/g·FW in the leaves and roots under 100 mg/L AgNO₃ stress condition. Root fresh weight, root dry weight, root water content, and photosynthetic pigments contents were significantly decreased with elevating AgNO₃ concentration. Obvious withered plant tissue, microstructure disorder, and disrupted organelles were observed. In vitro evaluations (e.g., PI and DCFH-DA staining) represented that AgNO₃ at high concentration (100 mg/L) exhibited obvious inhibition on cell viability, which was due possibly to the induction of reactive oxygen species (ROS) accumulation. A total of 44643 differentially expressed genes (DEGs) were identified under Ag stress, covering 27155 upregulated and 17488 downregulated genes. 12 stress-responsive DEGs was authenticated utilizing real-time quantitative PCR (qRT-PCR). Gene ontology (GO) analysis revealed that the DEGs were mostly related to metal ion binding (molecular function), nucleus (cellular component), and defense response (biological process). Involved DEGs in sequence-specific DNA binding transcription factor activity, response to various hormones (e.g., abscisic acid, IAA/Auxin, salicylic acid, and etc), calcium signal transduction, and protein ubiquitination were concluded to play crucial roles in Ag tolerance of red clover. On the other hand, Kyoto Encyclopedia of Genes and Genomes (KEGG) database annotated several stress responsive pathways such as plant-pathogen interaction, phenylpropanoid biosynthesis, ubiquitin mediated proteolysis, hormone signal transduction, and autophagy. Several down-regulated genes (e.g., RSF2, RCD1, DOX1, and etc) were identified indicating possible metabolic disturbance. Besides, protein-protein interaction network (PPI) identified several pivotal genes such as ribosomal proteins, TIR, and ZAT.

AtMIF1 increases seed oil content by attenuating GL2 inhibition

Vegetable oil is a major edible oil and an important industrial raw material. However, breeders have found it challenging to improve the oil content of crop seeds, and little is known about regulators with the potential to increase oil content via molecular engineering in modern oil crop breeding. We reported an F-box protein, Arabidopsis thaliana MYB Interaction Factor 1 (AtMIF1), which is a member of the ubiquitin-protein ligase E3 complex involved in the 26S proteasome protein degradation pathway. AtMIF1 physically interacts with MYB domain protein 5 (MYB5), which results in MYB5 degradation, so that transcriptional activation of the MYB/bHLH/WD-repeat (MBW) complex does not occur normally and GLABRA2 (GL2), encoding an inhibitor of oil content and functioning as a direct downstream gene of MBW, is not properly transcribed. AtMIF1 functioned as a positive regulator that increases oil content by attenuating GL2 inhibition. We overexpressed AtMIF1 and obtained transgenic plants with significantly higher seed oil contents. Importantly, both vegetative and reproductive growth of the transgenic plants appeared normal. In summary, this work reveals a novel regulator, AtMIF1, and a new regulatory pathway, 26S proteasome-AtMIF1-MYB5, for increasing the oil content of seeds without affecting plant growth, thus facilitating oil crop breeding.

Generation of a fertile ask1 mutant uncovers a comprehensive set of SCF-mediated intracellular functions

Many eukaryotic intracellular processes employ protein ubiquitylation by ubiquitin E3 ligases for functional regulation or protein quality control. In plants, the multi-subunit Skp1-Cullin1-F-box (SCF) complexes compose the largest group of E3 ligases whose specificity is determined by a diverse array of F-box proteins. Although both sequence divergence and polymorphism of F-box genes well support a broad spectrum of SCF functions, experimental evidence is scarce due to the low number of identified SCF substrates. Taking advantage of the bridge role of Skp1 between F-box and Cullin1 in the complex, we systematically analyzed the functional influence of a well-characterized Arabidopsis Skp1-Like1 (ASK1) Ds insertion allele, ask1, in different Arabidopsis accessions. Through 10 generations of backcrossing with Columbia-0 (Col-0), we partially rescued the fertility of this otherwise sterile ask1 allele in Landsberg erecta, thus providing experimental evidence showing the polymorphic roles of SCF complexes. This ask1 mutant produces twisted rosette leaves, a reduced number of petals, fewer viable pollen grains, and larger embryos and seeds compared to Col-0. RNA-Seq-based transcriptome analysis of ask1 uncovered a large spectrum of SCF functions, which is greater than a 10-fold increase compared with previous studies. We also identified its hyposensitive responses to auxin and abscisic acid treatments and enhanced far-red light/phyA-mediated photomorphogenesis. Such diverse roles are consistent with the 20-30% reduction of ubiquitylation events in ask1 estimated by immunoblotting analysis in this work. Collectively, we conclude that ASK1 is a predominant Skp1 protein in Arabidopsis and that the fertile ask1 mutant allowed us to uncover a comprehensive set of SCF functions.

Regulation of micoRNA2111 and its target IbFBK in sweet potato on wounding

Plant microRNAs (miRNAs) are non-coding RNAs, which are composed of 20-24 nucleotides. MiRNAs play important roles in plant growth and responses to biotic and abiotic stresses. Wounding is one of the most serious stresses for plants; however, the regulation of miRNAs in plants upon wounding is not well studied. In this study, miR2111, a wound-repressed miRNA, identified previously in sweet potato (Ipomoea batatas cv Tainung 57) by small RNA deep sequencing was chosen for further analysis. Based on sweet potato transcriptome database, F-box/kelch repeat protein (IbFBK), a target gene of miR2111, was identified. IbFBK is a wound-inducible gene, and the miR2111-induced cleavage site in IbFBK mRNA is between the 10th and 11th nucleotides of miR2111. IbFBK is a component of the E3 ligase SCF (SKP1-Cullin-F-box) complex participating in protein ubiquitination and degradation. The results of yeast two-hybrid and bimolecular fluorescence complementation assays demonstrate that IbFBK was conjugated with IbSKP1 through the F-box domain in IbFBK N-terminus to form SCF complex, and interacted with IbCNR8 through the kelch-repeat domain in IbFBK C-terminus. The interaction of IbFBK and IbCNR8 may lead to the ubiquitination and degradation of IbCNR8. In conclusion, the suppression of miR2111 resulted in the increase of IbFBK, and may regulate protein degradation of IbCNR8 in sweet potato responding to wounding.

Diverse and dynamic roles of F-box proteins in plant biology

The SCF complex is a widely studied multi-subunit ring E3 ubiquitin ligase that tags targeted proteins with ubiquitin for protein degradation by the ubiquitin 26S-proteasome system (UPS). The UPS is an important system that generally keeps cellular events tightly regulated by purging misfolded or damaged proteins and selectively degrading important regulatory proteins. The specificity of this post-translational regulation is controlled by F-box proteins (FBPs) via selective recognition of a protein-protein interaction motif at the C-terminal domain. Hence, FBPs are pivotal proteins in determining the plant response in multiple scenarios. It is not surprising that the FBP family is one of the largest protein families in the plant kingdom. In this review, the roles of FBPs, specifically in plants, are compiled to provide insights into their involvement in secondary metabolites, plant stresses, phytohormone signalling, plant developmental processes and miRNA biogenesis.

Overexpression of rice F-box protein OsFBX322 confers increased sensitivity to gamma irradiation in Arabidopsis

Ionizing radiation has a substantial effect on physiological and biochemical processes in plants via induction of transcriptional changes and diverse genetic alterations. Previous microarray analysis showed that rice OsFBX322, which encodes a rice F-box protein, was downregulated in response to three types of ionizing radiation: gamma irradiation, ion beams, and cosmic rays. In order to characterize the radiation-responsive genes in rice, OsFBX322 was selected for further analysis. OsFBX322 expression patterns in response to radiation were confirmed using quantitative RT-PCR. Transient expression of a GFP-OsFBX322 fusion protein in tobacco leaf epidermis indicated that OsFBX322 is localized to the nucleus. To determine the effect of OsFBX322 expression on radiation response, OsFBX322 was overexpressed in Arabidopsis. Transgenic overexpression lines were more sensitive to gamma irradiation than control plants. These results suggest that OsFBX322 plays a negative role in the defense response to radiation in plants. In addition, we obtained four co-expression genes of OsFBX322 by specific co-expression networks using the ARANCE. Quantitative RT-PCR showed that the four genes were also downregulated after exposure to the three types of radiation. These results imply that the co-expressed genes may serve as key regulators in the radiation response pathway in plants.

Jasmonate and auxin perception: how plants keep F-boxes in check

Phytohormones regulate the plasticity of plant growth and development, and responses to biotic and abiotic stresses. Many hormone signal transduction cascades involve ubiquitination and subsequent degradation of proteins by the 26S proteasome. The conjugation of ubiquitin to a substrate is facilitated by the E1 activating, E2 conjugating, and the substrate-specifying E3 ligating enzymes. The most prevalent type of E3 ligase in plants is the Cullin-RING ligase (CRL)-type, with F-box proteins (FBPs) as the substrate recognition component. The activity of these SKP-Cullin-F-box (SCF) complexes needs to be tightly regulated in time and place. Here, we review the regulation of SCF function in plants on multiple levels, with a focus on the auxin and jasmonate SCF-type receptor complexes. We discuss in particular the relevance of protein-protein interactions and post-translational modifications as mechanisms to keep SCF functioning under control. Additionally, we highlight the unique property of SCFTIR1/AFB and SCFCOI1 to recognize substrates by forming co-receptor complexes. Finally, we explore how engineered selective agonists can be used to study and uncouple the outcomes of the complex auxin and jasmonate signaling networks that are governed by these FBPs.

Neofunctionalization of Mitochondrial Proteins and Incorporation into Signaling Networks in Plants

Because of their symbiotic origin, many mitochondrial proteins are well conserved across eukaryotic kingdoms. It is however less obvious how specific lineages have obtained novel nuclear-encoded mitochondrial proteins. Here, we report a case of mitochondrial neofunctionalization in plants. Phylogenetic analysis of genes containing the Domain of Unknown Function 295 (DUF295) revealed that the domain likely originated in Angiosperms. The C-terminal DUF295 domain is usually accompanied by an N-terminal F-box domain, involved in ubiquitin ligation via binding with ASK1/SKP1-type proteins. Due to gene duplication, the gene family has expanded rapidly, with 94 DUF295-related genes in Arabidopsis thaliana alone. Two DUF295 family subgroups have uniquely evolved and quickly expanded within Brassicaceae. One of these subgroups has completely lost the F-box, but instead obtained strongly predicted mitochondrial targeting peptides. We show that several representatives of this DUF295 Organellar group are effectively targeted to plant mitochondria and chloroplasts. Furthermore, many DUF295 Organellar genes are induced by mitochondrial dysfunction, whereas F-Box DUF295 genes are not. In agreement, several Brassicaceae-specific DUF295 Organellar genes were incorporated in the evolutionary much older ANAC017-dependent mitochondrial retrograde signaling pathway. Finally, a representative set of DUF295 T-DNA insertion mutants was created. No obvious aberrant phenotypes during normal growth and mitochondrial dysfunction were observed, most likely due to the large extent of gene duplication and redundancy. Overall, this study provides insight into how novel mitochondrial proteins can be created via “intercompartmental” gene duplication events. Moreover, our analysis shows that these newly evolved genes can then be specifically integrated into relevant, pre-existing coexpression networks.

Adaptive and degenerative evolution of the S-Phase Kinase-Associated Protein 1-Like family in Arabidopsis thaliana

Genome sequencing has uncovered tremendous sequence variation within and between species. In plants, in addition to large variations in genome size, a great deal of sequence polymorphism is also evident in several large multi-gene families, including those involved in the ubiquitin-26S proteasome protein degradation system. However, the biological function of this sequence variation is yet not clear. In this work, we explicitly demonstrated a single origin of retroposed Arabidopsis Skp1-Like (ASK) genes using an improved phylogenetic analysis. Taking advantage of the 1,001 genomes project, we here provide several lines of polymorphism evidence showing both adaptive and degenerative evolutionary processes in ASK genes. Yeast two-hybrid quantitative interaction assays further suggested that recent neutral changes in the ASK2 coding sequence weakened its interactions with some F-box proteins. The trend that highly polymorphic upstream regions of ASK1 yield high levels of expression implied negative expression regulation of ASK1 by an as-yet-unknown transcriptional suppression mechanism, which may contribute to the polymorphic roles of Skp1-CUL1-F-box complexes. Taken together, this study provides new evolutionary evidence to guide future functional genomic studies of SCF-mediated protein ubiquitylation.

Sugar-Recognizing Ubiquitin Ligases: Action Mechanisms and Physiology

F-box proteins, the substrate recognition subunits of SKP1–CUL1–F-box protein (SCF) E3 ubiquitin ligase complexes, play crucial roles in various cellular events mediated by ubiquitination. Several sugar-recognizing F-box proteins exist in both mammalian and plant cells. Although glycoproteins generally reside outside of cells, or in organelles of the secretory pathway, these lectin-type F-box proteins reside in the nucleocytoplasmic compartment. Mammalian sugar-recognizing F-box proteins commonly bind to the innermost position of N-glycans through a unique small hydrophobic pocket in their loops. Two cytosolic F-box proteins, Fbs1 and Fbs2, recognize high-mannose glycans synthesized in the ER, and SCF^Fbs1 and SCF^Fbs2 ubiquitinate excess unassembled or misfolded glycoproteins in the ERAD pathway by recognizing the innermost glycans, which serve as signals for aberrant proteins. On the other hand, endomembrane-bound Fbs3 recognizes complex glycans as well as high-mannose glycans, and SCF^Fbs3 ubiquitinates exposed glycoproteins in damaged lysosomes fated for elimination by selective autophagy. Plants express stress-inducible lectin-type F-box proteins recognizing a wider range of N- and O-glycans, suggesting that the roles of mammalian and plant lectin-type F-box proteins have diverged over the course of evolution to recognize species-specific targets with distinct functions. These sugar-recognizing F-box proteins interpret glycans in the cytosol as markers of unwanted proteins and organelles, and degrade them via the proteasome or autophagy.

An update to database TraVA: organ-specific cold stress response in Arabidopsis thaliana

BACKGROUND

Transcriptome map is a powerful tool for a variety of biological studies; transcriptome maps that include different organs, tissues, cells and stages of development are currently available for at least 30 plants. Some of them include samples treated by environmental or biotic stresses. However, most studies explore only limited set of organs and developmental stages (leaves or seedlings). In order to provide broader view of organ-specific strategies of cold stress response we studied expression changes that follow exposure to cold (+ 4 °C) in different aerial parts of plant: cotyledons, hypocotyl, leaves, young flowers, mature flowers and seeds using RNA-seq.

RESULTS

The results on differential expression in leaves are congruent with current knowledge on stress response pathways, in particular, the role of CBF genes. In other organs, both essence and dynamics of gene expression changes are different. We show the involvement of genes that are confined to narrow expression patterns in non-stress conditions into stress response. In particular, the genes that control cell wall modification in pollen, are activated in leaves. In seeds, predominant pattern is the change of lipid metabolism.

CONCLUSIONS

Stress response is highly organ-specific; different pathways are involved in this process in each type of organs. The results were integrated with previously published transcriptome map of Arabidopsis thaliana and used for an update of a public database TraVa: http://travadb.org/browse/Species=AthStress .

Arabidopsis SKP1-like protein13 (ASK13) positively regulates seed germination and seedling growth under abiotic stress

SKP1 (S-phase kinase-associated protein1) proteins are key members of the SCF (SKP-cullin-F-box protein) E3 ligase complexes that ubiquitinate target proteins and play diverse roles in plant biology. However, in comparison with other members of the SCF complex, knowledge of SKP1-like proteins is very limited in plants. In the present work, we report that Arabidopsis SKP1-like protein13 (ASK13) is differentially regulated in different organs during seed development and germination and is up-regulated in response to abiotic stress. Yeast two-hybrid library screening and subsequent assessment of in vivo interactions through bimolecular fluorescence complementation analysis revealed that ASK13 not only interacts with F-box proteins but also with other proteins that are not components of SCF complexes. Biochemical analysis demonstrated that ASK13 not only exists as a monomer but also as a homo-oligomer or heteromer with other ASK proteins. Functional analysis using ASK13 overexpression and knockdown lines showed that ASK13 positively influences seed germination and seedling growth, particularly under abiotic stress. Taken together, our data strongly suggest that apart from participation to form SCF complexes, ASK13 interacts with several other proteins and is implicated in different cellular processes distinct from protein degradation.

Regulation of SCFTIR1/AFBs E3 ligase assembly by S-nitrosylation of Arabidopsis SKP1-like1 impacts on auxin signaling

The F-box proteins (FBPs) TIR1/AFBs are the substrate recognition subunits of SKP1–cullin–F-box (SCF) ubiquitin ligase complexes and together with Aux/IAAs form the auxin co-receptor. Although tremendous knowledge on auxin perception and signaling has been gained in the last years, SCF^TIR1/AFBs complex assembly and stabilization are emerging as new layers of regulation. Here, we investigated how nitric oxide (NO), through S-nitrosylation of ASK1 is involved in SCF^TIR1/AFBs assembly. We demonstrate that ASK1 is S-nitrosylated and S-glutathionylated in cysteine (Cys) 37 and Cys118 residues in vitro. Both, in vitro and in vivo protein-protein interaction assays show that NO enhances ASK1 binding to CUL1 and TIR1/AFB2, required for SCF^TIR1/AFB2 assembly. In addition, we demonstrate that Cys37 and Cys118 are essential residues for proper activation of auxin signaling pathway in planta. Phylogenetic analysis revealed that Cys37 residue is only conserved in SKP proteins in Angiosperms, suggesting that S-nitrosylation on Cys37 could represent an evolutionary adaption for SKP1 function in flowering plants. Collectively, these findings indicate that multiple events of redox modifications might be part of a fine-tuning regulation of SCF^TIR1/AFBs for proper auxin signal transduction.

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ASK1 adaptor protein of the SCF^TIR1/AFB E3 ligase complex is redox regulated.

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NO regulates ASK1 function by S-nitrosylation in Cys37 and Cys118 residues.

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NO enhances ASK1-CUL1 and ASK1-TIR1/AFB2 protein-protein interactions required for SCF^TIR1/AFB2 assembly in vitro and in vivo.

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S-nitrosylated residues in ASK1 are essential for activation of auxin signaling pathway in plants.

GmSK1, an SKP1 homologue in soybean, is involved in the tolerance to salt and drought

In plants, various proteins are regulated by the ubiquitin-mediated system in response to different environmental stresses, such as drought, cold and heat. The Skp1-Cullin-F-box (SCF) complex, one of the multisubunit E3 ligases, has been shown to be involved in abiotic response pathways. In this study, Glycine max SKP1-like 1 (GmSK1), which had the typical characteristics of an SKP1 protein, with an alpha/beta structure, targeted to the cytoplasm and nucleus, was isolated from soybean [Glycine max (L.)]. GmSK1 was constitutively expressed in all the tested tissues, especially in the roots. Furthermore, the expression of GmSK1 was simultaneously induced by abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), NaCl, low temperatures and drought, which suggests important roles for GmSK1 in plant responses to hormone treatments and abiotic stress. GmSK1-overexpressing transgenic tobacco (Nicotiana tobacum cv. Samsun) plants showed enhanced tolerance to high salinity and drought stress; exhibited significantly reduced inhibition of growth, greenness and water loss; and exhibited increased MDA accumulation compared with wild-type controls. Our results suggest that GmSK1 might play a role in the crosstalk between ubiquitination and abiotic stress responses in plants.

Efficient ASK-assisted system for expression and purification of plant F-box proteins

Ubiquitin-mediated protein degradation plays an essential role in plant growth and development as well as responses to environmental and endogenous signals. F-box protein is one of the key components of the SCF (SKP1-CUL1-F-box protein) E3 ubiquitin ligase complex, which recruit specific substrate proteins for subsequent ubiquitination and 26S proteasome-mediated degradation to regulate developmental processes and signaling networks. However, it is not easy to obtain purified F-box proteins with high activity due to their unstable protein structures. Here, we found that Arabidopsis SKP-like proteins (ASKs) can significantly improve soluble expression of F-box proteins and maintain their bioactivity. We established an efficient ASK-assisted method to express and purify plant F-box proteins. The method meets a broad range of criteria required for the biochemical analysis or protein crystallization of plant F-box proteins.

A photo-responsive F-box protein FOF2 regulates floral initiation by promoting FLC expression in Arabidopsis

Floral initiation is regulated by various genetic pathways in response to light, temperature, hormones and developmental status; however, the molecular mechanisms underlying the interactions between different genetic pathways are not fully understood. Here, we show that the photoresponsive gene FOF2 (F-box of flowering 2) negatively regulates flowering. FOF2 encodes a putative F-box protein that interacts specifically with ASK14, and its overexpression results in later flowering under both long-day and short-day photoperiods. Conversely, transgenic plants expressing the F-box domain deletion mutant of FOF2 (FOF2ΔF), or double loss of function mutant of FOF2 and FOL1 (FOF2-LIKE 1) present early flowering phenotypes. The late flowering phenotype of the FOF2 overexpression lines is suppressed by the flc-3 loss-of-function mutation. Furthermore, FOF2 mRNA expression is regulated by autonomous pathway gene FCA, and the repressive effect of FOF2 in flowering can be overcome by vernalization. Interestingly, FOF2 expression is regulated by light. The protein level of FOF2 accumulates in response to light, whereas it is degraded under dark conditions via the 26S proteasome pathway. Our findings suggest a possible mechanistic link between light conditions and the autonomous floral promotion pathway in Arabidopsis.

Transcriptome analysis uncovers Arabidopsis F-BOX STRESS INDUCED 1 as a regulator of jasmonic acid and abscisic acid stress gene expression

Background

The ubiquitin 26S proteasome system (UPS) selectively degrades cellular proteins, which results in physiological changes to eukaryotic cells. F-box proteins are substrate adaptors within the UPS and are responsible for the diversity of potential protein targets. Plant genomes are enriched in F-box genes, but the vast majority of these have unknown roles. This work investigated the Arabidopsis F-box gene F-BOX STRESS INDUCED 1 (FBS1) for its effects on gene expression in order elucidate its previously unknown biological function.

Results

Using publically available Affymetrix ATH1 microarray data, we show that FBS1 is significantly co-expressed in abiotic stresses with other well-characterized stress response genes, including important stress-related transcriptional regulators. This gene suite is most highly expressed in roots under cold and salt stresses. Transcriptome analysis of fbs1–1 knock-out plants grown at a chilling temperature shows that hundreds of genes require FBS1 for appropriate expression, and that these genes are enriched in those having roles in both abiotic and biotic stress responses. Based on both this genome-wide expression data set and quantitative real-time PCR (qPCR) analysis, it is apparent that FBS1 is required for elevated expression of many jasmonic acid (JA) genes that have established roles in combatting environmental stresses, and that it also controls a subset of JA biosynthesis genes. FBS1 also significantly impacts abscisic acid (ABA) regulated genes, but this interaction is more complex, as FBS1 has both positive and negative effects on ABA-inducible and ABA-repressible gene modules. One noteworthy effect of FBS1 on ABA-related stress processes, however, is the restraint it imposes on the expression of multiple class I LIPID TRANSFER PROTEIN (LTP) gene family members that have demonstrated protective effects in water deficit-related stresses.

Conclusion

FBS1 impacts plant stress responses by regulating hundreds of genes that respond to the plant stress hormones JA and ABA. The positive effect that FBS1 has on JA processes and the negative effect it has on at least some ABA processes indicates that it in part regulates cellular responses balanced between these two important stress hormones. More broadly then, FBS1 may aid plant cells in switching between certain biotic (JA) and abiotic (ABA) stress responses. Finally, because FBS1 regulates a subset of JA biosynthesis and response genes, we conclude that it might have a role in tuning hormone responses to particular circumstances at the transcriptional level.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3864-6) contains supplementary material, which is available to authorized users.

Genome-Wide Screening for Lectin Motifs in Arabidopsis thaliana

For more than three decades, served as a model for plant biology research. At present only a few protein families have been studied in detail in . This study focused on all sequences with lectin motifs in the genome of . Based on amino acid sequence similarity (BLASTp searches), 217 putative lectin genes were retrieved belonging to 9 out of 12 different lectin families. The domain organization and genomic distribution for each lectin family were analyzed. Domain architecture analysis revealed that most of these lectin gene sequences are linked to other domains, often belonging to protein families with catalytic activity. Many protein domains identified are known to play a role in stress signaling and defense, suggesting a major contribution of the putative lectins in development and plant defense. This genome-wide screen for different lectin motifs will help to unravel the functional characteristics of lectins. In addition, phylogenetic trees and WebLogos were created and showed that most lectin sequences that share the same domain architecture evolved together. Furthermore, the amino acids responsible for carbohydrate binding are largely conserved. Our results provide information about the evolutionary relationships and functional divergence of the lectin motifs in .

Comparative Study of Lectin Domains in Model Species: New Insights into Evolutionary Dynamics

Lectins are present throughout the plant kingdom and are reported to be involved in diverse biological processes. In this study, we provide a comparative analysis of the lectin families from model species in a phylogenetic framework. The analysis focuses on the different plant lectin domains identified in five representative core angiosperm genomes (Arabidopsisthaliana, Glycine max, Cucumis sativus, Oryza sativa ssp. japonica and Oryza sativa ssp. indica). The genomes were screened for genes encoding lectin domains using a combination of Basic Local Alignment Search Tool (BLAST), hidden Markov models, and InterProScan analysis. Additionally, phylogenetic relationships were investigated by constructing maximum likelihood phylogenetic trees. The results demonstrate that the majority of the lectin families are present in each of the species under study. Domain organization analysis showed that most identified proteins are multi-domain proteins, owing to the modular rearrangement of protein domains during evolution. Most of these multi-domain proteins are widespread, while others display a lineage-specific distribution. Furthermore, the phylogenetic analyses reveal that some lectin families evolved to be similar to the phylogeny of the plant species, while others share a closer evolutionary history based on the corresponding protein domain architecture. Our results yield insights into the evolutionary relationships and functional divergence of plant lectins.

Analysis of CFB, a cytokinin-responsive gene of Arabidopsis thaliana encoding a novel F-box protein regulating sterol biosynthesis

Protein degradation by the ubiquitin-26S proteasome pathway is important for the regulation of cellular processes, but the function of most F-box proteins relevant to substrate recognition is unknown. We describe the analysis of the gene Cytokinin-induced F-box encoding (CFB, AT3G44326), identified in a meta-analysis of cytokinin-related transcriptome studies as one of the most robust cytokinin response genes. F-box domain-dependent interaction with the E3 ubiquitin ligase complex component ASK1 classifies CFB as a functional F-box protein. Apart from F-box and transmembrane domains, CFB contains no known functional domains. CFB is expressed in all plant tissues, predominantly in root tissue. A ProCFB:GFP-GUS fusion gene showed strongest expression in the lateral root cap and during lateral root formation. CFB-GFP fusion proteins were mainly localized in the nucleus and the cytosol but also at the plasma membrane. cfb mutants had no discernible phenotype, but CFB overexpressing plants showed several defects, such as a white upper inflorescence stem, similar to the hypomorphic cycloartenol synthase mutant cas1-1. Both CFB overexpressing plants and cas1-1 mutants accumulated the CAS1 substrate 2,3-oxidosqualene in the white stem tissue, the latter even more after cytokinin treatment, indicating impairment of CAS1 function. This suggests that CFB may link cytokinin and the sterol biosynthesis pathway.