Identification and analysis of a differentially expressed wheat RING-type E3 ligase in spike primordia development during post-vernalization

The E3 ligase TaE3V-B1 ubiquitinates proteins encoded by the vernalization gene TaVRN1 and regulates developmental processes in wheat

In wheat (Triticum aestivum), early maturity is desired to avoid the hot and dry summer season, especially in view of climate change. Here, we report that TaE3V1, a C3H2C3 RING-type E3 ligase that interacts with TaVRN1, is associated with early development. Aside from its RING domain, TaE3V1 does not harbor any domains that are conserved in other RING-type or other E3 ligase proteins. TaE3V-B1b, encoded by the functional TaE3V1 allele, interacts with and ubiquitinates TaVRN1. In contrast, TaE3V-B1a, encoded by a natural nonfunctional TaE3V1 allele, neither interacts with TaVRN1 nor has E3 ligase activity. TaE3V-B1b activity decreases with plant age under warmer temperatures, but not under the low temperatures required for vernalization. We employed a gene editing method to simultaneously inactivate the 3 homoeologous TaE3V1 genes to validate their functions. Overall, our results suggest that the naturally mutated and edited TaE3V1 alleles can accelerate wheat development and aid adaptation to warming climates.

RING/U-box E3 protein BIR1 interacts with and ubiquitinates barley growth repressor BROAD LEAF1

Establishment of final leaf size in plants relies on the precise regulation of 2 interconnected processes, cell division and cell expansion. The barley (Hordeum vulgare) protein BROAD LEAF1 (BLF1) limits cell proliferation and leaf growth in the width direction. However, how the levels of this potent repressor of leaf growth are controlled remains unclear. Here, we used a yeast 2-hybrid screen to identify the BLF1-INTERACTING RING/U-BOX 1 (BIR1) E3 ubiquitin ligase that interacts with BLF1 and confirmed the interaction of the 2 proteins in planta. Inhibiting the proteasome caused overaccumulation of a BLF1-eGFP fusion protein when co-expressed with BIR1, and an in vivo ubiquitination assay in bacteria confirmed that BIR1 can mediate ubiquitination of BLF1 protein. Consistent with regulation of endogenous BLF1 in barley by proteasomal degradation, inhibition of the proteasome in BLF1-vYFP-expressing barley plants caused an accumulation of the BLF1 protein. The BIR1 protein co-localized with BLF1 in nuclei and appeared to reduce BLF1 protein levels. Analysis of bir1-1 knockout mutants suggested the involvement of BIR1 in leaf growth control, although mainly on leaf length. Together, our results suggest that proteasomal degradation, in part mediated by BIR1, helps fine-tune BLF1 protein levels in barley.

Overexpression of a TaATL1 encoding RING-type E3 ligase negatively regulates cell division and flowering time in Arabidopsis thaliana

The transition of food crops from the vegetative to reproductive stages is an important process that affects the final yield. Despite extensive characterization of E3 ligases in model plants, their roles in wheat development remain unknown. In this study, we elucidated the molecular function of wheat TaATL1 (Arabidopsis thaliana Toxicos EN Levadura), which acts as a negative regulator of flowering time and cell division. TaATL1 amino acid residues contain a RING domain and exist mainly in a beta-turn form. The expression level of TaATL1 was highly reduced during the transition from vegetative to reproductive stages. TaATL1 is localized in the nucleus and exhibits E3 ligase activity. Transgenic Arabidopsis plants, in which the TaATL1 gene is constitutively overexpressed under the control of the cauliflower mosaic virus 35 S promoter, exhibited regulation of cell numbers, thereby influencing both leaf and root growth. Moreover, TaATL1 overexpression plants showed a late-flowering phenotype compared to wild-type (WT) plants. Following transcriptome analysis, it was discovered that 1661 and 901 differentially expressed genes were down- or up- regulated, respectively, in seedling stages between WT and TaATL1 overexpression. TaATL1 transcripts are involved in cell division, flowering, and signaling. Overall, our findings demonstrated that the regulatory mechanism of wheat TaATL1 gene plays a significant role in cell division-mediated flowering in Arabidopsis.

The wheat TaF-box3, SCF ubiquitin ligase component, participates in the regulation of flowering time in transgenic Arabidopsis

Histone methylation is actively involved in plant flowering time and is regulated by a myriad of genetic pathways that integrate endogenous and exogenous signals. We identified an F-box gene from wheat (Triticum aestivum L.) and named it TaF-box3. Transcript expression analysis showed that TaF-box3 expression was gradually induced during the floret development and anthesis stages (WS2.5-10). Furthermore, ubiquitination assays have shown that TaF-box3 is a key component of the SCF ubiquitin ligase complex. TaF-box3 overexpression in Arabidopsis resulted in an early flowering phenotype and different cell sizes in leaves compared to the WT. Furthermore, the transcript level of a flowering time-related gene was significantly reduced in TaF-box3 overexpressing plants, which was linked with lower histone H3 Lys4 trimethylation (H3K4me3) and H3 Lys36 trimethylation (H3K36me3). Overexpression of TaF-box3 in Arabidopsis was shown to be involved in the regulation of flowering time by demethylating FLC chromatin, according to ChIP experiments. Protein analysis confirmed that TaMETS interacts with TaF-box3 and is ubiquitinated and degraded in a TaF-box3-dependnent manner. Based on these findings, we propose that TaF-box3 has a positive role in flowering time, which leads to a better understanding of TaF-box3 physiological mechanism in Arabidopsis.

Genome-wide analysis and identification of TaRING-H2 gene family and TaSDIR1 positively regulates salt stress tolerance in wheat

Salt stress is an abiotic stress factor that limits high yields, and thus identifying salt tolerance genes is very important for improving the tolerance of salt in wheat. In this study we identified 274 TaRING-H2 family members and analyzed their gene positions, gene structures, conserved structural domains, promoter cis-acting elements and covariance relationships. And we investigated TaRING-H2-120 (TaSDIR1) in salt stress. Transgenic lines exhibited higher salt tolerance in the germination and seedling stages. Compared with the wild type, overexpression of TaSDIR1 upregulated the expression of genes encoding enzymes related to the control of reactive oxygen species (ROS), thereby reducing the accumulation of ROS, as well as increased the expression of ion transport-related genes to limit the inward flow of Na⁺ in vivo and maintain a higher K⁺/Na⁺ ratio. The expression levels of these genes were opposite in lines where TaSDIR1 was silenced by BSMV-VIGS, and the silenced wheat exhibited higher salt sensitivity. Arabidopsis mutants and heterologous TaSDIR1 overexpressing lines had similar salt stress tolerance phenotypes. We also demonstrated that TaSDIR1 interacted with TaSDIR1P2 in vivo and in vitro. A sequence of 80-100 amino acids in TaSDIR1P2 encoded a coiled coil domain that was important for the activity of E3 ubiquitin ligase, and it was also the core region for the interaction between TaSDIR1 and TaSDIR1P2. Overall, our results suggest that TaSDIR1 positively regulates salt stress tolerance in wheat.

Overexpression of a plant U-box gene TaPUB4 confers drought stress tolerance in Arabidopsis thaliana

Drought stress frequently results in significant reductions in crop production and yield. Plant U-box proteins (PUB) play a key role in the response to abiotic stress. Despite extensive characterization of PUB in model plants, their roles in wheat abiotic stress response remains unknown. In this study, we identified the physiological function of TaPUB4, a gene encoding the U-box and nuclear localization domains. The transcription level of TaPUB4 was induced by drought (mannitol) and abscisic acid. TaPUB4 displays E3 ubiquitin ligase activity and is located in the nucleus. Overexpression of TaPUB4 in Arabidopsis plants enhanced sensitivity with under ABA condition during early seedling developmental stages. In addition, the stomatal conductance of TaPUB4 was closer to that of WT under ABA conditions. Moreover, TaPUB4 facilitated stomatal response to elevated CO₂ emission rates under ABA conditions. TaPUB4-overexpressing Arabidopsis, on the other hand, was more resistant to drought stress in plant development, demonstrating that TaPUB4 positively regulates drought-mediated control of plant growth. Moreover, the ectopic expression of the TaPUB4 gene was significant influential in drought sensitive metrics including survival rate, chlorophyll content, water loss, proline content and the expression of drought stress-response genes. Collectively, our results demonstrate that TaPUB4 may regulate drought stress response and ABA conditions.

Molecular characterization of wheat floret development-related F-box protein (TaF-box2): Possible involvement in regulation of Arabidopsis flowering

In wheat (Triticum aestivum L.), the floret development stage is an important step in determining grain yield per spike; however, the molecular mechanisms underlying floret development remain unclear. In this study, we elucidated the role of TaF-box2, a member of the F-box-containing E3 ubiquitin protein ligases, which is involved in floret development and anthesis of wheat. TaF-box2 was transiently expressed in the plasma membrane and cytoplasm of both tobacco and wheat. We also found that the SCF^F-box2 (Skp1-Cul1-Rbx1-TaF-box2) ubiquitin ligase complex mediated self-ubiquitination activity. Transgenic Arabidopsis plants that constitutively overexpressed TaF-box2 showed markedly greater hypocotyl and root length than wild-type plants, and produced early flowering phenotypes. Flowering-related genes were significantly upregulated in TaF-box2-overexpressing Arabidopsis plants. Further protein interaction analyses such as yeast two-hybrid, in vitro pull-down, and bimolecular fluorescence complementation assays confirmed that TaF-box2 physically interacted with TaCYCL1 (Triticum aestivum cyclin-L1-1). Ubiquitination and degradation assays demonstrated that TaCYCL1 was ubiquitinated by SCF^F-box2 and degraded through the 26S proteasome complex. The physiological functions of the TaF-box2 protein remain unclear; however, we discuss several potential routes of involvement in various physiological mechanisms which counteract flowering in transgenic Arabidopsis plants.