Deciphering the role of the AT-rich interaction domain and the HMG-box domain of ARID-HMG proteins of Arabidopsis thaliana

The Arabidopsis ARID-HMG DNA-BINDING PROTEIN 15 modulates jasmonic acid signaling by regulating MYC2 during pollen development

The intricate process of male gametophyte development in flowering plants is regulated by jasmonic acid (JA) signaling. JA signaling initiates with the activation of the basic helix-loop-helix transcription factor (TF), MYC2, leading to the expression of numerous JA-responsive genes during stamen development and pollen maturation. However, the regulation of JA signaling during different stages of male gametophyte development remains less understood. This study focuses on the characterization of the plant ARID-HMG DNA-BINDING PROTEIN 15 (AtHMGB15) and its role in pollen development in Arabidopsis (Arabidopsis thaliana). Phenotypic characterization of a T-DNA insertion line (athmgb15-4) revealed delayed bolting, shorter siliques, and reduced seed set in mutant plants compared to the wild type. Additionally, AtHMGB15 deletion resulted in defective pollen morphology, delayed pollen germination, aberrant pollen tube growth, and a higher percentage of nonviable pollen grains. Molecular analysis indicated the downregulation of JA biosynthesis and signaling genes in the athmgb15-4 mutant. Quantitative analysis demonstrated that JA and its derivatives were ∼10-fold lower in athmgb15-4 flowers. Exogenous application of methyl jasmonate could restore pollen morphology and germination, suggesting that the low JA content in athmgb15-4 impaired JA signaling during pollen development. Furthermore, our study revealed that AtHMGB15 physically interacts with MYC2 to form a transcription activation complex. This complex promotes the transcription of key JA signaling genes, the R2R3-MYB TFs MYB21 and MYB24, during stamen and pollen development. Collectively, our findings highlight the role of AtHMGB15 as a positive regulator of the JA pathway, controlling the spatiotemporal expression of key regulators involved in Arabidopsis stamen and pollen development.

Plant-TFClass: a structural classification for plant transcription factors

Transcription factors (TFs) bind DNA at specific sequences to regulate gene expression. This universal process is achieved via their DNA-binding domain (DBD). In mammals, the vast diversity of DBD structural conformations and the way in which they contact DNA has been used to organize TFs in the TFClass hierarchical classification. However, the numerous DBD types present in plants but absent from mammalian genomes were missing from this classification. We reviewed DBD 3D structures and models available for plant TFs to classify most of the 56 recognized plant TF types within the TFClass framework. This extended classification adds eight new classes and 37 new families corresponding to DBD structures absent in mammals. Plant-TFClass provides a unique resource for TF comparison across families and organisms.

Overexpression of OsHMGB707, a High Mobility Group Protein, Enhances Rice Drought Tolerance by Promoting Stress-Related Gene Expression

Drought stress adversely affects crop growth and productivity worldwide. In response, plants have evolved several strategies in which numerous genes are induced to counter stress. High mobility group (HMG) proteins are the second most abundant family of chromosomal proteins. They play a crucial role in gene transcriptional regulation by modulating the chromatin/DNA structure. In this study, we isolated a novel HMG gene, OsHMGB707, one of the candidate genes localized in the quantitative trait loci (QTL) interval of rice drought tolerance, and examined its function on rice stress tolerance. The expression of OsHMGB707 was up-regulated by dehydration and high salt treatment. Its overexpression significantly enhanced drought tolerance in transgenic rice plants, whereas its knockdown through RNA interference (RNAi) did not affect the drought tolerance of the transgenic rice plants. Notably, OsHMGB707-GFP is localized in the cell nucleus, and OsHMGB707 is protein-bound to the synthetic four-way junction DNA. Several genes were up-regulated in OsHMGB707-overexpression (OE) rice lines compared to the wild-type rice varieties. Some of the genes encode stress-related proteins (e.g., DREB transcription factors, heat shock protein 20, and heat shock protein DnaJ). In summary, OsHMGB707 encodes a stress-responsive high mobility group protein and regulates rice drought tolerance by promoting the expression of stress-related genes.

Identification of genome-wide targets and DNA recognition sequence of the Arabidopsis HMG-box protein AtHMGB15 during cold stress response

AtHMGB15 belongs to a group of ARID-HMG proteins which are plant specific. The presence of two known DNA binding domains: AT rich interacting domain (ARID) and High Mobility Group (HMG)-box, in one polypeptide, makes this protein intriguing. Although proteins containing individual HMG and ARID domains have been characterized, not much is known about the role of ARID-HMG proteins. Promoter analysis of AtHMGB15 showed the presence of various stress responsive cis regulatory elements along with MADS-box containing transcription factors. Our result shows that the expression of AtHMGB15 increased significantly upon application of cold stress. Using ChIP-chip approach, we have identified 6128 and 4689 significantly enriched loci having AtHMGB15 occupancy under control and cold stressed condition respectively. GO analysis shows genes belonging to abiotic stress response, cold response and root development were AtHMGB15 targets during cold stress. DNA binding and footprinting assays further identified A(A/C)--ATA---(A/T)(A/T) as AtHMGB15 binding motif. The enriched probe distribution in both control and cold condition shows a bias of AtHMGB15 binding towards the transcribed (gene body) region. Further, the expression of cold stress responsive genes decreased in athmgb15 knockout plants compared to wild-type. Taken together, binding enrichment of AtHMGB15 to the promoter and upstream to stress loci suggest an unexplored role of the protein in stress induced transcription regulation.

Overexpression of CsHMGB Alleviates Phytotoxicity and Propamocarb Residues in Cucumber

Cucumber (Cucumis sativus L.) is one of the most economically important fruits of the Cucurbitaceae family, therefore consideration of potential pesticide residues in the fruit in the context of cucumber breeding and production programs is important. Propamocarb (a pesticide commonly used to prevent downy mildew) is widely used in cucumber cultivation, but the molecular mechanism underlying the degradation and metabolism of propamocarb in cucumber is not well understood. We screened a candidate CsHMGB gene (CsaV3-5G28190) for response to propamocarb exposure using transcriptome data. The coding region of CsHMGB was 624 bp in length and encoded the conserved HMB-box region. CsHMGB expression differed significantly between the "D0351" genotype, which accumulated low levels of propamocarb, and the "D9320" genotype, which accumulated high levels of propamocarb. CsHMGB expression was positively correlated with propamocarb levels in the cucumber peel. CsHMGB expression was upregulated in the fruit peels of the "D0351" genotype following exposure to propamocarb stress for 3-120 h, but no difference was observed in expression between propamocarb treatment and control for the "D9320" genotype. For the "D0351" genotype, CsHMGB expression was higher in the fruit peels and leaves than that in female flowers; expression was moderate in the stems and fruit pulps, and weak in male flowers and roots. The CsHMGB protein was targeted to the nucleus in Arabidopsis protoplasts and in the epidermis of Nicotiana benthamiana leaves. We measured MDA, O₂ ^-, and H₂O₂ levels in cucumber plants and found that they were likely to accumulate reactive oxygen species (ROS) in response to propamocarb stress. Analysis of antioxidant enzyme activity (SOD, POD, CAT, APX, GPX, GST, and GR) and the ascorbate-glutathione (AsA-GSH) system showed that the resistance of the plants was reduced and the levels of propamocarb residue was increased in CsHMGB-silenced plants in response to propamocarb stress. Conversely, overexpression of CsHMGB promoted glutathione-dependent detoxification by AsA-GSH system and improved the antioxidant potential, reduced the accumulation of ROS. Ultimately, the metabolism of propamocarb in cucumber was increased via increase in the wax levels and the stomatal conductance.

Diversity of cis-regulatory elements associated with auxin response in Arabidopsis thaliana

The phytohormone auxin regulates virtually every developmental process in land plants. This regulation is mediated via de-repression of DNA-binding auxin response factors (ARFs). ARFs bind TGTC-containing auxin response cis-elements (AuxREs), but there is growing evidence that additional cis-elements occur in auxin-responsive regulatory regions. The repertoire of auxin-related cis-elements and their involvement in different modes of auxin response are not yet known. Here we analyze the enrichment of nucleotide hexamers in upstream regions of auxin-responsive genes associated with auxin up- or down-regulation, with early or late response, ARF-binding domains, and with different chromatin states. Intriguingly, hexamers potentially bound by basic helix-loop-helix (bHLH) and basic leucine zipper (bZIP) factors as well as a family of A/T-rich hexamers are more highly enriched in auxin-responsive regions than canonical TGTC-containing AuxREs. We classify and annotate the whole spectrum of enriched hexamers and discuss their patterns of enrichment related to different modes of auxin response.