Basic helix-loop-helix transcription factor PxbHLH02 enhances drought tolerance in Populus (Populus simonii × P. nigra)

Functions of Basic Helix-Loop-Helix (bHLH) Proteins in the Regulation of Plant Responses to Cold, Drought, Salt, and Iron Deficiency: A Comprehensive Review

Abiotic stresses including cold, drought, salt, and iron deficiency severely impair plant development, crop productivity, and geographic distribution. Several bodies of research have shed light on the pleiotropic functions of BASIC HELIX-LOOP-HELIX (bHLH) proteins in plant responses to these abiotic stresses. In this review, we mention the regulatory roles of bHLH TFs in response to stresses such as cold, drought, salt resistance, and iron deficiency, as well as in enhancing grain yield in plants, especially crops. The bHLH proteins bind to E/G-box motifs in the target promoter and interact with various other factors to form a complex regulatory network. Through this network, they cooperatively activate or repress the transcription of downstream genes, thereby regulating various stress responses. Finally, we present some perspectives for future research focusing on the molecular mechanisms that integrate and coordinate these abiotic stresses. Understanding these molecular mechanisms is crucial for the development of stress-tolerant crops.

Transcription factors as molecular switches regulating plant responses to drought stress

Plants often experience abiotic stress, which severely affects their growth. With the advent of global warming, drought stress has become a pivotal factor affecting crop yield and quality. Increasing numbers of studies have focused on elucidating the molecular mechanisms underlying plant responses to drought stress. As molecular switches, transcription factors (TFs) are key participants in drought-resistance regulatory networks in crops. TFs regulate the transcription of downstream genes and are regulated by various upstream regulatory factors. Therefore, understanding the mechanisms of action of TFs in regulating drought stress can help enhance the adaptive capacity of crops under drought conditions. In this review, we summarize the structural characteristics of several common TFs, their multiple drought-response pathways, and recently employed research strategies. We describe the application of new technologies such as analysis of stress granule dynamics and function, multi-omics data, gene editing, and molecular crosstalk between TFs in drought resistance. This review aims to familiarize readers with the regulatory network of TFs in drought resistance and to provide a reference for examining the molecular mechanisms of drought resistance in plants and improving agronomic traits.

MdbHLH93 confers drought tolerance by activating MdTyDC expression and promoting dopamine biosynthesis

Dopamine and its biosynthesis-limiting enzyme tyrosine decarboxylase (TyDC) play a vital part in mediating plant growth and the response to drought stress. However, the underlying molecular mechanism remains poorly understood. Here, drought stress markedly induced the expression of Malus domestica bHLH93 (MdbHLH93), the apple basic helix-loop-helix transcription factor. Moreover, MdbHLH93 directly bound to the Malus domestica TyDC (MdTyDC) promoter and positively regulated its expression, which resulted in dopamine synthesis and enhanced drought tolerance. Furthermore, the additive effect of overexpressing MdbHLH93 and MdTyDC simultaneously promoted dopamine synthesis and drought tolerance in apples, while the interference of MdbHLH93 inhibited this effect, indicating that MdTyDC-regulated dopamine synthesis and drought tolerance were positively regulated by MdbHLH93. Taken together, these findings suggest the positive regulation of dopamine accumulation by MdbHLH93 through its transcriptional regulation of MdTyDC and show that increased dopamine content confers drought tolerance in apples.

Genome-Wide Identification of the ABC Gene Family and Its Expression in Response to the Wood Degradation of Poplar in Trametes gibbosa

Wood-rotting fungi's degradation of wood not only facilitates the eco-friendly treatment of organic materials, decreasing environmental pollution, but also supplies crucial components for producing biomass energy, thereby reducing dependence on fossil fuels. The ABC gene family, widely distributed in wood-rotting fungi, plays a crucial role in the metabolism of lignin, cellulose, and hemicellulose. Trametes gibbosa, as a representative species of wood-rotting fungi, exhibits robust capabilities in wood degradation. To investigate the function of the ABC gene family in wood degradation by T. gibbosa, we conducted a genome-wide analysis of T. gibbosa's ABC gene family. We identified a total of 12 Tg-ABCs classified into four subfamilies (ABCA, ABCB, ABCC, and ABCG). These subfamilies likely play significant roles in wood degradation. Scaffold localization and collinearity analysis results show that Tg-ABCs are dispersed on scaffolds and there is no duplication of gene sequences in the Tg-ABCs in the genome sequence of T. gibbosa. Phylogenetic and collinearity analyses of T. gibbosa along with four other wood-rotting fungi show that T. gibbosa shares a closer phylogenetic relationship with its same-genus fungus (Trametes versicolor), followed by Ganoderma leucocontextum, Laetiporus sulphureus, and Phlebia centrifuga in descending order of phylogenetic proximity. In addition, we conducted quantitative analyses of Tg-ABCs from T. gibbosa cultivated in both woody and non-woody environments for 10, 15, 20, 25, 30, and 35 days using an RT-qPCR analysis. The results reveal a significant difference in the expression levels of Tg-ABCs between woody and non-woody environments, suggesting an active involvement of the ABC gene family in wood degradation. During the wood degradation period of T. gibbosa, spanning from 10 to 35 days, the relative expression levels of most Tg-ABCs exhibited a trend of increasing, decreasing, and then increasing again. Additionally, at 20 and 35 days of wood degradation by T. gibbosa, the relative expression levels of Tg-ABCs peak, suggesting that at these time points, Tg-ABCs exert the most significant impact on the degradation of poplar wood by T. gibbosa. This study systematically reveals the biological characteristics of the ABC gene family in T. gibbosa and their response to woody environments. It establishes the foundation for a more profound comprehension of the wood-degradation mechanism of the ABC gene family and provides strong support for the development of more efficient wood-degradation strategies.

Functional analysis of sweet cherry PavbHLH106 in the regulation of cold stress

KEY MESSAGE

Sweet cherry PavbHLH106 was up-regulated under cold induction and overexpressed to enhance the cold resistance in tobacco by mediating the scavenging of ROS through increasing of antioxidant enzyme activity. Sweet cherry (Prunus avium L.) is an economically important fruit. Chilling requirements are critical during dormancy, but abnormally low temperatures unfavorably affect fruit growth and development. Differences were found in the transcript level of PavbHLH106 under salt, dehydration, and low-temperature treatments, especially in response to cold stress, suggesting that this gene is involved in the regulation of different abiotic stresses. PavbHLH106 is homologous to Arabidopsis thaliana AtbHLH106 with a conserved bHLH domain, and transient expression in tobacco suggests that the protein is localized in the nucleus and has transcriptional activity in yeast. The PavbHLH106 overexpression in tobacco resulted in weaker electrolyte leakages, lower malondialdehyde, and higher proline content than the wild type at low-temperature treatment. Reactive oxygen species accumulation was significantly reduced in the overexpressed lines, negatively correlated with the antioxidant enzyme activity. In addition, overexpression of PavbHLH106 delayed the germination of tobacco seeds and promoted plant growth. Resistance-related genes were expressed more in the overexpressed plants compared to the wild type. PavbHLH106 bound to the PavACO promoter in yeast and potentially interacted with a bHLH162-like transcription factor. These results indicate that PavbHLH106 has various functions and is particularly active in controlling low-temperature stress.

Genome-Wide Characterization of bHLH Family Genes and Expression Analysis in Response to Osmotic Stress in Betula platyphylla

The bHLH family, as a superfamily of transcription factors (TFs), has special functional characteristics in plants and plays a crucial role in a plant's growth and development and helping the plant cope with various stresses. In this study, 128 bHLH family genes were screened in the birch (B. platyphylla) genome using conservative domain scan and blast analysis. These genes are clustered into 21 subfamilies based on the phylogenetic tree construction and are unevenly distributed among the 14 birch chromosomes. In all, 22 segmental duplication pairs with 27 BpbHLH genes were identified. The duplications were distributed on eight chromosomes. Analysis of gene structures and protein motifs revealed intra-group conservation of BpbHLHs. Of the BpbHLH family genes, 16 contain only one intron each. The BPChr14G06667 gene contains the most introns, that is, 19. The cis-elements, which respond to plant hormones, light, defense, and stress, were found on the promoter of BHLH family genes. As per RNA-seq data analysis, under PEG osmotic stress, most BpbHLH genes were differentially expressed, and eight were highly differentially expressed. The qRT-PCR analysis results further indicated that BPChr06G09475 was the gene with the highest expression level in leaves, roots, and stems, and that the expression of these eight genes was higher in leaves than in roots and stems and upregulated in all three tissues under osmotic stress compared to the controls. The above analysis suggests that the BpbHLH family genes have a certain biological effect under drought stress that provides a basis for molecular breeding for stress resistance in birch.

Co-Expression of JcNAC1- and JcZFP8-Improved Agronomic Traits of Tobacco and Enhanced Drought Resistance through NbbHLH1 and NbbHLH2

Previous studies have identified numerous transcription factors involved in drought response, each of which play different roles in plants. The objective of the present study was to evaluate the effectiveness of two transcription factors on drought response in Jatropha curcas L., JcNAC1 and JcZFP8. The overexpression of these transcription factors in tobacco (Nicotiana benthamiana L.) improved drought resistance, but JcZFP8 delayed germination and JcNAC1 reduced biomass and yield. By constitutively co-expressing these two genes in tobacco, drought resistance was improved, and the negative effects of each of them were overcome. The transgenic plants with double-gene co-expression showed stronger drought tolerance with 1.76-fold greater accumulation of proline and lower H2O2 and malondialdehyde (MDA) content to 43 and 65% of wildtype (WT) levels, respectively. The expression levels of NbbHLH1 and NbbHLH2 genes upregulated linearly with the increased drought tolerance of double genes co-expression plants. In drought conditions, the leaf water contents of bhlh1, bhlh2, and bhlh1bhlh2 deletion mutants obtained by CRISPR-CAS9 knockout technique were maintained at 99%, 97%, and 97% of WT. The bhlh1bhlh2 was found with lower germination rate but with higher reactive oxygen levels (1.64-fold H2O2 and 1.41-fold MDA levels). Thus, the co-expression of two transcription factors with different functions overcame the adverse traits brought by a single gene and enhanced the shared drought-tolerant traits, which can provide guidance on theory and selection of gene combinations for the application of multi-gene co-expression in agriculture in the future.

Transcriptional profiling analysis providing insights into desiccation tolerance mechanisms of the desert moss Syntrichia caninervis

Syntrichia caninervis is a desiccation tolerant moss and is the dominant bryophyte found in biological soil crusts in the Gurbantunggut desert. In this study, we assessed the transcriptome profiles of S. caninervis gametophytes during the dehydration-rehydration (D-R) process (across 9 time points) using Illumina sequencing. In total, 22489 transcripts were identified, including 5337 novel transcripts, that mapped to the reference genome. A total of 12548 transcripts exhibited significant alterations in the D-R samples compared with the control samples. The differentially expressed transcripts (DETs) possessed several enriched Gene Ontology terms, such as "water stress response", "oxidation-reduction process", "membrane metabolism", "photosynthesis", and "transcription factor activity". Moreover, during early dehydration stress, the DETs were significantly enriched in stress-related pathways from the Kyoto Encyclopedia of Genes and Genomes, such as "phenylpropanoid biosynthesis", "alpha-linolenic acid metabolism", and "fructose and mannose metabolism". Photosynthesis-related transcripts (e.g., ScPsa H, ScRubisco, and ScLhcb1) were inhibited during the dehydration treatment and significantly accumulated during the late rehydration period. Most transcripts from the late embryogenesis abundant proteins (LEA) and early light-inducible protein (ELIP) families strongly accumulated at the late dehydration stage. These pathways were positively correlated with the content changes of absolute water content and Fv/Fm values, alongside peroxidase and superoxide dismutase activities. Seven transcription factor families, including AP2-ERF, bHLH, G2-like, MYB, NAC, WRKY, and bZIP, were enriched in DETs during D-R treatment. This study is the first transcriptome analysis using the S. caninervis genome for gene annotation and multigroup D-R treatment points. Our results demonstrated the detailed dynamic changes in the transcriptome of S. caninervis during the D-R process. These results also improve understanding of desiccation tolerant plants' adaptations to desiccation stress at the transcription level and provide promising gene resources for transgenic crop breeding.