Methylation of a MITE insertion in the MdRFNR1-1 promoter is positively associated with its allelic expression in apple in response to drought stress

DNA Methylation Dynamics in Response to Drought Stress in Crops

Drought is one of the most hazardous environmental factors due to its severe damage on plant growth, development and productivity. Plants have evolved complex regulatory networks and resistance strategies for adaptation to drought stress. As a conserved epigenetic regulation, DNA methylation dynamically alters gene expression and chromosome interactions in plants' response to abiotic stresses. The development of omics technologies on genomics, epigenomics and transcriptomics has led to a rapid increase in research on epigenetic variation in non-model crop species. In this review, we summarize the most recent findings on the roles of DNA methylation under drought stress in crops, including methylating and demethylating enzymes, the global methylation dynamics, the dual regulation of DNA methylation on gene expression, the RNA-dependent DNA methylation (RdDM) pathway, alternative splicing (AS) events and long non-coding RNAs (lnc RNAs). We also discuss drought-induced stress memory. These epigenomic findings provide valuable potential for developing strategies to improve crop drought tolerance.

The BTB-BACK-TAZ domain protein MdBT2 reduces drought resistance by weakening the positive regulatory effect of MdHDZ27 on apple drought tolerance via ubiquitination

Drought stress is one of the dominating challenges to the growth and productivity in crop plants. Elucidating the molecular mechanisms of plants responses to drought stress is fundamental to improve fruit quality. However, such molecular mechanisms are poorly understood in apple (Malus domestica Borkh.). In this study, we explored that the BTB-BACK-TAZ protein, MdBT2, negatively modulates the drought tolerance of apple plantlets. Moreover, we identified a novel Homeodomain-leucine zipper (HD-Zip) transcription factor, MdHDZ27, using a yeast two-hybrid (Y2H) screen with MdBT2 as the bait. Overexpression of MdHDZ27 in apple plantlets, calli, and tomato plantlets enhanced their drought tolerance by promoting the expression of drought tolerance-related genes [responsive to dehydration 29A (MdRD29A) and MdRD29B]. Biochemical analyses demonstrated that MdHDZ27 directly binds to and activates the promoters of MdRD29A and MdRD29B. Furthermore, in vitro and in vivo assays indicate that MdBT2 interacts with and ubiquitinates MdHDZ27, via the ubiquitin/26S proteasome pathway. This ubiquitination results in the degradation of MdHDZ27 and weakens the transcriptional activation of MdHDZ27 on MdRD29A and MdRD29B. Finally, a series of transgenic analyses in apple plantlets further clarified the role of the relationship between MdBT2 and MdHDZ27, as well as the effect of their interaction on drought resistance in apple plantlets. Collectively, our findings reveal a novel mechanism by which the MdBT2-MdHDZ27 regulatory module controls drought tolerance, which is of great significance for enhancing the drought resistance of apple and other plants.

Dynamic DNA methylation modifications in the cold stress response of cassava

Cassava, a crucial tropical crop, faces challenges from cold stress, necessitating an exploration of its molecular response. Here, we investigated the role of DNA methylation in moderating the response to moderate cold stress (10 °C) in cassava. Using whole-genome bisulfite sequencing, we examined DNA methylation patterns in leaf blades and petioles under control conditions, 5 h, and 48 h of cold stress. Tissue-specific responses were observed, with leaf blades exhibiting subtle changes, while petioles displayed a pronounced decrease in methylation levels under cold stress. We identified cold stress-induced differentially methylated regions (DMRs) that demonstrated both tissue and treatment specificity. Importantly, these DMRs were enriched in genes with altered expression, implying functional relevance. The cold-response transcription factor ERF105 associated with DMRs emerged as a significant and conserved regulator across tissues and treatments. Furthermore, we investigated DNA methylation dynamics in transposable elements, emphasizing the sensitivity of MITEs with bHLH binding motifs to cold stress. These findings provide insights into the epigenetic regulation of response to cold stress in cassava, contributing to an understanding of the molecular mechanisms underlying stress adaptation in this tropical plant.

A pan-TE map highlights transposable elements underlying domestication and agronomic traits in Asian rice

Transposable elements (TEs) are ubiquitous genomic components and hard to study due to being highly repetitive. Here we assembled 232 chromosome-level genomes based on long-read sequencing data. Coupling the 232 genomes with 15 existing assemblies, we developed a pan-TE map comprising both cultivated and wild Asian rice. We detected 177 084 high-quality TE variations and inferred their derived state using outgroups. We found TEs were one source of phenotypic variation during rice domestication and differentiation. We identified 1246 genes whose expression variation was associated with TEs but not single-nucleotide polymorphisms (SNPs), such as OsRbohB, and validated OsRbohB's relative expression activity using a dual-Luciferase (LUC) reporter assays system. Our pan-TE map allowed us to detect multiple novel loci associated with agronomic traits. Collectively, our findings highlight the contributions of TEs to domestication, differentiation and agronomic traits in rice, and there is massive potential for gene cloning and molecular breeding by the high-quality Asian pan-TE map we generated.

The microRNA miR482 regulates NBS-LRR genes in response to ALT1 infection in apple

Plants are frequently attacked by a variety of pathogens and thus have evolved a series of defense mechanisms, one important mechanism is resistance gene (R gene)-mediated disease resistance, but its expression is tightly regulated. NBS-LRR genes are the largest gene family of R genes. microRNAs (miRNAs) target to a number of NBS-LRR genes and trigger the production of phased small interfering RNAs (phasiRNAs) from these transcripts. phasiRNAs cis or trans regulate NBS-LRR genes, which can result in the repression of R gene expression. In this study, we screened for upregulated miR482 in the susceptible apple cultivar 'Golden Delicious' (GD) after inoculation with the fungal pathogen Alternaria alternata f. sp. mali (ALT1). Additionally, through combined degradome sequencing, we identified a gene targeted by miR482, named MdTNL1, a gene encoding a TIR-NBS-LRR (Toll/interleukin1 receptor-nucleotide binding site-leucine-rich repeat) protein. This gene exhibited a significant down-regulation post ALT1 inoculation, suggesting an impact on gene expression mediated by miRNA regulation. miR482 could cleave MdTNL1 and generate phasiRNAs at the cleavage site. We found that overexpression of miR482 inhibited the expression of MdTNL1 and thus reduced the disease resistance of GD, while silencing of miR482 increased the expression of MdTNL1 and thus improved the disease resistance of GD. This work elucidates key mechanisms underlying the immune response to Alternaria infection in apple. Identification of the resistance genes involved will enable molecular breeding for prevention and control of Alternaria leaf spot disease in this important fruit crop.

Long noncoding RNAs underlie multiple domestication traits and leafhopper resistance in soybean

The origin and functionality of long noncoding RNA (lncRNA) remain poorly understood. Here, we show that multiple quantitative trait loci modulating distinct domestication traits in soybeans are pleiotropic effects of a locus composed of two tandem lncRNA genes. These lncRNA genes, each containing two inverted repeats, originating from coding sequences of the MYB genes, function in wild soybeans by generating clusters of small RNA (sRNA) species that inhibit the expression of their MYB gene relatives through post-transcriptional regulation. By contrast, the expression of lncRNA genes in cultivated soybeans is severely repressed, and, consequently, the corresponding MYB genes are highly expressed, shaping multiple distinct domestication traits as well as leafhopper resistance. The inverted repeats were formed before the divergence of the Glycine genus from the Phaseolus-Vigna lineage and exhibit strong structure-function constraints. This study exemplifies a type of target for selection during plant domestication and identifies mechanisms of lncRNA formation and action.

Transposable elements in Rosaceae: insights into genome evolution, expression dynamics, and syntenic gene regulation

Transposable elements (TEs) exert significant influence on plant genomic structure and gene expression. Here, we explored TE-related aspects across 14 Rosaceae genomes, investigating genomic distribution, transposition activity, expression patterns, and nearby differentially expressed genes (DEGs). Analyses unveiled distinct long terminal repeat retrotransposon (LTR-RT) evolutionary patterns, reflecting varied genome size changes among nine species over the past million years. In the past 2.5 million years, Rubus idaeus showed a transposition rate twice as fast as Fragaria vesca, while Pyrus bretschneideri displayed significantly faster transposition compared with Crataegus pinnatifida. Genes adjacent to recent TE insertions were linked to adversity resistance, while those near previous insertions were functionally enriched in morphogenesis, enzyme activity, and metabolic processes. Expression analysis revealed diverse responses of LTR-RTs to internal or external conditions. Furthermore, we identified 3695 pairs of syntenic DEGs proximal to TEs in Malus domestica cv. 'Gala' and M. domestica (GDDH13), suggesting TE insertions may contribute to varietal trait differences in these apple varieties. Our study across representative Rosaceae species underscores the pivotal role of TEs in plant genome evolution within this diverse family. It elucidates how these elements regulate syntenic DEGs on a genome-wide scale, offering insights into Rosaceae-specific genomic evolution.

Recent Advances in Studies of Genomic DNA Methylation and Its Involvement in Regulating Drought Stress Response in Crops

As global arid conditions worsen and groundwater resources diminish, drought stress has emerged as a critical impediment to plant growth and development globally, notably causing declines in crop yields and even the extinction of certain cultivated species. Numerous studies on drought resistance have demonstrated that DNA methylation dynamically interacts with plant responses to drought stress by modulating gene expression and developmental processes. However, the precise mechanisms underlying these interactions remain elusive. This article consolidates the latest research on the role of DNA methylation in plant responses to drought stress across various species, focusing on methods of methylation detection, mechanisms of methylation pattern alteration (including DNA de novo methylation, DNA maintenance methylation, and DNA demethylation), and overall responses to drought conditions. While many studies have observed significant shifts in genome-wide or gene promoter methylation levels in drought-stressed plants, the identification of specific genes and pathways involved remains limited. This review aims to furnish a reference for detailed research into plant responses to drought stress through epigenetic approaches, striving to identify drought resistance genes regulated by DNA methylation, specific signaling pathways, and their molecular mechanisms of action.

High-quality genome assembly enables prediction of allele-specific gene expression in hybrid poplar

Poplar (Populus) is a well-established model system for tree genomics and molecular breeding, and hybrid poplar is widely used in forest plantations. However, distinguishing its diploid homologous chromosomes is difficult, complicating advanced functional studies on specific alleles. In this study, we applied a trio-binning design and PacBio high-fidelity long-read sequencing to obtain haplotype-phased telomere-to-telomere genome assemblies for the 2 parents of the well-studied F1 hybrid "84K" (Populus alba × Populus tremula var. glandulosa). Almost all chromosomes, including the telomeres and centromeres, were completely assembled for each haplotype subgenome apart from 2 small gaps on one chromosome. By incorporating information from these haplotype assemblies and extensive RNA-seq data, we analyzed gene expression patterns between the 2 subgenomes and alleles. Transcription bias at the subgenome level was not uncovered, but extensive-expression differences were detected between alleles. We developed machine-learning (ML) models to predict allele-specific expression (ASE) with high accuracy and identified underlying genome features most highly influencing ASE. One of our models with 15 predictor variables achieved 77% accuracy on the training set and 74% accuracy on the testing set. ML models identified gene body CHG methylation, sequence divergence, and transposon occupancy both upstream and downstream of alleles as important factors for ASE. Our haplotype-phased genome assemblies and ML strategy highlight an avenue for functional studies in Populus and provide additional tools for studying ASE and heterosis in hybrids.

Multi-omics analysis provides new insights into the changes of important nutrients and fructose metabolism in loquat bud sport mutant

Bud sport is a common and stable somatic variation in perennial fruit trees, and often leads to significant modification of fruit traits and affects the breeding value. To investigate the impact of bud sport on the main metabolites in the fruit of white-fleshed loquat, we conducted a multi-omics analysis of loquat fruits at different developmental stages of a white-fleshed bud sport mutant of Dongting loquat (TBW) and its wild type (TBY). The findings from the detection of main fruit quality indices and metabolites suggested that bud sport resulted in a reduction in the accumulation of carotenoids, fructose, titratable acid and terpenoids at the mature stage of TBW, while leading to the accumulation of flavonoids, phenolic acids, amino acids and lipids. The comparably low content of titratable acid further enhances the balanced and pleasent taste profile of TBW. Expression patterns of differentially expressed genes involved in fructose metabolism exhibited a significant increase in the expression level of S6PDH (EVM0006243, EVM0044405) prior to fruit maturation. The comparison of protein sequences and promoter region of S6PDH between TBY and TBW revealed no structural variations that would impact gene function or expression, indicating that transcription factors may be responsible for the rapid up-regulation of S6PDH before maturation. Furthermore, correlation analysis helped to construct a comprehensive regulatory network of fructose metabolism in loquat, including 23 transcription factors, six structural genes, and nine saccharides. Based on the regulatory network and existing studies, it could be inferred that transcription factors such as ERF, NAC, MYB, GRAS, and bZIP may promote fructose accumulation in loquat flesh by positively regulating S6PDH. These findings improve our understanding of the nutritional value and breeding potential of white-fleshed loquat bud sport mutant, as well as serve as a foundation for exploring the genes and transcription factors that regulate fructose metabolism in loquat.

Genomic insight into the origin, domestication, dispersal, diversification and human selection of Tartary buckwheat

BACKGROUND

Tartary buckwheat, Fagopyrum tataricum, is a pseudocereal crop with worldwide distribution and high nutritional value. However, the origin and domestication history of this crop remain to be elucidated.

RESULTS

Here, by analyzing the population genomics of 567 accessions collected worldwide and reviewing historical documents, we find that Tartary buckwheat originated in the Himalayan region and then spread southwest possibly along with the migration of the Yi people, a minority in Southwestern China that has a long history of planting Tartary buckwheat. Along with the expansion of the Mongol Empire, Tartary buckwheat dispersed to Europe and ultimately to the rest of the world. The different natural growth environments resulted in adaptation, especially significant differences in salt tolerance between northern and southern Chinese Tartary buckwheat populations. By scanning for selective sweeps and using a genome-wide association study, we identify genes responsible for Tartary buckwheat domestication and differentiation, which we then experimentally validate. Comparative genomics and QTL analysis further shed light on the genetic foundation of the easily dehulled trait in a particular variety that was artificially selected by the Wa people, a minority group in Southwestern China known for cultivating Tartary buckwheat specifically for steaming as a staple food to prevent lysine deficiency.

CONCLUSIONS

This study provides both comprehensive insights into the origin and domestication of, and a foundation for molecular breeding for, Tartary buckwheat.

DNA methylation: an emerging paradigm of gene regulation under drought stress in plants

Drought is an enormous threat to global crop production. In order to ensure food security for the burgeoning population, we must develop drought tolerant crop varieties. This necessitates the identification of drought-responsive genes and understanding the mechanisms involved in their regulation. DNA methylation is a widely studied mechanism of epigenetic regulation of gene expression, which is known to play vital role in conferring tolerance to various biotic and abiotic stress factors. The recent advances in next-generation sequencing (NGS) technologies, has allowed unprecedented access to genome-wide methylation marks, with single base resolution. The most important roles of DNA methylation have been studied in terms of gene body methylation (gbM), which is associated with regulation of both transcript abundance and its stability. The availability of mutants for the various genes encoding enzymes involved in methylation of DNA has allowed ascertainment of the biological significance of methylation. Even though a vast number of reports have emerged in the recent past, where both genome-wide methylation landscape and locus specific changes in DNA methylation have been studied, a conclusive picture with regards to the biological role of DNA methylation is still lacking. Compounding this, is the lack of sufficient evidence supporting the heritability of these epigenetic changes. Amongst the various epigenetic variations, the DNA methylation changes are observed to be the most stable. This review describes the drought-induced changes in DNA methylation identified across different plant species. We also briefly describe the stress memory contributed by these changes. The identification of heritable, drought-induced methylation marks would broaden the scope of crop improvement in the future.

Regulatory mechanism of heat-active retrotransposons by the SET domain protein SUVH2

New transposon insertions are deleterious to genome stability. The RNA-directed DNA methylation (RdDM) pathway evolved to regulate transposon activity via DNA methylation. However, current studies have not yet clearly described the transposition regulation. ONSEN is a heat-activated retrotransposon that is activated at 37°C. The plant-specific SUPPRESSOR OF VARIEGATION 3-9 HOMOLOG (SUVH) family proteins function downstream of the RdDM pathway. The SUVH protein families are linked to TE silencing by two pathways, one through DNA methylation and the other through chromatin remodeling. In this study, we analyzed the regulation of ONSEN activity by SUVH2. We observed that ONSEN transcripts were increased; however, there was no transpositional activity in Arabidopsis suvh2 mutant. The suvh2 mutant produced siRNAs from the ONSEN locus under heat stress, suggesting that siRNAs are involved in suppressing transposition. These results provide new insights into the regulatory mechanisms of retrotransposons that involve siRNA in the RdDM pathway.

MdbHLH160 is stabilized via reduced MdBT2-mediated degradation to promote MdSOD1 and MdDREB2A-like expression for apple drought tolerance

Drought stress is a key environmental factor limiting the productivity, quality, and geographic distribution of crops worldwide. Abscisic acid (ABA) plays an important role in plant drought stress responses, but the molecular mechanisms remain unclear. Here, we report an ABA-responsive bHLH transcription factor, MdbHLH160, which promotes drought tolerance in Arabidopsis (Arabidopsis thaliana) and apple (Malus domestica). Under drought conditions, MdbHLH160 is directly bound to the MdSOD1 (superoxide dismutase 1) promoter and activated its transcription, thereby triggering reactive oxygen species (ROS) scavenging and enhancing apple drought tolerance. MdbHLH160 also promoted MdSOD1 enzyme activity and accumulation in the nucleus through direct protein interactions, thus inhibiting excessive nuclear ROS levels. Moreover, MdbHLH160 directly upregulated the expression of MdDREB2A-like, a DREB (dehydration-responsive element binding factor) family gene that promotes apple drought tolerance. Protein degradation and ubiquitination assays showed that drought and ABA treatment stabilized MdbHLH160. The BTB protein MdBT2 was identified as an MdbHLH160-interacting protein that promoted MdbHLH160 ubiquitination and degradation, and ABA treatment substantially inhibited this process. Overall, our findings provide insights into the molecular mechanisms of ABA-modulated drought tolerance at both the transcriptional and post-translational levels via the ABA-MdBT2-MdbHLH160-MdSOD1/MdDREB2A-like cascade.

SlSAMS1 enhances salt tolerance through regulation DNA methylation of SlGI in tomato

S-adenosylmethionine (SAM), which is synthesized from methionine and ATP catalyzed by S-adenosylmethionine synthetase (SAMS), is an important methyl donor in plants. SAMS and DNA methylation play an important role in the plant response to abiotic stresses. Previous studies have shown that SAMS improves salt tolerance in tomato plants, but it is not clear whether the DNA methylation pathway mediates SAMS-induced salt tolerance. This study confirmed that SlSAMS1-overexpressing plants exhibited improved salt tolerance. Through whole-genome bisulfite sequencing (WGBS) and transcriptome sequencing (RNA-seq) analysis, the study screened the circadian rhythm pathway and identified the gene SlGI in this pathway, which was regulated by SlSAMS1. The gene body region of SlGI, the core gene of the circadian rhythm pathway, was hypermethylated in SlSAMS1-overexpressing plants, and its expression level was significantly increased. Furthermore, the SlGI-overexpressing plants showed higher salt tolerance, less reduction in plant height and fresh weight, lower electrolyte leakage, malondialdehyde and H2O2 content, and higher antioxidant enzyme activity compared to wild type plants. Therefore, SlSAMS1-overexpressing plants regulated significant changes in CHG-type methylation sites of the SlGI gene body and its expression levels, leading to an enhanced salt tolerance of tomato plants.

Post-transcriptional and translational regulation of plant gene expression by transposons

Transposons are mobile DNA sequences that can move within the genome and integrate in new genomic locations. They are widespread in eukaryotes and prokaryotes and can influence gene expression when landing within or nearby a gene. Although transposon-induced regulation of gene expression at the transcriptional level has been extensively studied, there has been less focus on regulation at the post-transcriptional and translational levels. Recent studies in maize (Zea mays) and other plant species suggest that transposon insertions can affect RNA processing, RNA stability, protein translation and protein stability. We will describe the diverse mechanisms by which transposons can influence gene expression at the post-transcriptional and translational levels, and discuss the interactions between these mechanisms.

DNA cytosine methylation dynamics and functional roles in horticultural crops

Methylation of cytosine is a conserved epigenetic modification that maintains the dynamic balance of methylation in plants under the regulation of methyltransferases and demethylases. In recent years, the study of DNA methylation in regulating the growth and development of plants and animals has become a key area of research. This review describes the regulatory mechanisms of DNA cytosine methylation in plants. It summarizes studies on epigenetic modifications of DNA methylation in fruit ripening, development, senescence, plant height, organ size, and under biotic and abiotic stresses in horticultural crops. The review provides a theoretical basis for understanding the mechanisms of DNA methylation and their relevance to breeding, genetic improvement, research, innovation, and exploitation of new cultivars of horticultural crops.

Heterologous Overexpression of Apple MdKING1 Promotes Fruit Ripening in Tomato

Fruit ripening is governed by a complex regulatory network, and ethylene plays an important role in this process. MdKING1 is a γ subunit of SNF1-related protein kinases (SnRKs), but the function was unclear. Here, we characterized the role of MdKING1 during fruit ripening, which can promote fruit ripening through the ethylene pathway. Our findings reveal that MdKING1 has higher expression in early-ripening cultivars than late-ripening during the early stage of apple fruit development, and its transcription level significantly increased during apple fruit ripening. Overexpression of MdKING1 (MdKING1 OE) in tomatoes could promote early ripening of fruits, with the increase in ethylene content and the loss of fruit firmness. Ethylene inhibitor treatment could delay the fruit ripening of both MdKING1 OE and WT fruits. However, MdKING1 OE fruits turned fruit ripe faster, with an increase in carotenoid content compared with WT. In addition, the expression of genes involved in ethylene biosynthesis (SlACO1, SlACS2, and SlACS4), carotenoid biosynthesis (SlPSY1 and SlGgpps2a), and fruit firmness regulation (SlPG2a, SlPL, and SlCEL2) was also increased in the fruits of MdKING1 OE plants. In conclusion, our results suggest that MdKING1 plays a key role in promoting tomato fruit ripening, thus providing a theoretical basis for apple fruit quality improvement by genetic engineering in the future.

Insights into the molecular mechanisms underlying responses of apple trees to abiotic stresses

Apple (Malus[Formula: see text]domestica) is a popular temperate fruit crop worldwide. However, its growth, productivity, and quality are often adversely affected by abiotic stresses such as drought, extreme temperature, and high salinity. Due to the long juvenile phase and highly heterozygous genome, the conventional breeding approaches for stress-tolerant cultivars are time-consuming and resource-intensive. These issues may be resolved by feasible molecular breeding techniques for apples, such as gene editing and marker-assisted selection. Therefore, it is necessary to acquire a more comprehensive comprehension of the molecular mechanisms underpinning apples' response to abiotic stress. In this review, we summarize the latest research progress in the molecular response of apples to abiotic stressors, including the gene expression regulation, protein modifications, and epigenetic modifications. We also provide updates on new approaches for improving apple abiotic stress tolerance, while discussing current challenges and future perspectives for apple molecular breeding.

Haplotype-Resolution Transcriptome Analysis Reveals Important Responsive Gene Modules and Allele-Specific Expression Contributions under Continuous Salt and Drought in Camellia sinensis

The tea plant, Camellia sinensis (L.) O. Kuntze, is one of the most important beverage crops with significant economic and cultural value. Global climate change and population growth have led to increased salt and drought stress, negatively affecting tea yield and quality. The response mechanism of tea plants to these stresses remains poorly understood due to the lack of reference genome-based transcriptional descriptions. This study presents a high-quality genome-based transcriptome dynamic analysis of C. sinensis' response to salt and drought stress. A total of 2244 upregulated and 2164 downregulated genes were identified under salt and drought stress compared to the control sample. Most of the differentially expression genes (DEGs) were found to involve divergent regulation processes at different time points under stress. Some shared up- and downregulated DEGs related to secondary metabolic and photosynthetic processes, respectively. Weighted gene co-expression network analysis (WGCNA) revealed six co-expression modules significantly positively correlated with C. sinensis' response to salt or drought stress. The MEpurple module indicated crosstalk between the two stresses related to ubiquitination and the phenylpropanoid metabolic regulation process. We identified 1969 salt-responsive and 1887 drought-responsive allele-specific expression (ASE) genes in C. sinensis. Further comparison between these ASE genes and tea plant heterosis-related genes suggests that heterosis likely contributes to the adversity and stress resistance of C. sinensis. This work offers new insight into the underlying mechanisms of C. sinensis' response to salt and drought stress and supports the improved breeding of tea plants with enhanced salt and drought tolerance.

Interfering small ubiquitin modifiers (SUMO) improves the thermotolerance of apple by facilitating the activity of MdDREB2A

Heat stress, which is caused by global warming, threatens crops yield and quality across the world. As a kind of post-translation modification, SUMOylation involves in plants heat stress response with a rapid and wide pattern. Here, we identified small ubiquitin modifiers (SUMO), which affect drought tolerance in apple, also participated in thermotolerance. Six isoforms of SUMOs located on six chromosomes in apple genome, and all the SUMOs were up-regulated in response to heat stress condition. The MdSUMO2 RNAi transgenic apple plants exhibited higher survival rate, lower ion leakage, higher catalase (CAT) activity, and Malondialdehyde (MDA) content under heat stress. MdDREB2A, the substrate of MdSUMO2 in apple, was accumulated in MdSUMO2 RNAi transgenic plants than the wild type GL-3 at the protein level in response to heat stress treatment. Further, the inhibited SUMOylation level of MdDREB2A in MdSUMO2 RNAi plants might repress its ubiquitination, too. The accumulated MdDREB2A in MdSUMO2 RNAi plants further induced heat-responsive genes expression to strengthen plants thermotolerance, including MdHSFA3, MdHSP26.5, MdHSP18.2, MdHSP70, MdCYP18-1 and MdTLP1. In summary, these findings illustrate that interfering small ubiquitin modifiers (SUMO) in apple improves plants thermotolerance, partly by facilitating the stability and activity of MdDREB2A.

Polymorphic inverted repeats near coding genes impact chromatin topology and phenotypic traits in Arabidopsis thaliana

Transposons are mobile elements that are commonly silenced to protect eukaryotic genome integrity. In plants, transposable element (TE)-derived inverted repeats (IRs) are commonly found near genes, where they affect host gene expression. However, the molecular mechanisms of such regulation are unclear in most cases. Expression of these IRs is associated with production of 24-nt small RNAs, methylation of the IRs, and drastic changes in local 3D chromatin organization. Notably, many of these IRs differ between Arabidopsis thaliana accessions, causing variation in short-range chromatin interactions and gene expression. CRISPR-Cas9-mediated disruption of two IRs leads to a switch in genome topology and gene expression with phenotypic consequences. Our data show that insertion of an IR near a gene provides an anchor point for chromatin interactions that profoundly impact the activity of neighboring loci. This turns IRs into powerful evolutionary agents that can contribute to rapid adaptation.

Insertion of a TRIM-like sequence in MdFLS2-1 promoter is associated with its allele-specific expression in response to Alternaria alternata in apple

Alternaria blotch disease, caused by Alternaria alternata apple pathotype (AAAP), is one of the major fungal diseases in apple. Early field observations revealed, the anther-derived homozygote Hanfu line (HFTH1) was highly susceptible to AAAP, whereas Hanfu (HF) exhibited resistance to AAAP. To understand the molecular mechanisms underlying the difference in sensitivity of HF and HFTH1 to AAAP, we performed allele-specific expression (ASE) analysis and comparative transcriptomic analysis before and after AAAP inoculation. We reported an important immune gene, namely, MdFLS2, which displayed strong ASE in HF with much lower expression levels of HFTH1-derived alleles. Transient overexpression of the dominant allele of MdFLS2-1 from HF in GL-3 apple leaves could enhance resistance to AAAP and induce expression of genes related to salicylic acid pathway. In addition, MdFLS2-1 was identified with an insertion of an 85-bp terminal-repeat retrotransposon in miniature (TRIM) element-like sequence in the upstream region of the nonreference allele. In contrast, only one terminal direct repeat (TDR) from TRIM-like sequence was present in the upstream region of the HFTH1-derived allele MdFLS2-2. Furthermore, the results of luciferase and β-glucuronidase reporter assays demonstrated that the intact TRIM-like sequence has enhancer activity. This suggested that insertion of the TRIM-like sequence regulates the expression level of the allele of MdFLS2, in turn, affecting the sensitivity of HF and HFTH1 to AAAP.