Angiosperm-Wide and Family-Level Analyses of AP2/ERF Genes Reveal Differential Retention and Sequence Divergence After Whole-Genome Duplication

Genome-wide identification and analysis of ERF transcription factors related to abiotic stress responses in Nelumbo nucifera

BACKGROUND

Ethylene-responsive factor (ERF) transcription factors belong to the APETALA2/ERF (AP2/ERF) superfamily, and play crucial roles in plant development process and stress responses. However, the function of ERF proteins (especially for their role in response to abiotic stresses) remains scarce in Nelumbo nucifera, which is an important aquatic plant with high ornamental, economic, and ecological values.

RESULTS

A total of 107 ERF genes were identified from the N. nucifera genome, and phylogenetic analysis classified these genes into 11 groups. The NnERF genes in the same group exhibited similar gene structure and conserved motifs, and they were unevenly distributed across the 8 chromosomes, with three pairs of tandem duplications and 21 pairs of segmental duplications. Synteny analysis revealed 44 and 39 of NnERF genes were orthologous to those in Arabidopsis thaliana and Oryza sativa, respectively. Tissue-specific expression patterns analysis of NnERF showed that 26 NnERF genes were expressed in all tested tissues, in which five genes exhibited high expression levels. Furthermore, 16 NnERF genes were selected for exploring their responses to different abiotic stresses, including cold, salt, drought, and Cd stresses. qRT-PCR analysis revealed that all these 16 investigated genes were regulated by at least one stress treatment, and 12 genes responded to all the stress treatments with different expression patterns or levels, suggesting their potential roles in diverse abiotic stress tolerance of N. nucifera. Additionally, two representative stress-related NnERFs (Nn3g19628 and Nn1g06033) were confirmed to be nuclear-localized proteins and displayed transcriptional activation.

CONCLUSIONS

In this study, we conducted a genome-wide identification and analysis of NnERF gene family related to abiotic stress responses in N. nucifera, which provides valuable information for further functional validation of these genes in stress responses, and forms a foundation for stress tolerance breeding in N. nucifera and other aquatic ornamental plants.

Ancient Duplication and Lineage-Specific Transposition Determine Evolutionary Trajectory of ERF Subfamily across Angiosperms

AP2/ERF transcription factor family plays an important role in plant development and stress responses. Previous studies have shed light on the evolutionary trajectory of the AP2 and DREB subfamilies. However, knowledge about the evolutionary history of the ERF subfamily in angiosperms still remains limited. In this study, we performed a comprehensive analysis of the ERF subfamily from 107 representative angiosperm species by combining phylogenomic and synteny network approaches. We observed that the expansion of the ERF subfamily was driven not only by whole-genome duplication (WGD) but also by tandem duplication (TD) and transposition duplication events. We also found multiple transposition events in Poaceae, Brassicaceae, Poales, Brassicales, and Commelinids. These events may have had notable impacts on copy number variation and subsequent functional divergence of the ERF subfamily. Moreover, we observed a number of ancient tandem duplications occurred in the ERF subfamily across angiosperms, e.g., in Subgroup IX, IXb originated from ancient tandem duplication events within IXa. These findings together provide novel insights into the evolution of this important transcription factor family.

Models for the retention of duplicate genes and their biological underpinnings

Gene content in genomes changes through several different processes, with gene duplication being an important contributor to such changes. Gene duplication occurs over a range of scales from individual genes to whole genomes, and the dynamics of this process can be context dependent. Still, there are rules by which genes are retained or lost from genomes after duplication, and probabilistic modeling has enabled characterization of these rules, including their context-dependence. Here, we describe the biology and corresponding mathematical models that are used to understand duplicate gene retention and its contribution to the set of biochemical functions encoded in a genome.

Regulation of micro- and small-exon retention and other splicing processes by GRP20 for flower development

Pre-mRNA splicing is crucial for gene expression and depends on the spliceosome and splicing factors. Plant exons have an average size of ~180 nucleotides and typically contain motifs for interactions with spliceosome and splicing factors. Micro exons (<51 nucleotides) are found widely in eukaryotes and in genes for plant development and environmental responses. However, little is known about transcript-specific regulation of splicing in plants and about the regulators for micro exon splicing. Here we report that glycine-rich protein 20 (GRP20) is an RNA-binding protein and required for splicing of ~2,100 genes including those functioning in flower development and/or environmental responses. Specifically, GRP20 is required for micro-exon retention in transcripts of floral homeotic genes; these micro exons are conserved across angiosperms. GRP20 is also important for small-exon (51-100 nucleotides) splicing. In addition, GRP20 is required for flower development. Furthermore, GRP20 binds to poly-purine motifs in micro and small exons and a spliceosome component; both RNA binding and spliceosome interaction are important for flower development and micro-exon retention. Our results provide new insights into the mechanisms of micro-exon retention in flower development.

Expectations of duplicate gene retention under the gene duplicability hypothesis

BACKGROUND

Gene duplication is an important process in evolution. What causes some genes to be retained after duplication and others to be lost is a process not well understood. The most prevalent theory is the gene duplicability hypothesis, that something about the function and number of interacting partners (number of subunits of protein complex, etc.), determines whether copies have more opportunity to be retained for long evolutionary periods. Some genes are also more susceptible to dosage balance effects following WGD events, making them more likely to be retained for longer periods of time. One would expect these processes that affect the retention of duplicate copies to affect the conditional probability ratio after consecutive whole genome duplication events. The probability that a gene will be retained after a second whole genome duplication event (WGD2), given that it was retained after the first whole genome duplication event (WGD1) versus the probability a gene will be retained after WGD2, given it was lost after WGD1 defines the probability ratio that is calculated.

RESULTS

Since duplicate gene retention is a time heterogeneous process, the time between the events (t1) and the time since the most recent event (t2) are relevant factors in calculating the expectation for observation in any genome. Here, we use a survival analysis framework to predict the probability ratio for genomes with different values of t1 and t2 under the gene duplicability hypothesis, that some genes are more susceptible to selectable functional shifts, some more susceptible to dosage compensation, and others only drifting. We also predict the probability ratio with different values of t1 and t2 under the mutational opportunity hypothesis, that probability of retention for certain genes changes in subsequent events depending upon how they were previously retained. These models are nested such that the mutational opportunity model encompasses the gene duplicability model with shifting duplicability over time. Here we present a formalization of the gene duplicability and mutational opportunity hypotheses to characterize evolutionary dynamics and explanatory power in a recently developed statistical framework.

CONCLUSIONS

This work presents expectations of the gene duplicability and mutational opportunity hypotheses over time under different sets of assumptions. This expectation will enable formal testing of processes leading to duplicate gene retention.

Overexpression of TgERF1, a Transcription Factor from Tectona grandis, Increases Tolerance to Drought and Salt Stress in Tobacco

Teak (Tectona grandis) is one of the most important wood sources, and it is cultivated in tropical regions with a significant market around the world. Abiotic stresses are an increasingly common and worrying environmental phenomenon because it causes production losses in both agriculture and forestry. Plants adapt to these stress conditions by activation or repression of specific genes, and they synthesize numerous stress proteins to maintain their cellular function. For example, APETALA2/ethylene response factor (AP2/ERF) was found to be involved in stress signal transduction. A search in the teak transcriptome database identified an AP2/ERF gene named TgERF1 with a key AP2/ERF domain. We then verified that the TgERF1 expression is rapidly induced by Polyethylene Glycol (PEG), NaCl, and exogenous phytohormone treatments, suggesting a potential role in drought and salt stress tolerance in teak. The full-length coding sequence of TgERF1 gene was isolated from teak young stems, characterized, cloned, and constitutively overexpressed in tobacco plants. In transgenic tobacco plants, the overexpressed TgERF1 protein was localized exclusively in the cell nucleus, as expected for a transcription factor. Furthermore, functional characterization of TgERF1 provided evidence that TgERF1 is a promising candidate gene to be used as selective marker on plant breeding intending to improve plant stress tolerance.

Developing future heat-resilient vegetable crops

Climate change seriously impacts global agriculture, with rising temperatures directly affecting the yield. Vegetables are an essential part of daily human consumption and thus have importance among all agricultural crops. The human population is increasing daily, so there is a need for alternative ways which can be helpful in maximizing the harvestable yield of vegetables. The increase in temperature directly affects the plants' biochemical and molecular processes; having a significant impact on quality and yield. Breeding for climate-resilient crops with good yields takes a long time and lots of breeding efforts. However, with the advent of new omics technologies, such as genomics, transcriptomics, proteomics, and metabolomics, the efficiency and efficacy of unearthing information on pathways associated with high-temperature stress resilience has improved in many of the vegetable crops. Besides omics, the use of genomics-assisted breeding and new breeding approaches such as gene editing and speed breeding allow creation of modern vegetable cultivars that are more resilient to high temperatures. Collectively, these approaches will shorten the time to create and release novel vegetable varieties to meet growing demands for productivity and quality. This review discusses the effects of heat stress on vegetables and highlights recent research with a focus on how omics and genome editing can produce temperature-resilient vegetables more efficiently and faster.

Evolution of the DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEIN subfamily in green plants

Adapting to unfavorable environments is a necessary step in plant terrestrialization and radiation. The dehydration-responsive element-binding (DREB) protein subfamily plays a pivotal role in plant abiotic stress regulation. However, relationships between the origin and expansion of the DREB subfamily and adaptive evolution of land plants are still being elucidated. Here, we constructed the evolutionary history of the DREB subfamily by compiling APETALA2/ethylene-responsive element-binding protein superfamily genes from 169 representative species of green plants. Through extensive phylogenetic analyses and comparative genomic analysis, our results revealed that the DREB subfamily diverged from the ethylene-responsive factor (ERF) subfamily in the common ancestor of Zygnemophyceae and Embryophyta during the colonization of land by plants, followed by expansions to form three different ancient archetypal genes in Zygnemophyceae species, designated as groups archetype-I, archetype-II/III, and archetype-IV. Four large-scale expansions paralleling the evolution of land plants led to the nine-subgroup divergence of group archetype-II/III in angiosperms, and five whole-genome duplications during Brassicaceae and Poaceae radiation shaped the diversity of subgroup IIb-1. We identified a Poaceae-specific gene in subgroup IIb-1, ERF014, remaining in a Poaceae-specific microsynteny block and co-evolving with a small heat shock protein cluster. Expression analyses demonstrated that heat acclimation may have driven the neofunctionalization of ERF014s in Pooideae by engaging in the conserved heat-responsive module in Poaceae. This study provides insights into lineage-specific expansion and neofunctionalization in the DREB subfamily, together with evolutionary information valuable for future functional studies of plant stress biology.

Pervasive misannotation of microexons that are evolutionarily conserved and crucial for gene function in plants

It is challenging to identify the smallest microexons (≤15-nt) due to their small size. Consequently, these microexons are often misannotated or missed entirely during genome annotation. Here, we develop a pipeline to accurately identify 2,398 small microexons in 10 diverse plant species using 990 RNA-seq datasets, and most of them have not been annotated in the reference genomes. Analysis reveals that microexons tend to have increased detained flanking introns that require post-transcriptional splicing after polyadenylation. Examination of 45 conserved microexon clusters demonstrates that microexons and associated gene structures can be traced back to the origin of land plants. Based on these clusters, we develop an algorithm to genome-wide model coding microexons in 132 plants and find that microexons provide a strong phylogenetic signal for plant organismal relationships. Microexon modeling reveals diverse evolutionary trajectories, involving microexon gain and loss and alternative splicing. Our work provides a comprehensive view of microexons in plants.

The small size (≤15-nt) of micorexons poses difficulties for genome annotation and identification using standard RNA sequence mapping approaches. Here, the authors develop computational pipelines to discover and predict microexons in plants and reveal diverse evolutionary trajectories via genomewide microexon modeling.

Lineage-Specific Genes and Family Expansions in Dictyostelid Genomes Display Expression Bias and Evolutionary Diversification during Development

Gene duplications generate new genes that can contribute to expression changes and the evolution of new functions. Genomes often consist of gene families that undergo expansions, some of which occur in specific lineages that reflect recent adaptive diversification. In this study, lineage-specific genes and gene family expansions were studied across five dictyostelid species to determine when and how they are expressed during multicellular development. Lineage-specific genes were found to be enriched among genes with biased expression (predominant expression in one developmental stage) in each species and at most developmental time points, suggesting independent functional innovations of new genes throughout the phylogeny. Biased duplicate genes had greater expression divergence than their orthologs and paralogs, consistent with subfunctionalization or neofunctionalization. Lineage-specific expansions in particular had biased genes with both molecular signals of positive selection and high expression, suggesting adaptive genetic and transcriptional diversification following duplication. Our results present insights into the potential contributions of lineage-specific genes and families in generating species-specific phenotypes during multicellular development in dictyostelids.

Transcriptome analysis of rice response to blast fungus identified core genes involved in immunity

Rice blast disease caused by the filamentous Ascomycetous fungus Magnaporthe oryzae is a major threat to rice production worldwide. The mechanisms underlying rice resistance to M. oryzae, such as transcriptional reprogramming and signalling networks, remain elusive. In this study, we carried out an in-depth comparative transcriptome study on the susceptible and resistant rice cultivars in response to M. oryzae. Our analysis highlighted that rapid, high-amplitude transcriptional reprogramming was important for rice defence against blast fungus. Ribosome- and protein translation-related genes were significantly enriched among differentially expressed genes (DEGs) at 12 hpi in both cultivars, indicating that the protein translation machinery is regulated in the activation of immunity in rice. Furthermore, we identified a core set of genes that are involved in the rice response to both biotic and abiotic stress. More importantly, among the core genes, we demonstrated that the metallothionein OsMT1a and OsMT1b genes positively regulated rice resistance while a peroxidase gene Perox4 negatively regulated rice resistance to M. oryzae. Our study provides novel insight into transcriptional reprogramming and serves as a valuable resource for functional studies on rice immune signalling components in resistance to blast disease.

Genome-wide analysis, identification, evolution and genomic organization of dehydration responsive element-binding (DREB) gene family in Solanum tuberosum

Background

The dehydration responsive element-binding (DREB) gene family plays a crucial role as transcription regulators and enhances plant tolerance to abiotic stresses. Although the DREB gene family has been identified and characterized in many plants, knowledge about it in Solanum tuberosum (Potato) is limited.

Results

In the present study, StDREB gene family was comprehensively analyzed using bioinformatics approaches. We identified 66 StDREB genes through genome wide screening of the Potato genome based on the AP2 domain architecture and amino acid conservation analysis (Valine at position 14th). Phylogenetic analysis divided them into six distinct subgroups (A1–A6). The categorization of StDREB genes into six subgroups was further supported by gene structure and conserved motif analysis. Potato DREB genes were found to be distributed unevenly across 12 chromosomes. Gene duplication proved that StDREB genes experienced tandem and segmental duplication events which led to the expansion of the gene family. The Ka/Ks ratios of the orthologous pairs also demonstrated the StDREB genes were under strong purification selection in the course of evolution. Interspecies synteny analysis revealed 45 and 36 StDREB genes were orthologous to Arabidopsis and Solanum lycopersicum, respectively. Moreover, subcellular localization indicated that StDREB genes were predominantly located within the nucleus and the StDREB family’s major function was DNA binding according to gene ontology (GO) annotation.

Conclusions

This study provides a comprehensive and systematic understanding of precise molecular mechanism and functional characterization of StDREB genes in abiotic stress responses and will lead to improvement in Solanum tuberosum.

Plant Transcription Factors Involved in Drought and Associated Stresses

Transcription factors (TFs) play a significant role in signal transduction networks spanning the perception of a stress signal and the expression of corresponding stress-responsive genes. TFs are multi-functional proteins that may simultaneously control numerous pathways during stresses in plants-this makes them powerful tools for the manipulation of regulatory and stress-responsive pathways. In recent years, the structure-function relationships of numerous plant TFs involved in drought and associated stresses have been defined, which prompted devising practical strategies for engineering plants with enhanced stress tolerance. Vast data have emerged on purposely basic leucine zipper (bZIP), WRKY, homeodomain-leucine zipper (HD-Zip), myeloblastoma (MYB), drought-response elements binding proteins/C-repeat binding factor (DREB/CBF), shine (SHN), and wax production-like (WXPL) TFs that reflect the understanding of their 3D structure and how the structure relates to function. Consequently, this information is useful in the tailored design of variant TFs that enhances our understanding of their functional states, such as oligomerization, post-translational modification patterns, protein-protein interactions, and their abilities to recognize downstream target DNA sequences. Here, we report on the progress of TFs based on their interaction pathway participation in stress-responsive networks, and pinpoint strategies and applications for crops and the impact of these strategies for improving plant stress tolerance.

Gene co-expression analysis reveals transcriptome divergence between wild and cultivated chickpea under drought stress

Ancestral adaptations in crop wild relatives can provide a genetic reservoir for crop improvement. Here we document physiological changes to mild and severe drought stress, and the associated transcriptome dynamics in both wild and cultivated chickpea. Over 60% of transcriptional changes were related to metabolism, indicating that metabolic plasticity is a core and conserved drought response. In addition, changes in RNA processing and protein turnover were predominant in the data, suggestive of broad restructuring of the chickpea proteome in response to drought. While 12% of the drought-responsive transcripts have similar dynamics in cultivated and wild accessions, numerous transcripts had expression patterns unique to particular genotypes, or that distinguished wild from cultivated genotypes and whose divergence may be a consequence of domestication. These and other comparisons provide a transcriptional correlate of previously described species' genetic diversity, with wild accessions well differentiated from each other and from cultivars, and cultivars essentially indistinguishable at the broad transcriptome level. We identified metabolic pathways such as phenylpropanoid metabolism, and biological processes such as stomatal development, which are differentially regulated across genotypes with potential consequences on drought tolerance. These data indicate that wild Cicer reticulatum may provide both conserved and divergent mechanisms as a resource in breeding for drought tolerance in cultivated chickpea.

Gene duplication and stress genomics in Brassicas: Current understanding and future prospects

Polyploidy or whole genome duplication (WGD) is an evolutionary phenomenon that happened in all angiosperms multiple times over millions of years. Extensive studies on the model plant Arabidopsis thaliana genome have revealed that it has undergone five rounds of WGDs followed, in the Brassicaceae tribe, by a characteristic whole genome triplication (WGT). In addition, small-scale events such as tandem or segmental duplications and retrotransposition also enable plants to reshape their genomes. Over the decades, extensive research efforts have been undertaken to understand the evolutionary significance of polyploidy. On the other hand, much less attention has been paid to understanding the impact of gene duplication on the diversification of important stress response genes. The main objective of this review is to discuss key aspects of gene and genome duplications with a focus on genes primarily regulated by osmotic stresses. The focal family is the Brassicaceae, since it (i) underwent multiple rounds of WGDs plus WGTs, (ii) hosts many economically important crops and wild relatives that are tolerant to a range of stresses, and (iii) comprises many species that have already been sequenced. Diverse molecular mechanisms that lead to structural and regulatory alterations of duplicated genes are discussed. Examples are drawn from recent literature to elucidate expanded, stress responsive gene families identified from different Brassica crops. A combined bioinformatic and transcriptomic method has been proposed and tested on a known stress-responsive gene pair to prove that stress-responsive duplicated allelic variants can be identified by this method. Finally, future prospects for engineering these genes into crops to enhance stress tolerance are discussed, and important resources for Brassica genome research are provided.

Genome-wide investigation and expression analysis of APETALA-2 transcription factor subfamily reveals its evolution, expansion and regulatory role in abiotic stress responses in Indica Rice (Oryza sativa L. ssp. indica)

Rice is an important cereal crop that serves as staple food for more than half of the world population. Abiotic stresses resulting from changing climatic conditions are continuously threating its yield and production. Genes in APETALA-2 (AP2) family encode transcriptional regulators implicated during regulation of developmental processes and abiotic stress responses but their identification and characterization in indica rice was still missing. In this context, twenty-six genes distributed among eleven chromosomes in Indica rice encoding AP2 transcription-factor subfamily were identified and their diverse haplotypes were studied. Phylogenetic analysis of OsAP2 TF family-members grouped them into three clades indicating conservation of clades among cereals. Segmental duplications were observed to be principal route of evolution, supporting the higher positive selection-pressure, which were estimated to be originated about 10.57 to 56.72 million years ago (MYA). Conserved domain analysis and intron-exon distribution pattern of identified OsAP2s revealed their exclusive distribution among the specific clades of the phylogenetic tree. Moreover, the members of osa-miR172 family were also identified potentially targeting four OsAP2 genes. The real-time quantitative expression profiling of OsAP2s under heat stress conditions in contrasting indica rice genotypes revealed the differential expression pattern of OsAP2s (6 genes up-regulated and 4 genes down-regulated) in stress- and genotype-dependent manner. These findings unveiled the evolutionary pathways of AP2-TF in rice, and can help the functional characterization under developmental and stress responses.

Phylogenomic analysis of the APETALA2 transcription factor subfamily across angiosperms reveals both deep conservation and lineage-specific patterns

The APETALA2 (AP2) subfamily of transcription factors are key regulators of angiosperm root, shoot, flower and embryo development. The broad diversity of anatomical and morphological structures is potentially associated with the genomic dynamics of the AP2 subfamily. However, a comprehensive phylogenomic analysis of the AP2 subfamily across angiosperms is lacking. We combined phylogenetic and synteny analysis of distinct AP2 subclades in the completed genomes of 107 angiosperm species. We identified major changes in copy number variation and genomic context within subclades across lineages, and discuss how these changes may have contributed to the evolution of lineage-specific traits. Multiple AP2 subclades show highly conserved patterns of copy number and synteny across angiosperms, while others are more dynamic and show distinct lineage-specific patterns. As examples of lineage-specific morphological divergence due to AP2 subclade dynamics, we hypothesize that loss of PLETHORA1/2 in monocots correlates with the absence of taproots, whereas independent lineage-specific changes of PLETHORA4/BABY BOOM and WRINKLED1 genes in Brassicaceae and monocots point towards regulatory divergence of embryogenesis between these lineages. Additionally, copy number expansion of TOE1 and TOE3/AP2 in asterids is implicated with differential regulation of flower development. Moreover, we show that the genomic context of AP2s is in general highly specialized per angiosperm lineage. To our knowledge, this study is the first to shed light on the evolutionary divergence of the AP2 subfamily subclades across major angiosperm lineages and emphasizes the need for lineage-specific characterization of developmental networks to understand trait variability further.

Genome-Wide Analysis of the DREB Subfamily in Saccharum spontaneum Reveals Their Functional Divergence During Cold and Drought Stresses

Drought and cold stresses are the main environmental factors that affect the yield of sugarcane, and DREB genes play very important roles in tolerance to drought, cold, and other environmental stresses. In this study, bioinformatics analysis was performed to characterize Saccharum spontaneum SsDREB genes. RNA sequencing (RNA-seq) was used to detect the expression profiles of SsDREBs induced by cold and drought stresses. According to our results, there are 110 SsDREB subfamily proteins in S. spontaneum, which can be classified into six groups; 106 of these genes are distributed among 29 chromosomes. Inter- and intraspecies synteny analyses suggested that all DREB groups have undergone gene duplication, highlighting the polyploid events that played an important role in the expansion of the DREB subfamily. Furthermore, RNA-seq results showed that 45 SsDREBs were up- or downregulated under cold stress; 35 of them were found to be involved in responding to drought stress. According to protein–protein interaction analysis, SsDREB100, SsDREB102, and SsDREB105 play key roles during the response to cold stress. These results reveal that functional divergence exists between collinear homologous genes or among common origin genes in the DREB subfamily of S. spontaneum. This study presents a comprehensive analysis and systematic understanding of the precise mechanism of SsDREBs in response to abiotic stress and will lead to improvements in sugarcane.