Genome-wide investigation and expression profiling of AP2/ERF transcription factor superfamily in foxtail millet (Setaria italica L.)

Drought resistance strategies in minor millets: a review

MAIN CONCLUSION

The review discusses growth and drought-response mechanisms in minor millets under three themes: drought escape, drought avoidance and drought tolerance. Drought is one of the most prominent abiotic stresses impacting plant growth, performance, and productivity. In the context of climate change, the prevalence and severity of drought is expected to increase in many agricultural regions worldwide. Millets (coarse grains) are a group of small-seeded grasses cultivated in arid and semi-arid regions throughout the world and are an important source of food and feed for humans and livestock. Although minor millets, i.e., foxtail millet, finger millet, proso millet, barnyard millet, kodo millet and little millet are generally hardier and more drought-resistant than cereals and major millets (sorghum and pearl millet), understanding their responses, processes and strategies in response to drought is more limited. Here, we review drought resistance strategies in minor millets under three themes: drought escape (e.g., short crop cycle, short vegetative period, developmental plasticity and remobilization of assimilates), drought avoidance (e.g., root traits for better water absorption and leaf traits to control water loss), and drought tolerance (e.g., osmotic adjustment, maintenance of photosynthetic ability and antioxidant potential). Data from 'omics' studies are summarized to provide an overview of the molecular mechanisms important in drought tolerance. In addition, the final section highlights knowledge gaps and challenges to improving minor millets. This review is intended to enhance major cereals and millet per se in light of climate-related increases in aridity.

Genome-wide identification and expression profiling of the ABI5 gene family in foxtail millet (Setaria italica)

BACKGROUND

ABA Insensitive 5 (ABI5) is a basic leucine zipper transcription factor that crucially influences plant growth, development, and stress response. However, there is minimal research on the ABI5 family in foxtail millet.

RESULTS

In this study, 16 ABI5 genes were identified in foxtail millet, and their sequence composition, gene structures, cis-acting elements, chromosome positions, and gene replication events were analyzed. To more thoroughly evaluate the developmental mechanisms of the SiABI5 family during evolution, we selected three dicotyledons (S. lycopersicum, A. thaliana, F. tataricum) and three (Z. mays, O. sativa, S. bicolor) specific representative monocotyledons associated with foxtail millet for comparative homology mapping. The results showed that foxtail millet ABI5 genes had the best homology with maize. A promoter sequence analysis showed that the SiABI5s contain numerous cis-acting elements related to hormone and stress responses, indicating that the regulation of SiABI5 expression was complex. The expression responses of 16 genes in different tissues, seed germination, and ear development were analyzed. A total of six representative genes were targeted from five subfamilies to characterize their gene expression responses to four different abiotic stresses. Overexpression of SiABI5.12 confers tolerance to osmotic stress in transgenic Arabidopsis thaliana, which demonstrated the function of SiABI5 responded to abiotic stress.

CONCLUSIONS

In summary, our research results comprehensively characterized the SiABI5 family and can provide a valuable reference for demonstrating the role of SiABI5s in regulating abiotic stress responses in foxtail millet.

Genome-Wide Identification and Transcriptional Analysis of AP2/ERF Gene Family in Pearl Millet (Pennisetum glaucum)

The apetala2/ethylene response factor (AP2/ERF) gene family plays a crucial role in regulating plant growth and development and responding to different abiotic stresses (e.g., drought, heat, cold, and salinity). However, the knowledge of the ERF family in pearl millet remains limited. Here, a total of 167 high-confidence PgERF genes are identified and divided into five subgroups based on gene-conserved structure and phylogenetic analysis. Forty-one pairs of segmental duplication are found using collinear analysis. Nucleotide substitution analysis reveals these duplicated pairs are under positive purification, indicating they are actively responding to natural selection. Comprehensive transcriptomic analysis reveals that PgERF genesare preferentially expressed in the imbibed seeds and stem (tilling stage) and respond to heat, drought, and salt stress. Prediction of the cis-regulatory element by the PlantCARE program indicates that PgERF genes are involved in responses to environmental stimuli. Using reverse transcription quantitative real-time PCR (RT-qPCR), expression profiles of eleven selected PgERF genes are monitored in various tissues and during different abiotic stresses. Transcript levels of each PgERF gene exhibit significant changes during stress treatments. Notably, the PgERF7 gene is the only candidate that can be induced by all adverse conditions. Furthermore, four PgERF genes (i.e., PgERF22, PgERF37, PgERF88, and PgERF155) are shown to be involved in the ABA-dependent signaling pathway. These results provide useful bioinformatic and transcriptional information for understanding the roles of the pearl millet ERF gene family in adaptation to climate change.

Transcriptome Profiles Reveals ScDREB10 from Syntrichia caninervis Regulated Phenylpropanoid Biosynthesis and Starch/Sucrose Metabolism to Enhance Plant Stress Tolerance

Desiccation is a kind of extreme form of drought stress and desiccation tolerance (DT) is an ancient trait of plants that allows them to survive tissue water potentials reaching -100 MPa or lower. ScDREB10 is a DREB A-5 transcription factor gene from a DT moss named Syntrichia caninervis, which has strong comprehensive tolerance to osmotic and salt stresses. This study delves further into the molecular mechanism of ScDREB10 stress tolerance based on the transcriptome data of the overexpression of ScDREB10 in Arabidopsis under control, osmotic and salt treatments. The transcriptional analysis of weight gene co-expression network analysis (WGCNA) showed that "phenylpropanoid biosynthesis" and "starch and sucrose metabolism" were key pathways in the network of cyan and yellow modules. Meanwhile, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of differentially expressed genes (DEGs) also showed that "phenylpropanoid biosynthesis" and "starch and sucrose metabolism" pathways demonstrate the highest enrichment in response to osmotic and salt stress, respectively. Quantitative real-time PCR (qRT-PCR) results confirmed that most genes related to phenylpropanoid biosynthesis" and "starch and sucrose metabolism" pathways in overexpressing ScDREB10 Arabidopsis were up-regulated in response to osmotic and salt stresses, respectively. In line with the results, the corresponding lignin, sucrose, and trehalose contents and sucrose phosphate synthase activities were also increased in overexpressing ScDREB10 Arabidopsis under osmotic and salt stress treatments. Additionally, cis-acting promoter element analyses and yeast one-hybrid experiments showed that ScDREB10 was not only able to bind with classical cis-elements, such as DRE and TATCCC (MYBST1), but also bind with unknown element CGTCCA. All of these findings suggest that ScDREB10 may regulate plant stress tolerance by effecting phenylpropanoid biosynthesis, and starch and sucrose metabolism pathways. This research provides insights into the molecular mechanisms underpinning ScDREB10-mediated stress tolerance and contributes to deeply understanding the A-5 DREB regulatory mechanism.

Genome-wide characterization and evolutionary analysis of the AP2/ERF gene family in lettuce (Lactuca sativa)

The APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF) gene family plays vital roles in plants, serving as a key regulator in responses to abiotic stresses. Despite its significance, a comprehensive understanding of this family in lettuce remains incomplete. In this study, we performed a genome-wide search for the AP2/ERF family in lettuce and identified a total of 224 members. The duplication patterns provided evidence that both tandem and segmental duplications contributed to the expansion of this family. Ka/Ks ratio analysis demonstrated that, following duplication events, the genes have been subjected to purifying selection pressure, leading to selective constraints on their protein sequence. This selective pressure provides a dosage benefit against stresses in plants. Additionally, a transcriptome analysis indicated that some duplicated genes gained novel functions, emphasizing the contribution of both dosage effect and functional divergence to the family functionalities. Furthermore, an orthologous relationship study showed that 60% of genes descended from a common ancestor of Rosid and Asterid lineages, 28% from the Asterid ancestor, and 12% evolved in the lettuce lineage, suggesting lineage-specific roles in adaptive evolution. These results provide valuable insights into the evolutionary mechanisms of the AP2/ERF gene family in lettuce, with implications for enhancing abiotic stress tolerance, ultimately contributing to the genetic improvement of lettuce crop production.

Comprehensive and evolutionary analysis of Spodoptera litura-inducible Cytochrome P450 monooxygenase gene family in Glycine max elucidate their role in defense

Plants being sessile organisms and lacking both circulating phagocytic cells and somatic adaptive immune response, have thrived on various defense mechanisms to fend off insect pests and invasion of pathogens. CYP450s are the versatile enzymes, which thwart plants against insect pests by ubiquitous biosynthesis of phytohormones, antioxidants, and secondary metabolites, utilizing them as feeding deterrents and direct toxins. Therefore, a comprehensive analysis of biotic stress-responsive CYPs from Glycine max was performed to ascertain their function against S. litura-infestation. Phylogenetic analysis and evolutionary studies on conserved domains and motifs disclosed the evolutionary correspondence of these GmCYPs with already characterized members of the CYP450 superfamily and close relatedness to Medicago truncatula. These GmCYPs were mapped on 13 chromosomes; they possess 1-8 exons; they have evolved due to duplication and are localized in endoplasmic reticulumn. Further, identification of methyl-jasmonate, salicylic acid, defense responsive and flavonoid biosynthesis regulating cis-acting elements, their interaction with biotic stress regulating proteins and their differential expression in diverse types of tissues, and during herbivory, depicted their responsiveness to biotic stress. Three-dimensional homology modelling of GmCYPs, docking with heme cofactor required for their catalytic activity and enzyme-substrate interactions were performed to understand the functional mechanism of their action. Moreover, to gain insight into their involvement in plant defense, gene expression analysis was evaluated, which revealed differential expression of 11 GmCYPs upon S. litura-infestation, 12 GmCYPs on wounding while foliar spray of ethylene, methyl-jasmonate and salicylic acid differentially regulated 11 GmCYPs, 6 GmCYPs, and 10 GmCYPs respectively. Our study comprehensively analysed the underlying mechanism of GmCYPs function during S. litura-infestation, which can be further utilized for functional characterization to develop new strategies for enhancing soybean resistance to insect pests.

Deciphering mitogen activated protein kinase pathway activated during insect attack in Nicotiana attenuata

Plant yields are compromised due to abiotic and biotic stresses. A crucial biotic stress instigated by insect attack, is a major concern that limits crop production. To overcome the deleterious effect of herbivory, pesticides are used but long-term usage of pesticides can be harmful to the environment and human health. Understanding the plants' inherent defense mechanism by interpreting the interaction pattern of defense-related proteins and signalling components and manipulating them to strengthen defense status, is one of the alternative approaches of green biotechnology. During insect attack, host plants initiate innumerable signalling pathways to activate defense response; Mitogen Activated Protein Kinase (MAPK) Pathway is a crucial component of signalling pathway that regulate the expression of downstream defense-related genes. MAPK pathway has three components: MAPKKK, MAPKK and MAPK. Earlier studies have shown participation of SIPK and WIPK (MAPKs) as well as MEK2 (MAPKK) during insect infestation and its association with plant defense. However, information on the third component and elucidation of the complete MAPK pathway are still elusive. Therefore, this study aims to identify the unknown component and decipher MAPK pathway in Nicotiana attenuata involved in plant defense against herbivory by identifying herbivory-inducible MAPKKKs and and their interaction with known partners of the MAPK pathway by docking and MD simulation. The possible pathway was predicted to be MAPKKK Na12134/Na04522-MEK2-SIPK/WIPK. Further, validation of the above interaction by in vitro and in vivo methods is highly recommended.Communicated by Ramaswamy H. Sarma.

Genetic enhancement of climate-resilient traits in small millets: A review

Agriculture is facing the challenge of feeding the ever-growing population that is projected to reach ten billion by 2050. While improving crop yield and productivity can address this challenge, the increasing effects of global warming and climate change seriously threaten agricultural productivity. Thus, genomics and genome modification technologies are crucial to improving climate-resilient traits to enable sustained yield and productivity; however, significant research focuses on staple crops such as rice, wheat, and maize. Crops that are naturally climate-resilient and nutritionally superior to staple cereals, such as small millets, remain neglected and underutilized by mainstream research. The ability of small millets to grow in marginal regions having limited irrigation and poor soil fertility makes these crops a better choice for cultivation in arid and semi-arid areas. Hence, mainstreaming small millets for cultivation and using omics technologies to dissect the climate-resilient traits to identify the molecular determinants underlying these traits are imperative for addressing food and nutritional security. In this context, the review discusses the genomics and genome modification approaches for dissecting key traits in small millets and their application for improving these traits in cultivated germplasm. The review also discusses biofortification for nutritional security and machine-learning approaches for trait improvement in small millets. Altogether, the review provides a roadmap for the effective use of next-generation approaches for trait improvement in small millets. This will lead to the development of improved varieties for addressing multiple insecurities prevailing in the present climate change scenario.

CsAP2-09 confers resistance against citrus bacterial canker by regulating CsGH3.1L-mediated phytohormone biosynthesis

Citrus bacterial canker (CBC) is a serious bacterial disease affecting citrus plantations and the citrus industry all over the world. We have previously shown that an apetala 2/ethylene response factor in Citrus sinensis, CsAP2-09, positively regulated resistance to CBC, although the regulatory mechanisms remained undetermined. Here, we demonstrated that CsAP2-09 positively and sustainably controlled resistance to CBC in three-year transgenic plants. CsAP2-09 was found to be a transcriptional activator, and qRT-PCR and dual luciferase assays showed that it controlled the expression CsGH3.1L. CsAP2-09 bound directly to the promotor of CsGH3.1L, shown by yeast one-hybrid assay, with the binding site confirmed by electrophoretic mobility shift assay. Biochemical assays showed that CsAP2-09 negatively regulated the biosynthesis of indole acetic acid (IAA) and positively regulated that of salicylic acid (SA) and ethylene, verified with transient overexpression of CsGH3.1L. The combination of these results with those of previous reports indicated that SA, ethylene, and IAA can directly regulate CBC resistance. Overall, we revealed a pathway whereby CsAP2-09 conferred CBC resistance by direct binding to the CsGH3.1L promoter, activating its expression and modulating IAA, SA, and ethylene biosynthesis. Our study indicates the potential value of manipulating CsAP2-09 and CsGH3.1L in the breeding of CBC-resistant citrus.

Wide-Range Portrayal of AP2/ERF Transcription Factor Family in Maize (Zea mays L.) Development and Stress Responses

The APETALA2/Ethylene-Responsive Transcriptional Factors containing conservative AP2/ERF domains constituted a plant-specific transcription factor (TF) superfamily, called AP2/ERF. The configuration of the AP2/ERF superfamily in maize has remained unresolved. In this study, we identified the 229 AP2/ERF genes in the latest (B73 RefGen_v5) maize reference genome. Phylogenetic classification of the ZmAP2/ERF family members categorized it into five clades, including 27 AP2 (APETALA2), 5 RAV (Related to ABI3/VP), 89 DREB (dehydration responsive element binding), 105 ERF (ethylene responsive factors), and a soloist. The duplication events of the paralogous genes occurred from 1.724-25.855 MYA, a key route to maize evolution. Structural analysis reveals that they have more introns and few exons. The results showed that 32 ZmAP2/ERFs regulate biotic stresses, and 24 ZmAP2/ERFs are involved in responses towards abiotic stresses. Additionally, the expression analysis showed that DREB family members are involved in plant sex determination. The real-time quantitative expression profiling of ZmAP2/ERFs in the leaves of the maize inbred line B73 under ABA, JA, salt, drought, heat, and wounding stress revealed their specific expression patterns. Conclusively, this study unveiled the evolutionary pathway of ZmAP2/ERFs and its essential role in stress and developmental processes. The generated information will be useful for stress resilience maize breeding programs.

Genome-Wide Analysis of AP2/ERF Gene Superfamily in Ramie (Boehmeria nivea L.) Revealed Their Synergistic Roles in Regulating Abiotic Stress Resistance and Ramet Development

AP2/ERF transcription factors (TFs) are one of the largest superfamilies in plants, and play vital roles in growth and response to biotic/abiotic stresses. Although the AP2/ERF family has been extensively characterized in many species, very little is known about this family in ramie (Boehmeria nivea L.). In this study, 138 AP2/ERF TFs were identified from the ramie genome and were grouped into five subfamilies, including the AP2 (19), RAV (5), Soloist (1), ERF (77), and DREB (36). Unique motifs were found in the DREB/ERF subfamily members, implying significance to the AP2/ERF TF functions in these evolutionary branches. Segmental duplication events were found to play predominant roles in the BnAP2/ERF TF family expansion. Light-, stress-, and phytohormone-responsive elements were identified in the promoter region of BnAP2/ERF genes, with abscisic acid response elements (ABRE), methyl jasmonate response elements, and the dehydration response element (DRE) being dominant. The integrated transcriptome and quantitative real-time PCR (qPCR) revealed 12 key BnAP2/ERF genes positively responding to waterlogging. Five of the genes are also involved in ramet development, with two (BnERF-30 and BnERF-32) further showing multifunctional roles. The protein interaction prediction analysis further verified their crosstalk mechanism in coordinating waterlogging resistance and ramet development. Our study provides new insights into the presence of AP2/ERF TFs in ramie, and provides candidate AP2/ERF TFs for further studies on breeding varieties with coupling between water stress tolerance and high yield.

Genome-wide identification, molecular evolution and expression analysis of the non-specific lipid transfer protein (nsLTP) family in Setaria italica

BACKGROUND

Foxtail millet (Setaria italica L.) is a millet species with high tolerance to stressful environments. Plant non-specific lipid transfer proteins (nsLTPs) are a kind of small, basic proteins involved in many biological processes. So far, the genome of S. italica has been fully sequenced, and a comprehensive understanding of the evolution and expression of the nsLTP family is still lacking in foxtail millet.

RESULTS

Forty-five nsLTP genes were identified in S. italica and clustered into 5 subfamilies except three single genes (SinsLTP38, SinsLTP7, and SinsLTP44). The proportion of SinsLTPs was different in each subfamily, and members within the same subgroup shared conserved exon-intron structures. Besides, 5 SinsLTP duplication events were investigated. Both tandem and segmental duplication contributed to nsLTP expansion in S. italica, and the duplicated SinsLTPs had mainly undergone purifying selection pressure, which suggested that the function of the duplicated SinsLTPs might not diverge much. Moreover, we identified the nsLTP members in 5 other monocots, and 41, 13, 10, 4, and 1 orthologous gene pairs were identified between S. italica and S. viridis, S. bicolor, Z. mays, O. sativa, and B. distachyon, respectively. The functional divergence within the nsLTP orthologous genes might be limited. In addition, the tissue-specific expression patterns of the SinsLTPs were investigated, and the expression profiles of the SinsLTPs in response to abiotic stress were analyzed, all the 10 selected SinsLTPs were responsive to drought, salt, and cold stress. Among the selected SinsLTPs, 2 paired duplicated genes shared almost equivalent expression profiles, suggesting that these duplicated genes might retain some essential functions during subsequent evolution.

CONCLUSIONS

The present study provided the first systematic analysis for the phylogenetic classification, conserved domain and gene structure, expansion pattern, and expression profile of the nsLTP family in S. italica. These findings could pave a way for further comparative genomic and evolution analysis of nsLTP family in foxtail millet and related monocots, and lay the foundation for the functional analysis of the nsLTPs in S. italica.

Multi-omics intervention in Setaria to dissect climate-resilient traits: Progress and prospects

Millets constitute a significant proportion of underutilized grasses and are well known for their climate resilience as well as excellent nutritional profiles. Among millets, foxtail millet (Setaria italica) and its wild relative green foxtail (S. viridis) are collectively regarded as models for studying broad-spectrum traits, including abiotic stress tolerance, C₄ photosynthesis, biofuel, and nutritional traits. Since the genome sequence release, the crop has seen an exponential increase in omics studies to dissect agronomic, nutritional, biofuel, and climate-resilience traits. These studies have provided first-hand information on the structure, organization, evolution, and expression of several genes; however, knowledge of the precise roles of such genes and their products remains elusive. Several open-access databases have also been instituted to enable advanced scientific research on these important crops. In this context, the current review enumerates the contemporary trend of research on understanding the climate resilience and other essential traits in Setaria, the knowledge gap, and how the information could be translated for the crop improvement of related millets, biofuel crops, and cereals. Also, the review provides a roadmap for studying other underutilized crop species using Setaria as a model.

Identification of AP2/ERF transcription factors in Tetrastigma hemsleyanum revealed the specific roles of ERF46 under cold stress

Tetrastigma hemsleyanum (T. hemsleyanum) is a traditional medicinal plant that is widely used in China. Cultivated T. hemsleyanum usually encounters cold stress, limiting its growth and quality at key developmental stages. APETALA2 (AP2)/ethylene-responsive factor (ERF) transcription factors (TFs) comprise one of the largest gene superfamilies in plants and are widely involved in biotic and abiotic stresses. To reveal the roles of AP2/ERF TFs during T. hemsleyanum development, 70 AP2/ERF TFs were identified in T. hemsleyanum. Among them, 18 and 2 TFs were classified into the AP2 and RAV families, respectively. The other 50 TFs belonged to the ERF family and were further divided into the ERF and (dehydration reaction element binding factor) DREB subfamilies. The ERF subfamily contained 46 TFs, while the DREB subfamily contained 4 TFs. Phylogenetic analysis indicated that AP2/ERF TFs could be classified into five groups, in which 10 conserved motifs were confirmed. Several motifs were group- or subgroup-specific, implying that they were significant for the functions of the AP2/ERF TFs of these clades. In addition, 70 AP2/ERF TFs from the five groups were used for an expression pattern analysis under three low-temperature levels, namely, -4, 0, and 4°C. The majority of these AP2/ERF TFs exhibited a positive response to cold stress conditions. Specifically, ThERF5, ThERF31, ThERF46, and ThERF55 demonstrated a more sensitive response to cold stress. Moreover, AP2/ERF TFs exhibited specific expression patterns under cold stress. Transient overexpression and RNA interference indicated that ThERF46 has a specific tolerance to cold stress. These new insights provide the basis for further studies on the roles of AP2/ERF TFs in cold stress tolerance in T. hemsleyanum.

Genome‑wide identification, phylogenetic and expression pattern analysis of GATA family genes in foxtail millet (Setaria italica)

Background

Transcription factors (TFs) play important roles in plants. Among the major TFs, GATA plays a crucial role in plant development, growth, and stress responses. However, there have been few studies on the GATA gene family in foxtail millet (Setaria italica). The release of the foxtail millet reference genome presents an opportunity for the genome-wide characterization of these GATA genes.

Results

In this study, we identified 28 GATA genes in foxtail millet distributed on seven chromosomes. According to the classification method of GATA members in Arabidopsis, SiGATA was divided into four subfamilies, namely subfamilies I, II, III, and IV. Structural analysis of the SiGATA genes showed that subfamily III had more introns than other subfamilies, and a large number of cis-acting elements were abundant in the promoter region of the SiGATA genes. Three tandem duplications and five segmental duplications were found among SiGATA genes. Tissue-specific results showed that the SiGATA genes were mainly expressed in foxtail millet leaves, followed by peels and seeds. Many genes were significantly induced under the eight abiotic stresses, such as SiGATA10, SiGATA16, SiGATA18, and SiGATA25, which deserve further attention.

Conclusions

Collectively, these findings will be helpful for further in-depth studies of the biological function of SiGATA, and will provide a reference for the future molecular breeding of foxtail millet.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08786-0.

Genome-wide identification and characterization of the SPL gene family and its expression in the various developmental stages and stress conditions in foxtail millet (Setaria italica)

BACKGROUND

Among the major transcription factors, SPL plays a crucial role in plant growth, development, and stress response. Foxtail millet (Setaria italica), as a C4 crop, is rich in nutrients and is beneficial to human health. However, research on the foxtail millet SPL (SQUAMOSA PROMOTER BINDING-LIKE) gene family is limited.  RESULTS: In this study, a total of 18 SPL genes were identified for the comprehensive analysis of the whole genome of foxtail millet. These SiSPL genes were divided into seven subfamilies (I, II, III, V, VI, VII, and VIII) according to the classification of the Arabidopsis thaliana SPL gene family. Structural analysis of the SiSPL genes showed that the number of introns in subfamilies I and II were much larger than others, and the promoter regions of SiSPL genes were rich in different cis-acting elements. Among the 18 SiSPL genes, nine genes had putative binding sites with foxtail millet miR156. No tandem duplication events were found between the SiSPL genes, but four pairs of segmental duplications were detected. The SiSPL genes expression were detected in different tissues, which was generally highly expressed in seeds development process, especially SiSPL6 and SiSPL16, which deserve further study. The results of the expression levels of SiSPL genes under eight types of abiotic stresses showed that many stress responsive genes, especially SiSPL9, SiSPL10, and SiSPL16, were highly expressed under multiple stresses, which deserves further attention.

CONCLUSIONS

In this research, 18 SPL genes were identified in foxtail millet, and their phylogenetic relationships, gene structural features, duplication events, gene expression and potential roles in foxtail millet development were studied. The findings provide a new perspective for the mining of the excellent SiSPL gene and the molecular breeding of foxtail millet.

Time-series transcriptomics reveals a drought-responsive temporal network and crosstalk between drought stress and the circadian clock in foxtail millet

Drought stress is a serious factor affecting crop growth and production worldwide. The circadian clock has been identified as key to improving regional adaptability of plants. However, our understanding of the contribution of the circadian clock to drought response and the impacts of drought stress on the circadian clock in plants is still limited. To explore the interactions between the circadian clock and drought stress, foxtail millet seedlings were treated with simulated drought (20% polyethylene glycol-6000) treatment starting at the day (DD) onset zeitgeber time 0 (ZT0, lights on) and at the night (DN) onset zeitgeber time 16 (ZT16, lights off). A high temporal-resolution transcriptomic investigation was performed using DD and DN samples collected at intervals of 2 or 4 h within a 24-h drought-treatment period. Overall, we identified 13 294 drought-responsive genes (DRGs). Among these DRGs, 7931 were common between DD and DN samples, 2638 were specific to DD, and 2725 were specific to DN. Additionally, we identified 1257 circadian genes, of which 67% were DRGs. Interestingly, with drought treatment starting at the day for 8, 12 or 16 h, the circadian phase shifted to 12 h. We also found that the circadian clock led to different day and night drought-responsive pathways. The identification of DRG_Clock (DRG and circadian clock) and DRG_NonClock (DRG and not circadian clock) genes provides a reference for selecting candidate drought resistance genes. Our work reveals the temporal drought-response process and crosstalk between drought stress and the circadian clock in foxtail millet.

Genome-Wide Analysis of AP2/ERF Superfamily Genes in Contrasting Wheat Genotypes Reveals Heat Stress-Related Candidate Genes

The AP2/ERF superfamily is one of the largest groups of transcription factors (TFs) in plants, which plays important roles in regulating plant growth and development under heat stress. A complete genome-wide identification, characterization, and expression analysis of AP2/ERF superfamily genes focusing on heat stress response were conducted in bread wheat. This study identified 630 putative AP2/ERF superfamily TF genes in wheat, with 517 genes containing well-defined AP2-protein domains. They were classified into five sub-families, according to domain content, conserved motif, and gene structure. The unique genes identified in this study were 112 TaERF genes, 77 TaDREB genes, four TaAP2 genes, and one TaRAV gene. The chromosomal distribution analysis showed the unequal distribution of TaAP2/ERF genes in 21 wheat chromosomes, with 127 pairs of segmental duplications and one pair of tandem duplication, highly concentrated in TaERF and TaDREB sub-families. The qRT-PCR validation of differentially expressed genes (DEGs) in contrasting wheat genotypes under heat stress conditions revealed that significant DEGs in tolerant and susceptible genotypes could unequivocally differentiate tolerant and susceptible wheat genotypes. This study provides useful information on TaAP2/ERF superfamily genes and reveals candidate genes in response to heat stress, which forms a foundation for heat tolerance breeding in wheat.

A Comprehensive Analysis of Calmodulin-Like Proteins of Glycine max Indicates Their Role in Calcium Signaling and Plant Defense Against Insect Attack

The calcium (Ca²⁺) signaling is a crucial event during plant-herbivore interaction, which involves a transient change in cytosolic Ca²⁺ concentration, which is sensed by Ca²⁺-sensors, and the received message is transduced to downstream target proteins leading to appropriate defense response. Calmodulin-like proteins (CMLs) are calcium-sensing plant-specific proteins. Although CMLs have been identified in a few plants, they remained uncharacterized in leguminous crop plants. Therefore, a wide-range analysis of CMLs of soybean was performed, which identified 41 true CMLs with greater than 50% similarity with Arabidopsis CMLs. The phylogenetic study revealed their evolutionary relatedness with known CMLs. Further, the identification of conserved motifs, gene structure analysis, and identification of cis-acting elements strongly supported their identity as members of this family and their involvement in stress responses. Only a few Glycine max CMLs (GmCMLs) exhibited differential expression in different tissue types, and rest of them had minimal expression. Additionally, differential expression patterns of GmCMLs were observed during Spodoptera litura-feeding, wounding, and signaling compound treatments, indicating their role in plant defense. The three-dimensional structure prediction, identification of interactive domains, and docking with Ca²⁺ ions of S. litura-inducible GmCMLs, indicated their identity as calcium sensors. This study on the characterization of GmCMLs provided insights into their roles in calcium signaling and plant defense during herbivory.

Genome-wide identification and expression analysis of the bHLH transcription factor family and its response to abiotic stress in foxtail millet (Setaria italica L.)

Background

Members of the basic helix-loop-helix (bHLH) transcription factor family perform indispensable functions in various biological processes, such as plant growth, seed maturation, and abiotic stress responses. However, the bHLH family in foxtail millet (Setaria italica), an important food and feed crop, has not been thoroughly studied.

Results

In this study, 187 bHLH genes of foxtail millet (SibHLHs) were identified and renamed according to the chromosomal distribution of the SibHLH genes. Based on the number of conserved domains and gene structure, the SibHLH genes were divided into 21 subfamilies and two orphan genes via phylogenetic tree analysis. According to the phylogenetic tree, the subfamilies 15 and 18 may have experienced stronger expansion in the process of evolution. Then, the motif compositions, gene structures, chromosomal spread, and gene duplication events were discussed in detail. A total of sixteen tandem repeat events and thirty-eight pairs of segment duplications were identified in bHLH family of foxtail millet. To further investigate the evolutionary relationship in the SibHLH family, we constructed the comparative syntenic maps of foxtail millet associated with representative monocotyledons and dicotyledons species. Finally, the gene expression response characteristics of 15 typical SibHLH genes in different tissues and fruit development stages, and eight different abiotic stresses were analysed. The results showed that there were significant differences in the transcription levels of some SibHLH members in different tissues and fruit development stages, and different abiotic stresses, implying that SibHLH members might have different physiological functions.

Conclusions

In this study, we identified 187 SibHLH genes in foxtail millet and further analysed the evolution and expression patterns of the encoded proteins. The findings provide a comprehensive understanding of the bHLH family in foxtail millet, which will inform further studies on the functional characteristics of SibHLH genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08095-y.

Genome-wide identification, phylogeny and expression analysis of AP2/ERF transcription factors family in sweet potato

Background

In recent years, much attention has been given to AP2/ERF transcription factors because they play indispensable roles in many biological processes, such as plant development and biotic and abiotic stress responses. Although AP2/ERFs have been thoroughly characterised in many plant species, the knowledge about this family in the sweet potato, which is a vital edible and medicinal crop, is still limited. In this study, a comprehensive genome-wide investigation was conducted to characterise the AP2/ERF gene family in the sweet potato.

Results

Here, 198 IbAP2/ERF transcription factors were obtained. Phylogenetic analysis classified the members of the IbAP2/ERF family into three groups, namely, ERF (172 members), AP2 (21 members) and RAV (5 members), which was consistent with the analysis of gene structure and conserved protein domains. The evolutionary characteristics of these IbAP2/ERF genes were systematically investigated by analysing chromosome location, conserved protein motifs and gene duplication events, indicating that the expansion of the IbAP2/ERF gene family may have been caused by tandem duplication. Furthermore, the analysis of cis-acting elements in IbAP2/ERF gene promoters implied that these genes may play crucial roles in plant growth, development and stress responses. Additionally, the available RNA-seq data and quantitative real-time PCR (qRT-PCR) were used to investigate the expression patterns of IbAP2/ERF genes during sweet potato root development as well as under multiple forms of abiotic stress, and we identified several developmental stage-specific and stress-responsive IbAP2/ERF genes. Furthermore, g59127 was differentially expressed under various stress conditions and was identified as a nuclear protein, which was in line with predicted subcellular localization results.

Conclusions

This study originally revealed the characteristics of the IbAP2/ERF superfamily and provides valuable resources for further evolutionary and functional investigations of IbAP2/ERF genes in the sweet potato.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08043-w.

Genome-Wide Identification and Expression Analysis of AP2/ERF Transcription Factor Related to Drought Stress in Cultivated Peanut (Arachis hypogaea L.)

APETALA2/ethylene response element-binding factor (AP2/ERF) transcription factors (TFs) have been found to regulate plant growth and development and response to various abiotic stresses. However, detailed information of AP2/ERF genes in peanut against drought has not yet been performed. Herein, 185 AP2/ERF TF members were identified from the cultivated peanut (A. hypogaea cv. Tifrunner) genome, clustered into five subfamilies: AP2 (APETALA2), ERF (ethylene-responsive-element-binding), DREB (dehydration-responsive-element-binding), RAV (related to ABI3/VP), and Soloist (few unclassified factors)). Subsequently, the phylogenetic relationship, intron-exon structure, and chromosomal location of AhAP2/ERF were further characterized. All of these AhAP2/ERF genes were distributed unevenly across the 20 chromosomes, and 14 tandem and 85 segmental duplicated gene pairs were identified which originated from ancient duplication events. Gene evolution analysis showed that A. hypogaea cv. Tifrunner were separated 64.07 and 66.44 Mya from Medicago truncatula L. and Glycine max L., respectively. Promoter analysis discovered many cis-acting elements related to light, hormones, tissues, and stress responsiveness process. The protein interaction network predicted the exitance of functional interaction among families or subgroups. Expression profiles showed that genes from AP2, ERF, and dehydration-responsive-element-binding subfamilies were significantly upregulated under drought stress conditions. Our study laid a foundation and provided a panel of candidate AP2/ERF TFs for further functional validation to uplift breeding programs of drought-resistant peanut cultivars.

Evaluation of transcription factor and aquaporin gene expressions in response to Al2O3 and ZnO nanoparticles during barley germination

Aluminum oxide and zinc oxide nanoparticles (NPs) are two of the mostly produced engineered metal oxide NPs. Here, barley germination and root elongation as well as gene expressions of the selected aquaporins (HvTip1;1 and HvPip1;1) and transcription factors (HvERFs and HvNFX1) were investigated after exposure to Al₂O₃ and ZnO NPs for foreseeing the effect of NP exposure. ICP-MS analysis showed that the nanoparticles were taken up into root and leaves. Even the germination analysis and seedling establishment data indicate that the applied NPs do not have any observable inhibitory effects except on root length, the gene expression analysis revealed that these nanoparticle applications lead to a response at the molecular level. The gene expression profiling indicated that aquaporins and transcription factor genes were differentially regulated in leaves and roots in response to NPs treatments. The expressions of aquaporin genes were higher especially in leaves in compared to the control plants. Gradual decrease was obtained in roots by application of the increased levels of Al₂O₃ NPs. The effects of ZnO NPs on gene expression levels of barley TFs were dramatically more distinctive in comparison with that of Al₂O₃ NPs. The expression profiles of HvERFs and HvNFX1 transcription factors in response to the Al₂O₃ and ZnO NPs suggest that these selected TFs can play important roles in shaping abiotic stress tolerance in young barley roots and leaves. Outcomes of the study will allow us to predict complex stress response of barley in response to the nanoparticles.

Identification and functional analysis of the MdLTPG gene family in apple

Cuticular wax is synthesized from intracellular lipids that are exported by epidermal cells, and plant lipid transfer proteins (LTPs) play an important role in this process. The glycosylphosphatidylinositol (GPI)-anchored LTPs (LTPGs) are a large subgroup within the LTP family and function in lipid transport and wax formation. Although LTPG family members have been identified in several plant species, the LTPG gene family of apple (Malus domestica) remains uncharacterized. In this paper, we identified 26 potential LTPG genes by searching apple whole-genome annotation files using "GPI-anchored" and "lipid transferase" as keywords. Twenty of the 26 putative LTPG genes were confirmed as MdLTPG family members based on their subcellular localization predictions. The MdLTPGs were divided into four classes based on phylogenetic analysis and functional domain prediction. One member of each class was analyzed for subcellular localization, and all identified members were located on the plasma membrane. Most MdLTPG genes were induced by abiotic stress treatments such as low temperature, NaCl, and ABA. Finally, the MdLTPG17 protein was shown to interact with the lysine-rich arabinogalactan protein MdAGP18 to perform its function in wax transport during plant growth and development.

Genetic dissection of drought resistance based on root traits at the bud stage in common bean

A whole-genome resequencing-derived SNP dataset used for genome-wide association analysis revealed 196 loci significantly associated with drought stress based on root traits. Candidate genes identified in the regions of these loci include homologs of known drought resistance genes in A. thaliana. Drought is the main abiotic constraint of the production of common bean. Improved adaptation to drought environments has become a main goal of crop breeding due to the increasing scarcity of water that will occur in the future. The overall objective of our study was to identify genomic regions associated with drought resistance based on root traits using genome-wide association analysis. A natural population of 438 common bean accessions was evaluated for root traits: root surface area, root average diameter, root volume, total root length, taproot length, lateral root number, root dry weight, lateral root length, special root weight/length, using seed germination pouches under drought conditions and in well-watered environments. The coefficient of variation ranged from 11.24% (root average diameter) to 38.19% (root dry weight) in the well-watered environment and from 9.61% (root average diameter) to 39.05% (lateral root length) under drought stress. A whole-genome resequencing-derived SNP dataset revealed 196 loci containing 230 candidate SNPs associated with drought resistance. Seventeen candidate SNPs were simultaneously associated with more than two traits. Forty-one loci were simultaneously associated with more than two traits, and eleven loci were colocated with loci previously reported to be related to drought resistance. Candidate genes of the associated loci included the ABA-responsive element-binding protein family, MYB, NAC, the protein kinase superfamily, etc. These results revealed promising alleles linked to drought resistance or root traits, providing insights into the genetic basis of drought resistance and roots, which will be useful for common bean improvement.

Expansion and Molecular Characterization of AP2/ERF Gene Family in Wheat (Triticum aestivum L.)

The AP2/ERF is a large protein family of transcription factors, playing an important role in signal transduction, plant growth, development, and response to various stresses. AP2/ERF super-family is identified and functionalized in a different plant but no comprehensive and systematic analysis in wheat (Triticum aestivum L.) has been reported. However, a genome-wide and functional analysis was performed and identified 322 TaAP2/ERF putative genes from the wheat genome. According to the phylogenetic and structural analysis, TaAP2/ERF genes were divided into 12 subfamilies (Ia, Ib, Ic, IIa, IIb, IIc, IIIa, IIIb, IIIc, IVa, IVb, and IVc). Furthermore, conserved motifs and introns/exons analysis revealed may lead to functional divergence within clades. Cis-Acting analysis indicated that many elements were involved in stress-related and plant development. Chromosomal location showed that 320 AP2/ERF genes were distributed among 21 chromosomes and 2 genes were present in a scaffold. Interspecies microsynteny analysis revealed that maximum orthologous between Arabidopsis, rice followed by wheat. Segment duplication events have contributed to the expansion of the AP2/ERF family and made this family larger than rice and Arabidopsis. Additionally, AP2/ERF genes were differentially expressed in wheat seedlings under the stress treatments of heat, salt, and drought, and expression profiles were verified by qRT-PCR. Remarkably, the RNA-seq data exposed that AP2/ERF gene family might play a vital role in stress-related. Taken together, our findings provided useful and helpful information to understand the molecular mechanism and evolution of the AP2/ERF gene family in wheat.

Digital gene expression analysis of the response to Ralstonia solanacearum between resistant and susceptible tobacco varieties

Tobacco bacterial wilt (TBW) caused by Ralstonia solanacearum is the most serious soil-borne disease of tobacco. However, molecular mechanism information of R. solanacearum resistance is limited to tobacco, hindering better breeding of resistant tobacco. In this study, the expression profiles of the rootstalks of Yunyan87 (susceptible cultivar) and Fandi3 (resistant cultivar) at different stages after R. solanacearum infection were compared to explore molecular mechanisms of tobacco resistance against the bacterium. Findings from gene-expression profiling indicated that the number of upregulated differentially expressed genes (DEGs) at 3 and 7 days post-inoculation (dpi) increased significantly in the resistant cultivar. WRKY6 and WRKY11 family genes in WRKY transcription factors, ERF5 and ERF15 family genes in ERFs transcription factors, and genes encoding PR5 were significantly upregulated in the resistant cultivar response to the infection. For the first time, WRKY11 and ERF15 were found to be possibly involved in disease-resistance. The Kyoto Encyclopedia of Genes and Genomes analysis demonstrated glutathione metabolism and phenylpropane pathways as primary resistance pathways to R. solanacearum infection. In the resistant cultivar, DEGs encoding CYP450, TCM, CCoAOMT, 4CL, PAL, CCR, CSE, and CADH, involved in the synthesis of plant antitoxins such as flavonoids, stilbenoids, and lignins, enriched in the phenylpropane pathway were upregulated at 3 and 7 dpi. Furthermore, a pot experiment was performed to verify the role of flavonoids in controlling TBW. This study will strongly contribute to a better understanding of molecular interactions between tobacco plants and R. solanacearum.

Whole-genome characterization of Rosa chinensis AP2/ERF transcription factors and analysis of negative regulator RcDREB2B in Arabidopsis

BACKGROUND

Rose (Rosa chinensis) is a traditional famous flower with valuable ornamental characteristics. However, drought stress restricts its growth and development, leading to an abnormal phenotype. One of the main transcription factor (TF) protein groups in the plant kingdom are the APETALA2/ethylene-responsive factor (AP2/ERF) proteins and are potentially involved in the growth and stress responses of various plants.

RESULTS

Our investigation mainly focused on exploring the genome of rose and thereby we discovered 135 apparent AP2/ERF TFs. Phylogenic analyses revealed that RcAP2/ERF genes are categorized into DREB, Soloist, AP2, and ERF subfamilies, and are further classified these into 17 groups, with the same as Malus domestica and Arabidopsis thaliana. The analysis of the gene structure revealed that the introns ranged from 0 to 9 in number. Pattern examination demonstrated that the RcAP2/ERF predominantly consists of typical AP2 domains, of which the 2nd motif is the most ubiquitous. Distributions of cis-acting elements indicated that members of the AP2/ERF family are frequently involved in growth and development, phytohormone and stress response in rose species. Also, the distribution mapping of the rose chromosomes indicated that AP2/ERF class genes are dispersed among all seven chromosomes. Additionally, we isolated a novel DREB A2 subgroup gene and named it RcDREB2B. Subsequently, the RcDREB2B transcript accumulation was repressed under the mild and severe drought stress in the root samples of rose. RcDREB2B was targeted to the nucleus and exhibited transactivation in yeast cells. The overexpression of RcDREB2B was found to promote sensitivity to a higher salt concentration, ABA, and PEG at the germination and post-germination stages. Twelve putative osmotic and ABA-related genes were impaired in RcDREB2B-overexpressing plants.

CONCLUSIONS

The results provide comprehensive information regarding the gene structure, phylogenic, and distribution of the rose AP2/ERF family and bring insight into the complex transcriptional gene regulation of RcAP2/ERF. Findings in this study would also contribute to further understanding of the RcDREB2B gene in rose.

Genome-wide analysis of AP2/ERF transcription factors in pineapple reveals functional divergence during flowering induction mediated by ethylene and floral organ development

The APETALA2/ethylene-responsive factor (AP2/ERF) has important roles in regulating developmental processes and hormone signaling transduction in plants. Pineapple demonstrates a special sensitivity to ethylene, and AP2/ERFs may contribute to this distinct sensitivity of pineapples to ethylene. However, little information is available on the AP2/ERF of pineapple. In this study, 97 AP2/ERF family members were identified from the pineapple genome. The AcAP2/ERF superfamily could be further divided into five subfamilies, and different subfamily existed functional divergence in multifarious biological processes. ERF and RAV subfamily genes might play important roles in the process of ethylene response of pineapple; ERF and DREB subfamily genes had particular functions in the floral organ development. This study is the first to provide detailed information on the features of AP2/ERFs in pineapple, provide new insights into the potential functional roles of the AP2/ERF superfamily members, and will facilitate a better understanding of the molecular mechanism of flower in pineapple.

In Silico Identification of the Complex Interplay between Regulatory SNPs, Transcription Factors, and Their Related Genes in Brassica napus L. Using Multi-Omics Data

Regulatory SNPs (rSNPs) are a special class of SNPs which have a high potential to affect the phenotype due to their impact on DNA-binding of transcription factors (TFs). Thus, the knowledge about such rSNPs and TFs could provide essential information regarding different genetic programs, such as tissue development or environmental stress responses. In this study, we use a multi-omics approach by combining genomics, transcriptomics, and proteomics data of two different Brassica napus L. cultivars, namely Zhongshuang11 (ZS11) and Zhongyou821 (ZY821), with high and low oil content, respectively, to monitor the regulatory interplay between rSNPs, TFs and their corresponding genes in the tissues flower, leaf, stem, and root. By predicting the effect of rSNPs on TF-binding and by measuring their association with the cultivars, we identified a total of 41,117 rSNPs, of which 1141 are significantly associated with oil content. We revealed several enriched members of the TF families DOF, MYB, NAC, or TCP, which are important for directing transcriptional programs regulating differential expression of genes within the tissues. In this work, we provide the first genome-wide collection of rSNPs for B. napus and their impact on the regulation of gene expression in vegetative and floral tissues, which will be highly valuable for future studies on rSNPs and gene regulation.

Genome-Wide Analysis and the Expression Pattern of the ERF Gene Family in Hypericum perforatum

Hypericum perforatum is a well-known medicinal herb currently used as a remedy for depression as it contains many high levels of secondary metabolites. The ethylene response factor (ERF) family encodes transcriptional regulators with multiple functions that play a vital role in the diverse developmental and physiological processes of plants, which can protect plants from various stresses by regulating the expression of genes. Although the function of several ERF genes from other plants has been further confirmed, H. perforatum is the first sequenced species in Malpighiales, and no information regarding the ERFs has been reported thus far. In this study, a total of 101 ERF genes were identified from H. perforatum. A systematic and thorough bioinformatic analysis of the ERF family was performed using the genomic database of H. perforatum. According to the phylogenetic tree analysis, HpERFs were further classified into 11 subfamilies. Gene ontology (GO) analysis suggested that most of the HpERFs likely participate in the biological processes of plants. The cis-elements were mainly divided into five categories, associated with the regulation of gene transcription, response to various stresses, and plant development. Further analysis of the expression patterns showed that the stress-responsive HpERFs responded to different treatments. This work systematically analyzed HpERFs using the genome sequences of H. perforatum. Our results provide a theoretical basis for further investigation of the function of stress-related ERFs in H. perforatum.

Biotechnological approaches to dissect climate-resilient traits in millets and their application in crop improvement

'Small millets' is a generic term that includes all the millets except pearl millet and sorghum. These small or minor millets constitute eleven species that are marginally cultivated and consumed worldwide. These small millets possess excellent agronomic-, climate-resilient, and nutritional traits, although they lack popularity. Small millets withstand a broad spectrum of environmental stresses and possess better water-use and nitrogen-use efficiencies. Of note, small millets are five- to seven-fold nutritionally rich in terms of protein, bioactive compounds, micro- and macro-nutrients as compared to major cereals. Irrespective of these merits, small millets have received little research attention compared to major millets and cereals. However, the knowledge generated from such studies is significant for the improvement of millets per se and for translating the information to improve major cereals through breeding and transgene-based approaches. Given this, the review enumerates the efforts invested in dissecting the climate-resilient traits in small millets and provides a roadmap for deploying the information in crop improvement of millets as well as cereals in the scenario of climate change.

Re-examination of the APETALA2/Ethylene-Responsive Factor Gene Family in Barley (Hordeum vulgare L.) Indicates a Role in the Regulation of Starch Synthesis

The APETALA2/Ethylene-Responsive factor (AP2/ERF) gene family is a large plant-specific transcription factor family, which plays important roles in regulating plant growth and development. A role in starch synthesis is among the multiple functions of this family of transcription factors. Barley (Hordeum vulgare L.) is one of the most important cereals for starch production. However, there are limited data on the contribution of AP2 transcription factors in barley. In this study, we used the recently published barley genome database (Morex) to identify 185 genes of the HvAP2/ERF family. Compared with previous work, we identified 64 new genes in the HvAP2/ERF gene family and corrected some previously misannotated and duplicated genes. After phylogenetic analysis, HvAP2/ERF genes were classified into four subfamilies and 18 subgroups. Expression profiling showed different patterns of spatial and temporal expression for HvAP2/ERF genes. Most of the 12 HvAP2/ERF genes analyzed using quantitative reverse transcription-polymerase chain reaction had similar expression patterns when compared with those of starch synthase genes in barley, except for HvAP2-18 and HvERF-73. HvAP2-18 is homologous to OsRSR1, which negatively regulates the synthesis of rice starch. Luciferase reporter gene, and yeast one-hybrid assays showed that HvAP2-18 bound the promoter of AGP-S and SBE1 in vitro. Thus, HvAP2-18 might be an interesting candidate gene to further explore the mechanisms involved in the regulation of starch synthesis in barley.

Molecular Manipulation of the miR399/PHO2 Expression Module Alters the Salt Stress Response of Arabidopsis thaliana

In Arabidopsis thaliana (Arabidopsis), the microRNA399 (miR399)/PHOSPHATE2 (PHO2) expression module is central to the response of Arabidopsis to phosphate (PO₄) stress. In addition, miR399 has been demonstrated to also alter in abundance in response to salt stress. We therefore used a molecular modification approach to alter miR399 abundance to investigate the requirement of altered miR399 abundance in Arabidopsis in response to salt stress. The generated transformant lines, MIM399 and MIR399 plants, with reduced and elevated miR399 abundance respectively, displayed differences in their phenotypic and physiological response to those of wild-type Arabidopsis (Col-0) plants following exposure to a 7-day period of salt stress. However, at the molecular level, elevated miR399 abundance, and therefore, altered PHO2 target gene expression in salt-stressed Col-0, MIM399 and MIR399 plants, resulted in significant changes to the expression level of the two PO₄ transporter genes, PHOSPHATE TRANSPORTER1;4 (PHT1;4) and PHT1;9. Elevated PHT1;4 and PHT1;9 PO₄ transporter levels in salt stressed Arabidopsis would enhance PO₄ translocation from the root to the shoot tissue which would supply additional levels of this precious cellular resource that could be utilized by the aerial tissues of salt stressed Arabidopsis to either maintain essential biological processes or to mount an adaptive response to salt stress.

The AP2/ERF Gene Family in Triticum durum: Genome-Wide Identification and Expression Analysis under Drought and Salinity Stresses

Members of the AP2/ERF transcription factor family play critical roles in plant development, biosynthesis of key metabolites, and stress response. A detailed study was performed to identify TtAP2s/ERFs in the durum wheat (Triticum turgidum ssp. durum) genome, which resulted in the identification of 271 genes distributed on chromosomes 1A-7B. By carrying 27 genes, chromosome 6A had the highest number of TtAP2s/ERFs. Furthermore, a duplication assay of TtAP2s/ERFs demonstrated that 70 duplicated gene pairs had undergone purifying selection. According to RNA-seq analysis, the highest expression levels in all tissues and in response to stimuli were associated with DRF and ERF subfamily genes. In addition, the results revealed that TtAP2/ERF genes have tissue-specific expression patterns, and most TtAP2/ERF genes were significantly induced in the root tissue. Additionally, 13 TtAP2/ERF genes (six ERFs, three DREBs, two DRFs, one AP2, and one RAV) were selected for further analysis via qRT-PCR of their potential in coping with drought and salinity stresses. The TtAP2/ERF genes belonging to the DREB subfamily were markedly induced under both drought-stress and salinity-stress conditions. Furthermore, docking simulations revealed several residues in the pocket sites of the proteins associated with the stress response, which may be useful in future site-directed mutagenesis studies to increase the stress tolerance of durum wheat. This study could provide valuable insights for further evolutionary and functional assays of this important gene family in durum wheat.

Genome-wide identification and expression analysis of Arabidopsis GRAM-domain containing gene family in response to abiotic stresses and PGPR treatment

Characterization of stress-responsive genes is important to understand the genomics perspective of stress tolerance. In this purview, several gene-families are being identified and characterized in the model and non-model plant species, which has greatly enhanced the knowledge of molecular intricacies associated with stress tolerance. One such gene family is the GRAM-domain containing which have been found to be upregulated in response to plant growth-promoting rhizobacteria (PGPR) treatment followed by salinity stress. Thus, we aimed at understanding the involvement of GRAM domain-containing proteins in abiotic stress response under the influence of rhizobacteria in Arabidopsis thaliana. The study identified fourteen AtGRAM genes in A. thaliana. Further, comprehensive analyses of domain family including phylogenetic studies, domain architecture, gene structure and genomic composition analysis, promoter analysis, homology modelling, and duplication and divergence rates estimation was performed. RNA-Seq derived expression profiling of AtGRAM genes using GENVESTIGATOR in different stresses, developmental stages and hormonal treatments was performed, followed by qRT-PCR analysis under abiotic stresses in response to PGPR. Altogether, the study provided insights into the structure, organization, and evolutionary properties of AtGRAM gene family. Modulation in expression pattern in response to stresses influenced by PGPR-treatment suggests its multifaceted role in cross-talk among abiotic stresses and phytohormones. Further functional characterization of the selected candidate genes would enable understanding of the precise roles of GRAM-genes underlying stress tolerance.

Genome-Wide Gene Expression Profiles Analysis Reveal Novel Insights into Drought Stress in Foxtail Millet (Setaria italica L.)

Foxtail millet (Setaria italica (L.) P. Beauv) is an important food and forage crop because of its health benefits and adaptation to drought stress; however, reports of transcriptomic analysis of genes responding to re-watering after drought stress in foxtail millet are rare. The present study evaluated physiological parameters, such as proline content, p5cs enzyme activity, anti-oxidation enzyme activities, and investigated gene expression patterns using RNA sequencing of the drought-tolerant foxtail millet variety (Jigu 16) treated with drought stress and rehydration. The results indicated that drought stress-responsive genes were related to many multiple metabolic processes, such as photosynthesis, signal transduction, phenylpropanoid biosynthesis, starch and sucrose metabolism, and osmotic adjustment. Furthermore, the Δ1-pyrroline-5-carboxylate synthetase genes, SiP5CS1 and SiP5CS2, were remarkably upregulated in foxtail millet under drought stress conditions. Foxtail millet can also recover well on rehydration after drought stress through gene regulation. Our data demonstrate that recovery on rehydration primarily involves proline metabolism, sugar metabolism, hormone signal transduction, water transport, and detoxification, plus reversal of the expression direction of most drought-responsive genes. Our results provided a detailed description of the comparative transcriptome response of foxtail millet variety Jigu 16 under drought and rehydration environments. Furthermore, we identify SiP5CS2 as an important gene likely involved in the drought tolerance of foxtail millet.

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.

Genetic and genomic resources, and breeding for accelerating improvement of small millets: current status and future interventions

Current agricultural and food systems encourage research and development on major crops, neglecting regionally important minor crops. Small millets include a group of small- seeded cereal crops of the grass family Poaceae. This includes finger millet, foxtail millet, proso millet, barnyard millet, kodo millet, little millet, teff, fonio, job’s tears, guinea millet, and browntop millet. Small millets are an excellent choice to supplement major staple foods for crop and dietary diversity because of their diverse adaptation on marginal lands, less water requirement, lesser susceptibility to stresses, and nutritional superiority compared to major cereal staples. Growing interest among consumers about healthy diets together with climate-resilient features of small millets underline the necessity of directing more research and development towards these crops. Except for finger millet and foxtail millet, and to some extent proso millet and teff, other small millets have received minimal research attention in terms of development of genetic and genomic resources and breeding for yield enhancement. Considerable breeding efforts were made in finger millet and foxtail millet in India and China, respectively, proso millet in the United States of America, and teff in Ethiopia. So far, five genomes, namely foxtail millet, finger millet, proso millet, teff, and Japanese barnyard millet, have been sequenced, and genome of foxtail millet is the smallest (423-510 Mb) while the largest one is finger millet (1.5 Gb). Recent advances in phenotyping and genomics technologies, together with available germplasm diversity, could be utilized in small millets improvement. This review provides a comprehensive insight into the importance of small millets, the global status of their germplasm, diversity, promising germplasm resources, and breeding approaches (conventional and genomic approaches) to accelerate climate-resilient and nutrient-dense small millets for sustainable agriculture, environment, and healthy food systems.

Identification of a novel ERF gene, TaERF8, associated with plant height and yield in wheat

BACKGROUND

Ethylene Responsive Factor (ERF) is involved in various processes of plant development and stress responses. In wheat, several ERFs have been identified and their roles in mediating biotic or abiotic stresses have been elucidated. However, their effects on wheat plant architecture and yield-related traits remain poorly studied.

RESULTS

In this study, TaERF8, a new member of the ERF family, was isolated in wheat (Triticum aestivum L.). Three homoeologous TaERF8 genes, TaERF8-2A, TaERF8-2B and TaERF8-2D (named according to sub-genomic origin), were cloned from the common wheat cultivar Chinese Spring. The three homoeologs showed highly similar protein sequences, with identical AP2 domain. Whereas homoeologs sequence polymorphism analysis allowed the establishment of ten, two and three haplotypes, respectively. Expression analysis revealed that TaERF8s were constitutively expressed through entire wheat developmental stages. Analysis of related agronomic traits of TaERF8-2B overexpressing transgenic lines showed that TaERF8-2B plays a role in regulating plant architecture and yield-related traits. Association analysis between TaERF8-2B haplotypes (Hap-2B-1 and Hap-2B-2) and agronomic traits showed that TaERF8-2B was associated with plant height, heading date and 1000 kernel weight (TKW). The TaERF8-2B haplotypes distribution analysis revealed that Hap-2B-2 frequency increased in domesticated emmer wheat and modern varieties, being predominant in five major China wheat producing zones.

CONCLUSION

These results indicated that TaERF8s are differentially involved in the regulation of wheat growth and development. Haplotype Hap-2B-2 was favored during domestication and in Chinese wheat breeding. Unveiling that the here described molecular marker TaERF8-2B-InDel could be used for marker-assisted selection, plant architecture and TKW improvement in wheat breeding.

Characterization of the AP2/ERF Transcription Factor Family and Expression Profiling of DREB Subfamily under Cold and Osmotic Stresses in Ammopiptanthus nanus

APETALA2/ethylene-responsive factor (AP2/ERF) is one of the largest transcription factor (TF) families in plants, which play important roles in regulating plant growth, development, and response to environmental stresses. Ammopiptanthus nanus, an unusual evergreen broad-leaved shrub in the arid region in the northern temperate zone, demonstrates a strong tolerance to low temperature and drought stresses, and AP2/ERF transcription factors may contribute to the stress tolerance of A. nanus. In the current study, 174 AP2/ERF family members were identified from the A. nanus genome, and they were divided into five subfamilies, including 92 ERF members, 55 dehydration-responsive element binding (DREB) members, 24 AP2 members, 2 RAV members, and 1 Soloist member. Compared with the other leguminous plants, A. nanus has more members of the DREB subfamily and the B1 group of the ERF subfamily, and gene expansion in the AP2/ERF family is primarily driven by tandem and segmental duplications. Promoter analysis showed that many stress-related cis-acting elements existed in promoter regions of the DREB genes, implying that MYB, ICE1, and WRKY transcription factors regulate the expression of DREB genes in A. nanus. Expression profiling revealed that the majority of DREB members were responsive to osmotic and cold stresses, and several DREB genes such as EVM0023336.1 and EVM0013392.1 were highly induced by cold stress, which may play important roles in cold response in A. nanus. This study provided important data for understanding the evolution and functions of AP2/ERF and DREB transcription factors in A. nanus.

Insights into Barley Root Transcriptome under Mild Drought Stress with an Emphasis on Gene Expression Regulatory Mechanisms

Root systems play a pivotal role in coupling with drought stress, which is accompanied with a substantial transcriptome rebuilding in the root tissues. Here, we present the results of global gene expression profiling of roots of two barley genotypes with contrasting abilities to cope with drought that were subjected to a mild level of the stress. We concentrate our analysis on gene expression regulation processes, which allowed the identification of 88 genes from 39 families involved in transcriptional regulation in roots upon mild drought. They include 13 genes encoding transcription factors (TFs) from AP2 family represented by ERFs, DREB, or B3 domain-containing TFs, eight WRKYs, six NACs, five of the HD-domain, MYB or MYB-related, bHLH and bZIP TFs. Also, the representatives of C3H, CPP, GRAS, LOB-domain, TCP, Tiffy, Tubby, and NF-Ys TFs, among others were found to be regulated by the mild drought in barley roots. We found that drought tolerance is accompanied with a lower number of gene expression changes than the amount observed in a susceptible genotype. The better drought acclimation may be related to the activation of transcription factors involved in the maintenance of primary root growth and in the epigenetic control of chromatin and DNA methylation. In addition, our analysis pointed to fives TFs from ERF, LOB, NAC, WRKY and bHLH families that may be important in the mild but not the severe drought response of barley roots.

Besides and Beyond Flowering: Other roles of EuAP2 Genes in Plant Development

EuAP2 genes are well-known for their role in flower development, a legacy of the founding member of this subfamily of transcription factors, whose mutants lacked petals in Arabidopsis. However, studies of euAP2 genes in several species have accumulated evidence highlighting the diverse roles of euAP2 genes in other aspects of plant development. Here, we emphasize other developmental roles of euAP2 genes in various species and suggest a shift from regarding euAP2 genes as just flowering genes to consider the global role they may be playing in plant development. We hypothesize that their almost universal expression profile and pleiotropic effects of their mutation suggest their involvement in fundamental plant development processes.

The expanding roles of APETALA2/Ethylene Responsive Factors and their potential applications in crop improvement

Understanding the molecular basis of the gene-regulatory networks underlying agronomic traits or plant responses to abiotic/biotic stresses is very important for crop improvement. In this context, transcription factors, which either singularly or in conjugation directly control the expression of many target genes, are suitable candidates for improving agronomic traits via genetic engineering. In this regard, members of one of the largest class of plant-specific APETALA2/Ethylene Response Factor (AP2/ERF) superfamily, which is implicated in various aspects of development and plant stress adaptation responses, are considered high-value targets for crop improvement. Besides their long-known regulatory roles in mediating plant responses to abiotic stresses such as drought and submergence, the novel roles of AP2/ERFs during fruit ripening or secondary metabolites production have also recently emerged. The astounding functional plasticity of AP2/ERF members is considered to be achieved by their interplay with other regulatory networks and signalling pathways. In this review, we have integrated the recently accumulated evidence from functional genomics studies and described their newly emerged functions in plants. The key structural features of AP2/ERF proteins and the modes of their action are briefly summarized. The importance of AP2/ERFs in plant development and stress responses and a summary of the event of their successful applications in crop improvement programs are also provided. Altogether, we envisage that the synthesized information presented in this review will be useful to design effective strategies for improving agronomic traits in crop plants.

Transcription Factors Associated with Abiotic and Biotic Stress Tolerance and Their Potential for Crops Improvement

In field conditions, crops are adversely affected by a wide range of abiotic stresses including drought, cold, salt, and heat, as well as biotic stresses including pests and pathogens. These stresses can have a marked effect on crop yield. The present and future effects of climate change necessitate the improvement of crop stress tolerance. Plants have evolved sophisticated stress response strategies, and genes that encode transcription factors (TFs) that are master regulators of stress-responsive genes are excellent candidates for crop improvement. Related examples in recent studies include TF gene modulation and overexpression approaches in crop species to enhance stress tolerance. However, much remains to be discovered about the diverse plant TFs. Of the >80 TF families, only a few, such as NAC, MYB, WRKY, bZIP, and ERF/DREB, with vital roles in abiotic and biotic stress responses have been intensively studied. Moreover, although significant progress has been made in deciphering the roles of TFs in important cereal crops, fewer TF genes have been elucidated in sorghum. As a model drought-tolerant crop, sorghum research warrants further focus. This review summarizes recent progress on major TF families associated with abiotic and biotic stress tolerance and their potential for crop improvement, particularly in sorghum. Other TF families and non-coding RNAs that regulate gene expression are discussed briefly. Despite the emphasis on sorghum, numerous examples from wheat, rice, maize, and barley are included. Collectively, the aim of this review is to illustrate the potential application of TF genes for stress tolerance improvement and the engineering of resistant crops, with an emphasis on sorghum.

Genome-wide identification and expression analysis reveal the potential function of ethylene responsive factor gene family in response to Botrytis cinerea infection and ovule development in grapes (Vitis vinifera L.)

The prevention of Botrytis cinerea infection and the study of grape seedlessness are very important for grape industries. Finding correlated regulatory genes is an important approach towards understanding their molecular mechanisms. Ethylene responsive factor (ERF) gene family play critical roles in defence networks and the growth of plants. To date, no large-scale study of the ERF proteins associated with pathogen defence and ovule development has been performed in grape (Vitis vinifera L.). In the present study, we identified 113 ERF genes (VvERF) and named them based on their chromosome locations. The ERF genes could be divided into 11 groups based on a multiple sequence alignment and a phylogenetic comparison with homologues from Arabidopsis thaliana. Synteny analysis and Ka/Ks ratio calculation suggested that segmental and tandem duplications contributed to the expansion of the ERF gene family. The evolutionary relationships between the VvERF genes were investigated by exon-intron structure characterisation, and an analysis of the cis-acting regulatory elements in their promoters suggested potential regulation after stress or hormone treatments. Expression profiling after infection with the fungus, B. cinerea, indicated that ERF genes function in responses to pathogen attack. In addition, the expression levels of most ERF genes were much higher during ovule development in seedless grapes, suggesting a role in ovule abortion related to seedlessness. Taken together, these results indicate that VvERF proteins are involved in responses to Botrytis cinerea infection and in grape ovule development. This information may help guide strategies to improve grape production.