Overexpression of a novel chrysanthemum NAC transcription factor gene enhances salt tolerance in tobacco

A pervasive phosphorylation cascade modulation of plant transcription factors in response to abiotic stress

MAIN CONCLUSION

Transcriptional regulation of stress-responsive genes is a crucial step in establishing the mechanisms behind plant abiotic stress tolerance. A sensitive method of regulating transcription factors activity, stability, protein interaction, and subcellular localization is through phosphorylation. This review highlights a widespread regulation mechanism that involves phosphorylation of plant TFs in response to abiotic stress. Abiotic stress is one of the main components limiting crop yield and sustainability on a global scale. It greatly reduces the land area that is planted and lowers crop production globally. In all living organisms, transcription factors (TFs) play a crucial role in regulating gene expression. They participate in cell signaling, cell cycle, development, and plant stress response. Plant resilience to diverse abiotic stressors is largely influenced by TFs. Transcription factors modulate gene expression by binding to their target gene's cis-elements, which are impacted by genomic characteristics, DNA structure, and TF interconnections. In this review, we focus on the six major TFs implicated in abiotic stress tolerance, namely, DREB, bZIP, WRKY, ABF, MYB, and NAC, and the cruciality of phosphorylation of these transcription factors in abiotic stress signaling, as protein phosphorylation has emerged as one of the key post-translational modifications, playing a critical role in cell signaling, DNA amplification, gene expression and differentiation, and modification of other biological configurations. These TFs have been discovered after extensive study as stress-responsive transcription factors which may be major targets for crop development and important contributors to stress tolerance and crop production.

Manipulation of the transcription factor SlNAC1 for improved tolerance to abiotic stress in tomato

The tomato transcription factor SlNAC1 plays an important role in abiotic stress response and is fine-tuned at both transcriptional and posttranslational levels. The SlNAC1 gene is strongly induced by multiple abiotic stresses and the SlNAC1 protein is subjected to ubiquitin proteasome-mediated degradation. We found here that SlNAC1 possesses two distinct transactivation domains (TADs), TAD1 and TAD2. Significantly, the instability of SlNAC1 was attributed to the acidic amino acid-rich TAD1, in which the instability and transcriptional potential of TAD1 functionally overlapped; whereas the glutamine-rich TAD2 was stable and accounted for the abiotic stress signalling mediated by SlNAC1. Towards the goal of enhanced tolerance to abiotic stress in tomatoes, we manipulated SlNAC1 at both gene and protein levels: we generated a stable and functional SlNAC1 mutant SlNAC1^∆191-270 by removing TAD1 and further engineered it to be stress-controllable by fusing the corresponding cDNA with the abiotic stress-inducible promoter Pro_StNAC1 . Transgenic tomato plants expressing the Pro_StNAC1 ::SlNAC1^∆191-270 transgene did not display any undesired traits and exhibited enhanced tolerance to cold, drought and salt stresses. Taken together, our manipulation of the stress-related transcription factor via conditional expression of its derived stable and functional mutant provides a successful example for developing crops dynamically adapted to abiotic stress.

Ectopic expression of NAC transcription factor HaNAC3 from Haloxylon ammodendron increased abiotic stress resistance in tobacco

HaNAC3 is a transcriptional activator located in the nucleus that may be involved in the response to high temperature, high salt and drought stresses as well as phytohormone IAA and ABA treatments. Our study demonstrated that HaNAC3 increased the tolerance of transgenic tobacco to abiotic stress and was involved in the regulation of a range of downstream genes and metabolic pathways. This also indicates the potential application of HaNAC3 as a plant tolerance gene. NAC transcription factors play a key role in plant growth and development and plant responses to biotic and abiotic stresses. However, the biological functions of NAC transcription factors in the desert plant Haloxylon ammodendron are still poorly understood. In this study, the NAC transcription factor HaNAC3 was isolated and cloned from a typical desert plant H. ammodendron, and its possible biological functions were investigated. Bioinformatics analysis showed that HaNAC3 has the unique N-terminal NAC structural domain of NAC transcription factor. Quantitative real-time fluorescence analysis showed that HaNAC3 was able to participate in the response to simulated drought, high temperature, high salt, and phytohormone IAA and ABA treatments, and was very sensitive to simulated high temperature and phytohormone ABA treatments. Subcellular localization analysis showed that the GFP-HaNAC3 fusion protein was localized in the nucleus of tobacco epidermal cells. The transcriptional self-activation assay showed that HaNAC3 had transcriptional self-activation activity, and the truncation assay confirmed that the transcriptional activation activity was located at the C-terminus. HaNAC3 gene was expressed exogenously in wild-type Nicotiana benthamiana, and the physiological function of HaNAC3 was verified by simulating drought and other abiotic stresses. The results indicated that transgenic tobacco had better resistance to abiotic stresses than wild-type B. fuminata. Further transcriptome analysis showed that HaNAC3 was involved in the regulation of a range of downstream resistance genes, wax biosynthesis and other metabolic pathways. These results suggest that HaNAC3 may have a stress resistance role in H. ammodendron and has potential applications in plant molecular breeding.

Joint GWAS and WGCNA uncover the genetic control of calcium accumulation under salt treatment in maize seedlings

Soil salinization is an important factor threatening the yield and quality of maize. Ca²⁺ plays a considerable role in regulating plant growth under salt stress. Herein, we examined the shoot Ca²⁺ concentrations, root Ca²⁺ concentrations, and transport coefficients of seedlings in an association panel composed of 305 maize inbred lines under normal and salt conditions. A genome-wide association study was conducted by using the investigated phenotypes and 46,408 single-nucleotide polymorphisms of the panel. As a result, 53 significant SNPs were specifically detected under salt treatment, and 544 genes were identified in the linkage disequilibrium regions of these SNPs. According to the expression data of the 544 genes, we carried out a weighted coexpression network analysis. Combining the enrichment analyses and functional annotations, four hub genes (GRMZM2G051032, GRMZM2G004314, GRMZM2G421669, and GRMZM2G123314) were finally determined, which were then used to evaluate the genetic variation effects by gene-based association analysis. Only GRMZM2G123314, which encodes a pentatricopeptide repeat protein, was significantly associated with Ca²⁺ transport and the haplotype G-CT was identified as the superior haplotype. Our study brings novel insights into the genetic and molecular mechanisms of salt stress response and contributes to the development of salt-tolerant varieties in maize.

Silencing GhJUB1L1 (JUB1-like 1) reduces cotton (Gossypium hirsutum) drought tolerance

Drought stress massively restricts plant growth and the yield of crops. Reducing the deleterious effects of drought is necessary for agricultural industry. The plant-specific NAC (NAM, ATAF1/2 and CUC2) transcription factors (TFs) are widely involved in the regulation of plant development and stress response. One of the NAC TF, JUNGBRUNNEN1 (JUB1), has been reported to involve in drought resistance in Arabidopsis. However, little is known of how the JUB1 gene respond to drought stress in cotton. In the present study, we cloned GhJUB1L1, a homologous gene of JUB1 in upland cotton. GhJUB1L1 is preferentially expressed in stem and leaf and could be induced by drought stress. GhJUB1L1 protein localizes to the cell nucleus, and the transcription activation region of which is located in the C-terminal region. Silencing GhJUB1L1 gene via VIGS () reduced cotton drought tolerance, and retarded secondary cell wall (SCW) development. Additionally, the expression of some drought stress-related genes and SCW synthesis-related genes were altered in the GhJUB1L1 silencing plants. Collectively, our findings indicate that GhJUB1L1 may act as a positive regulator in response to drought stress and SCW development in cotton. Our results enriched the roles of NAC TFs in cotton drought tolerance and laid a foundation for the cultivation of transgenic cotton with higher drought tolerance.

Identification and analysis of Chrysanthemum nankingense NAC transcription factors and an expression analysis of OsNAC7 subfamily members

NAC (NAM, ATAF1-2, and CUC2) transcription factors (TFs) play a vital role in plant growth and development, as well as in plant response to biotic and abiotic stressors (Duan et al., 2019; Guerin et al., 2019). Chrysanthemum is a plant with strong stress resistance and adaptability; therefore, a systematic study of NAC TFs in chrysanthemum is of great significance for plant breeding. In this study, 153 putative NAC TFs were identified based on the Chrysanthemum nankingense genome. According to the NAC family in Arabidopsis and rice, a rootless phylogenetic tree was constructed, in which the 153 CnNAC TFs were divided into two groups and 19 subfamilies. Moreover, the expression levels of 12 CnNAC TFs belonging to the OsNAC7 subfamily were analyzed in C. nankingense under osmotic and salt stresses, and different tissues were tested during different growth periods. The results showed that these 12 OsNAC7 subfamily members were involved in the regulation of root and stem growth, as well as in the regulation of drought and salt stresses. Finally, we investigated the function of the CHR00069684 gene, and the results showed that CHR00069684 could confer improved salt and low temperature resistance, enhance ABA sensitivity, and lead to early flowering in tobacco. It was proved that members of the OsNAC7 subfamily have dual functions including the regulation of resistance and the mediation of plant growth and development. This study provides comprehensive information on analyzing the function of CnNAC TFs, and also reveals the important role of OsNAC7 subfamily genes in response to abiotic stress and the regulation of plant growth. These results provide new ideas for plant breeding to control stress resistance and growth simultaneously.

Ectopic expression of a poplar gene NAC13 confers enhanced tolerance to salinity stress in transgenic Nicotiana tabacum

NACs are one of the major transcription factor families in plants which play an important role in plant growth and development, as well as in adverse stress responses. In this study, we cloned a salt-inducible NAC transcription factor gene (NAC13) from a poplar variety 84K, followed by transforming it into both Nicotiana tabacum and Arabidopsis thaliana. Stable expression analysis of 35S::NAC13-GFP fusion protein in Arabidopsis indicated that NAC13 protein was localized to the nucleus. We also obtained five transgenic tobacco lines. Evidence from morphological and physiological characterization and salt treatment analyses indicated that in the transgenic tobacco the salt tolerance was enhanced, suggesting that NAC13 gene may function as a positive regulator in tobacco responses to salt stress.

The role of NAC transcription factor in plant cold response

The NAC transcription factor (TF) is one of the largest families of TFs in plants and plays an important role in plant growth, development, and response to environmental stress. The structural and functional characteristics of NAC TFs have been uncovered in the past years, including sequence binding features of the DNA-binding domain located in the N-terminus and dynamic interplay between the domain located at the C-terminus and other proteins. Studies on NAC TF are increasing in number; these studies distinctly contribute to our understanding of the regulatory networks of NAC-mediated complex signaling and transcriptional reprogramming. Previous studies have indicated that NAC TFs are key regulators of the plant stress response. However, these studies have been for six years so far and mainly focused on drought and salt stress. There are relatively few reports about NAC TFs in plant cold signal pathway and no related reviews have been published. In this review article, we summarize the structural features of NAC TFs, the target genes, upstream regulators and interaction proteins of stress-responsive NAC TFs, and the roles NAC TFs play in plant cold stress signal pathway.

Identification and Expression Analysis of the NAC Gene Family in Coffea canephora

The NAC gene family is one of the largest families of transcriptional regulators in plants, and it plays important roles in the regulation of growth and development as well as in stress responses. Genome-wide analyses have been performed in diverse plant species, but there is still no systematic analysis of the NAC genes of Coffea canephora Pierre ex A. Froehner. In this study, we identified 63 NAC genes from the genome of C. canephora. The basic features and comparison analysis indicated that the NAC gene members increased via duplication events during the evolution of the plant. Phylogenetic analysis divided the NAC proteins from C. canephora, Arabidopsis and rice into 16 subgroups. Analysis of the expression patterns of CocNACs under cold stress and coffee bean development indicated that 38 CocNACs were differentially expressed under cold stress; six genes may play important roles in the process of cold acclimation, and four genes among 54 CocNACs showing a variety of expression patterns during different developmental stages of coffee beans may be positively related to the bean development. This study can expand our understanding of the functions of the CocNAC gene family in cold responses and bean development, thereby potentially intensifying the molecular breeding programs of Coffea spp. plants.

The chrysanthemum leaf and root transcript profiling in response to salinity stress

RNA-Seq was applied to capture the transcriptome of the leaf and root of non-treated and salinity-treated chrysanthemum cv. 'Jinba' plants. A total of 206,868 unigenes of mean length 849 nt and of N50 length 1363 nt was identified; of these about 64% (> 132,000) could be functionally assigned. Depending on the severity of the salinity stress, differential transcription was observed for genes encoding proteins involved in osmotic adjustment, in ion transport, in reactive oxygen species scavenging and in the regulation of abscisic acid (ABA) signaling. The root stress response was dominated by the up-regulation of genes involved in ion transport, while that of the leaf reflected the plant's effort to make osmotic adjustments and to regulate Ca²⁺ transport. An array of known transcription factors (WRKY, AP2/ERF, MYB, bHLH and NAC) were differentially transcribed.

Lateral Root Development in Potato Is Mediated by Stu-mi164 Regulation of NAC Transcription Factor

The NAC designation is derived from petunia (Petunia hybrida) gene NO APICAL MERISTEM (NAM) and Arabidopsis genes ATAF1/ATAF2 and CUP-SHAPED COTYLEDON2 (CUC2), which belongs to the family of plant-specific transcription factors (TFs), and plays important role in plant development processes, such as response to biotic and abiotic stress, and hormone signaling. MicroRNAs (miRNAs) are a class of small, non-coding endogenous RNAs which play versatile and significant role in plant stress response and development via negatively affecting gene expression at a post-transcriptional level. Here, we showed that Stu-mi164 had a complementary sequence in the CDS sequence of potato NAC TFs, and that NAC expression exhibited significant differences under osmotic stress. We measured expression levels of the Stu-mi164 target gene StNAC262 between control and PEG-treated plants using real-time PCR, and the results demonstrated that they had inverse relationship. We suggested that Stu-miR164 might drive overexpression of NAC gene under osmotic stress in potato. To confirm the regulation of NAC TFs by Stu-mi164, we developed transgenic plants, using Agrobacterium tumefaciens-mediated transformation, of the potato cultivars "Gannongshu 2" and "Kexin 3" overexpressing the Stu-mi164 or the TF StNAC262. Real-time PCR analysis of transgenic potato plants under osmotic (PEG) stress, showed that potato plants overexpressing Stu-mi164 had reduced expression of StNAC262 and their osmotic resistance decreased. Furthermore, these plants had low number of lateral roots although the same length as the control. Our findings support the regulatory role of Stu-miRNAs in controlling plant response to osmotic stress via StNAC262.

Transcriptome analysis of Crossostephium chinensis provides insight into the molecular basis of salinity stress responses

Soil salinization is becoming a limitation to the utilization of ornamental plants worldwide. Crossostephium chinensis (Linnaeus) Makino is often cultivated along the southeast coast of China for its desirable ornamental qualities and high salt tolerance. However, little is known about the genomic background of the salt tolerance mechanism in C. chinensis. In the present study, we used Illumina paired-end sequencing to systematically investigate leaf transcriptomes derived from C. chinensis seedlings grown under normal conditions and under salt stress. A total of 105,473,004 bp of reads were assembled into 163,046 unigenes, of which 65,839 (40.38% of the total) and 54,342 (33.32% of the total) were aligned in Swiss-Prot and Nr protein, respectively. A total of 11,331 (6.95%) differentially expressed genes (DEGs) were identified among three comparisons, including 2,239 in ‘ST3 vs ST0’, 5,880 in ‘ST9 vs ST3’ and 9,718 in ‘ST9 vs ST0’, and they were generally classified into 26 Gene Ontology terms and 58 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway terms. Many genes encoding important transcription factors (e.g., WRKY, MYB, and AP2/EREBP) and proteins involved in starch and sucrose metabolism, arginine and proline metabolism, plant hormone signal transduction, amino acid biosynthesis, plant-pathogen interactions and carbohydrate metabolism, among others, were substantially up-regulated under salt stress. These genes represent important candidates for studying the salt-response mechanism and molecular biology of C. chinensis and its relatives. Our findings provide a genomic sequence resource for functional genetic assignments in C. chinensis. These transcriptome datasets will help elucidate the molecular mechanisms responsible for salt-stress tolerance in C. chinensis and facilitate the breeding of new stress-tolerant cultivars for high-saline areas using this valuable genetic resource.

Overexpression of a chrysanthemum transcription factor gene DgNAC1 improves the salinity tolerance in chrysanthemum

DgNAC1, a transcription factor of chrysanthemum, was functionally verified to confer salt stress responses by regulating stress-responsive genes. NAC transcription factors play effective roles in resistance to different abiotic stresses, and overexpressions of NAC TFs in Arabidopsis have been proved to be conducive in improving salinity tolerance. However, functions of NAC genes in chrysanthemum continue to be poorly understood. Here, we performed physiology and molecular experiments to evaluate roles of DgNAC1 in chrysanthemum salt stress responses. In this study, DgNAC1-overexpressed chrysanthemum was obviously more resistant to salt over the WT (wild type). Specifically, the transgenic chrysanthemum showed a higher survival rate and lower EC (electrolyte conductivity) than WT under salt stress. The transgenic chrysanthemum also showed fewer accumulations of MDA (malondialdehyde) and reactive oxygen species (H₂O₂ and O₂^-), greater activities of SOD (superoxide dismutase), POD (peroxidase) and CAT (catalase), as well as more proline content than WT under salt stress. Furthermore, stress-responsive genes in transgenic chrysanthemum were greater up-regulated than in WT under salinity stress. Thus, all results revealed that DgNAC1 worked as a positive regulator in responses to salt stress and it may be an essential gene for molecular breeding of salt-tolerant plants.

Whole-transcriptome sequence analysis of differentially expressed genes in Phormium tenax under drought stress

Phormium tenax is a kind of drought resistant garden plant with its rich and colorful leaves. To clarify the molecular mechanism of drought resistance in Phormium tenax, transcriptome was sequenced by the Illumina sequencing technology under normal and drought stress, respectively. A large number of contigs, transcripts and unigenes were obtained. Among them, only 30,814 unigenes were annotated by comparing with the protein databases. A total of 4,380 genes were differentially expressed, 2,698 of which were finally annotated under drought stress. Differentially expression analysis was also performed upon drought treatment. In KEGG pathway, the mechanism of drought resistance in Phormium tenax was explained from three aspects of metabolism and signaling of hormones, osmotic adjustment and reactive oxygen species metabolism. These results are helpful to understand the drought tolerance mechanism of Phormium tenax and will provide a precious genetic resource for drought-resistant vegetation breeding and research.

Characterization, Expression, and Functional Analysis of a Novel NAC Gene Associated with Resistance to Verticillium Wilt and Abiotic Stress in Cotton

Elucidating the mechanism of resistance to biotic and abiotic stress is of great importance in cotton. In this study, a gene containing the NAC domain, designated GbNAC1, was identified from Gossypium barbadense L. Homologous sequence alignment indicated that GbNAC1 belongs to the TERN subgroup. GbNAC1 protein localized to the cell nucleus. GbNAC1 was expressed in roots, stems, and leaves, and was especially highly expressed in vascular bundles. Functional analysis showed that cotton resistance to Verticillium wilt was reduced when the GbNAC1 gene was silenced using the virus-induced gene silencing (VIGS) method. GbNAC1-overexpressing Arabidopsis showed enhanced resistance to Verticillium dahliae compared to wild-type. Thus, GbNAC1 is involved in the positive regulation of resistance to Verticillium wilt. In addition, analysis of GbNAC1-overexpressing Arabidopsis under different stress treatments indicated that it is involved in plant growth, development, and response to various abiotic stresses (ABA, mannitol, and NaCl). This suggests that GbNAC1 plays an important role in resistance to biotic and abiotic stresses in cotton. This study provides a foundation for further study of the function of NAC genes in cotton and other plants.

Overexpression of a Novel NAC Domain-Containing Transcription Factor Gene (AaNAC1) Enhances the Content of Artemisinin and Increases Tolerance to Drought and Botrytis cinerea in Artemisia annua

The NAC (NAM, ATAF and CUC) superfamily is one of the largest plant-specific transcription factor families. NAC transcription factors always play important roles in response to various abiotic stresses. A NAC transcription factor gene AaNAC1 containing a complete open reading frame (ORF) of 864 bp was cloned from Artemisia annua. The expression of AaNAC1 could be induced by dehydration, cold, salicylic acid (SA) and methyl jasmonate (MJ), suggesting that it might be a key regulator of stress signaling pathways in A. annua. AaNAC1 was shown to be localized to the nuclei by transforming tobacco leaf epidermal cells. When AaNAC1 was overexpressed in A. annua, the content of artemisinin and dihydroartemisinic acid was increased by 79% and 150%, respectively. The expression levels of artemisinin biosynthetic pathway genes, i.e. amorpha-4,11-diene synthase (ADS), artemisinic aldehyde Δ11(13) reductase (DBR2) and aldehyde dehydrogenase 1 (ALDH1), were increased. Dual luciferase (dual-LUC) assays showed that AaNAC1 could activate the transcription of ADS in vivo. The transgenic A. annua exhibited increased tolerance to drought and resistance to Botrytis cinerea. When AaNAC1 was overexpressed in Arabidopsis, the transgenic Arabidopsis were markedly more tolerant to drought. The transgenic Arabidopsis showed increased resistance to B. cinerea. These results indicate that AaNAC1 can potentially be used in transgenic breeding for improving the content of artemisinin and drought tolerance in A. annua.

Identification and characterization of a novel NAC-like gene in chrysanthemum (Dendranthema lavandulifolium)

KEY MESSAGE

A NAC -like gene named DlNAC1 was identified in chrysanthemum and characterized; it may be involved in regulation of response to abiotic stressors, especially in tolerance to drought and salinity. NAC transcription factors in plants play crucial roles in tolerance to abiotic stressors, and overexpression of the NAC gene in Arabidopsis has been demonstrated to lead to improved drought tolerance. Functions of the NAC genes in chrysanthemum, however, remain poorly understood. In this study, a NAC-like gene named DlNAC1 was identified in chrysanthemum (Dendranthema lavandulifolium) and characterized. Phylogenetic analysis indicated that DlNAC1 contains a typical NAC domain and belongs to the ONAC022 subgroup. According to the subcellular localization and yeast one-hybrid assay, the DlNAC1 protein is localized to nuclei and has a transcription activation ability. Moreover, quantitative real-time PCR analyses showed that DlNAC1 was induced by low-temperature, high-salinity, and drought conditions (separately), but not by abscisic acid (ABA) and heat shock. In these experiments, the downstream genes of NAC transcription factors were found to be up-regulated, including stress-responsive genes KIN1 and AMY1. To further explore the effects of DlNAC1 in response to abiotic stressors, DlNAC1 was overexpressed in tobacco, and these transgenic plants showed significantly enhanced tolerance to drought and salinity. This study suggests that in chrysanthemum, the DlNAC1 gene is involved in regulation of the response to abiotic stressors, especially in tolerance to drought and salinity.

NAC transcription factors in plant multiple abiotic stress responses: progress and prospects

Abiotic stresses adversely affect plant growth and agricultural productivity. According to the current climate prediction models, crop plants will face a greater number of environmental stresses, which are likely to occur simultaneously in the future. So it is very urgent to breed broad-spectrum tolerant crops in order to meet an increasing demand for food productivity due to global population increase. As one of the largest families of transcription factors (TFs) in plants, NAC TFs play vital roles in regulating plant growth and development processes including abiotic stress responses. Lots of studies indicated that many stress-responsive NAC TFs had been used to improve stress tolerance in crop plants by genetic engineering. In this review, the recent progress in NAC TFs was summarized, and the potential utilization of NAC TFs in breeding abiotic stress tolerant transgenic crops was also be discussed. In view of the complexity of field conditions and the specificity in multiple stress responses, we suggest that the NAC TFs commonly induced by multiple stresses should be promising candidates to produce plants with enhanced multiple stress tolerance. Furthermore, the field evaluation of transgenic crops harboring NAC genes, as well as the suitable promoters for minimizing the negative effects caused by over-expressing some NAC genes, should be considered.

Overexpression of a novel chrysanthemum SUPERMAN-like gene in tobacco affects lateral bud outgrowth and flower organ development

Previous studies have shown that the SUP genes play important roles in flower development and plant growth and morphogenesis. In this study, we isolated and characterized a SUPERMAN-like gene DgSZFP from chrysanthemum. DgSZFP contains one conserved Cys2/His2-type zinc finger motifs in the N-terminal region and an EAR-box in C-terminus. Its expression was significantly higher in nodes, flower buds, disc stamens, and petals than in the other tissues. Overexpression of DgSZFP in tobacco resulted in enhanced branching, reduced plant height, increased the width of petal tubes, produced the staminoid petals and petaloid stamens in flowers, and enhanced the seed weight and size. In addition, DgSZFP-overexpression tobacco plants accumulated high concentrations of cytokinin and chlorophyll. These results suggest that DgSZFP may be the candidate gene for regulating branching and floral organ development in chrysanthemum.

A rapid, highly efficient and economical method of Agrobacterium-mediated in planta transient transformation in living onion epidermis

Transient transformation is simpler, more efficient and economical in analyzing protein subcellular localization than stable transformation. Fluorescent fusion proteins were often used in transient transformation to follow the in vivo behavior of proteins. Onion epidermis, which has large, living and transparent cells in a monolayer, is suitable to visualize fluorescent fusion proteins. The often used transient transformation methods included particle bombardment, protoplast transfection and Agrobacterium-mediated transformation. Particle bombardment in onion epidermis was successfully established, however, it was expensive, biolistic equipment dependent and with low transformation efficiency. We developed a highly efficient in planta transient transformation method in onion epidermis by using a special agroinfiltration method, which could be fulfilled within 5 days from the pretreatment of onion bulb to the best time-point for analyzing gene expression. The transformation conditions were optimized to achieve 43.87% transformation efficiency in living onion epidermis. The developed method has advantages in cost, time-consuming, equipment dependency and transformation efficiency in contrast with those methods of particle bombardment in onion epidermal cells, protoplast transfection and Agrobacterium-mediated transient transformation in leaf epidermal cells of other plants. It will facilitate the analysis of protein subcellular localization on a large scale.

Cloning and characterization of a novel vacuolar Na+/H+ antiporter gene (Dgnhx1) from chrysanthemum

Plant vacuolar Na(+)/H(+) antiporter genes play significant roles in salt tolerance. However, the roles of the chrysanthemum vacuolar Na(+)/H(+) antiporter genes in salt stress response remain obscure. In this study, we isolated and characterized a novel vacuolar Na(+)/H(+) antiporter gene DgNHX1 from chrysanthemum. The DgNHX1 sequence contained 1920 bp with a complete open reading frame of 1533 bp encoding a putative protein of 510 amino acids with a predicted protein molecular weight of 56.3 kDa. DgNHX1 was predicted containing nine transmembrane domains. Its expression in the chrysanthemum was up-regulated by salt stress, but not by abscisic acid (ABA). To assess roles of DgNHX1 in plant salt stress responses, we performed gain-of-function experiment. The DgNHX1-overexpression tobacco plants showed significant salt tolerance than the wild type (WT). The transgenic lines exhibited more accumulation of Na(+) and K(+) under salt stress. These findings suggest that DgNHX1 plays a positive regulatory role in salt stress response.

Bioengineering for salinity tolerance in plants: state of the art

Genetic engineering of plants for abiotic stress tolerance is a challenging task because of its multifarious nature. Comprehensive studies for developing abiotic stress tolerance are in progress, involving genes from different pathways including osmolyte synthesis, ion homeostasis, antioxidative pathways, and regulatory genes. In the last decade, several attempts have been made to substantiate the role of "single-function" gene(s) as well as transcription factor(s) for abiotic stress tolerance. Since, the abiotic stress tolerance is multigenic in nature, therefore, the recent trend is shifting towards genetic transformation of multiple genes or transcription factors. A large number of crop plants are being engineered by abiotic stress tolerant genes and have shown the stress tolerance mostly at laboratory level. This review presents a mechanistic view of different pathways and emphasizes the function of different genes in conferring salt tolerance by genetic engineering approach. It also highlights the details of successes achieved in developing salt tolerance in plants thus far.

Roles of NAC transcription factors in the regulation of biotic and abiotic stress responses in plants

NAC transcription factors are one of the largest families of transcriptional regulators in plants, and members of the NAC gene family have been suggested to play important roles in the regulation of the transcriptional reprogramming associated with plant stress responses. A phylogenetic analysis of NAC genes, with a focus on rice and Arabidopsis, was performed. Herein, we present an overview of the regulation of the stress responsive NAC SNAC/(IX) group of genes that are implicated in the resistance to different stresses. SNAC factors have important roles for the control of biotic and abiotic stresses tolerance and that their overexpression can improve stress tolerance via biotechnological approaches. We also review the recent progress in elucidating the roles of NAC transcription factors in plant biotic and abiotic stresses. Modification of the expression pattern of transcription factor genes and/or changes in their activity contribute to the elaboration of various signaling pathways and regulatory networks. However, a single NAC gene often responds to several stress factors, and their protein products may participate in the regulation of several seemingly disparate processes as negative or positive regulators. Additionally, the NAC proteins function via auto-regulation or cross-regulation is extensively found among NAC genes. These observations assist in the understanding of the complex mechanisms of signaling and transcriptional reprogramming controlled by NAC proteins.

Overexpression of a chrysanthemum transcription factor gene, DgWRKY3, in tobacco enhances tolerance to salt stress

WRKY transcription factor genes (TFs) play important roles in response to various abiotic stresses. However, the roles of the chrysanthemum WRKY genes in abiotic stress response remain obscure. In this study, we functionally characterized a novel WRKY gene, DgWRKY3, from chrysanthemum (Dendranthema grandiflorum). Its expression in the chrysanthemum was up-regulated by salinity or dehydration stress, but not by abscisic acid (ABA). The DgWRKY3-overexpression tobacco plants increase salt tolerance compared with wild-type (WT) tobacco plants. The increased levels of proline were observed in transgenic plants compared to WT plants under salt stress. In addition, the DgWRKY3 transgenic plants reduced accumulation of malondialdehyde (MDA) and hydrogen peroxide (H2O2) compared with WT plants, accompanied by higher activities of antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) and the greater accumulation of antioxidants including ascorbate (AsA) and glutathione (GSH) under salt stress. Moreover, the DgWRKY3 transgenic plants enhanced the expression of stress-related genes involved in osmotic adjustment and membrane protection (NtP5CS, NtLEA5, and NtERD10D) and oxidative stress response (NtSOD, NtPOD, NtCAT, and NtAPX) under salt stress. However, no significant difference in the expression of stress-related genes (NtP5CS, NtLEA5, NtERD10D, NtSOD, NtPOD, NtCAT, and NtAPX) was found between the DgWRKY3-overexpression and WT tobacco plants under normal conditions, despite the fact that the constitutive promoter was used to drive DgWRKY3. These findings suggest that DgWRKY3 functions as a positive regulator to mediate tolerance of plants to salt stress.

Comprehensive genome-wide survey, genomic constitution and expression profiling of the NAC transcription factor family in foxtail millet (Setaria italica L.)

The NAC proteins represent a major plant-specific transcription factor family that has established enormously diverse roles in various plant processes. Aided by the availability of complete genomes, several members of this family have been identified in Arabidopsis, rice, soybean and poplar. However, no comprehensive investigation has been presented for the recently sequenced, naturally stress tolerant crop, Setaria italica (foxtail millet) that is famed as a model crop for bioenergy research. In this study, we identified 147 putative NAC domain-encoding genes from foxtail millet by systematic sequence analysis and physically mapped them onto nine chromosomes. Genomic organization suggested that inter-chromosomal duplications may have been responsible for expansion of this gene family in foxtail millet. Phylogenetically, they were arranged into 11 distinct sub-families (I-XI), with duplicated genes fitting into one cluster and possessing conserved motif compositions. Comparative mapping with other grass species revealed some orthologous relationships and chromosomal rearrangements including duplication, inversion and deletion of genes. The evolutionary significance as duplication and divergence of NAC genes based on their amino acid substitution rates was understood. Expression profiling against various stresses and phytohormones provides novel insights into specific and/or overlapping expression patterns of SiNAC genes, which may be responsible for functional divergence among individual members in this crop. Further, we performed structure modeling and molecular simulation of a stress-responsive protein, SiNAC128, proffering an initial framework for understanding its molecular function. Taken together, this genome-wide identification and expression profiling unlocks new avenues for systematic functional analysis of novel NAC gene family candidates which may be applied for improvising stress adaption in plants.

SlNAC1, a stress-related transcription factor, is fine-tuned on both the transcriptional and the post-translational level

The plant-specific NAC (NAM, ATAF1,2, CUC2) transcription factors play significant roles in diverse physiological processes. In this study, we determined the regulation of a stress-related tomato (Solanum lycopersicum) NAC1 (SlNAC1) transcription factor at both the transcriptional and the post-translational level. The SlNAC1 protein was found to be stable in the presence of proteasome-specific inhibitor MG132 or MG115 and ubiquitinated in plant cells, suggesting that the SlNAC1 is subject to the ubiquitin-proteasome system-mediated degradation. Deletion analysis identified a short segment of 10 amino acids (aa261-270) that was required for ubiquitin-proteasome system-mediated degradation, among which two leucine residues (L268 and L269) were critical for the protein instability of SlNAC1. Fusion of the degron (SlNAC1(191-270) ) containing these 10 amino acids to green fluorescent protein was found to be sufficient to trigger the degradation of the fusion protein. In addition, the SlNAC1 gene is strongly upregulated during Pseudomonas infection, while repression of the NAC1 ortholog in Nicotiana benthamiana resulted in enhanced susceptibility to Pseudomonas bacteria. These results suggest that rapid upregulation of the NAC1 gene resulting in more protein production is likely one of the strategies plants use to defend themselves against pathogen infection.

microRNAs associated with drought response in the bioenergy crop sugarcane (Saccharum spp.)

Sugarcane (Saccharum spp.) is one of the most important crops in the world. Drought stress is a major abiotic stress factor that significantly reduces sugarcane yields. However the gene network that mediates plant responses to water stress remains largely unknown in several crop species. Although several microRNAs that mediate post-transcriptional regulation during water stress have been described in other species, the role of the sugarcane microRNAs during drought stress has not been studied. The objective of this work was to identify sugarcane miRNAs that are differentially expressed under drought stress and to correlate this expression with the behavior of two sugarcane cultivars with different drought tolerances. The sugarcane cultivars RB867515 (higher drought tolerance) and RB855536 (lower drought tolerance) were cultivated in a greenhouse for three months and then subjected to drought for 2, 4, 6 or 8 days. By deep sequencing of small RNAs, we were able to identify 18 miRNA families. Among all of the miRNAs thus identified, seven were differentially expressed during drought. Six of these miRNAs were differentially expressed at two days of stress, and five miRNAs were differentially expressed at four days. The expression levels of five miRNAs (ssp-miR164, ssp-miR394, ssp-miR397, ssp-miR399-seq 1 and miR528) were validated by RT-qPCR (quantitative reverse transcriptase PCR). Six precursors and the targets of the differentially expressed miRNA were predicted using an in silico approach and validated by RT-qPCR; many of these targets may play important roles in drought tolerance. These findings constitute a significant increase in the number of identified miRNAs in sugarcane and contribute to the elucidation of the complex regulatory network that is activated by drought stress.

NAC proteins: regulation and role in stress tolerance

The plant-specific NAC (NAM, ATAF1,2 and CUC2) proteins constitute a major transcription factor family renowned for their roles in several developmental programs. Despite their highly conserved DNA-binding domains, their remarkable diversification across plants reflects their numerous functions. Lately, they have received much attention as regulators in various stress signaling pathways which may include interplay of phytohormones. This review summarizes the recent progress in research on NACs highlighting the proteins' potential for engineering stress tolerance against various abiotic and biotic challenges. We discuss regulatory components and targets of NAC proteins in the context of their prospective role for crop improvement strategies via biotechnological intervention.