Identification and characterization of plant-specific NAC gene family in canola (Brassica napus L.) reveal novel members involved in cell death

Genome-Wide Identification of NAC Family Genes and Their Expression Analyses in Response to Osmotic Stress in Cannabis sativa L

NAC (NAM, ATAF1/2, and CUC2) transcription factors are unique and essential for plant growth and development. Although the NAC gene family has been identified in a wide variety of plants, its chromosomal location and function in Cannabis sativa are still unknown. In this study, a total of 69 putative CsNACs were obtained, and chromosomal location analysis indicated that the CsNAC genes mapped unevenly to 10 chromosomes. Phylogenetic analyses showed that the 69 CsNACs could be divided into six subfamilies. Additionally, the CsNAC genes in group IV-a are specific to Cannabis sativa and contain a relatively large number of exons. Promoter analysis revealed that most CsNAC promoters contained cis-elements related to plant hormones, the light response, and abiotic stress. Furthermore, transcriptome expression profiling revealed that 24 CsNAC genes in two Cannabis sativa cultivars (YM1 and YM7) were significantly differentially expressed under osmotic stress, and these 12 genes presented differential expression patterns across different cultivars according to quantitative real-time PCR (RT-qPCR) analysis. Among these, the genes homologous to the CsNAC18, CsNAC24, and CsNAC61 genes have been proven to be involved in the response to abiotic stress and might be candidate genes for further exploration to determine their functions. The present study provides a comprehensive insight into the sequence characteristics, structural properties, evolutionary relationships, and expression patterns of NAC family genes under osmotic stress in Cannabis sativa and provides a basis for further functional characterization of CsNAC genes under osmotic stress to improve agricultural traits in Cannabis sativa.

A comprehensive analysis of transcriptomic data for comparison of cold tolerance in two Brassica napus genotypes

Brassica napus is an important oil crop and cold stress severely limits its productivity. To date, several studies have reported the regulatory genes and pathways involved in cold-stress responses in B. napus. However, transcriptome-scale identification of the regulatory genes is still lacking. In this study, we performed comparative transcriptome analysis of cold-tolerant C18 (CT - C18) and cold-sensitive C6 (CS - C6) Brassica napus genotypes under cold stress for 7 days, with the primary purpose of identifying cold-responsive transcription in B. napus. A total of 6061 TFs belonging to 58 families were annotated in the B. napus genome, of which 3870 were expressed under cold stress in both genotypes. Among these, 451 TFs were differentially expressed (DE), with 21 TF genes expressed in both genotypes. Most TF members of the MYB (26), bHLH (23), and NAC (17) families were significantly expressed in the CT - C18 genotype compared with the CS - C6 B. napus genotype. GO classification showed a significant role in transcription regulation, DNA-binding transcription factor activity, response to chitin, and the ethylene-activated signaling pathway. KEGG pathway annotation revealed these TFs are involved in regulating more pathways, resulting in more tolerance. In conclusion, the results provide insights into the molecular regulation mechanisms of B. napus in response to freezing treatment, expanding our understanding of the complex molecular mechanisms in plants' response to freezing stress.

ANAC087 transcription factor positively regulates age-dependent leaf senescence through modulating the expression of multiple target genes in Arabidopsis

Leaf senescence is the final stage of leaf development and appropriate onset and progression of leaf senescence are critical for reproductive success and fitness. Although great progress has been made in identifying key genes regulating leaf senescence and elucidating the underlining mechanisms in the model plant Arabidopsis, there is still a gap to understanding the complex regulatory network. In this study, we discovered that Arabidopsis ANAC087 transcription factor (TF) positively modulated leaf senescence. Expression of ANAC087 was induced in senescing leaves and the encoded protein acted as a transcriptional activator. Both constitutive and inducible overexpression lines of ANAC087 showed earlier senescence than control plants, whereas T-DNA insertion mutation and dominant repression of the ANAC087 delayed senescence rate. A quantitative reverse transcription-polymerase chain reaction (qRT-PCR) profiling showed that the expression of an array of senescence-associated genes was upregulated in inducible ANAC087 overexpression plants including BFN1, NYE1, CEP1, RbohD, SAG13, SAG15, and VPEs, which are involved in programmed cell death (PCD), chlorophyll degradation and reactive oxygen species (ROS) accumulation. In addition, electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation-quantitative polymerase chain reaction (ChIP-qPCR) assays demonstrated that ANAC087 directly bound to the canonical NAC recognition sequence (NACRS) motif in promoters of its target genes. Moreover, mutation of two representative target genes, BFN1 or NYE1 alleviated the senescence rate of ANAC087-overexpression plants, suggesting their genetic regulatory relationship. Taken together, this study indicates that ANAC087 serves as an important regulator linking PCD, ROS, and chlorophyll degradation to leaf senescence.

Transcriptomic Analysis of Distal Parts of Roots Reveals Potentially Important Mechanisms Contributing to Limited Flooding Tolerance of Canola (Brassica napus) Plants

Since most of the root metabolic activities as well as root elongation and the uptake of water and mineral nutrients take place in the distal parts of roots, we aimed to gain insight into the physiological and transcriptional changes induced by root hypoxia in the distal parts of roots in canola (Brassica napus) plants, which are relatively sensitive to flooding conditions. Plants were subject to three days of root hypoxia via lowering oxygen content in hydroponic medium, and various physiological and anatomical features were examined to characterize plant responses. Untargeted transcriptomic profiling approaches were also applied to investigate changes in gene expression that took place in the distal root tissues in response to hypoxia. Plants responded to three days of root hypoxia by reducing growth and gas exchange rates. These changes were accompanied by decreases in leaf water potential (Ψ_leaf) and root hydraulic conductivity (L_pr). Increased deposition of lignin and suberin was also observed in the root tissues of hypoxic plants. The transcriptomic data demonstrated that the effect of hypoxia on plant water relations involved downregulation of most BnPIPs in the root tissues with the exception of BnPIP1;3 and BnPIP2;7, which were upregulated. Since some members of the PIP1 subfamily of aquaporins are known to transport oxygen, the increase in BnPIP1;3 may represent an important hypoxia tolerance strategy in plants. The results also demonstrated substantial rearrangements of different signaling pathways and transcription factors (TFs), which resulted in alterations of genes involved in the regulation of L_pr, TCA (tricarboxylic acid) cycle-related enzymes, antioxidant enzymes, and cell wall modifications. An integration of these data enabled us to draft a comprehensive model of the molecular pathways involved in the responses of distal parts of roots in B. napus. The model highlights systematic transcriptomic reprogramming aimed at explaining the relative sensitivity of Brassica napus to root hypoxia.

DNA and coding/non-coding RNA methylation analysis provide insights into tomato fruit ripening

Ripening is the last, irreversible developmental stage during which fruit become palatable, thus promoting seed dispersal by frugivory. In Alisa Craig fruit, mRNAs with increasing m5C levels, such as STPK and WRKY 40, were identified as being involved in response to biotic and abiotic stresses. Furthermore, two mRNAs involved in cell wall metabolism, PG and EXP-B1, also presented increased m5C levels. In the Nr mutant, several m5C-modified mRNAs involved in fruit ripening, including those encoding WRKY and MADS-box proteins, were found. Targets of long non-coding RNAs and circular RNAs with different m5C sites were also found; these targets included 2-alkenal reductase, soluble starch synthase 1, WRKY, MADS-box, and F-box/ketch-repeat protein SKIP11. A combined analysis of changes in 5mC methylation and mRNA revealed many differentially expressed genes with differentially methylated regions encoding transcription factors and key enzymes related to ethylene biosynthesis and signal transduction; these included ERF084, EIN3, AP2/ERF, ACO5, ACS7, EIN3/4, EBF1, MADS-box, AP2/ERF, and ETR1. Taken together, our findings contribute to the global understanding of the mechanisms underlying fruit ripening, thereby providing new information for both fruit and post-harvest behavior.

Rapeseed NAM transcription factor positively regulates leaf senescence via controlling senescence-associated gene expression

Leaf senescence is one of the most visible forms of programmed cell death in plants. It can be a seasonal adaptation in trees or the final stage in crops ensuring efficient translocation of nutrients to seeds. Along with developmental cues, various environmental factors could also trigger the onset of senescence through transcriptional cascades. Rapeseed (Brassica napus L.) is an important oil crop with its yielding affected by significant falling leaves as a result of leaf senescence, compared to many other crops. Therefore, a better understanding of leaf senescence and developing strategies controlling the progress of leaf senescence in rapeseed is necessary for warranting vegetable oil security. Here we functionally characterized the gene BnaNAM encoding No Apical Meristem (NAM) homologue to identify transcriptional regulation of leaf senescence in rapeseed. A combination of transient and stable expression techniques revealed overexpression of BnaNAM induced ROS production and leaf chlorosis. Quantitative evaluation of up-regulated genes in BnaNAM overexpression lines identified genes related to ROS production (RbohD, RbohF), proteases (βVPE, γVPE, SAG12, SAG15), chlorophyll catabolism (PaO, PPH) and nucleic acid degradation (BFN1) as the putative downstream targets. A dual luciferase-based transcriptional activation assay of selected promoters further confirmed BnaNAM mediated transactivation of promoters of the downstream genes. Finally, an electrophoretic mobility shift assay further confirmed direct binding of BnaNAM to promoters of βVPE, γVPE, SAG12, SAG15 and BFN1. Our results therefore demonstrate a novel role of BnaNAM in leaf senescence.

A De Novo Transcriptome Analysis Identifies Cold-Responsive Genes in the Seeds of Taxillus chinensis (DC.) Danser

Taxillus chinensis (DC.) Danser, a parasitic plant of the Loranthaceae family, grows by attacking other plants. It has a long history of being used in Chinese medicine to treat multiple chronic diseases. We previously observed that T. chinensis seeds are sensitive to cold. In this study, we performed transcriptome sequencing for T. chinensis seeds treated by cold (0°C) for 0 h, 12 h, 24 h, and 36 h. TRINITY assembled 257,870 transcripts from 223,512 genes. The GC content and N50 were calculated as 42.29% and 1,368, respectively. Then, we identified 42,183 CDSs and 35,268 likely proteins in the assembled transcriptome, which contained 1,622 signal peptides and 6,795 transmembrane domains. Next, we identified 17,217 genes (FPKM > 5) and 2,333 differentially expressed genes (DEGs) in T. chinensis seeds under cold stress. The MAPK pathway, as an early cold response, was significantly enriched by the DEGs in the T. chinensis seeds after 24 h of cold treatment. Known cold-responsive genes encoding abscisic acid-associated, aquaporin, C-repeat binding factor (CBF), cold-regulated protein, heat shock protein, protein kinase, ribosomal protein, transcription factor (TF), zinc finger protein, and ubiquitin were deregulated in the T. chinensis seeds under cold stress. Notably, the upregulation of CBF gene might be the consequences of the downregulation of MYB and GATA TFs. Additionally, we identified that genes encoding CDC20, YLS9, EXORDIUM, and AUX1 and wound-responsive family protein might be related to novel mechanisms of T. chinensis seeds exposed to cold. This study is first to report the differential transcriptional induction in T. chinensis seeds under cold stress. It will improve our understanding of parasitic plants in response to cold and provide a valuable resource for future studies.

Genome-wide analysis of the rose (Rosa chinensis) NAC family and characterization of RcNAC091

A genome-wide analysis identified 116 NAC genes in rose, including stress-related ones with different expression patterns under drought and salt stress. Silencing of RcNAC091, a member of the ATAF subfamily, decreased dehydration tolerance in rose. The NAC (NAM, ATAF, and CUC) transcription factors (TFs) are plant-specific proteins that regulate various developmental processes and stress responses. However, knowledge of the NAC TFs in rose (Rosa chinensis), one of the most important horticultural crops, is limited. In this study, 116 NAC genes were identified from the rose genome and classified into 16 subfamilies based on protein phylogenetic analysis. Chromosomal mapping revealed that the RcNAC genes were unevenly distributed on the seven chromosomes of rose. Gene structure and motif analysis identified a total of ten conserved motifs, of which motifs 1-7 were highly conserved and present in most rose NACs, while motifs 8-10 were present only in a few subfamilies. Further study of the stress-related RcNACs based on the transcriptome data showed differences in the expression patterns among the organs, at various floral developmental stages, and under drought and salt stress in rose leaves and roots. The stress-related RcNACs possessed cis-regulatory elements (CREs) categorized into three groups corresponding to plant growth and development, phytohormone response, and abiotic and biotic stress response. Reverse transcription-quantitative real-time PCR (RT-qPCR) analysis of 11 representative RcNACs revealed their differential expression in rose leaves and roots under abscisic acid (ABA), polyethylene glycol (PEG), and sodium chloride (NaCl) treatments. Furthermore, the silencing of RcNAC091 verified its role in positively regulating the dehydration stress response. Overall, the present study provides valuable insights into stress-related RcNACs and paves the way for stress tolerance in rose.

Genome-wide identification and characterization of NAC genes in Brassica juncea var. tumida

Background

NAC (NAM, ATAF1/2, and CUC2) transcription factors play an important role in plant growth and development. However, in tumorous stem mustard (Brassica juncea var. tumida), one of the economically important crops cultivated in southwest China and some southeast Asian countries, reports on the identification of NAC family genes are lacking. In this study, we conducted a genome-wide investigation of the NAC family genes in B. juncea var. tumida, based on its recently published genome sequence data.

Methods

The NAC genes were identified in B. juncea var. tumida using the bioinformatics approach on the whole genome level. Additionally, the expression of BjuNAC genes was analyzed under high- and low-temperature stresses by quantitative real-time PCR (qRT-PCR).

Results

A total of 300 BjuNAC genes were identified, of which 278 were mapped to specific chromosomes. Phylogenetic analysis of B. juncea var. tumida, Brassica rapa, Brassica nigra, rice and Arabidopsis thaliana NAC proteins revealed that all NAC genes were divided into 18 subgroups. Furthermore, gene structure analysis showed that most of the NAC genes contained two or three exons. Conserved motif analysis revealed that BjuNAC genes contain a conserved NAM domain. Additionally, qRT-PCR data indicated that thirteen BjuNAC genes with a varying degree of up-regulation during high-temperature stress. Conversely, four BjuNAC genes (BjuNAC006, BjuNAC083, BjuNAC170 and BjuNAC223) were up-regulated and two BjuNAC genes (BjuNAC074 and BjuNAC295) down-regulated under low temperature, respectively. Together, the results of this study provide a strong foundation for future investigation of the biological function of NAC genes in B. juncea var. tumida.

Membrane-Bound Transcriptional Activator NTL1 from Rapeseed Positively Modulates Leaf Senescence through Targeting Genes Involved in Reactive Oxygen Species Production and Programmed Cell Death

Leaf senescence is the last stage of leaf development and is determined by various environmental and endogenous signals. Leaf senescence can determine plant productivity and fitness. Transcription factors (TFs) with the transmembrane domain constitute a special group of regulatory proteins that can translocate from the membrane system into nuclei to exert the transcriptional function upon endogenous or exogenous stimuli. Reactive oxygen species (ROSs) play an important role in numerous processes throughout the life cycle of plants including leaf senescence. Leaf senescence is characterized by massive programmed cell death (PCD) and is a type of developmental PCD. The transcriptional regulatory relationships between membrane-bound TFs and leaf senescence remain largely uncharacterized, especially in rapeseed (Brassica napus L.), an important oil crop. Here, we show that BnaNTL1 is a membrane-bound NAC (NAM, ATAF, and CUC) TF, which is predominantly expressed in senescent leaves. Expression of BnaNTL1ΔTM, a form of BnaNTL1 devoid of the transmembrane domain, can induce serious HR-like cell death symptoms and ROS accumulation in cells. Plants overexpressing BnaNTL1ΔTM show earlier leaf senescence compared with the control, accompanied by chlorophyll degradation and electrolyte leakage. Genes involved in ROS production (RbohD), PCD (VPEs and CEP1), and leaf senescence (BFN1) are significantly induced and activated by BnaNTL1ΔTM according to the quantitative reverse transcription PCR (qRT-PCR) analysis and dual luciferase reporter (Dual-LUC) assay. Moreover, electrophoretic mobility shift assay revealed that BnaNTL1 directly bound to the NTLBS elements in promoters of RbohD, γVPE, and BFN1. In conclusion, these results demonstrate that BnaNTL1 positively modulates ROS production and HR-like cell death to induce leaf senescence.

Metabolomics Intervention Towards Better Understanding of Plant Traits

The majority of the most economically important plant and crop species are enriched with the availability of high-quality reference genome sequences forming the basis of gene discovery which control the important biochemical pathways. The transcriptomics and proteomics resources have also been made available for many of these plant species that intensify the understanding at expression levels. However, still we lack integrated studies spanning genomics–transcriptomics–proteomics, connected to metabolomics, the most complicated phase in phenotype expression. Nevertheless, for the past few decades, emphasis has been more on metabolome which plays a crucial role in defining the phenotype (trait) during crop improvement. The emergence of modern high throughput metabolome analyzing platforms have accelerated the discovery of a wide variety of biochemical types of metabolites and new pathways, also helped in improving the understanding of known existing pathways. Pinpointing the causal gene(s) and elucidation of metabolic pathways are very important for development of improved lines with high precision in crop breeding. Along with other-omics sciences, metabolomics studies have helped in characterization and annotation of a new gene(s) function. Hereby, we summarize several areas in the field of crop development where metabolomics studies have made its remarkable impact. We also assess the recent research on metabolomics, together with other omics, contributing toward genetic engineering to target traits and key pathway(s).

Ectopic overexpression of a membrane-tethered transcription factor gene NAC60 from oilseed rape positively modulates programmed cell death and age-triggered leaf senescence

Senescence is an integrative final stage of plant development that is governed by internal and external cues. The NAM, ATAF1/2, CUC2 (NAC) transcription factor (TF) family is specific to plants and membrane-tethered NAC TFs (MTTFs) constitute a unique and sophisticated mechanism in stress responses and development. However, the function of MTTFs in oilseed rape (Brassica napus L.) remains unknown. Here, we report that BnaNAC60 is an MTTF associated with the endoplasmic reticulum (ER) membrane. Expression of BnaNAC60 was induced during the progression of leaf senescence. Translocation of BnaNAC60 into nuclei was induced by ER stress and oxidative stress treatments. It binds to the NTLBS motif, rather than the canonical NAC recognition site. Overexpression of BnaNAC60 devoid of the transmembrane domain, but not the full-length BnaNAC60, induces significant reactive oxygen species (ROS) accumulation and hypersensitive response-like cell death in both tobacco (Nicotiana benthamiana) and oilseed rape protoplasts. Moreover, ectopic overexpression of BnaNAC60 devoid of the transmembrane domain, but not the full-length BnaNAC60, in Arabidopsis also induces precocious leaf senescence. Furthermore, screening and expression profiling identified an array of functional genes that are significantly induced by BnaNAC60 expression. Further it was found that BnaNAC60 can activate the promoter activities of BnaNYC1, BnaRbohD, BnaBFN1, BnaZAT12, and multiple BnaVPEs in a dual-luciferase reporter assay. Electrophoretic mobility shift assay and chromatin immunoprecipitation coupled to quantitative PCR assays revealed that BnaNAC60 directly binds to the promoter regions of these downstream target genes. To summarize, our data show that BnaNAC60 is an MTTF that modulates cell death, ROS accumulation, and leaf senescence.

WRKY55 transcription factor positively regulates leaf senescence and the defense response by modulating the transcription of genes implicated in the biosynthesis of reactive oxygen species and salicylic acid in Arabidopsis

Reactive oxygen species (ROS) and salicylic acid (SA) are two factors regulating leaf senescence and defense against pathogens. However, how a single gene integrates both ROS and SA pathways remains poorly understood. Here, we show that Arabidopsis WRKY55 transcription factor positively regulates ROS and SA accumulation, and thus leaf senescence and resistance against the bacterial pathogen Pseudomonas syringae WRKY55 is predominantly expressed in senescent leaves and encodes a transcriptional activator localized to nuclei. Both inducible and constitutive overexpression of WRKY55 accelerates leaf senescence, whereas mutants delay it. Transcriptomic sequencing identified 1448 differentially expressed genes, of which 1157 genes are upregulated by WRKY55 expression. Accordingly, the ROS and SA contents in WRKY55-overexpressing plants are higher than those in control plants, whereas the opposite occurs in mutants. Moreover, WRKY55 positively regulates defense against P. syringae Finally, we show that WRKY55 activates the expression of RbohD, ICS1, PBS3 and SAG13 by binding directly to the W-box-containing fragments. Taken together, our work has identified a new WRKY transcription factor that integrates both ROS and SA pathways to regulate leaf senescence and pathogen resistance.

Engineering Multiple Abiotic Stress Tolerance in Canola, Brassica napus

Impacts of climate change like global warming, drought, flooding, and other extreme events are posing severe challenges to global crop production. Contribution of Brassica napus towards the oilseed industry makes it an essential component of international trade and agroeconomics. Consequences from increasing occurrences of multiple abiotic stresses on this crop are leading to agroeconomic losses making it vital to endow B. napus crop with an ability to survive and maintain yield when faced with simultaneous exposure to multiple abiotic stresses. For an improved understanding of the stress sensing machinery, there is a need for analyzing regulatory pathways of multiple stress-responsive genes and other regulatory elements such as non-coding RNAs. However, our understanding of these pathways and their interactions in B. napus is far from complete. This review outlines the current knowledge of stress-responsive genes and their role in imparting multiple stress tolerance in B. napus. Analysis of network cross-talk through omics data mining is now making it possible to unravel the underlying complexity required for stress sensing and signaling in plants. Novel biotechnological approaches such as transgene-free genome editing and utilization of nanoparticles as gene delivery tools are also discussed. These can contribute to providing solutions for developing climate change resilient B. napus varieties with reduced regulatory limitations. The potential ability of synthetic biology to engineer and modify networks through fine-tuning of stress regulatory elements for plant responses to stress adaption is also highlighted.

Genome-wide identification of cold responsive transcription factors in Brassica napus L

BACKGROUND

Cold stress is one of the primary environmental factors that affect plant growth and productivity, especially for crops like Brassica napus that live through cold seasons. Till recently, although a number of genes and pathways involved in B. napus cold response have been revealed by independent studies, a genome-wide identification of the key regulators and the regulatory networks is still lack. In this study, we investigated the transcriptomes of cold stressed semi-winter and winter type rapeseeds in short day condition, mainly with the purpose to systematically identify the functional conserved transcription factors (TFs) in cold response of B. napus.

RESULTS

Global modulation of gene expression was observed in both the semi-winter type line (158A) and the winter type line (SGDH284) rapeseeds, in response to a seven-day chilling stress in short-day condition. Function analysis of differentially expressed genes (DEGs) revealed enhanced stresses response mechanisms and inhibited photosynthesis in both lines, as well as a more extensive inhibition of some primary biological processes in the semi-winter type line. Over 400 TFs were differentially expressed in response to cold stress, including 56 of them showed high similarity to the known cold response TFs and were consistently regulated in 158A and SGDH284, as well as 25 TFs which targets were over-represented in the total DEGs. A further investigation based on their interactions indicated the critical roles of several TFs in cold response of B. napus.

CONCLUSION

In summary, our results revealed the alteration of gene expression in cold stressed semi-winter and winter ecotype B. napus lines and provided a valuable collection of candidate key regulators involved in B. napus response to cold stress, which could expand our understanding of plant stress response and benefit the future improvement of the breed of rapeseeds.

The R2R3-MYB transcription factor BnaMYB111L from rapeseed modulates reactive oxygen species accumulation and hypersensitive-like cell death

As one of the largest families of transcription factors in plants, the R2R3-MYB proteins play important roles in diverse biological processes including growth and development, primary and secondary metabolism such as flavonoid and anthocyanin biosynthesis as well as abiotic and biotic stress responses. However, functions of R2R3-MYB genes in rapeseed (Brassica napus L.) remain elusive. Here, we characterized BnaMYB111L, which is homologous to Arabidopsis MYB111 and encodes an R2R3-MYB protein in rapeseed. BnaMYB111L is responsive to abscisic acid (ABA), heat, cold, hydrogen peroxide and fungal pathogen Sclerotinia scelerotiorum treatments through quantitative RT-PCR assay. BnaMYB111L encodes a transcriptional activator and is localized exclusively to nuclei. Interestingly, overexpression of BnaMYB111L in tobacco (Nicotiana benthamiana) and rapeseed protoplasts promoted reactive oxygen species (ROS) production and hypersensitive response-like cell death, accumulation of malondialdehyde (MDA) as well as degradation of chlorophyll. Furthermore, BnaMYB111L expression evoked the alterations of transcript levels of genes encoding ROS-producing enzyme, vacuolar processing enzymes and genes implicated in defense responses. A further dual luciferase reporter assay indicated that BnaMYB111L activated the expression of RbohB, MC4 and ACRE132, which are involved in ROS generation, cell death as well as defense responses. Taken together, this study characterized the function of rapeseed MYB111L and identified its putative target genes involved in ROS production and cell death.

Transcriptional regulation of Lonicera japonica Thunb. during flower development as revealed by comprehensive analysis of transcription factors

BACKGROUND

Lonicera japonica Thunb. flower has been used for the treatment of various diseases for a long time and attracted many studies on its potential effects. Transcription factors (TFs) regulate extensive biological processes during plant development. As the restricted reports of L. japonica on TFs, our work was carried out to better understand the TFs' regulatory roles under different developmental stages in L. japonica.

RESULTS

In this study, 1316 TFs belonging to 52 families were identified from the transcriptomic data, and corresponding expression profiles during the L. japonica flower development were comprehensively analyzed. 917 (69.68%) TFs were differentially expressed. TFs in bHLH, ERF, MYB, bZIP, and NAC families exhibited obviously altered expression during flower growth. Based on the analysis of differentially expressed TFs (DETFs), TFs in MYB, WRKY, NAC and LSD families that involved in phenylpropanoids biosynthesis, senescence processes and antioxidant activity were detected. The expression of MYB114 exhibited a positive correlation with the contents of luteoloside; Positive correlation was observed among the expression of MYC12, chalcone synthase (CHS) and flavonol synthase (FLS), while negative correlation was observed between the expression of MYB44 and the synthases; The expression of LSD1 was highly correlated with the expression of SOD and the total antioxidant capacity, while the expression of LOL1 and LOL2 exhibited a negative correlation with them; Many TFs in NAC and WRKY families may be potentially involved in the senescence process regulated by hormones and reactive oxygen species (ROS). The expression of NAC19, NAC29, and NAC53 exhibited a positive correlation with the contents of ABA and H₂O₂, while the expression of WRKY53, WRKY54, and WRKY70 exhibited a negative correlation with the contents of JA, SA and ABA.

CONCLUSIONS

Our study provided a comprehensive characterization of the expression profiles of TFs during the developmental stages of L. japonica. In addition, we detected the key TFs that may play significant roles in controlling active components biosynthesis, antioxidant activity and flower senescence in L. japonica, thereby providing valuable insights into the molecular networks underlying L. japonica flower development.

Exploring transcription factors reveals crucial members and regulatory networks involved in different abiotic stresses in Brassica napus L

BACKGROUND

Brassica napus (B. napus) encompasses diverse transcription factors (TFs), but thorough identification and characterization of TF families, as well as their transcriptional responsiveness to multifarious stresses are still not clear.

RESULTS

Totally 2167 TFs belonging to five families were genome-widely identified in B. napus, including 518 BnAP2/EREBPs, 252 BnbZIPs, 721 BnMYBs, 398 BnNACs and 278 BnWRKYs, which contained some novel members in comparison with existing results. Sub-genome distributions of BnAP2/EREBPs and BnMYBs indicated that the two families might have suffered from duplication and divergence during evolution. Synteny analysis revealed strong co-linearity between B. napus and its two ancestors, although chromosomal rearrangements have occurred and 85 TFs were lost. About 7.6% and 9.4% TFs of the five families in B. napus were novel genes and conserved genes, which both showed preference on the C sub-genome. RNA-Seq revealed that more than 80% TFs were abiotic stress inducible and 315 crucial differentially expressed genes (DEGs) were screened out. Network analysis revealed that the 315 DEGs are highly co-expressed. The homologous gene network in A. thaliana revealed that a considerable amount of TFs could trigger the differential expression of targeted genes, resulting in a complex clustered network with clusters of genes responsible for targeted stress responsiveness.

CONCLUSIONS

We identified and characterized five TF families in B. napus. Some crucial members and regulatory networks involved in different abiotic stresses have been explored. The investigations deepen our understanding of TFs for stress tolerance in B. napus.

Application of protoplast technology to CRISPR/Cas9 mutagenesis: from single-cell mutation detection to mutant plant regeneration

Plant protoplasts are useful for assessing the efficiency of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) mutagenesis. We improved the process of protoplast isolation and transfection of several plant species. We also developed a method to isolate and regenerate single mutagenized Nicotianna tabacum protoplasts into mature plants. Following transfection of protoplasts with constructs encoding Cas9 and sgRNAs, target gene DNA could be amplified for further analysis to determine mutagenesis efficiency. We investigated N. tabacum protoplasts and derived regenerated plants for targeted mutagenesis of the phytoene desaturase (NtPDS) gene. Genotyping of albino regenerants indicated that all four NtPDS alleles were mutated in amphidiploid tobacco, and no Cas9 DNA could be detected in most regenerated plants.

A Novel NAC-Type Transcription Factor, NAC87, from Oilseed Rape Modulates Reactive Oxygen Species Accumulation and Cell Death

Reactive oxygen species (ROS) are thought to play a dual role in plants by functioning as signaling molecules and toxic by-products of aerobic metabolism. The hypersensitive response (HR) is a typical feature of immune responses in plants and also a type of programmed cell death (PCD). How these two processes are regulated in oilseed rape (Brassica napus L.) at the transcriptional level remains largely unknown. In this study, we report that an oilseed rape (Brassica napus L.) NAM-ATAF-CUC (NAC)-type transcription factor NAC87 modulates ROS and cell death accompanied by typical changes at the morphological and cellular levels. The BnaNAC87 gene was induced by multiple stress and hormone treatments and was highly expressed in senescent leaves by quantitative reverse transcription-PCR (qRT-PCR). BnaNAC87 is located in nuclei and has transcriptional activation activity. Expression of BnaNAC87 promoted significant ROS production, cell death as well as death of protoplasts, as indicated by histological staining. In addition, putative downstream target genes of NAC87 were identified through both qRT-PCR and dual luciferase reporter assays. We found that genes implicated in ROS generation (RbohB), cell death (VPE1a, ZEN1), leaf senescence (WRKY6, ZAT12) and defense (PR2, PR5 and HIN1) were significantly induced. Through an electrophoretic mobility shift assay (EMSA), we confirmed that BnaNAC87 directly binds to the NACRS-containing promoter fragments of ZEN1, ZAT12, HIN1 and PR5 genes. From these results, we conclude that oilseed rape NAC87 is a novel NAC transcription factor that acts as a positive regulator of ROS metabolism and cell death.

The Glycine soja NAC transcription factor GsNAC019 mediates the regulation of plant alkaline tolerance and ABA sensitivity

Overexpression of Gshdz4 or GsNAC019 enhanced alkaline tolerance in transgenic Arabidopsis. We proved that Gshdz4 up-regulated both GsNAC019 and GsRD29B but GsNAC019 may repress the GsRD29B expression under alkaline stress. Wild soybean (Glycine soja) has a high tolerance to environmental challenges. It is a model species for dissecting the molecular mechanisms of salt-alkaline stresses. Although many NAC transcription factors play important roles in response to multiple abiotic stresses, such as salt, osmotic and cold, their mode of action in alkaline stress resistance is largely unknown. In our study, we identified a G. soja NAC gene, GsNAC019, which is a homolog of the Arabidopsis AtNAC019 gene. GsNAC019 was highly up-regulated by 50 mM NaHCO₃ treatment in the roots of wild soybean. Further investigation showed that a well-characterized transcription factor, Gshdz4 protein, bound the cis-acting element sequences (CAATA/TA), which are located in the promoter of the AtNAC019/GsNAC019 genes. Overexpression of Gshdz4 positively regulated AtNAC019 expression in transgenic Arabidopsis, implying that AtNAC019/GsNAC019 may be the target genes of Gshdz4. GsNAC019 was demonstrated to be a nuclear-localized protein in onion epidermal cells and possessed transactivation activity in yeast cells. Moreover, overexpression of GsNAC019 in Arabidopsis resulted in enhanced tolerance to alkaline stress at the seedling and mature stages, but reduced ABA sensitivity. The closest Arabidopsis homolog mutant plants of Gshdz4, GsNAC019 and GsRD29B containing athb40, atnac019 and atrd29b were sensitive to alkaline stress. Overexpression or the closest Arabidopsis homolog mutant plants of the GsNAC019 gene in Arabidopsis positively or negatively regulated the expression of stress-related genes, such as AHA2, RD29A/B and KIN1. Moreover, this mutation could phenotypically promoted or compromised plant growth under alkaline stress, implying that GsNAC019 may contribute to alkaline stress tolerance via the ABA signal transduction pathway and regulate expression of the downstream stress-related genes.

Genome-wide survey of switchgrass NACs family provides new insights into motif and structure arrangements and reveals stress-related and tissue-specific NACs

NAC proteins comprise of a plant-specific transcription factor (TF) family and play important roles in plant development and stress responses. Switchgrass (Panicum virgatum) is the prime candidate and model bioenergy grass across the world. Excavating agronomically valuable genes is important for switchgrass molecular breeding. In this study, a total of 251 switchgrass NAC (PvNACs) family genes clustered into 19 subgroups were analyzed, and those potentially involved in stress response or tissue-specific expression patterns were pinpointed. Specifically, 27 PvNACs were considered as abiotic stress-related including four membrane-associated ones. Among 40 tissue-specific PvNACs expression patterns eight factors were identified that might be relevant for lignin biosynthesis and/or secondary cell wall formation. Conserved functional domains and motifs were also identified among the PvNACs and potential association between these motifs and their predicted functions were proposed, that might encourage experimental studies to use PvNACs as possible targets to improve biomass production and abiotic stress tolerance.

Oilseed rape NAC56 transcription factor modulates reactive oxygen species accumulation and hypersensitive response-like cell death

The NAC (NAM, ATAF1/2, CUC2) transcription factor gene family is plant-specific and plays diverse roles in development and responses to abiotic stresses and pathogen challenge. Oilseed rape (Brassica napus) or canola is an important oil crop worldwide, however, the function of NAC genes in it remains largely elusive. In the present study, we identified and characterized the NAC56 gene isolated from oilseed rape. Expression of BnaNAC56 was induced by abscisic acid (ABA), jasmonic acid (JA), methyl viologen (MV) and a necrotrophic fungal pathogen Sclerotinia sclerotiorum, but repressed by cold. BnaNAC56 is a transcription activator and localized to nuclei. Overexpression of BnaNAC56 induced reactive oxygen species (ROS) accumulation and hypersensitive response (HR)-like cell death, with various physiological measurements supporting these. Furthermore, BnaNAC56 expression caused evident nuclear DNA fragmentation. Moreover, quantitative reverse transcription PCR (qRT-PCR) analysis identified that the expression levels of multiple genes regulating ROS homeostasis, cell death and defense response were significantly induced. Using a dual luciferase reporter assay, we further confirmed that BnaNAC56 could activate the expression of a few ROS- and cell death-related genes. In summary, our data demonstrate that BnaNAC56 functions as a stress-responsive transcriptional activator and plays a role in modulating ROS accumulation and cell death.

LcMCII-1 is involved in the ROS-dependent senescence of the rudimentary leaves of Litchi chinensis

KEY MESSAGE

LcMCII - 1 is a type II metacaspase. Over-expression of LcMCII- 1 in Arabidopsis promoted ROS-dependent and natural senescence. Virus-induced LcMCII- 1 silencing delayed the ROS-dependent senescence of the rudimentary leaves of Litchi chinensis . Litchi is an evergreen woody fruit tree that is widely cultivated in subtropical and tropical regions. Its floral buds are mixed with axillary or apical panicle primordia, leaf primordia and rudimentary leaves. A low spring temperature is vital for litchi production as it promotes the abscission of the rudimentary leaves, which could otherwise prevent panicle development. Hence, climate change could present additional challenges for litchi production. We previously reported that reactive oxygen species (ROS) can substitute low-temperature treatment to induce the senescence of rudimentary leaves. We have now identified from RNA-Seq data a litchi type II metacaspase gene, LcMCII-1, that is responsive to ROS. Silencing LcMCII-1 by virus-induced gene silencing delayed ROS-dependent senescence. The ectopic over-expression of LcMCII-1 in transgenic Arabidopsis promoted ROS-dependent and natural senescence. Consistently, the transient expression of LcMCII-1 in tobacco leaf by agroinfiltration resulted in leaf yellowing. Our findings demonstrate that LcMCII-1 is positively involved in the regulation of rudimentary leaf senescence in litchi and provide a new target for the future molecular breeding of new cultivars that can set fruit in warmer climates.

An NAM Domain Gene, GhNAC79, Improves Resistance to Drought Stress in Upland Cotton

Plant-specific NAC proteins comprise one of the largest transcription factor families in plants and play important roles in plant development and the stress response. Gossypium hirsutum L. is a major source of fiber, but its growth and productivity are limited by many biotic and abiotic stresses. In this study, the NAC domain gene GhNAC79 was functionally characterized in detail, and according to information about the cotton genome sequences, it was located on scaffold42.1, containing three exons and two introns. Promoter analysis indicated that the GhNAC79 promoter contained both basic and stress-related elements, and it was especially expressed in the cotyledon of Arabidopsis. A transactivation assay in yeast demonstrated that GhNAC79 was a transcription activator, and its activation domain was located at its C-terminus. The results of qRT-PCR proved that GhNAC79 was preferentially expressed at later stages of cotyledon and fiber development, and it showed high sensitivity to ethylene and meJA treatments. Overexpression of GhNAC79 resulted in an early flowering phenotype in Arabidopsis, and it also improved drought tolerance in both Arabidopsis and cotton. Furthermore, VIGS-induced silencing of GhNAC79 in cotton led to a drought-sensitive phenotype. In summary, GhNAC79 positively regulates drought stress, and it also responds to ethylene and meJA treatments, making it a candidate gene for stress studies in cotton.

Functional characterization of NAC55 transcription factor from oilseed rape (Brassica napus L.) as a novel transcriptional activator modulating reactive oxygen species accumulation and cell death

NAC transcription factors (TFs) are plant-specific and play important roles in development, responses to biotic and abiotic cues and hormone signaling. So far, only a few NAC genes have been reported to regulate cell death. In this study, we identified and characterized a NAC55 gene isolated from oilseed rape (Brassica napus L.). BnaNAC55 responds to multiple stresses, including cold, heat, abscisic acid (ABA), jasmonic acid (JA) and a necrotrophic fungal pathogen Sclerotinia sclerotiorum. BnaNAC55 has transactivation activity and is located in the nucleus. BnaNAC55 is able to form homodimers in planta. Unlike ANAC055, full-length BnaNAC55, but not either the N-terminal NAC domain or C-terminal regulatory domain, induces ROS accumulation and hypersensitive response (HR)-like cell death when expressed both in oilseed rape protoplasts and Nicotiana benthamiana. Furthermore, BnaNAC55 expression causes obvious nuclear DNA fragmentation. Moreover, quantitative reverse transcription PCR (qRT-PCR) analysis identified that the expression levels of multiple genes regulating ROS production and scavenging, defense response as well as senescence are significantly induced. Using a dual luciferase reporter assay, we further confirm that BnaNAC55 could activate the expression of a few ROS and defense-related gene expression. Taken together, our work has identified a novel NAC TF from oilseed rape that modulates ROS accumulation and cell death.

Identification, cloning and characterization of R2R3-MYB gene family in canola (Brassica napus L.) identify a novel member modulating ROS accumulation and hypersensitive-like cell death

The R2R3-MYB proteins comprise one of the largest families of transcription factors in plants. Although genome-wide analysis of this family has been carried out in some plant species, little is known about R2R3-MYB genes in canola (Brassica napus L.). In this study, we have identified 76 R2R3-MYB genes in the canola genome through mining of expressed sequence tags (ESTs). The cDNA sequences of 44 MYB genes were successfully cloned. The transcriptional activities of BnaMYB proteins encoded by these genes were assayed in yeast. The subcellular localizations of representative R2R3-MYB proteins were investigated through GFP fusion. Besides, the transcript abundance level analysis during abiotic conditions and ABA treatment identified a group of R2R3-MYB genes that responded to one or more treatments. Furthermore, we identified a previously functionally unknown MYB gene-BnaMYB78, which modulates reactive oxygen species (ROS)-dependent cell death in Nicotiana benthamiana, through regulating the transcription of a few ROS- and defence-related genes. Taken together, this study has provided a solid foundation for understanding the roles and regulatory mechanism of canola R2R3-MYB genes.

Biofuel Potential of Plants Transformed Genetically with NAC Family Genes

NAC genes contribute to enhance survivability of plants under conditions of environmental stress and in secondary growth of the plants, thereby building biomass. Thus, genetic transformation of plants using NAC genes provides a possibility to tailor biofuel plants. Over-expression studies have indicated that NAC family genes can provide tolerance to various biotic and abiotic stresses, either by physiological or biochemical changes at the cellular level, or by affecting visible morphological and anatomical changes, for example, by development of lateral roots in a number of plants. Over-expression of these genes also work as triggers for development of secondary cell walls. In our laboratory, we have observed a NAC gene from Lepidium latifolium contributing to both enhanced biomass as well as cold stress tolerance of model plants tobacco. Thus, we have reviewed all the developments of genetic engineering using NAC genes which could enhance the traits required for biofuel plants, either by enhancing the stress tolerance or by enhancing the biomass of the plants.

Mitogen-activated protein kinase kinase kinase (MAPKKK) 4 from rapeseed (Brassica napus L.) is a novel member inducing ROS accumulation and cell death

MAPKKK is the largest family of MAPK cascade, which is known to play important roles in plant growth, development and immune responses. So far, only a few have been functionally characterized even in the model plant, Arabidopsis due to the potential functional redundancy of MAPKKK. We previously identified and cloned a few MAPKKK family genes from rapeseed. In this study, BnaMAPKKK4 was characterized as a member in eliciting accumulation of reactive oxygen species (ROS) and hypersensitive response (HR)-like cell death. This is accompanied with accumulation of malondialdehyde (MDA), anthocyanin as well as nuclear DNA fragmentation. The transcript abundance of a series of ROS accumulation, cell death, and defense response related genes were up-regulated by the expression of MAPKKK4. Further investigation identified BnaMAPKKK4 elicited ROS through the downstream MPK3. These results indicate that BnaMAPKKK4 and its downstream components function in the ROS-induced cell death.