A membrane-associated NAC transcription factor regulates salt-responsive flowering via FLOWERING LOCUS T in Arabidopsis

CYCLIN-DEPENDENT KINASE G2 regulates salinity stress response and salt mediated flowering in Arabidopsis thaliana

Cyclin-dependent protein kinases are involved in many crucial cellular processes and aspects of plant growth and development, but their precise roles in abiotic stress responses are largely unknown. Here, Arabidopsis thaliana CYCLIN-DEPENDENT KINASE G2 (CDKG2) was shown to act as a negative regulator of the salinity stress response, as well as being involved in the control of flowering time. GUS expression experiments based on a pCDKG2::GUS transgene suggested that CDKG2 was expressed throughout plant development, with especially high expression levels recorded in the seed and in the flower. The loss-of-function of CDKG2 led to an increased tolerance of salinity stress and the up-regulation of the known stress-responsive genes SOS1, SOS2, SOS3, NHX3, RD29B, ABI2, ABI3, MYB15 and P5CS1. Flowering was accelerated in the cdkg2 mutants via the repression of FLC and the consequent up-regulation of FT, SOC1, AP1 and LFY. Transgenic lines constitutively expressing CDKG2 showed greater sensitivity to salinity stress and were delayed in flowering. Furthermore, the CDKG2 genotype affected the response of flowering time to salinity stress. Our data connect CDKG2 to undescribed functions related to salt stress tolerance and flowering time through the regulation of specific target genes.

Identification of novel and salt-responsive miRNAs to explore miRNA-mediated regulatory network of salt stress response in radish (Raphanus sativus L.)

Background

Salt stress is one of the most representative abiotic stresses that severely affect plant growth and development. MicroRNAs (miRNAs) are well known for their significant involvement in plant responses to abiotic stresses. Although miRNAs implicated in salt stress response have been widely reported in numerous plant species, their regulatory roles in the adaptive response to salt stress in radish (Raphanus sativus L.), an important root vegetable crop worldwide, remain largely unknown.

Results

Solexa sequencing of two sRNA libraries from NaCl-free (CK) and NaCl-treated (Na200) radish roots were performed for systematical identification of salt-responsive miRNAs and their expression profiling in radish. Totally, 136 known miRNAs (representing 43 miRNA families) and 68 potential novel miRNAs (belonging to 51 miRNA families) were identified. Of these miRNAs, 49 known and 22 novel miRNAs were differentially expressed under salt stress. Target prediction and annotation indicated that these miRNAs exerted a role by regulating specific stress-responsive genes, such as squamosa promoter binding-like proteins (SPLs), auxin response factors (ARFs), nuclear transcription factor Y (NF-Y) and superoxide dismutase [Cu-Zn] (CSD1). Further functional analysis suggested that these target genes were mainly implicated in signal perception and transduction, regulation of ion homeostasis, basic metabolic processes, secondary stress responses, as well as modulation of attenuated plant growth and development under salt stress. Additionally, the expression patterns of ten miRNAs and five corresponding target genes were validated by reverse-transcription quantitative PCR (RT-qPCR).

Conclusions

With the sRNA sequencing, salt-responsive miRNAs and their target genes in radish were comprehensively identified. The results provide novel insight into complex miRNA-mediated regulatory network of salt stress response in radish, and facilitate further dissection of molecular mechanism underlying plant adaptive response to salt stress in root vegetable crops.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1416-5) contains supplementary material, which is available to authorized users.

The abiotic stress-responsive NAC-type transcription factor SlNAC4 regulates salt and drought tolerance and stress-related genes in tomato (Solanum lycopersicum)

SlNAC4 functions as a stress-responsive transcription factor and might be useful for crop salt and drought tolerance improvement. Abiotic stresses, especially salinity and drought, are major factors that significantly limit crop growth and productivity. Plant-specific NAC transcription factors play crucial roles in various stress responses. However, to date only little information regarding stress-related NAC genes is available in tomato. Previously, we reported that tomato SlNAC4-SlNAC10 genes are involved in response of various abiotic stresses. Expression analysis revealed that SlNAC4 was also induced significantly by MeJA, but not by ABA. To further unravel the function of SlNAC4 in response to abiotic stress, we investigated the effects of salt and drought stress on wild-type and SlNAC4-RNAi transgenic tomato plants. The results demonstrated that the root and shoot growth of RNAi plants was more inhibited by salt stress than that of wild-type at post-germination stage. The leaf salt assay also showed less tolerance in transgenic plants by retaining lower chlorophyll content compared with wild-type plants. In addition, transgenic plants became less tolerant to salt and drought stress in soil, which were demonstrated by lower levels of water and chlorophyll contents, and higher water loss rate in their leaves as compared to wild-type plants under stressed conditions. Notably, the expressions of multiple stress-related genes were downregulated in SlNAC4-RNAi plants under both control and salt-stressed conditions. Collectively, these results highlight the important role of SlNAC4 functions as a stress-responsive transcription factor in positive modulation of abiotic stress tolerance through an ABA-independent signaling networks and possibly in response to biotic stress, and may hold promising applications in the engineering of salt- and drought-tolerant tomato.

NAC transcription factor genes: genome-wide identification, phylogenetic, motif and cis-regulatory element analysis in pigeonpea (Cajanus cajan (L.) Millsp.)

The NAC (NAM, ATAF and CUC) proteins are plant-specific transcription factors implicated in development and stress responses. In the present study 88 pigeonpea NAC genes were identified from the recently published draft genome of pigeonpea by using homology based and de novo prediction programmes. These sequences were further subjected to phylogenetic, motif and promoter analyses. In motif analysis, highly conserved motifs were identified in the NAC domain and also in the C-terminal region of the NAC proteins. A phylogenetic reconstruction using pigeonpea, Arabidopsis and soybean NAC genes revealed 33 putative stress-responsive pigeonpea NAC genes. Several stress-responsive cis-elements were identified through in silico analysis of the promoters of these putative stress-responsive genes. This analysis is the first report of NAC gene family in pigeonpea and will be useful for the identification and selection of candidate genes associated with stress tolerance.

Genomic analysis of NAC transcription factors in banana (Musa acuminata) and definition of NAC orthologous groups for monocots and dicots

Identifying the molecular mechanisms underlying tolerance to abiotic stresses is important in crop breeding. A comprehensive understanding of the gene families associated with drought tolerance is therefore highly relevant. NAC transcription factors form a large plant-specific gene family involved in the regulation of tissue development and responses to biotic and abiotic stresses. The main goal of this study was to set up a framework of orthologous groups determined by an expert sequence comparison of NAC genes from both monocots and dicots. In order to clarify the orthologous relationships among NAC genes of different species, we performed an in-depth comparative study of four divergent taxa, in dicots and monocots, whose genomes have already been completely sequenced: Arabidopsis thaliana, Vitis vinifera, Musa acuminata and Oryza sativa. Due to independent evolution, NAC copy number is highly variable in these plant genomes. Based on an expert NAC sequence comparison, we propose forty orthologous groups of NAC sequences that were probably derived from an ancestor gene present in the most recent common ancestor of dicots and monocots. These orthologous groups provide a curated resource for large-scale protein sequence annotation of NAC transcription factors. The established orthology relationships also provide a useful reference for NAC function studies in newly sequenced genomes such as M. acuminata and other plant species.

Identification of 32 full-length NAC transcription factors in ramie (Boehmeria nivea L. Gaud) and characterization of the expression pattern of these genes

NAM, ATAF, and CUC (NAC) genes are plant-specific transcription factors (TFs) that play key roles in plant growth, development, and stress tolerance. To date, none of the ramie NAC (BnNAC) genes had been identified, even though ramie is one of the most important natural fiber crops. In order to mine the BnNAC TFs and identify their potential function, the search for BnNAC genes against two pools of unigenes de novo assembled from the RNA-seq in our two previous studies was performed, and a total of 32 full-length BnNAC genes were identified in this study. Forty-seven function-known NAC proteins published in other species, in concert with these 32 BnNAC proteins were subjected to phylogenetic analysis, and the result showed that all the 79 NAC proteins can be divided into eight groups (NAC-I-VIII). Among the 32 BnNAC genes, 24, 2, and 1 gene showed higher expression in stem xylem, leaf, and flower, respectively. Furthermore, the expression of 14, 11 and 4 BnNAC genes was regulated by drought, cadmium stress, and infection by root lesion nematode, respectively. Interestingly, there were five BnNAC TFs which showed high homology with the NAC TFs of other species involved in regulating the secondary wall synthesis, and their expressions were not regulated by drought and cadmium stress. These results suggested that the BnNAC family might have a functional diversity. The identification of these 32 full-length BnNAC genes and the characterization of their expression pattern provide a basis for future clarification of their functions in ramie growth and development.

Recent advances in plant membrane-bound transcription factor research: emphasis on intracellular movement

Transcription factors constitute numerous signal transduction networks and play a central role in gene expression regulation. Recent studies have shown that a limited portion of transcription factors are anchored in the cellular membrane, storing as dormant forms. Upon exposure to environmental and developmental cues, these transcription factors are released from the membrane and translocated to the nucleus, where they regulate associated target genes. As this process skips both transcriptional and translational regulations, it guarantees prompt response to external and internal signals. Membrane-bound transcription factors (MTFs) undergo several unique steps that are not involved in the action of canonical nuclear transcription factors: proteolytic processing and intracellular movement. Recently, alternative splicing has also emerged as a mechanism to liberate MTFs from the cellular membranes, establishing an additional activation scheme independent of proteolytic processing. Multiple layers of MTF regulation add complexity to transcriptional regulatory scheme and ensure elaborate action of MTFs. In this review, we provide an overview of recent findings on MTFs in plants and highlight the molecular mechanisms underlying MTF liberation from cellular membranes with an emphasis on intracellular movement.

The NAC family transcription factor OsNAP confers abiotic stress response through the ABA pathway

Plants respond to environmental stresses by altering gene expression, and several genes have been found to mediate stress-induced expression, but many additional factors are yet to be identified. OsNAP is a member of the NAC transcription factor family; it is localized in the nucleus, and shows transcriptional activator activity in yeast. Analysis of the OsNAP transcript levels in rice showed that this gene was significantly induced by ABA and abiotic stresses, including high salinity, drought and low temperature. Rice plants overexpressing OsNAP did not show growth retardation, but showed a significantly reduced rate of water loss, enhanced tolerance to high salinity, drought and low temperature at the vegetative stage, and improved yield under drought stress at the flowering stage. Microarray analysis of transgenic plants overexpressing OsNAP revealed that many stress-related genes were up-regulated, including OsPP2C06/OsABI2, OsPP2C09, OsPP2C68 and OsSalT, and some genes coding for stress-related transcription factors (OsDREB1A, OsMYB2, OsAP37 and OsAP59). Our data suggest that OsNAP functions as a transcriptional activator that plays a role in mediating abiotic stress responses in rice.

The NAC-like gene ANTHER INDEHISCENCE FACTOR acts as a repressor that controls anther dehiscence by regulating genes in the jasmonate biosynthesis pathway in Arabidopsis

ANTHER INDEHISCENCE FACTOR (AIF), a NAC-like gene, was identified in Arabidopsis. In AIF:GUS flowers, β-glucuronidase (GUS) activity was detected in the anther, the upper parts of the filaments, and in the pollen of stage 7-9 young flower buds; GUS activity was reduced in mature flowers. Yellow fluorescent protein (YFP)+AIF-C fusion proteins, which lacked a transmembrane domain, accumulated in the nuclei of the Arabidopsis cells, whereas the YFP+AIF fusion proteins accumulated in the membrane and were absent in the nuclei. Further detection of a cleaved AIF protein in flowers revealed that AIF needs to be processed and released from the endoplasmic reticulum in order to function. The ectopic expression of AIF-C caused a male-sterile phenotype with indehiscent anthers throughout flower development in Arabidopsis. The presence of a repressor domain in AIF and the similar phenotype of indehiscent anthers in AIF-C+SRDX plants suggest that AIF acts as a repressor. The defect in anther dehiscence was due to the down-regulation of genes that participate in jasmonic acid (JA) biosynthesis, such as DAD1/AOS/AOC3/OPR3/OPCL1. The external application of JA rescued the anther indehiscence in AIF-C and AIF-C+SRDX flowers. In AIF-C+VP16 plants, which are transgenic dominant-negative mutants in which AIF is converted to a potent activator via fusion to a VP16-AD motif, the anther dehiscence was promoted, and the expression of DAD1/AOS/AOC3/OPR3/OPCL1 was up-regulated. Furthermore, the suppression of AIF through an antisense strategy resulted in a mutant phenotype similar to that observed in the AIF-C+VP16 flowers. The present data suggest a role for AIF in controlling anther dehiscence by suppressing the expression of JA biosynthesis genes in Arabidopsis.

The Arabidopsis floral repressor BFT delays flowering by competing with FT for FD binding under high salinity

Soil salinity is one of the most serious agricultural problems that significantly reduce crop yields in the arid and semi-arid regions. It influences various phases of plant growth and developmental processes, such as seed germination, leaf and stem growth, and reproductive propagation. Salt stress delays the onset of flowering in many plant species. We have previously reported that the Arabidopsis BROTHER OF FT AND TFL1 (BFT) acts as a floral repressor under salt stress. However, the molecular mechanisms underlying the BFT function in the salt regulation of flowering induction is unknown. In this work, we found that BFT delays flowering under high salinity by competing with FLOWERING LOCUS T (FT) for binding to the FD transcription factor. The flowering time of FD-deficient fd-2 mutant was insensitive to high salinity. BFT interacts with FD in the nucleus via the C-terminal domain of FD, which is also required for the interaction of FD with FT, and interferes with the FT-FD interaction. These observations indicate that BFT constitutes a distinct salt stress signaling pathway that modulates the function of the FT-FD module and possibly provides an adaptation strategy that fine-tunes photoperiodic flowering under high salinity.

Emerging roles for diverse intramembrane proteases in plant biology

Progress in the field of regulated intramembrane proteolysis (RIP) in recent years has made its impact on plant biology as well. Although this field within plant research is still in its infancy, some interesting observations have started to emerge. Gene encoding orthologs of rhomboid proteases, site-2 proteases (S2P), presenilin/γ-secretases, and signal peptide peptidases are found in plant genomes and some of these gene products were identified in different plant cell membranes. The lack of chloroplast-located rhomboid proteases was associated with reduced fertility and aberrations in flower morphology. Mutations in homologues of S2P resulted in chlorophyll deficiency and impaired chloroplast development. An S2P was also implicated in the response to ER stress through cleavage of ER-membrane bZIP transcription factors, allowing their migration to the nucleus and activation of the transcription of BiP chaperones. Other membrane-bound transcription factors of the NAC and PHD families were also demonstrated to undergo RIP and relocalization to the nucleus. These and other new data are expected to shed more light on the roles of intramembrane proteases in plant biology in the future. This article is part of a Special Issue entitled: Intramembrane Proteases.

Phenotypic effects of salt and heat stress over three generations in Arabidopsis thaliana

Current and predicted environmental change will force many organisms to adapt to novel conditions, especially sessile organisms such as plants. It is therefore important to better understand how plants react to environmental stress and to what extent genotypes differ in such responses. It has been proposed that adaptation to novel conditions could be facilitated by heritable epigenetic changes induced by environmental stress, independent of genetic variation. Here we assessed phenotypic effects of heat and salt stress within and across three generations using four highly inbred Arabidopsis thaliana genotypes (Col, Cvi, Ler and Sha). Salt stress generally decreased fitness, but genotypes were differently affected, suggesting that susceptibility of A. thaliana to salt stress varies among genotypes. Heat stress at an early rosette stage had less detrimental effects but accelerated flowering in three out of four accessions. Additionally, we found three different modes of transgenerational effects on phenotypes, all harboring the potential of being adaptive: heat stress in previous generations induced faster rosette growth in Sha, both under heat and control conditions, resembling a tracking response, while in Cvi, the phenotypic variance of several traits increased, resembling diversified bet-hedging. Salt stress experienced in earlier generations altered plant architecture of Sha under salt but not control conditions, similar to transgenerational phenotypic plasticity. However, transgenerational phenotypic effects depended on the type of stress as well as on genotype, suggesting that such effects may not be a general response leading to adaptation to novel environmental conditions in A. thaliana.

Genome-wide analysis and identification of stress-responsive genes of the NAM-ATAF1,2-CUC2 transcription factor family in apple

NAC (NAM, ATAF1,2, and CUC2) proteins constitute one of the largest families of plant-specific transcription factors. To date, little is known about the NAC genes in the apple (Malus domestica). In this study, a total of 180 NAC genes were identified in the apple genome and were phylogenetically clustered into six groups (I-VI) with the NAC genes from Arabidopsis and rice. The predicted apple NAC genes were distributed across all of 17 chromosomes at various densities. Additionally, the gene structure and motif compositions of the apple NAC genes were analyzed. Moreover, the expression of 29 selected apple NAC genes was analyzed in different tissues and under different abiotic stress conditions. All of the selected genes, with the exception of four genes, were expressed in at least one of the tissues tested, which indicates that the NAC genes are involved in various aspects of the physiological and developmental processes of the apple. Encouragingly, 17 of the selected genes were found to respond to one or more of the abiotic stress treatments, and these 17 genes included not only the expected 7 genes that were clustered with the well-known stress-related marker genes in group IV but also 10 genes located in other subgroups, none of which contains members that have been reported to be stress-related. To the best of our knowledge, this report describes the first genome-wide analysis of the apple NAC gene family, and the results should provide valuable information for understanding the classification and putative functions of this family.

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.

Genome-wide organization and expression profiling of the NAC transcription factor family in potato (Solanum tuberosum L.)

NAC [no apical meristem (NAM), Arabidopsisthaliana transcription activation factor [ATAF1/2] and cup-shaped cotyledon (CUC2)] proteins belong to one of the largest plant-specific transcription factor (TF) families and play important roles in plant development processes, response to biotic and abiotic cues and hormone signalling. Our genome-wide analysis identified 110 StNAC genes in potato encoding for 136 proteins, including 14 membrane-bound TFs. The physical map positions of StNAC genes on 12 potato chromosomes were non-random, and 40 genes were found to be distributed in 16 clusters. The StNAC proteins were phylogenetically clustered into 12 subgroups. Phylogenetic analysis of StNACs along with their Arabidopsis and rice counterparts divided these proteins into 18 subgroups. Our comparative analysis has also identified 36 putative TNAC proteins, which appear to be restricted to Solanaceae family. In silico expression analysis, using Illumina RNA-seq transcriptome data, revealed tissue-specific, biotic, abiotic stress and hormone-responsive expression profile of StNAC genes. Several StNAC genes, including StNAC072 and StNAC101that are orthologs of known stress-responsive Arabidopsis RESPONSIVE TO DEHYDRATION 26 (RD26) were identified as highly abiotic stress responsive. Quantitative real-time polymerase chain reaction analysis largely corroborated the expression profile of StNAC genes as revealed by the RNA-seq data. Taken together, this analysis indicates towards putative functions of several StNAC TFs, which will provide blue-print for their functional characterization and utilization in potato improvement.

Transcriptomic analysis of rice (Oryza sativa) endosperm using the RNA-Seq technique

The endosperm plays an important role in seed formation and germination, especially in rice (Oryza sativa). We used a high-throughput sequencing technique (RNA-Seq) to reveal the molecular mechanisms involved in rice endosperm development. Three cDNA libraries were taken from rice endosperm at 3, 6 and 10 days after pollination (DAP), which resulted in the detection of 21,596, 20,910 and 19,459 expressed gens, respectively. By ERANGE, we identified 10,371 differentially expressed genes (log(2)Ratio ≥1, FDR ≤0.001). The results were compared against three public databases (Gene Ontology, Kyoto Encyclopedia of Genes and Genomes and MapMan) in order to annotate the gene descriptions, associate them with gene ontology terms and to assign each to pathways. A large number of genes related to ribosomes, the spliceosome and oxidative phosphorylation were found to be expressed in the early and middle stages. Plant hormone, galactose metabolism and carbon fixation related genes showed a significant increase in expression at the middle stage, whereas genes for defense against disease or response to stress as well as genes for starch/sucrose metabolism were strongly expressed during the later stages of endosperm development. Interestingly, most metabolic pathways were down-regulated between 3 and 10 DAP except for those involved in the accumulation of material, such as starch/sucrose and protein metabolism. We also identified the expression of 1,118 putative transcription factor genes in endosperm development. The RNA-Seq results provide further systematic understanding of rice endosperm development at a fine scale and a foundation for future studies.

ENAC1, a NAC transcription factor, is an early and transient response regulator induced by abiotic stress in rice (Oryza sativa L.)

The plant-specific NAC (NAM, ATAF, and CUC)-domain proteins play important roles in plant development and stress responses. In this research, a full-length cDNA named ENAC1 (early NAC-domain protein induced by abiotic stress 1) was isolated from rice. ENAC1 possess one NAC domain in the N-terminus. Comparative time-course expression analysis indicated that ENAC1 expression, similar with OsDREB1A, was induced very quickly by various abiotic stresses including salt, drought, cold, and exogenous abscisic acid. However, the induction of ENAC1 by abiotic stress was transient and lasted up to 3 h, whereas that of OsDREB1A maintained longer. The promoter sequence of ENAC1 harbors several cis-elements including ABA response elements, but the well-known dehydration responsive element/C-repeat element is absent. The ENAC1-GFP (green fluorescent protein) fusion protein was localized in the nucleus of rice protoplast cell. Yeast hybrid assays revealed that ENAC1 was a transcription activator and bound to NAC recognition sequence (NACRS). Co-expression analysis suggested that ENAC1 co-expressed with a number of stress-related genes. Taken together, ENAC1 may be an early transcription activator of stress responses and function in the regulation of NACRS-mediated gene expression under abiotic stress.

Isolation and characterization of cold responsive NAC gene from Lepidium latifolium

Cold stress is one of the major limiting factor in crop productivity. Plants growing in colder regions acclimatize to severe conditions owing to the presence of 'cold stress tolerant genes'. Isolation and functional characterization of these genes are important before their exploitation in modern agricultural practices. Here, we have cloned full length NAC gene (1,388 bp) from Lepidium latifolium (LlaNAC). This gene belongs to NAP sub-group which also includes ANAC056 of Arabidopsis thaliana, nearest relative of LlaNAC. Upstream analysis and microarray data analysis of ANAC056 suggested that LlaNAC might also be ABA-regulated. However, quantitative transcript expression analysis revealed that LlaNAC transcript upregulated by cold stress and downregulated in response to varying concentrations of abscisic acid, salicylic acid, calcium chloride and ethylene. There is also a possibility that the gene may be getting regulated by a pathway whose components are still unknown. Any further investigations to understand the mechanism of regulation of LlaNAC gene expression are likely to find immense importance in plant biotechnology and crop improvement.

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.

Dissecting out the crosstalk between salinity and hormones in roots of Arabidopsis

Phytohormones are chemical messengers that play a leading role in regulating the vital activity of plants, including transcription, posttranscriptional pre-mRNA splicing, translation, and posttranslational modifications by interacting with specific protein receptors. Plant hormones are synthesized in one tissue and act on specific target sites in other tissues at vanishingly low concentrations. High salinity is one of the main factors limiting Arabidopsis growth and productivity. In this study, phytohormones including abscisic acid, auxin, ethylene, and cytokinin responsive genes regulating salinity stress in Arabidopsis roots were monitored using microarray data. We identified phytohormone responsive genes on the basis of their expression pattern at genomic level at various time points. Using publicly available microarray data, we analyzed the effect of salt stress on the transcription of phytohormone responsive genes. Gene ontology (GO) analysis of phytohormone responsive genes showed their role in important biological processes such as signal transduction, hormone metabolism, biosynthetic process, and gene expression. Gene enrichment terms also reveal that transcription regulator activity is the main class of ABA responsive genes under salinity stress. We conclude that expression of ABA responsive genes involves induction of several transcription factors under salt stress treatment in Arabidopsis roots.

Molecular characterization of novel TaNAC genes in wheat and overexpression of TaNAC2a confers drought tolerance in tobacco

Plant-specific NAC (NAM/ATAF/CUC) transcription factors (TFs) have been reported to play a role in diverse stress responses and developmental processes. We show here that six new genes encoding NAC TFs in wheat (Triticum aestivum) were identified (named as TaNAC2a, TaNAC4a, TaNAC6, TaNAC7, TaNAC13 and TaNTL5, respectively), and we classified them into three groups: stress-related NACs, development-related NACs and NTLs (membrane-associated TFs belonging to NAC) by phylogenetic analysis. All TaNACs were induced by one or several kinds of stress treatments including dehydration, salinity and low temperature, whereas different genes showed different expression levels. All these TaNACs, except TaNAC7, were proven to have transcriptional activation activity in the yeast strain AH109 by transactivation analysis. Furthermore, subcellular localization analysis revealed that four TaNAC:GFP (green fluorescent protein) fusion proteins were localized in the nucleus, TaNAC2a:GFP mainly located in the nucleus and the plasma membrane, TaNTL5:GFP was associated with the membrane, while truncated TaNTL5(ΔTM):GFP (lacking the transmembrane motif) was detected exclusively in the nucleus. Semi-quantitative reverse transcription polymerase chain reaction analysis demonstrated that five genes exhibited organ-specific expression. Transgenic tobacco plants overexpressing TaNAC2a showed higher fresh weight and dry weight than non-transgenic plants under drought condition, which indicated that the transgene improved tobacco tolerance to drought treatment. Together, these results provided a preliminary characterization of six TaNACs, which possessed a potential role in improving stress tolerance and the regulation of development in wheat, and suggested that TaNAC2a was potentially useful for engineering drought tolerant plants.

Phylogenetic analyses unravel the evolutionary history of NAC proteins in plants

NAC (NAM/ATAF/CUC) proteins are one of the largest groups of transcription factors in plants. Although many NAC proteins based on Arabidopsis and rice genomes have been reported in a number of species, a complete survey and classification of all NAC genes in plant species from disparate evolutionary groups is lacking. In this study, we analyzed whole-genome sequences from nine major lineages of land plants to unveil the relationships between these proteins. Our results show that there are fewer than 30 NAC proteins present in both mosses and lycophytes, whereas more than 100 were found in most of the angiosperms. Phylogenetic analyses suggest that NAC proteins consist of 21 subfamilies, most of which have highly conserved non-NAC domain motifs. Six of these subfamilies existed in early-diverged land plants, whereas the remainder diverged only within the angiosperms. We hypothesize that NAC proteins probably originated sometime more than 400 million years ago and expanded together with the differentiation of plants into organisms of increasing complexity possibly after the divergence of lycophytes from the other vascular plants.

A subcellular localization compendium of hydrogen peroxide-induced proteins

The signal transduction mechanisms of the oxidative stress response in plants remain largely unexplored. Previously, increased levels of cellular hydrogen peroxide (H(2)O(2)) had been shown to drastically affect the plant transcriptome. Genome-wide transcriptome analyses allowed us to build a comprehensive inventory of H(2)O(2)-induced genes in plants. Here, the primary objective was to determine the subcellular localization of these genes and to assess potential trafficking during oxidative stress. After high-throughput cloning in Gateway-derived vectors, the subcellular localization of 49 proteins fused to the green fluorescent protein (GFP) was identified in a transient assay in tobacco (Nicotiana benthamiana) by means of agro-infiltration and confirmed for a selection of genes in transgenic Arabidopsis thaliana plants. Whereas eight of the GFP-tagged proteins are exclusively localized in the nucleus, 23 reside both in the nucleus and cytosol, in which several classes of known transcription factors and proteins of unknown function can be recognized. In this study, the mapping of the subcellular localization of H(2)O(2) -induced proteins paves the way for future research to unravel the H(2)O(2) responses in plants. Furthermore, the effect of increased H(2)O(2) levels on the subcellular localization of a subset of proteins was assessed.

Molecular cloning and characterization of a membrane associated NAC family gene, SiNAC from foxtail millet [Setaria italica (L.) P. Beauv]

The plant-specific NAC (NAM, ATAF, and CUC) transcription factors have diverse role in development and stress regulation. A transcript encoding NAC protein, termed SiNAC was identified from a salt stress subtractive cDNA library of S. italica seedling (Puranik et al., J Plant Physiol 168:280-287, 2011). This single/low copy gene containing four exons and four introns within the genomic-sequence encoded a protein of 462 amino acids. Structural analysis revealed that highly divergent C terminus contains a transmembrane domain. The NAC domain consisted of a twisted antiparallel beta-sheet packing against N terminal alpha helix on one side and a shorter helix on the other side. The domain was predicted to homodimerize and control DNA-binding specificity. The physicochemical features of the SiNAC homodimer interface justified the dimeric form of the predicted model. A 1539 bp fragment upstream to the start codon of SiNAC gene was cloned and in silico analysis revealed several putative cis-acting regulatory elements within the promoter sequence. Transactivation analysis indicated that SiNAC activated expression of reporter gene and the activation domain lied at the C terminal. The SiNAC:GFP was detected in the nucleus and cytoplasm while SiNAC ΔC(1-158):GFP was nuclear localized in onion epidermal cells. SiNAC transcripts mostly accumulated in young spikes and were strongly induced by dehydration, salinity, ethephon, and methyl jasmonate. These results suggest that SiNAC encodes a membrane associated NAC-domain protein that may function as a transcriptional activator in response to stress and developmental regulation in plants.

The floral repressor BROTHER OF FT AND TFL1 (BFT) modulates flowering initiation under high salinity in Arabidopsis

Floral transition is coordinately regulated by both endogenous and exogenous cues to ensure reproductive success under fluctuating environmental conditions. Abiotic stress conditions, including drought and high salinity, also have considerable influence on this developmental process. However, the signaling components and molecular mechanisms underlying the regulation of floral transition by environmental factors have not yet been defined. In this work, we show that the Arabidopsis BROTHER OF FT AND TFL1 (BFT) gene, which encodes a member of the FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family, regulates floral transition under conditions of high salinity. The BFT gene was transcriptionally induced by high salinity in an abscisic acid (ABA)-dependent manner. Transgenic plants overexpressing the BFT gene (35S:BFT) and BFT-deficient mutant (bft-2) plants were phenotypically indistinguishable from Col-0 plants in seed germination and seedling growth under high salinity. In contrast, although the floral transition was delayed significantly in Col-0 plants under high salinity, that of the bft-2 mutant was not affected by high salinity. We also observed that expression of the APETALA1 (AP1) gene was suppressed to a lesser degree in the bft-2 mutant than in Col-0 plants. Taken together, our observations suggest that BFT mediates salt stress-responsive flowering, providing an adaptive strategy that ensures reproductive success under unfavorable stress conditions.

Membrane-tethered transcription factors provide a connection between stress response and developmental pathways

Membrane-tethered transcription factors (MTTFs) are proteins that are targeted to membranes and are capable of regulating gene expression. In this way, they are physically restrained from entering the nucleus and are innately dormant. Upon specific signal recognition cues, MTTFs are activated through cleavage by a protease that releases the transcription factor domain into the cytosol thus allowing it to translocate to the nucleus where it can regulate gene expression. MTTFs are classically thought to provide an advantage to an organism by allowing for rapid signal transduction in response to cellular and environmental stresses. However, recent findings suggest that MTTFs may not only act as a means to respond quickly to stress but also are able to regulate developmental pathways, illustrating a point of interaction between stress and development. 

Identification and expression pattern of one stress-responsive NAC gene from Solanum lycopersicum

NAC (for NAM, ATAF1, 2, and CUC2) family genes have been found to play an important role in diversified developmental processes and environmental responses. A new NAC-type transcription factor SlNAC3 was primarily identified and isolated from the cDNA libraries of tomato cultivar Ailsa Craig. It contains three exons and two introns within genomic DNA sequence and encodes a polypeptide of 329 amino acids. A plant-specific and conserved NAC domain is located in the N-terminus of SlNAC3. The protein SlNAC3 is subcellularly localized in the nucleus of onion epidemical cells and it has a transcriptional activation domain in the C-terminal region which shows extremely divergent among NACs. Phylogenetic analysis showed that SlNAC3 belonged to the OsNAC3 subgroup of the NAC protein family. Tissue expression profile analysis revealed that SlNAC3 was expressed mainly in flower, fruit and root. The transcription expression of SlNAC3 was inhibited by salt, drought stress and ABA treatment. These data demonstrate that SlNAC3 might interact with environmental and endogenous stimuli and probably function when plants response to salt and drought stresses through ABA signaling pathways as a transcriptional activator.

Fructose sensitivity is suppressed in Arabidopsis by the transcription factor ANAC089 lacking the membrane-bound domain

In living organisms sugars not only provide energy and carbon skeletons but also act as evolutionarily conserved signaling molecules. The three major soluble sugars in plants are sucrose, glucose, and fructose. Information on plant glucose and sucrose signaling is available, but to date no fructose-specific signaling pathway has been reported. In this study, sugar repression of seedling development was used to study fructose sensitivity in the Landsberg erecta (Ler)/Cape Verde Islands (Cvi) recombinant inbred line population, and eight fructose-sensing quantitative trait loci (QTLs) (FSQ1-8) were mapped. Among them, FSQ6 was confirmed to be a fructose-specific QTL by analyzing near-isogenic lines in which Cvi genomic fragments were introgressed in the Ler background. These results indicate the existence of a fructose-specific signaling pathway in Arabidopsis. Further analysis demonstrated that the FSQ6-associated fructose-signaling pathway functions independently of the hexokinase1 (HXK1) glucose sensor. Remarkably, fructose-specific FSQ6 downstream signaling interacts with abscisic acid (ABA)- and ethylene-signaling pathways, similar to HXK1-dependent glucose signaling. The Cvi allele of FSQ6 acts as a suppressor of fructose signaling. The FSQ6 gene was identified using map-based cloning approach, and FSQ6 was shown to encode the transcription factor gene Arabidopsis NAC (petunia No apical meristem and Arabidopsis transcription activation factor 1, 2 and Cup-shaped cotyledon 2) domain containing protein 89 (ANAC089). The Cvi allele of FSQ6/ANAC089 is a gain-of-function allele caused by a premature stop in the third exon of the gene. The truncated Cvi FSQ6/ANAC089 protein lacks a membrane association domain that is present in ANAC089 proteins from other Arabidopsis accessions. As a result, Cvi FSQ6/ANAC089 is constitutively active as a transcription factor in the nucleus.

A structural view of the conserved domain of rice stress-responsive NAC1

The importance of NAC (named as NAM, ATAF1, 2, and CUC2) proteins in plant development, transcription regulation and regulatory pathways involving protein-protein interactions has been increasingly recognized. We report here the high resolution crystal structure of SNAC1 (stress-responsive NAC) NAC domain at 2.5 Å. Although the structure of the SNAC1 NAC domain shares a structural similarity with the reported structure of the ANAC NAC1 domain, some key features, especially relating to two loop regions which potentially take the responsibility for DNA-binding, distinguish the SNAC1 NAC domain from other reported NAC structures. Moreover, the dimerization of the SNAC1 NAC domain is demonstrated by both soluble and crystalline conditions, suggesting this dimeric state should be conserved in this type of NAC family. Additionally, we discuss the possible NAC-DNA binding model according to the structure and reported biological evidences.

An Arabidopsis senescence-associated protein SAG29 regulates cell viability under high salinity

The plasma membrane is an important cellular organ that perceives incoming developmental and environmental signals and integrates these signals into cellular regulatory mechanisms. It also acts as a barrier against unfavorable extracellular factors to maintain cell viability. Despite its importance for cell viability, molecular components determining cell viability and underlying mechanisms are largely unknown. Here, we show that a plasma membrane-localized MtN3 protein SAG29 regulates cell viability under high salinity in Arabidopsis. The SAG29 gene is expressed primarily in senescing plant tissues. It is induced by osmotic stresses via an abscisic acid-dependent pathway. Whereas the SAG29-overexpressing transgenic plants (35S:SAG29) exhibited an accelerated senescence and were hypersensitive to salt stress, the SAG29-deficient mutants were less sensitive to high salinity. Consistent with this, the 35S:SAG29 transgenic plants showed reduced cell viability in the roots under normal growth condition. In contrast, cell viability in the SAG29-deficient mutant roots was indistinguishable from that in the roots of control plants. Notably, the mutant roots exhibited enhanced cell viability under high salinity. Our observations indicate that the senescence-associated SAG29 protein is associated with cell viability under high salinity and other osmotic stress conditions. We propose that the SAG29 protein may serve as a molecular link that integrates environmental stress responses into senescing process.

A membrane-tethered transcription factor ANAC089 negatively regulates floral initiation in Arabidopsis thaliana

The plant-specific NAC (NAM, ATAF1/2, and CUC2) transcription factors have a regulatory function in developmental processes and stress responses. Notably a group of NAC members named NTLs (NTM1-Like) are membrane-tethered, ensuring plants rapidly respond to developmental changes and environmental stimuli. Our results indicated that ANAC089 was a membrane-tethered transcription factor and its truncated form was responsible for the physiological function in flowering time control.

Comprehensive analysis of NAC domain transcription factor gene family in Populus trichocarpa

BACKGROUND

NAC (NAM, ATAF1/2 and CUC2) domain proteins are plant-specific transcriptional factors known to play diverse roles in various plant developmental processes. NAC transcription factors comprise of a large gene family represented by more than 100 members in Arabidopsis, rice and soybean etc. Recently, a preliminary phylogenetic analysis was reported for NAC gene family from 11 plant species. However, no comprehensive study incorporating phylogeny, chromosomal location, gene structure, conserved motifs, and expression profiling analysis has been presented thus far for the model tree species Populus.

RESULTS

In the present study, a comprehensive analysis of NAC gene family in Populus was performed. A total of 163 full-length NAC genes were identified in Populus, and they were phylogenetically clustered into 18 distinct subfamilies. The gene structure and motif compositions were considerably conserved among the subfamilies. The distributions of 120 Populus NAC genes were non-random across the 19 linkage groups (LGs), and 87 genes (73%) were preferentially retained duplicates that located in both duplicated regions. The majority of NACs showed specific temporal and spatial expression patterns based on EST frequency and microarray data analyses. However, the expression patterns of a majority of duplicate genes were partially redundant, suggesting the occurrence of subfunctionalization during subsequent evolutionary process. Furthermore, quantitative real-time RT-PCR (RT-qPCR) was performed to confirm the tissue-specific expression patterns of 25 NAC genes.

CONCLUSION

Based on the genomic organizations, we can conclude that segmental duplications contribute significantly to the expansion of Populus NAC gene family. The comprehensive expression profiles analysis provides first insights into the functional divergence among members in NAC gene family. In addition, the high divergence rate of expression patterns after segmental duplications indicates that NAC genes in Populus are likewise to have been retained by substantial subfunctionalization. Taken together, our results presented here would be helpful in laying the foundation for functional characterization of NAC gene family and further gaining an understanding of the structure-function relationship between these family members.

Genome-wide analysis of NAC transcription factor family in rice

We investigated 151 non-redundant NAC genes in rice and 117 in Arabidopsis. A complete overview of this gene family in rice is presented, including gene structures, phylogenies, genome localizations, and expression profiles. We also performed a comparative analysis of these genes in rice and Arabidopsis. Conserved amino acid residues and phylogeny construction using the NAC conserved domain sequence suggest that OsNAC gene family was classified broadly into two major groups (A and B) and sixteen subgroups in rice. We presented more specific phylogenetic analysis of OsNAC proteins based on the DNA-binding domain and known gene function, respectively. Loss of introns was observed in the segmental duplication. Homologous, paralogous, and orthologous searches of rice and Arabidopsis revealed that the major functional diversification within the NAC gene family predated the divergence of monocots and dicots. The chromosomal localizations of OsNAC genes indicated nine segmental duplication events involving 18 genes; 32 non-redundant OsNAC genes were involved in tandem duplications. Expression levels of this gene family were checked under various abiotic stresses (cold, drought, submergence, laid-down submergence, osmotic, salinity and hormone) and biotic stresses [infection with rice viruses such as RSV (rice stripe virus) and RTSV (rice tungro spherical virus)]. Biotic stresses are novel work and increase the possibilities for finding the best candidate genes. A preliminary search based on our microarray (22K and 44K) data suggested that more than 45 and 26 non-redundant genes in this family were upregulated in response to abiotic and biotic stresses, respectively. All of the genes were further investigated for their stress responsiveness by RT-PCR analysis. Six genes showed preferential expression under both biotic RSV and RTSV stress. Eleven genes were upregulated by at least three abiotic treatments. Our study provides a very useful reference for cloning and functional analysis of members of this gene family in rice.

Overexpression of the NAC transcription factor family gene ANAC036 results in a dwarf phenotype in Arabidopsis thaliana

NAC proteins comprise one of the largest families of transcription factors in the plant genome. They are known to be involved in various aspects of plant development, but the functions of most of them have not yet been determined. ANAC036, a member of the Arabidopsis NAC transcription factor family, contains unique sequences that are conserved among various NAC proteins found in other plant species. Expression analysis of the ANAC036 gene indicated that this gene was strongly expressed in leaves. Transgenic plants overexpressing the ANAC036 gene showed a semidwarf phenotype. The lengths of leaf blades, petioles and stems of these plants were smaller than those in wild-type plants. Microscopy revealed that cell sizes in leaves and stems of these plants were smaller than those in wild-type plants. These findings suggested that ANAC036 and its orthologues are involved in the growth of leaf cells.

Complete inventory of soybean NAC transcription factors: sequence conservation and expression analysis uncover their distinct roles in stress response

We performed an inventory of soybean NAC transcription factors, in which 101 NAC domain-containing proteins were annotated into 15 different subgroups, showing a clear relationship between structure and function. The six previously described GmNAC proteins (GmNAC1 to GmNAC6) were located in the nucleus and a transactivation assay in yeast confirmed that GmNAC2, GmNAC3, GmNAC4 and GmNAC5 function as transactivators. We also analyzed the expression of the six NAC genes in response to a variety of stress conditions. GmNAC2, GmNAC3 and GmNAC4 were strongly induced by osmotic stress. GmNAC3 and GmNAC4 were also induced by ABA, JA and salinity but differed in their response to cold. Consistent with an involvement in cell death programs, the transient expression of GmNAC1, GmNAC5 and GmNAC6 in tobacco leaves resulted in cell death and enhanced expression of senescence markers. Our results indicate that the described soybean NACs are functionally non-redundant transcription factors involved in response to abiotic stresses and in cell death events in soybean.

The low-oxygen-induced NAC domain transcription factor ANAC102 affects viability of Arabidopsis seeds following low-oxygen treatment

Low-oxygen stress imposed by field waterlogging is a serious impediment to plant germination and growth. Plants respond to waterlogging with a complex set of physiological responses regulated at the transcriptional, cellular, and tissue levels. The Arabidopsis (Arabidopsis thaliana) NAC domain-containing gene ANAC102 was shown to be induced under 0.1% oxygen within 30 min in both roots and shoots as well as in 0.1% oxygen-treated germinating seeds. Overexpression of ANAC102 altered the expression of a number of genes, including many previously identified as being low-oxygen responsive. Decreasing ANAC102 expression had no effect on global gene transcription in plants but did alter expression patterns in low-oxygen-stressed seeds. Increasing or decreasing the expression of ANAC102 did not affect adult plant survival of low-oxygen stress. Decreased ANAC102 expression significantly decreased germination efficiency following a 0.1% oxygen treatment, but increased expression had no effect on germination. This protective role during germination appeared to be specific to low-oxygen stress, implicating ANAC102 as an important regulator of seed germination under flooding.

Differential expression of miRNAs in response to salt stress in maize roots

BACKGROUND AND AIMS

Corn (Zea mays) responds to salt stress via changes in gene expression, metabolism and physiology. This adaptation is achieved through the regulation of gene expression at the transcriptional and post-transcriptional levels. MicroRNAs (miRNAs) have been found to act as key regulating factors of post-transcriptional gene expression. However, little is known about the role of miRNAs in plants' responses to abiotic stresses.

METHODS

A custom microparaflo microfluidic array containing release version 10.1 plant miRNA probes (http://microrna.sanger.ac.uk/) was used to discover salt stress-responsive miRNAs using the differences in miRNA expression between the salt-tolerant maize inbred line 'NC286' and the salt-sensitive maize line 'Huangzao4'. Key Results miRNA microarray hybridization revealed that a total of 98 miRNAs, from 27 plant miRNA families, had significantly altered expression after salt treatment. These miRNAs displayed different activities in the salt response, and miRNAs belonging to the same miRNA family showed the same behaviour. Interestingly, 18 miRNAs were found which were only expressed in the salt-tolerant maize line, and 25 miRNAs that showed a delayed regulation pattern in the salt-sensitive line. A gene model was proposed that showed how miRNAs could regulate the abiotic stress-associated process and the gene networks coping with the stress.

CONCLUSIONS

Salt-responsive miRNAs are involved in the regulation of metabolic, morphological and physiological adaptations of maize seedlings at the post-transcriptional level. The miRNA genotype-specific expression model might explain the distinct salt sensitivities between maize lines.

Membrane-tethered transcription factors in Arabidopsis thaliana: novel regulators in stress response and development

Membrane-tethered transcription factors (MTTFs) differ from cytosolic transcription factors (TF) in that they are innately membrane-bound. To attain TF activity, MTTFs are released from the membrane anchor as a result of proteolytic cleavage. This enables MTTFs to travel to the nucleus and modulate gene expression. Arabidopsis MTTFs characterized to date belong to either the bZIP or the NAC family. In this review, we highlight the most recent findings on Arabidopsis MTTFs that ascribe different yet important roles to these proteins: the MTTFs in the bZIP family appear to regulate stress signaling pathways, whereas members of the NAC family are involved in both development and stress response.

Membrane-bound transcription factors in plants

The ability to activate dormant transcription factors is an important molecular feature of the transcriptional regulatory networks that govern diverse cellular functions. An intriguing example is the controlled proteolytic activation of membrane-bound transcription factors (MTFs). Most MTFs are activated either by intramembrane proteases or by the ubiquitin-proteasome pathway. Recent studies have shown that several members of the bZIP and NAC families in Arabidopsis are membrane-associated and are activated by membrane-associated proteases during stress responses in the endoplasmic reticulum and when the plants experience environmental stresses. A genome-scale analysis shows that over 10% of all transcription factors are membrane bound, indicating that activation of MTFs occurs at the genomic level, allowing transcription to be regulated rapidly under stressful conditions.

Systematic sequence analysis and identification of tissue-specific or stress-responsive genes of NAC transcription factor family in rice

NAM, ATAF, and CUC (NAC) transcription factors comprise a large plant-specific gene family and a few members of this family have been characterized for their roles in plant growth, development, and stress tolerance. In this study, systematic sequence analysis revealed 140 putative NAC or NAC-like genes (ONAC) in rice. Phylogenetic analysis suggested that NAC family can be divided into five groups (I-V). Among them, all the published development-related genes fell into group I, and all the published stress-related NAC genes fell into the group III (namely stress-responsive NAC genes, SNAC). Distinct compositions of the putative motifs were revealed on the basis of NAC protein sequences in rice. Most members contained a complete NAC DNA-binding domain and a variable transcriptional regulation domain. Sequence analysis, together with the organization of putative motifs, indicated distinct structures and potential diverse functions of NAC family in rice. Yeast one-hybrid analysis confirmed that 12 NAC proteins representing different motif compositions can bind the NAC core DNA-binding site. Real-time polymerase chain reaction (PCR) analysis revealed 12 genes with different tissue-specific (such as callus, root, stamen, or immature endosperm) expression patterns, suggesting that these genes may play crucial regulatory roles during growth and development of rice. The expression levels of this family were also checked under various abiotic stresses including drought, salinity, and low temperature. A preliminary check based on our microarray data suggested that more than 40 genes of this family were responsive to drought and/or salt stresses. Among them, 20 genes were further investigated for their stress responsiveness in detail by real-time PCR analysis. Most of these stress-responsive genes belonged to the group III (SNAC). Considering the fact that a very limited number of genes of the NAC family have been characterized, our data provide a very useful reference for functional analysis of this family in rice.

A membrane-bound NAC transcription factor NTL8 regulates gibberellic acid-mediated salt signaling in Arabidopsis seed germination

Gibberellic acid (GA) plays a key role in seed germination through coordinate interactions with other growth hormones and external signals. However, the way in which external signals are incorporated into the GA-signaling pathway is largely unknown. Here, we demonstrate that a membrane-bound NAC transcription factor NTL8 mediates the salt regulation of seed germination via the GA pathway, primarily independently of ABA. NTL8 is induced by high salinity. Its expression is also elevated by a GA biosynthetic inhibitor paclabutrazol (PAC), but is repressed by GA. Notably, high salinity greatly represses the GA3 oxidase 1 (GA3ox1) gene, supporting the hypothesis that salt signals inhibit seed germination by repressing GA biosynthesis. Induction of NTL8 and repression of GA3ox1 by high salinity still occur in the ABA-deficient aba3-1 mutant. Accordingly, the germination of a T-DNA insertional ntl8-1 mutant seed is resistant to high salinity and PAC. Interestingly, NTL8 is significantly induced during cold imbibition, but the induction declines quickly in germinating seeds, like RGL2. NTL8 activity is also regulated by controlled proteolytic release of the membrane-bound NTL8 form. Its release from the membranes is activated by PAC and high salinity. Our data support that NTL8 modulates GA-mediated salt signaling in regulating seed germination. This regulatory scheme may provide an adaptative fitness, which delays seed germination under high salinity conditions.

Membrane-mediated salt stress signaling in flowering time control

More than 10% of the plant-specific NAC (NAM, ATAF1/2, CUC2) transcription factors have been predicted to have alpha-helical transmembrane (TM) domain in their C-terminal regions, among which at least three members have been proven to be membrane-associated and play a role in cell cycle control and stress responses. These observations suggest that membrane-mediated regulation would be an important molecular mechanism mediating rapid transcriptional responses to internal and external stimuli in plants. Recently, we showed that a salt-responsive NTL (NTM1-Like's) transcription factor NTL8 is localized primarily in plasma membranes as dormant form and subsequently processed into transcriptionally active, nuclear form. Overexpression of an active NTL8 form exhibited delayed flowering as well as reduced growth with small curled leaves. Consistent with this, expression of FLOWERING LOCUS T (FT) and its downstream genes was significantly reduced in the transgenic plants. Furthermore, FT was notably repressed by high salt. These results indicate that NTL8 mediates salt-responsive flowering via FT in Arabidopsis and that membrane-mediated transcription regulation underlies the salt signaling in mediating flowering initiation.