Erratum: Acetate-mediated novel survival strategy against drought in plants.

This corrects the article DOI: 10.1038/nplants.2017.97.

Acetate-mediated novel survival strategy against drought in plants.

Water deficit caused by global climate changes seriously endangers the survival of organisms and crop productivity, and increases environmental deterioration1,2. Plants' resistance to drought involves global reprogramming of transcription, cellular metabolism, hormone signalling and chromatin modification3-8. However, how these regulatory responses are coordinated via the various pathways, and the underlying mechanisms, are largely unknown. Herein, we report an essential drought-responsive network in which plants trigger a dynamic metabolic flux conversion from glycolysis into acetate synthesis to stimulate the jasmonate (JA) signalling pathway to confer drought tolerance. In Arabidopsis, the ON/OFF switching of this whole network is directly dependent on histone deacetylase HDA6. In addition, exogenous acetic acid promotes de novo JA synthesis and enrichment of histone H4 acetylation, which influences the priming of the JA signalling pathway for plant drought tolerance. This novel acetate function is evolutionarily conserved as a survival strategy against environmental changes in plants. Furthermore, the external application of acetic acid successfully enhanced the drought tolerance in Arabidopsis, rapeseed, maize, rice and wheat plants. Our findings highlight a radically new survival strategy that exploits an epigenetic switch of metabolic flux conversion and hormone signalling by which plants adapt to drought.

Sm-Like Protein-Mediated RNA Metabolism Is Required for Heat Stress Tolerance in Arabidopsis.

Sm-like proteins play multiple functions in RNA metabolism, which is essential for biological processes such as stress responses in eukaryotes. The Arabidopsis thaliana sad1 mutant has a mutation of sm-like protein 5 (LSM5) and shows impaired drought and salt stress tolerances. The lsm5/sad1 mutant also showed hypersensitivity to heat stress. GFP-fused LSM5/SAD1 was localized in the nucleus under optimal growth conditions. After heat stress treatment, GFP-fused LSM5/SAD1 fluorescence was also observed as small cytoplasmic dots, in addition to nuclear localization. Whole genome transcriptome analysis revealed that many genes in Arabidopsis were drastically changed in response to heat stress. More heat-responsive genes were highly expressed in lsm5/sad1 mutant at both 2 and 6 h after heat stress treatment. Additionally, intron-retained and capped transcripts accumulated in the lsm5/sad1 mutant after heat stress treatment. In this study, we also identified non-Arabidopsis Genome Initiative transcripts that were expressed from unannotated regions. Most of these transcripts were antisense transcripts, and many capped non-AGI transcripts accumulated in the lsm5/sad1 mutant during heat stress treatment. These results indicated that LSM5/SAD1 functions to degrade aberrant transcripts through appropriate mRNA splicing and decapping, and precise RNA metabolic machinery is required for heat stress tolerance.

A Stress-Activated Transposon in Arabidopsis Induces Transgenerational Abscisic Acid Insensitivity.

Transposable elements (TEs), or transposons, play an important role in adaptation. TE insertion can affect host gene function and provides a mechanism for rapid increases in genetic diversity, particularly because many TEs respond to environmental stress. In the current study, we show that the transposition of a heat-activated retrotransposon, ONSEN, generated a mutation in an abscisic acid (ABA) responsive gene, resulting in an ABA-insensitive phenotype in Arabidopsis, suggesting stress tolerance. Our results provide direct evidence that a transposon activated by environmental stress could alter the genome in a potentially positive manner. Furthermore, the ABA-insensitive phenotype was inherited when the transcription was disrupted by an ONSEN insertion, whereas ABA sensitivity was recovered when the effects of ONSEN were masked by IBM2. These results suggest that epigenetic mechanisms in host plants typically buffered the effect of a new insertion, but could selectively "turn on" TEs when stressed.

Analysis of differential expression patterns of mRNA and protein during cold-acclimation and de-acclimation in Arabidopsis.

Overwintering plants are capable of exhibiting high levels of cold tolerance, which is acquired through the process of cold acclimation (CA). In contrast to CA, the acquired freezing tolerance is rapidly reduced during cold de-acclimation (DA) and plants resume growth after sensing warm temperatures. In order to better understand plant growth and development, and to aid in the breeding of cold-tolerant plants, it is important to decipher the functional mechanisms of the DA process. In this study, we performed comparative transcriptomic and proteomic analyses during CA and DA. As revealed by shotgun proteomics, we identified 3987 peptides originating from 1569 unique proteins and the corresponding mRNAs were analyzed. Among the 1569 genes, 658 genes were specifically induced at the transcriptional level during the process of cold acclimation. In order to investigate the relationship between mRNA and the corresponding protein expression pattern, a Pearson correlation was analyzed. Interestingly, 199 genes showed a positive correlation of mRNA and protein expression pattern, indicating that both their transcription and translation occurred during CA. However, 226 genes showed a negative correlation of mRNA and protein expression pattern, indicating that their mRNAs were transcribed during CA and were stored for the subsequent DA step. Under this scenario, those proteins were specifically increased during DA without additional transcription of mRNA. In order to confirm the negative correlation of mRNA and protein expression patterns, qRT-PCR and western blot analyses were performed. Mitochondrial malate dehydrogenase 1 (mMDH1) exhibited a negative correlation of mRNA and protein levels, which was characterized by CA-specific mRNA induction and protein accumulation specifically during DA. These data indicate that the expression of specific mRNAs and subsequent accumulation of corresponding proteins are not always in accordance under low temperature stress conditions in plants.

Transcriptome analysis using a high-density oligomicroarray under drought stress in various genotypes of cassava: an important tropical crop.

Cassava is an important crop that provides food security and income generation in many tropical countries and is known for its adaptability to various environmental conditions. Despite its global importance, the development of cassava microarray tools has not been well established. Here, we describe the development of a 60-mer oligonucleotide Agilent microarray representing ∼20,000 cassava genes and how it can be applied to expression profiling under drought stress using three cassava genotypes (MTAI16, MECU72 and MPER417-003). Our results identified about 1300 drought stress up-regulated genes in cassava and indicated that cassava has similar mechanisms for drought stress response and tolerance as other plant species. These results demonstrate that our microarray is a useful tool for analysing the cassava transcriptome and that it is applicable for various cassava genotypes.

Surveillance of 3' Noncoding Transcripts Requires FIERY1 and XRN3 in Arabidopsis.

Eukaryotes possess several RNA surveillance mechanisms that prevent undesirable aberrant RNAs from accumulating. Arabidopsis XRN2, XRN3, and XRN4 are three orthologs of the yeast 5'-to-3' exoribonuclease, Rat1/Xrn2, that function in multiple RNA decay pathways. XRN activity is maintained by FIERY1 (FRY1), which converts the XRN inhibitor, adenosine 3', 5'-bisphosphate (PAP), into 5'AMP. To identify the roles of XRNs and FRY1 in suppression of non-coding RNAs, strand-specific genome-wide tiling arrays and deep strand-specific RNA-Seq analyses were carried out in fry1 and xrn single and double mutants. In fry1-6, about 2000 new transcripts were identified that extended the 3' end of specific mRNAs; many of these were also observed in genotypes that possess the xrn3-3 mutation, a partial loss-of-function allele. Mutations in XRN2 and XRN4 in combination with xrn3-3 revealed only a minor effect on 3' extensions, indicating that these genes may be partially redundant with XRN3. We also observed the accumulation of 3' remnants of many DCL1-processed microRNA (miRNA) precursors in fry1-6 and xrn3-3. These findings suggest that XRN3, in combination with FRY1, is required to prevent the accumulation of 3' extensions that arise from thousands of mRNA and miRNA precursor transcripts.

Arabidopsis HDA6 regulates locus-directed heterochromatin silencing in cooperation with MET1.

Heterochromatin silencing is pivotal for genome stability in eukaryotes. In Arabidopsis, a plant-specific mechanism called RNA-directed DNA methylation (RdDM) is involved in heterochromatin silencing. Histone deacetylase HDA6 has been identified as a component of such machineries; however, its endogenous targets and the silencing mechanisms have not been analyzed globally. In this study, we investigated the silencing mechanism mediated by HDA6. Genome-wide transcript profiling revealed that the loci silenced by HDA6 carried sequences corresponding to the RDR2-dependent 24-nt siRNAs, however their transcript levels were mostly unaffected in the rdr2 mutant. Strikingly, we observed significant overlap of genes silenced by HDA6 to those by the CG DNA methyltransferase MET1. Furthermore, regardless of dependence on RdDM pathway, HDA6 deficiency resulted in loss of heterochromatic epigenetic marks and aberrant enrichment for euchromatic marks at HDA6 direct targets, along with ectopic expression of these loci. Acetylation levels increased significantly in the hda6 mutant at all of the lysine residues in the H3 and H4 N-tails, except H4K16. Interestingly, we observed two different CG methylation statuses in the hda6 mutant. CG methylation was sustained in the hda6 mutant at some HDA6 target loci that were surrounded by flanking DNA-methylated regions. In contrast, complete loss of CG methylation occurred in the hda6 mutant at the HDA6 target loci that were isolated from flanking DNA methylation. Regardless of CG methylation status, CHG and CHH methylation were lost and transcriptional derepression occurred in the hda6 mutant. Furthermore, we show that HDA6 binds only to its target loci, not the flanking methylated DNA, indicating the profound target specificity of HDA6. We propose that HDA6 regulates locus-directed heterochromatin silencing in cooperation with MET1, possibly recruiting MET1 to specific loci, thus forming the foundation of silent chromatin structure for subsequent non-CG methylation.

Arabidopsis HDA6 is required for freezing tolerance.

Many plants exhibit altered gene expression patterns in response to low nonfreezing temperatures and an increase in freezing tolerance in a phenomenon known as cold acclimation. Here we show, for the first time, that the histone deacetylase gene HDA6 is required for cold acclimation and freezing tolerance in Arabidopsis. HDA6 is transcriptionally upregulated during long-term cold treatment. Cold-treated hda6 mutants showed reduced freezing tolerance compared with the cold-treated wild-type plants. Freezing-caused electrolyte leakage increased in the cold-treated hda6 mutant. In contrast, the non-cold-treated hda6 mutants showed no significant difference in survivability and electrolyte leakage compared to wild-type plants. Transcriptome analysis identified the genes that showed aberrant expression in the hda6 mutant after cold acclimation. We conclude that HDA6 plays a critical role in regulating cold acclimation process that confers freezing resistance on Arabidopsis.

ARTADE2DB: improved statistical inferences for Arabidopsis gene functions and structure predictions by dynamic structure-based dynamic expression (DSDE) analyses.

Recent advances in technologies for observing high-resolution genomic activities, such as whole-genome tiling arrays and high-throughput sequencers, provide detailed information for understanding genome functions. However, the functions of 50% of known Arabidopsis thaliana genes remain unknown or are annotated only on the basis of static analyses such as protein motifs or similarities. In this paper, we describe dynamic structure-based dynamic expression (DSDE) analysis, which sequentially predicts both structural and functional features of transcripts. We show that DSDE analysis inferred gene functions 12% more precisely than static structure-based dynamic expression (SSDE) analysis or conventional co-expression analysis based on previously determined gene structures of A. thaliana. This result suggests that more precise structural information than the fixed conventional annotated structures is crucial for co-expression analysis in systems biology of transcriptional regulation and dynamics. Our DSDE method, ARabidopsis Tiling-Array-based Detection of Exons version 2 and over-representation analysis (ARTADE2-ORA), precisely predicts each gene structure by combining two statistical analyses: a probe-wise co-expression analysis of multiple transcriptome measurements and a Markov model analysis of genome sequences. ARTADE2-ORA successfully identified the true functions of about 90% of functionally annotated genes, inferred the functions of 98% of functionally unknown genes and predicted 1,489 new gene structures and functions. We developed a database ARTADE2DB that integrates not only the information predicted by ARTADE2-ORA but also annotations and other functional information, such as phenotypes and literature citations, and is expected to contribute to the study of the functional genomics of A. thaliana. URL: http://artade.org.

Arabidopsis tiling array analysis to identify the stress-responsive genes.

Plants respond and adapt to drought, cold, and high-salinity stresses. Stress-inducible gene products function in the stress response and tolerance in plants. Using cDNA microarrays and oligonucleotide microarrays, stress-inducible genes have been identified in various plant species so far. Recently, tiling array technology has become a powerful tool for the whole-genome transcriptome analysis. We applied the Arabidopsis Affymetrix tiling arrays to study the whole-genome transcriptome under drought, cold, and high-salinity stresses and identified a large number of drought, cold, and high-salinity stress-inducible genes and transcriptional units (TUs).

Genome-wide analysis of endogenous abscisic acid-mediated transcription in dry and imbibed seeds of Arabidopsis using tiling arrays.

The phytohormone abscisic acid (ABA) plays important roles in the induction and maintenance of seed dormancy. Although application of exogenous ABA inhibits germination, the effects of exogenous ABA on ABA-mediated gene transcription differ from those of endogenous ABA. To understand how endogenous ABA regulates the transcriptomes in seeds, we performed comprehensive expression analyses using whole-genome Affymetrix tiling arrays in two ABA metabolism mutants - an ABA-deficient mutant (aba2) and an ABA over-accumulation mutant (cyp707a1a2a3 triple mutant). Hierarchical clustering and principal components analyses showed that differences in endogenous ABA levels do not influence global expression of stored mRNA in dry seeds. However, the transcriptome after seed imbibition was related to endogenous ABA levels in both types of mutant. Endogenous ABA-regulated genes expressed in imbibed seeds included those encoding key ABA signaling factors and gibberellin-related components. In addition, cohorts of ABA-upregulated genes partially resembled those of dormant genes, whereas ABA-downregulated genes were partially overlapped with after-ripening-regulated genes. Bioinformatic analyses revealed that 6105 novel genes [non-Arabidopsis Genome Initiative (AGI) transcriptional units (TUs)] were expressed from unannotated regions. Interestingly, approximately 97% of non-AGI TUs possibly encoded hypothetical non-protein-coding RNAs, including a large number of antisense RNAs. In dry and imbibed seeds, global expression profiles of non-AGI TUs were similar to those of AGI genes. For both non-AGI TUs and AGI code genes, we identified those that were regulated differently in embryo and endosperm tissues. Our results suggest that transcription in Arabidopsis seeds is more complex and dynamic than previously thought.

Transduction of RNA-directed DNA methylation signals to repressive histone marks in Arabidopsis thaliana.

RNA-directed modification of histones is essential for the maintenance of heterochromatin in higher eukaryotes. In plants, cytosine methylation is an additional factor regulating inactive chromatin, but the mechanisms regulating the coexistence of cytosine methylation and repressive histone modification remain obscure. In this study, we analysed the mechanism of gene silencing mediated by MORPHEUS' MOLECULE1 (MOM1) of Arabidopsis thaliana. Transcript profiling revealed that the majority of up-regulated loci in mom1 carry sequences related to transposons and homologous to the 24-nt siRNAs accumulated in wild-type plants that are the hallmarks of RNA-directed DNA methylation (RdDM). Analysis of a single-copy gene, SUPPRESSOR OF drm1 drm2 cmt3 (SDC), revealed that mom1 activates SDC with concomitant reduction of di-methylated histone H3 lysine 9 (H3K9me2) at the tandem repeats in the promoter region without changes in siRNA accumulation and cytosine methylation. The reduction of H3K9me2 is not observed in regions flanking the tandem repeats. The results suggest that MOM1 transduces RdDM signals to repressive histone modification in the core region of RdDM.

Transcriptome analyses revealed diverse expression changes in ago1 and hyl1 Arabidopsis mutants.

MicroRNAs (miRNAs) are 20-24 nucleotide endogenous regulatory molecules conserved in higher eukaryotes. In Arabidopsis, miRNAs are produced through step-wise cleavages of primary miRNA precursors (pri-miRNAs) by DICER-LIKE1 (DCL1). This cleavage step is also supported by a double-stranded RNA-binding protein, HYPONASTIC LEAVES1 (HYL1). In many cases, mature miRNA is predominantly incorporated into an endonuclease, ARGONAUTE1 (AGO1), which degrades miRNA-targeted mRNAs. Here, we examined and revealed whole genome transcriptomes in ago1-25 and hyl1-2 mutants using tiling arrays. The data in this paper are valuable for understanding the relationship between the miRNA pathway and its effect on transcriptomes.

Genome-wide suppression of aberrant mRNA-like noncoding RNAs by NMD in Arabidopsis.

The nonsense-mediated mRNA decay (NMD) pathway is a well-known eukaryotic surveillance mechanism that eliminates aberrant mRNAs that contain a premature termination codon (PTC). The UP-Frameshift (UPF) proteins, UPF1, UPF2, and UPF3, are essential for normal NMD function. Several NMD substrates have been identified, but detailed information on NMD substrates is lacking. Here, we noticed that, in Arabidopsis, most of the mRNA-like nonprotein-coding RNAs (ncRNAs) have the features of an NMD substrate. We examined the expression profiles of 2 Arabidopsis mutants, upf1-1 and upf3-1, using a whole-genome tiling array. The results showed that expression of not only protein-coding transcripts but also many mRNA-like ncRNAs (mlncRNAs), including natural antisense transcript RNAs (nat-RNAs) transcribed from the opposite strands of the coding strands, were up-regulated in both mutants. The percentage of the up-regulated mlncRNAs to all expressed mlncRNAs was much higher than that of the up-regulated protein-coding transcripts to all expressed protein- coding transcripts. This finding demonstrates that one of the most important roles of NMD is the genome-wide suppression of the aberrant mlncRNAs including nat-RNAs.

Identification of the candidate genes regulated by RNA-directed DNA methylation in Arabidopsis.

RNA-directed DNA methylation (RdDM) is a process in which 24 nucleotide (nt) small interfering RNAs (siRNAs) guide de novo cytosine methylation in the homologous genomic DNA region. Of several factors involving 24 nt siRNA accumulation, RNA-dependent RNA polymerase 2 (RDR2) is a key component, because accumulation of 24 nt siRNA disappears in the Arabidopsis rdr2 mutant. Here, we compared expression profiles among wild-type, rdr2-1 and ddc (drm1drm2cmt3), DNA methyltransferase triple mutant, using a whole genome tiling array to identify the candidate genes directly downregulated by RdDM-related 24 nt siRNAs. Of the transcripts upregulated in the mutants, we searched for those whose coding regions or flanking regions have siRNA-generating loci. We found upregulated expression of 18 transcripts with AGI codes and 19 predicted transcriptional units (TUs) with siRNA loci in both rdr2-1 and ddc. Our study provided important information for understanding the relationship between RdDM and the identified candidate genes.

Alterations of lysine modifications on the histone H3 N-tail under drought stress conditions in Arabidopsis thaliana.

Post-translational modification of histone N-tails affects eukaryotic gene activity. In Arabidopsis, the histone modification level correlates with gene activation and repression in vernalization and flowering processes, but there is little information on changes in histone modification status and nucleosome structure under abiotic stresses. We determined the temporal and spatial changes in nucleosome occupancy and levels of H3K4me3, H3K9ac, H3K14ac, H3K23ac and H3K27ac in the histone H3 N-tail on the regions of four Arabidopsis drought stress-inducible genes, RD29A, RD29B, RD20 and At2g20880 [corrected], under drought stress conditions by chromatin immunoprecipitation analysis. We found two types of regulatory mechanisms of nucleosome occupancy function in the drought stress response. For RD29A and RD29B genes, nucleosome occupancy of promoter regions is low compared with that of coding regions, and no notable nucleosome loss occurs under drought stress. In contrast, nucleosome density is gradually decreased in response to drought stress on RD20 and At2g20880 [corrected] genes. Enrichments of H3K4me3 and H3K9ac correlate with gene activation in response to drought stress in all four genes. Interestingly, establishment of H3K4me3 occurs after accumulation of RNAPII on the coding regions of RD29A and At2g20880 [corrected]. Enrichment of H3K23ac and H3K27ac occurs in response to drought stress on the coding regions of RD29B, RD20 and At2g20880 [corrected], but not on the coding region of At2g20880 [corrected]. Our results indicate that histone modifications on the H3 N-tail are altered with gene activation on the coding regions of drought stress-responsive genes under drought stress conditions and that several patterns of nucleosome changes function in the drought stress response.

Arabidopsis transcriptome analysis under drought, cold, high-salinity and ABA treatment conditions using a tiling array.

Plants respond and adapt to drought, cold and high-salinity stresses in order to survive. In this study, we applied Arabidopsis Affymetrix tiling arrays to study the whole genome transcriptome under drought, cold, high-salinity and ABA treatment conditions. The bioinformatic analysis using the tiling array data showed that 7,719 non-AGI transcriptional units (TUs) exist in the unannotated "intergenic" regions of Arabidopsis genome. These include 1,275 and 181 TUs that are induced and downregulated, respectively, by the stress or ABA treatments. Most of the non-AGI TUs are hypothetical non-protein-coding RNAs. About 80% of the non-AGI TUs belong to pairs of the fully overlapping sense-antisense transcripts (fSATs). Significant linear correlation between the expression ratios (treated/untreated) of the sense TUs and the ratios of the antisense TUs was observed in the SATs of AGI code/non-AGI TU. We studied the biogenesis mechanisms of the stress- or ABA-inducible antisense RNAs and found that the expression of sense TUs is necessary for the stress- or ABA-inducible expression of the antisense TUs in the fSATs (AGI code/non-AGI TU).