Tissue-specific transcriptome analysis reveals cell wall metabolism, flavonol biosynthesis and defense responses are activated in the endosperm of germinating Arabidopsis thaliana seeds

Proteomic analysis during seed development provides insight into the early establishment of seed dormancy in Xanthium strumarium

This experiment was carried out to provide a comprehensive insight into the protein activities involved in dormancy establishment in seeds of common cocklebur (Xanthium strumarium), an annual plant with two dimorphic seeds contained in one casing known as a burr. These consist of a smaller dormant seed and a larger non-dormant seed. The proteome profile was compared between developing dormant and non-dormant seeds of Xanthium strumarium at five consecutive stages including three, 10, 20, 30, and 45 days after burr emergence (stages 1 to 5). We identified 6524 proteins in total, and approximately 3.6% of these were differentially abundant proteins (DAPs) between the two seed types. Both seed types showed fundamental changes in developmental programs during the examined stages. More than 38% of all DAPs were observed at the first stage, supporting the importance of the early developmental stage in seed fate determination. The detected DAPs at stage 1 were mainly associated with the cell division phase, which showed a delay in the dormant seeds. Over-representation of proteins responsible for cell wall biosynthesis, cytokinesis, and seed development were detected for non-dormant seeds at the first stage, while dormancy-associated proteins showed less abundance. Stage 3 was the critical stage for switching processes toward seed maturation and abscisic acid (ABA) signaling. Interestingly, higher abundance proteins in the mature non-dormant seed were mainly involved in the facilitation of seed germination. Taken together, the temporal pattern of the accumulated proteins in developing dormant seeds demonstrated a delay in the initiation of active cell division, enriched response to ABA, and defects in seed maturation. Moreover, stored proteins in the mature dormant seed delay germination but not dormancy induction. Finally, our results suggest that dormancy may be established at a stage of seed development earlier than previously thought.

Aethionema arabicum dimorphic seed trait resetting during transition to seedlings

The transition from germinating seeds to emerging seedlings is one of the most vulnerable plant life cycle stages. Heteromorphic diaspores (seed and fruit dispersal units) are an adaptive bet-hedging strategy to cope with spatiotemporally variable environments. While the roles and mechanisms of seedling traits have been studied in monomorphic species, which produce one type of diaspore, very little is known about seedlings in heteromorphic species. Using the dimorphic diaspore model Aethionema arabicum (Brassicaceae), we identified contrasting mechanisms in the germination responses to different temperatures of the mucilaginous seeds (M+ seed morphs), the dispersed indehiscent fruits (IND fruit morphs), and the bare non-mucilaginous M- seeds obtained from IND fruits by pericarp (fruit coat) removal. What follows the completion of germination is the pre-emergence seedling growth phase, which we investigated by comparative growth assays of early seedlings derived from the M+ seeds, bare M- seeds, and IND fruits. The dimorphic seedlings derived from M+ and M- seeds did not differ in their responses to ambient temperature and water potential. The phenotype of seedlings derived from IND fruits differed in that they had bent hypocotyls and their shoot and root growth was slower, but the biomechanical hypocotyl properties of 15-day-old seedlings did not differ between seedlings derived from germinated M+ seeds, M- seeds, or IND fruits. Comparison of the transcriptomes of the natural dimorphic diaspores, M+ seeds and IND fruits, identified 2,682 differentially expressed genes (DEGs) during late germination. During the subsequent 3 days of seedling pre-emergence growth, the number of DEGs was reduced 10-fold to 277 root DEGs and 16-fold to 164 shoot DEGs. Among the DEGs in early seedlings were hormonal regulators, in particular for auxin, ethylene, and gibberellins. Furthermore, DEGs were identified for water and ion transporters, nitrate transporter and assimilation enzymes, and cell wall remodeling protein genes encoding enzymes targeting xyloglucan and pectin. We conclude that the transcriptomes of seedlings derived from the dimorphic diaspores, M+ seeds and IND fruits, undergo transcriptional resetting during the post-germination pre-emergence growth transition phase from germinated diaspores to growing seedlings.

A Surprising Diversity of Xyloglucan Endotransglucosylase/Hydrolase in Wheat: New in Sight to the Roles in Drought Tolerance

Drought has become a major limiting factor for wheat productivity, and its negative impact on crop growth is anticipated to increase with climate deterioration in arid areas. Xyloglucan endoglycosylases/hydrolases (XTHs) are involved in constructing and remodeling cell wall structures and play an essential role in regulating cell wall extensibility and stress responses. However, there are no systematic studies on the wheat XTH gene family. In this study, 71 wheat XTH genes (TaXTHs) were characterized and classified into three subgroups through phylogenetic analysis. Genomic replication promoted the expansion of TaXTHs. We found a catalytically active motif and a potential N-linked glycosylation domain in all TaXTHs. Further expression analysis revealed that many TaXTHs in the roots and shoots were significantly associated with drought stress. The wheat TaXTH12.5a gene was transferred into Arabidopsis to verify a possible role of TaXTHs in stress response. The transgenic plants possessed higher seed germination rates and longer roots and exhibited improved tolerance to drought. In conclusion, bioinformatics and gene expression pattern analysis indicated that the TaXTH genes played a role in regulating drought response in wheat. The expression of TaXTH12.5a enhanced drought tolerance in Arabidopsis and supported the XTH genes' role in regulating drought stress response in plants.

Transcriptome characteristics during cell wall formation of endosperm cellularization and embryo differentiation in Arabidopsis

Embryonic and endosperm development are important biological events during Arabidopsis seed development, and are controlled by dynamic changes in a range of gene expression. Nevertheless, the regulatory mechanisms of endosperm cellularization and embryo differentiation remain unclear. Here, we characterized the early embryo and endosperm development of the naa15 mutant that had abnormal embryo differentiation and incomplete endosperm cellularization compared to WT of Arabidopsis, and comparatively investigated the changes of gene expressions in WT seeds at 3, 4, and 5 days after pollination (3W, 4W, and 5W) and the white homozygous aborted naa15 seeds at 5, 6, and 7 DAP (5M, 6M, and 7M) from naa15-1/+ siliques using RNA sequencing and qPCR assays. The transcriptome analyses showed that there were 2040 and 3630 differentially expressed genes (DEGs) in 4W (at endosperm cellularization initiation stage and heart embryo stage) vs 3W (at syncytium stage and globular embryo stage), and 5W (at end of endosperm cellularization stage and torpedo embryo stage) vs 4W, respectively. The KEGG and GO analyses showed that lipid metabolic processes and transmembrane transport related to cell wall biogenesis, cell division and differentiation, the plant hormone signaling pathway, photosynthesis, and transcription regulator activity were evidently enriched in WT and naa15. The heatmap and qPCR analyses showed that auxin response genes (ARFs), auxin transport genes (PINs) cytokinin synthesis genes (LOGs), cytokinin dehydrogenase genes (CKXs), cytokinin receptor, transcription factors (MYB, bHLH, MADS-box, and ERF) were significantly downregulated in naa15 compared to WT. A series of cell wall genes annotated to xyloglucan endotransglycosylase/hydrolase, pectin methyl esterase, and pectin methyl esterase inhibitor were also identified in these DEGs. Moreover, using an immunofluorescent assay, the features of cell walls displayed that cellulose fluorescence signals in the embryo and endosperm of naa15 were significantly decreased, and the signals of low- and high- methyl esterification of pectin were also obviously decreased in the endosperm of naa15. In summary, we identified a large number of DEGs and investigated the features of cell walls during endosperm cellularization and embryonic differentiation, which provided important information on transcription and gene expression to reveal their regulatory mechanisms.

NAC transcription factors ATAF1 and ANAC055 affect the heat stress response in Arabidopsis

Pre-exposing (priming) plants to mild, non-lethal elevated temperature improves their tolerance to a later higher-temperature stress (triggering stimulus), which is of great ecological importance. ‘Thermomemory’ is maintaining this tolerance for an extended period of time. NAM/ATAF1/2/CUC2 (NAC) proteins are plant-specific transcription factors (TFs) that modulate responses to abiotic stresses, including heat stress (HS). Here, we investigated the potential role of NACs for thermomemory. We determined the expression of 104 Arabidopsis NAC genes after priming and triggering heat stimuli, and found ATAF1 expression is strongly induced right after priming and declines below control levels thereafter during thermorecovery. Knockout mutants of ATAF1 show better thermomemory than wild type, revealing a negative regulatory role. Differential expression analyses of RNA-seq data from ATAF1 overexpressor, ataf1 mutant and wild-type plants after heat priming revealed five genes that might be priming-associated direct targets of ATAF1: AT2G31260 (ATG9), AT2G41640 (GT61), AT3G44990 (XTH31), AT4G27720 and AT3G23540. Based on co-expression analyses applied to the aforementioned RNA-seq profiles, we identified ANAC055 to be transcriptionally co-regulated with ATAF1. Like ataf1, anac055 mutants show improved thermomemory, revealing a potential co-control of both NAC TFs over thermomemory. Our data reveals a core importance of two NAC transcription factors, ATAF1 and ANAC055, for thermomemory.

Genome-wide identification of xyloglucan endotransglucosylase/hydrolase gene family members in peanut and their expression profiles during seed germination

Seed germination marks the beginning of a new plant life cycle. Improving the germination rate of seeds and the consistency of seedling emergence in the field could improve crop yields. Many genes are involved in the regulation of seed germination. Our previous study found that some peanut XTHs (xyloglucan endotransglucosylases/hydrolases) were expressed at higher levels at the newly germinated stage. However, studies of the XTH gene family in peanut have not been reported. In this study, a total of 58 AhXTH genes were identified in the peanut genome. Phylogenetic analysis showed that these AhXTHs, along with 33 AtXTHs from Arabidopsis and 61 GmXTHs from soybean, were classified into three subgroups: the I/II, IIIA and IIIB subclades. All AhXTH genes were unevenly distributed on the 18 peanut chromosomes, with the exception of chr. 07 and 17, and they had relatively conserved exon-intron patterns, most with three to four introns. Through chromosomal distribution pattern and synteny analysis, it was found that the AhXTH family experienced many replication events, including 42 pairs of segmental duplications and 23 pairs of tandem duplications, during genome evolution. Conserved motif analysis indicated that their encoded proteins contained the conserved ExDxE domain and N-linked glycosylation sites and displayed the conserved secondary structural loops 1-3 in members of the same group. Expression profile analysis of freshly harvested seeds, dried seeds, and newly germinated seeds using transcriptome data revealed that 26 AhXTH genes, which account for 45% of the gene family, had relatively higher expression levels at the seed germination stage, implying the important roles of AhXTHs in regulating seed germination. The results of quantitative real-time PCR also confirmed that some AhXTHs were upregulated during seed germination. The results of GUS histochemical staining showed that AhXTH4 was mainly expressed in germinated seeds and etiolated seedlings and had higher expression levels in elongated hypocotyls. AhXTH4 was also verified to play a crucial role in the cell elongation of hypocotyls during seed germination.

Reconsidering the function of the xyloglucan endotransglucosylase/hydrolase family

Plants possess an outer cell layer called the cell wall. This matrix comprises various molecules, such as polysaccharides and proteins, and serves a wide array of physiologically important functions. This structure is not static but rather flexible in response to the environment. One of the factors responsible for this plasticity is the xyloglucan endotransglucosylase/hydrolase (XTH) family, which cleaves and reconnects xyloglucan molecules. Since xyloglucan molecules have been hypothesised to tether cellulose microfibrils forming the main load-bearing network in the primary cell wall, XTHs have been thought to play a central role in cell wall loosening for plant cell expansion. However, multiple lines of recent evidence have questioned this classic model. Nevertheless, reverse genetic analyses have proven the biological importance of XTHs; therefore, a major challenge at present is to reconsider the role of XTHs in planta. Recent advances in analytical techniques have allowed for gathering rich information on the structure of the primary cell wall. Thus, the integration of accumulated knowledge in current XTH studies may offer a turning point for unveiling the precise functions of XTHs. In the present review, we redefine the biological function of the XTH family based on the recent architectural model of the cell wall. We highlight three key findings regarding this enzyme family: (1) XTHs are not strictly required for cell wall loosening during plant cell expansion but play vital roles in response to specific biotic or abiotic stresses; (2) in addition to their transglycosylase activity, the hydrolase activity of XTHs is involved in physiological benefits; and (3) XTHs can recognise a wide range of polysaccharides other than xyloglucans.

Seed-derived defensins from Scots pine: structural and functional features

The recombinant PsDef5.1 defensin inhibits the growth of phytopathogenic fungi, Gram-positive and Gram-negative bacteria, and human pathogen Candida albicans. Expression of seed-derived Scots pine defensins is tissue-specific and developmentally regulated. Plant defensins are ubiquitous antimicrobial peptides that possess a broad spectrum of activities and multi-functionality. The genes for these antimicrobial proteins form a multigenic family in the plant genome and are expressed in every organ. Most of the known defensins have been isolated from seeds of various monocot and dicot species, but seed-derived defensins have not yet been characterized in gymnosperms. This study presents the isolation of two new 249 bp cDNA sequences from Scots pine seeds with 97.9% nucleotide homology named PsDef5.1 and PsDef5.2. Their deduced amino acid sequences have typical plant defensin features, including an endoplasmic reticulum signal sequence of 31 amino acids (aa), followed by a characteristic defensin domain of 51 aa. To elucidate the functional activity of new defensins, we expressed the mature form of PsDef5.1 in a prokaryotic system. The purified recombinant peptide exhibited activity against the phytopathogenic fungi and Gram-negative and Gram-positive bacteria with the IC₅₀ of 5-18 µM. Moreover, it inhibited the growth of the human pathogen Candida albicans with the IC₅₀ of 6.0 µM. Expression analysis showed that transcripts of PsDef5.1-2 genes were present in immature and mature pine seeds and different parts of seedlings at the early stage of germination. In addition, unlike the PsDef5.2, the PsDef5.1 gene was expressed in the reproductive organs. Our findings indicate that novel defensins are promising candidates for transgenic application and the development of new antimicrobial drugs.

Combinations of maternal-specific repressive epigenetic marks in the endosperm control seed dormancy

Polycomb Repressive Complex 2 (PRC2)-mediated trimethylation of histone H3 on lysine 27 (H3K27me3) and methylation of histone 3 on lysine 9 (H3K9me) are two repressive epigenetic modifications that are typically localized in distinct regions of the genome. For reasons unknown, however, they co-occur in some organisms and special tissue types. In this study, we show that maternal alleles marked by H3K27me3 in the Arabidopsis endosperm were targeted by the H3K27me3 demethylase REF6 and became activated during germination. In contrast, maternal alleles marked by H3K27me3, H3K9me2, and CHG methylation (CHGm) are likely to be protected from REF6 targeting and remained silenced. Our study unveils that combinations of different repressive epigenetic modifications time a key adaptive trait by modulating access of REF6.

A Genomic Perspective on the Evolutionary Diversity of the Plant Cell Wall

The plant cell wall is a complex and dynamic structure composed of numerous different molecules that play multiple roles in all aspects of plant life. Currently, a new frontier in biotechnology is opening up, which is providing new insights into the structural and functional diversity of cell walls, and is thus serving to re-emphasize the significance of cell wall divergence in the evolutionary history of plant species. The ever-increasing availability of plant genome datasets will thus provide an invaluable basis for enhancing our knowledge regarding the diversity of cell walls among different plant species. In this review, as an example of a comparative genomics approach, I examine the diverse patterns of cell wall gene families among 100 species of green plants, and illustrate the evident benefits of using genome databases for studying cell wall divergence. Given that the growth and development of all types of plant cells are intimately associated with cell wall dynamics, gaining a further understanding of the functional diversity of cell walls in relation to diverse biological events will make significant contributions to a broad range of plant sciences.

EARLY BUD BREAK 1 triggers bud break in peach trees by regulating hormone metabolism, the cell cycle, and cell wall modifications

In a previous study we identified EARLY BUD BREAK 1 (EBB1), an ERF transcription factor, in peach (Prunus persica var. nectarina cultivar Zhongyou 4); however, little is known of how PpEBB1 may regulate bud break. To verify the function of PpEBB1 in bud break, PpEBB1 was transiently transformed into peach buds, resulting in early bud break. Bud break occurred earlier in PpEBB1-oe poplar (Populus trichocarpa) obtained by heterologous transformation than in wild type (WT), consistent with the peach bud results, indicating that PpEBB1 can promote bud break. To explore how PpEBB1 affects bud break, differentially expressed genes (DEGs) between WT and PpEBB1-oe poplar plants were identified by RNA-sequencing. The expression of DEGs associated with hormone metabolism, cell cycle, and cell wall modifications changed substantially according to qRT-PCR. Auxin, ABA, and total trans-zeatin-type cytokinin levels were higher in the PpEBB1-oe plants than in WT plants, while the total N6-(Δ 2-isopentenyl)-adenine-type cytokinins was lower. Yeast two-hybrid and bimolecular fluorescence complementation assays verified that a cell wall modification-related protein (PpEXBL1) interacted with PpEBB1 suggesting that PpEBB1 could interact with these cell wall modification proteins directly. Overall, our study proposed a multifaceted explanation for how PpEBB1 regulates bud break and showed that PpEBB1 promotes bud break by regulating hormone metabolism, the cell cycle, and cell wall modifications.

An Updated Overview on the Regulation of Seed Germination

The ability of a seed to germinate and establish a plant at the right time of year is of vital importance from an ecological and economical point of view. Due to the fragility of these early growth stages, their swiftness and robustness will impact later developmental stages and crop yield. These traits are modulated by a continuous interaction between the genetic makeup of the plant and the environment from seed production to germination stages. In this review, we have summarized the established knowledge on the control of seed germination from a molecular and a genetic perspective. This serves as a “backbone” to integrate the latest developments in the field. These include the link of germination to events occurring in the mother plant influenced by the environment, the impact of changes in the chromatin landscape, the discovery of new players and new insights related to well-known master regulators. Finally, results from recent studies on hormone transport, signaling, and biophysical and mechanical tissue properties are underscoring the relevance of tissue-specific regulation and the interplay of signals in this crucial developmental process.

Vacuolar processing enzymes in the plant life cycle

Vacuolar processing enzyme (VPE) is a cysteine-type endopeptidase that has a substrate-specificity for asparagine or aspartic acid residues and cleaves peptide bonds at their carboxyl-terminal side. Various vacuolar proteins are synthesized as larger proprotein precursors, and VPE is an important initiator of maturation and activation of these proteins. It mediates programmed cell death (PCD) by provoking vacuolar rupture and initiating the proteolytic cascade leading to PCD. Vacuolar processing enzyme also possesses a peptide ligation activity, which is responsible for producing cyclic peptides in several plant species. These unique functions of VPE support developmental and environmental responses in plants. The number of VPE homologues is higher in angiosperm species, indicating that there has been differentiation and specialization of VPE function over the course of evolution. Angiosperm VPEs are separated into two major types: the γ-type VPEs, which are expressed mainly in vegetative organs, and the β-type VPEs, whose expression occurs mainly in storage organs; in eudicots, the δ-type VPEs are further separated within γ-type VPEs. This review also considers the importance of processing and peptide ligation by VPE in vacuolar protein maturation.

Temporal tissue-specific regulation of transcriptomes during barley (Hordeum vulgare) seed germination

The distinct functions of individual cell types require cells to express specific sets of genes. The germinating seed is an excellent model to study genome regulation between cell types since the majority of the transcriptome is differentially expressed in a short period, beginning from a uniform, metabolically inactive state. In this study, we applied laser-capture microdissection RNA-sequencing to small numbers of cells from the plumule, radicle tip and scutellum of germinating barley seeds every 8 h, over a 48 h time course. Tissue-specific gene expression was notably common; 25% (910) of differentially expressed transcripts in plumule, 34% (1876) in radicle tip and 41% (2562) in scutellum were exclusive to that organ. We also determined that tissue-specific storage of transcripts occurs during seed development and maturation. Co-expression of genes had strong spatiotemporal structure, with most co-expression occurring within one organ and at a subset of specific time points during germination. Overlapping and distinct enrichment of functional categories were observed in the tissue-specific profiles. We identified candidate transcription factors amongst these that may be regulators of spatiotemporal gene expression programs. Our findings contribute to the broader goal of generating an integrative model that describes the structure and function of individual cells within seeds during germination.

Asparagus racemosus bZIP transcription factor-regulated squalene epoxidase (ArSQE) promotes germination and abiotic stress tolerance in transgenic tobacco

A. racemosus is a rich source of pharmacologically active steroidal saponins. Most of the studies are related to its chemistry and pharmacology, but the pathway involved in the biosynthesis of steroidal saponin is not much emphasized. Squalene epoxidase acts as a rate-limiting enzyme in this biosynthesis. In this study, we have selected root specific squalene epoxidase ArSQE from A. racemosus for its characterization. ArSQE was able to complement ergosterol auxotrophy in erg1 yeast mutants. Mutants were sensitive to the antifungal drug terbinafine, whereas ArSQE complementation made them tolerant to the same drug. ArSQE plays a significant role in early germination in transgenic tobacco. The transgenic tobacco seedlings overexpressing ArSQE were tolerant to terbinafine and abiotic stress. Expression analysis of transcripts in ArSQE transgenic lines suggests that it mostly affects ABA, GA, stress, and sterol related functions in transgenic tobacco. Further, root specific MeJA responsive A. racemosus bZIP transcription factors (TFs), ArTGA1 and ArTGA2, were identified that bind to MeJA responsive cis-element present in the promoter region of ArSQE. Characterization of ArSQE of A. racemosus provides new information about its regulation through MeJA responsive bZIP TF along with its role in the development and abiotic stress response in transgenic tobacco.

Overexpression of the trehalose-6-phosphate phosphatase family gene AtTPPF improves the drought tolerance of Arabidopsis thaliana

BACKGROUND

Trehalose-6-phosphate phosphatases (TPPs), which are encoded by members of the TPP gene family, can improve the drought tolerance of plants. However, the molecular mechanisms underlying the dynamic regulation of TPP genes during drought stress remain unclear. In this study, we explored the function of an Arabidopsis TPP gene by conducting comparative analyses of a loss-of-function mutant and overexpression lines.

RESULTS

The loss-of-function mutation of Arabidopsis thaliana TPPF, a member of the TPP gene family, resulted in a drought-sensitive phenotype, while a line overexpressing TPPF showed significantly increased drought tolerance and trehalose accumulation. Compared with wild-type plants, tppf1 mutants accumulated more H₂O₂ under drought, while AtTPPF-overexpressing plants accumulated less H₂O₂ under drought. Overexpression of AtTPPF led to increased contents of trehalose, sucrose, and total soluble sugars under drought conditions; these compounds may play a role in scavenging reactive oxygen species. Yeast one-hybrid and luciferase activity assays revealed that DREB1A could bind to the DRE/CRT element within the AtTPPF promoter and activate the expression of AtTPPF. A transcriptome analysis of the TPPF-overexpressing plants revealed that the expression levels of drought-repressed genes involved in electron transport activity and cell wall modification were upregulated, while those of stress-related transcription factors related to water deprivation were downregulated. These results indicate that, as well as its involvement in regulating trehalose and soluble sugars, AtTPPF is involved in regulating the transcription of stress-responsive genes.

CONCLUSION

AtTPPF functions in regulating levels of trehalose, reactive oxygen species, and sucrose levels during drought stress, and the expression of AtTPPF is activated by DREB1A in Arabidopsis. These findings shed light on the molecular mechanism by which AtTPPF regulates the response to drought stress.

Comparative Phosphoproteomic Analysis of Barley Embryos with Different Dormancy during Imbibition

Dormancy is the mechanism that allows seeds to become temporally quiescent in order to select the right time and place to germinate. Like in other species, in barley, grain dormancy is gradually reduced during after-ripening. Phosphosignaling networks in barley grains were investigated by a large-scale analysis of phosphoproteins to examine potential changes in response pathways to after-ripening. We used freshly harvested (FH) and after-ripened (AR) barley grains which showed different dormancy levels. The LC-MS/MS analysis identified 2346 phosphopeptides in barley embryos, with 269 and 97 of them being up- or downregulated during imbibition, respectively. A number of phosphopeptides were differentially regulated between FH and AR samples, suggesting that phosphoproteomic profiles were quite different between FH and AR grains. Motif analysis suggested multiple protein kinases including SnRK2 and MAPK could be involved in such a difference between FH and AR samples. Taken together, our results revealed phosphosignaling pathways in barley grains during the water imbibition process.

A Regulatory Module Controlling GA-Mediated Endosperm Cell Expansion Is Critical for Seed Germination in Arabidopsis

A key component of seed germination is the interplay of mechanical forces governing embryo growth and the surrounding restraining endosperm tissue. Endosperm cell separation is therefore thought to play a critical role in the control of this developmental transition. Here we demonstrate that in Arabidopsis thaliana seeds, endosperm cell expansion is a key component of germination. Endosperm cells expand to accommodate embryo growth prior to germination. We show that this is an actively regulated process supported by spatiotemporal control of the cell expansion gene EXPANSIN 2 (EXPA2). The NAC transcription factors NAC25 and NAC1L were identified as upstream regulators of EXPA2 expression, gibberellin-mediated endosperm expansion, and seed germination. The DELLA protein RGL2 repressed activation of the EXPA2 promoter by NAC25/NAC1L. Taken together, our findings uncover a key role of the GA/DELLA-NAC25/NAC1L-EXPA2 network in regulating endosperm cell expansion to control the seed-to-seedling transition.

Characterization of the XTH Gene Family: New Insight to the Roles in Soybean Flooding Tolerance

Xyloglucan endotransglycosylases/hydrolases (XTHs) are a class of enzymes involved in the construction and remodeling of cellulose/xyloglucan crosslinks and play an important role in regulating cell wall extensibility. However, little is known about this class of enzymes in soybean. Here, 61 soybean XTH genes (GmXTHs) were identified and classified into three subgroups through comparative phylogenetic analysis. Genome duplication greatly contributed to the expansion of GmXTH genes in soybean. A conserved amino acid motif responsible for the catalytic activity was identified in all GmXTHs. Further expression analysis revealed that most GmXTHs exhibited a distinct organ-specific expression pattern, and the expression level of many GmXTH genes was significantly associated with ethylene and flooding stress. To illustrate a possible role of XTH genes in regulating stress responses, the ArabidopsisAtXTH31 gene was overexpressed in soybean. The generated transgenic plants exhibited improved tolerance to flooding stress, with a higher germination rate and longer roots/hypocotyls during the seedling stage and vegetative growth stages. In summary, our combined bioinformatics and gene expression pattern analyses suggest that GmXTH genes play a role in regulating soybean stress responses. The enhanced soybean flooding tolerance resulting from the expression of an Arabidopsis XTH also supports the role of XTH genes in regulating plant flooding stress responses.

CK2 and PI3K are direct molecular targets of quercetin in chronic lymphocytic leukaemia

Despite the encouraging results of the innovative therapeutic treatments, complete remission is uncommon in patients affected by chronic lymphocytic leukaemia, which remains an essentially incurable disease. Recently, clinical trials based on BH3-mimetic drugs showed positive outcomes in subjects with poor prognostic features. However, resistance to treatments occurs in a significant number of patients. We previously reported that the multi-kinase inhibitor quercetin, a natural flavonol, restores sensitivity to ABT-737, a BH3-mimetic compound, in both leukemic cell lines and B-cells isolated from patients. To identify the molecular target of quercetin, we employed a new cell line, HG3, obtained by immortalization of B-cells from a chronic lymphocytic leukaemia patient at the later stage of disease. We confirmed that quercetin in association with ABT-737 synergistically enhances apoptosis in HG3 (combination index < 1 for all fractions affected). We also reported that the cellular uptake of quercetin is extremely rapid, with an intracellular concentration of about 38.5 ng/10⁶ cells, after treatment with 25 μM for 5 min. We demonstrated that the activity of protein kinase CK2, which positively triggers PI₃K/Akt pathway by inactivating PTEN phosphatase, is inhibited by quercetin immediately after its addition to HG3 cells (0–2 min). PI₃K activity was also inhibited by quercetin within 60 min from the treatment. The combined inhibition of CK2 and PI₃K kinase activities by quercetin restored ABT-737 sensitivity and increased lethality in human leukemia cells.

Transcriptome assembly in Suaeda aralocaspica to reveal the distinct temporal gene/miRNA alterations between the dimorphic seeds during germination

BACKGROUND

Dimorphic seeds from Suaeda aralocaspica exhibit different germination behaviors that are thought to be a bet-hedging strategy advantageous in harsh and unpredictable environments. To understand the molecular mechanisms of Suaeda aralocaspica dimorphic seed germination, we applied RNA sequencing and small RNA sequencing for samples collected at three germination stages.

RESULTS

A total of 79,414 transcripts were assembled using Trinity, of which 57.67% were functionally annotated. KEGG enrichment unveiled that photosynthesis and flavonol biosynthesis pathways were activated earlier in brown seed compared with black seed. Gene expression analysis revealed that nine candidate unigenes in gibberellic acid and abscisic acid signal transduction and 23 unigenes in circadian rhythm-plant pathway showed distinct expression profiles to promote dimorphic seed germination. 194 conserved miRNAs comprising 40 families and 21 novel miRNAs belonging to 20 families in Suaeda aralocaspica were identified using miRDeep-P and Mfold. The expression of miRNAs in black seed was suppressed at imbibition stage. Among the identified miRNAs, 59 conserved and 13 novel miRNAs differentially expressed during seed germination. Of which, 43 conserved and nine novel miRNAs showed distinct expression patterns between black and brown seed. Using TAPIR, 208 unigenes were predicted as putative targets of 35 conserved miRNA families and 17 novel miRNA families. Among functionally annotated targets, genes participated in transcription regulation constituted the dominant category, followed by genes involved in signaling and stress response. Seven of the predicted targets were validated using 5' rapid amplification of cDNA ends or real-time quantitative reverse transcription-PCR.

CONCLUSIONS

Our results indicate that specific genes and miRNAs are regulated differently between black and brown seed during germination, which may contribute to the different germination behaviors of Suaeda aralocaspica dimorphic seeds in unpredictable variable environments. Our results lay a solid foundation for further studying the roles of candidate genes and miRNAs in Suaeda aralocaspica dimorphic seed germination.

α-Xylosidase plays essential roles in xyloglucan remodelling, maintenance of cell wall integrity, and seed germination in Arabidopsis thaliana

Regulation and maintenance of cell wall physical properties are crucial for plant growth and environmental response. In the germination process, hypocotyl cell expansion and endosperm weakening are prerequisites for dicot seeds to complete germination. We have identified the Arabidopsis mutant thermoinhibition-resistant germination 1 (trg1), which has reduced seed dormancy and insensitivity to unfavourable conditions for germination owing to a loss-of-function mutation of TRG1/XYL1, which encodes an α-xylosidase. Compared to those of wild type, the elongating stem of trg1 showed significantly lower viscoelasticity, and the fruit epidermal cells were longitudinally shorter and horizontally enlarged. Actively growing tissues of trg1 over-accumulated free xyloglucan oligosaccharides (XGOs), and the seed cell wall had xyloglucan with a greatly reduced molecular weight. These observations suggest that XGOs reduce xyloglucan size by serving as an acceptor in transglycosylation and eventually enhancing cell wall loosening. TRG1/XYL1 gene expression was abundant in growing wild-type organs and tissues but relatively low in cells at most actively elongating part of the tissues, suggesting that α-xylosidase contributes to maintaining the mechanical integrity of the primary cell wall in the growing and pre-growing tissues. In germinating seeds of trg1, expression of genes encoding specific abscisic acid and gibberellin metabolism enzymes was altered in accordance with the aberrant germination phenotype. Thus, cell wall integrity could affect seed germination not only directly through the physical properties of the cell wall but also indirectly through the regulation of hormone gene expression.

Dormant and after-Ripened Arabidopsis thaliana Seeds are Distinguished by Early Transcriptional Differences in the Imbibed State

Seed dormancy is a genetically controlled block preventing the germination of imbibed seeds in favorable conditions. It requires a period of dry storage (after-ripening) or certain environmental conditions to be overcome. Dormancy is an important seed trait, which is under selective pressure, to control the seasonal timing of seed germination. Dormant and non-dormant (after-ripened) seeds are characterized by large sets of differentially expressed genes. However, little information is available concerning the temporal and spatial transcriptional changes during early stages of rehydration in dormant and non-dormant seeds. We employed genome-wide transcriptome analysis on seeds of the model plant Arabidopsis thaliana to investigate transcriptional changes in dry seeds upon rehydration. We analyzed gene expression of dormant and after-ripened seeds of the Cvi accession over four time points and two seed compartments (the embryo and surrounding single cell layer endosperm), during the first 24 h after sowing. This work provides a global view of gene expression changes in dormant and non-dormant seeds with temporal and spatial detail, and these may be visualized via a web accessible tool (http://www.wageningenseedlab.nl/resources). A large proportion of transcripts change similarly in both dormant and non-dormant seeds upon rehydration, however, the first differences in transcript abundances become visible shortly after the initiation of imbibition, indicating that changes induced by after-ripening are detected and responded to rapidly upon rehydration. We identified several gene expression profiles which contribute to differential gene expression between dormant and non-dormant samples. Genes with enhanced expression in the endosperm of dormant seeds were overrepresented for stress-related Gene Ontology categories, suggesting a protective role for the endosperm against biotic and abiotic stress to support persistence of the dormant seed in its environment.

Synergism between ozone and light stress: structural responses of polyphenols in a woody Brazilian species

Microscopic studies on isolated ozone (O3) effects or on those in synergy with light stress commonly report the induction of polyphenols that exhibit different aspects within the vacuole of photosynthesizing cells. It has been assumed that these different aspects are randomly spread in the symptomatic (injured) regions of the leaf blade. Interestingly, secretory ducts that constitutively produce polyphenols also exhibit these same variations in their vacuolar aspect, in a spatial sequence related to the destiny of these cells (e.g., programmed cell death (PCD) in lytic secretion processes). Here, we demonstrate that the deposition pattern of polyphenols prior to the establishment of the hypersensitive-like response, a type of PCD caused by O3, follows the same one observed in the epithelial cells of the constitutive lysigenous secretory ducts. Astronium graveolens, an early secondary Brazilian woody species, was selected based on its susceptibility to high light and presence of secretory ducts. The synergism effects were assessed by exposing plants to the high O3 concentrations at an urban site in São Paulo City. Confocal, widefield and light microscopies were used to examine polyphenols' occurrence and aspects. The spatial pattern of polyphenols distribution along the leaflets of plants submitted to the synergism condition, in which a dense vacuolar aspect is the target of a cell destined to death, was also observed in the constitutive secretory cells prior to lysis. This similar structural pattern may be a case of homology of process involving both the constitutive (secretory ducts) and the induced (photosynthesizing cells) defenses.

Mannans and endo-β-mannanases (MAN) in Brachypodium distachyon: expression profiling and possible role of the BdMAN genes during coleorhiza-limited seed germination

Immunolocalization of mannans in the seeds of Brachypodium distachyon reveals the presence of these polysaccharides in the root embryo and in the coleorhiza in the early stages of germination (12h), decreasing thereafter to the point of being hardly detected at 27h. Concurrently, the activity of endo-β-mannanases (MANs; EC 3.2.1.78) that catalyse the hydrolysis of β-1,4 bonds in mannan polymers, increases as germination progresses. The MAN gene family is represented by six members in the Brachypodium genome, and their expression has been explored in different organs and especially in germinating seeds. Transcripts of BdMAN2, BdMAN4 and BdMAN6 accumulate in embryos, with a maximum at 24-30h, and are detected in the coleorhiza and in the root by in situ hybridization analyses, before root protrusion (germination sensu stricto). BdMAN4 is not only present in the embryo root and coleorhiza, but is abundant in the de-embryonated (endosperm) imbibed seeds, while BdMAN2 and BdMAN6 are faintly expressed in endosperm during post-germination (36-42h). BdMAN4 and BdMAN6 transcripts are detected in the aleurone layer. These data indicate that BdMAN2, BdMAN4 and BdMAN6 are important for germination sensu stricto and that BdMAN4 and BdMAN6 may also influence reserve mobilization. Whether the coleorhiza in monocots and the micropylar endosperm in eudicots have similar functions, is discussed.

Germination Potential of Dormant and Nondormant Arabidopsis Seeds Is Driven by Distinct Recruitment of Messenger RNAs to Polysomes

Dormancy is a complex evolutionary trait that temporally prevents seed germination, thus allowing seedling growth at a favorable season. High-throughput analyses of transcriptomes have led to significant progress in understanding the molecular regulation of this process, but the role of posttranscriptional mechanisms has received little attention. In this work, we have studied the dynamics of messenger RNA association with polysomes and compared the transcriptome with the translatome in dormant and nondormant seeds of Arabidopsis (Arabidopsis thaliana) during their imbibition at 25 °C in darkness, a temperature preventing germination of dormant seeds only. DNA microarray analysis revealed that 4,670 and 7,028 transcripts were differentially abundant in dormant and nondormant seeds in the transcriptome and the translatome, respectively. We show that there is no correlation between transcriptome and translatome and that germination regulation is also largely translational, implying a selective and dynamic recruitment of messenger RNAs to polysomes in both dormant and nondormant seeds. The study of 5' untranslated region features revealed that GC content and the number of upstream open reading frames could play a role in selective translation occurring during germination. Gene Ontology clustering showed that the functions of polysome-associated transcripts differed between dormant and nondormant seeds and revealed actors in seed dormancy and germination. In conclusion, our results demonstrate the essential role of selective polysome loading in this biological process.

Transcriptomic changes during tuber dormancy release process revealed by RNA sequencing in potato

Potato tuber dormancy release is a critical development process that allows potato to produce new plant. The first Illumina RNA sequencing to generate the expressed mRNAs at dormancy tuber (DT), dormancy release tuber (DRT) and sprouting tuber (ST) was performed. We identified 26,639 genes including 5,912 (3,450 up-regulated while 2,462 down-regulated) and 3,885 (2,141 up-regulated while 1,744 down-regulated) genes were differentially expressed from DT vs DRT and DRT vs ST. The RNA-Seq results were further verified using qRT-PCR. We found reserve mobilization events were activated before the bud emergence (DT vs DRT) and highlighted after dormancy release (DRT vs ST). Overexpressed genes related to metabolism of auxin, gibberellic acid, cytokinin and barssinosteriod were dominated in DT vs DRT, whereas overexpressed genes involved in metabolism of ethylene, jasmonate and salicylate were prominent in DRT vs ST. Various histone and cyclin isoforms associated genes involved in cell division/cycle were mainly up-regulated in DT vs DRT. Dormancy release process was also companied by stress response and redox regulation, those genes related to biotic stress, cell wall and second metabolism was preferentially overexpressed in DRT vs ST, which might accelerate dormancy breaking and sprout outgrowth. The metabolic processes activated during tuber dormancy release were also supported by plant seed models. These results represented the first comprehensive picture of a large number of genes involved in tuber dormancy release process.

A systems biology approach toward understanding seed composition in soybean

BACKGROUND

The molecular, biochemical, and genetic mechanisms that regulate the complex metabolic network of soybean seed development determine the ultimate balance of protein, lipid, and carbohydrate stored in the mature seed. Many of the genes and metabolites that participate in seed metabolism are unknown or poorly defined; even more remains to be understood about the regulation of their metabolic networks. A global omics analysis can provide insights into the regulation of seed metabolism, even without a priori assumptions about the structure of these networks.

RESULTS

With the future goal of predictive biology in mind, we have combined metabolomics, transcriptomics, and metabolic flux technologies to reveal the global developmental and metabolic networks that determine the structure and composition of the mature soybean seed. We have coupled this global approach with interactive bioinformatics and statistical analyses to gain insights into the biochemical programs that determine soybean seed composition. For this purpose, we used Plant/Eukaryotic and Microbial Metabolomics Systems Resource (PMR, http://www.metnetdb.org/pmr, a platform that incorporates metabolomics data to develop hypotheses concerning the organization and regulation of metabolic networks, and MetNet systems biology tools http://www.metnetdb.org for plant omics data, a framework to enable interactive visualization of metabolic and regulatory networks.

CONCLUSIONS

This combination of high-throughput experimental data and bioinformatics analyses has revealed sets of specific genes, genetic perturbations and mechanisms, and metabolic changes that are associated with the developmental variation in soybean seed composition. Researchers can explore these metabolomics and transcriptomics data interactively at PMR.

Promotion of testa rupture during garden cress germination involves seed compartment-specific expression and activity of pectin methylesterases

Pectin methylesterase (PME) controls the methylesterification status of pectins and thereby determines the biophysical properties of plant cell walls, which are important for tissue growth and weakening processes. We demonstrate here that tissue-specific and spatiotemporal alterations in cell wall pectin methylesterification occur during the germination of garden cress (Lepidium sativum). These cell wall changes are associated with characteristic expression patterns of PME genes and resultant enzyme activities in the key seed compartments CAP (micropylar endosperm) and RAD (radicle plus lower hypocotyl). Transcriptome and quantitative real-time reverse transcription-polymerase chain reaction analysis as well as PME enzyme activity measurements of separated seed compartments, including CAP and RAD, revealed distinct phases during germination. These were associated with hormonal and compartment-specific regulation of PME group 1, PME group 2, and PME inhibitor transcript expression and total PME activity. The regulatory patterns indicated a role for PME activity in testa rupture (TR). Consistent with a role for cell wall pectin methylesterification in TR, treatment of seeds with PME resulted in enhanced testa permeability and promoted TR. Mathematical modeling of transcript expression changes in germinating garden cress and Arabidopsis (Arabidopsis thaliana) seeds suggested that group 2 PMEs make a major contribution to the overall PME activity rather than acting as PME inhibitors. It is concluded that regulated changes in the degree of pectin methylesterification through CAP- and RAD-specific PME and PME inhibitor expression play a crucial role during Brassicaceae seed germination.

Translatome profiling in dormant and nondormant sunflower (Helianthus annuus) seeds highlights post-transcriptional regulation of germination

Seed dormancy, which blocks germination in apparently favourable conditions, is a key regulatory control point of plant population establishment. As germination requires de novo translation, its regulation by dormancy is likely to be related to the association of individual transcripts to polysomes. Here, the polysome-associated mRNAs, that is, the translatome, were fractionated and characterized with microarrays in dormant and nondormant sunflower (Helianthus annuus) embryos during their imbibition at 10°C, a temperature preventing germination of dormant embryos. Profiling of mRNAs in polysomal complexes revealed that the translatome differs between germinating and nongerminating embryos. Association of transcripts with polysomes reached a maximum after 15 h of imbibition; at this time-point 194 polysome-associated transcripts were specifically found in nondormant embryos and 47 in dormant embryos only. The proteins corresponding to the polysomal mRNAs in nondormant embryos appeared to be very pertinent for germination and were involved mainly in transport, regulation of transcription or cell wall modifications. This work demonstrates that seed germination results from a timely regulated and selective recruitment of mRNAs to polysomes, thus opening novel fields of investigation for the understanding of this developmental process.

Mining tissue-specific contigs from peanut (Arachis hypogaea L.) for promoter cloning by deep transcriptome sequencing

Peanut (Arachis hypogaea L.), one of the most important oil legumes in the world, is heavily damaged by white grubs. Tissue-specific promoters are needed to incorporate insect resistance genes into peanut by genetic transformation to control the subterranean pests. Transcriptome sequencing is the most effective way to analyze differential gene expression in this non-model species and contribute to promoter cloning. The transcriptomes of the roots, seeds and leaves of peanut were sequenced using Illumina technology. A simple digital expression profile was established based on number of transcripts per million clean tags (TPM) from different tissues. Subsequently, 584 root-specific candidate transcript assembly contigs (TACs) and 316 seed-specific candidate TACs were identified. Among these candidate TACs, 55.3% were root-specific and 64.6% were seed-specific by semi-quantitative RT-PCR analysis. Moreover, the consistency of semi-quantitative RT-PCR with the simple digital expression profile was correlated with the length and TPM value of TACs. The results of gene ontology showed that some root-specific TACs are involved in stress resistance and respond to auxin stimulus, whereas, seed-specific candidate TACs are involved in embryo development, lipid storage and long-chain fatty acid biosynthesis. One root-specific promoter was cloned and characterized. We developed a high-yield screening system in peanut by establishing a simple digital expression profile based on Illumina sequencing. The feasible and rapid method presented by this study can be used for other non-model crops to explore tissue-specific or spatially specific promoters.

The functions of the endosperm during seed germination

In angiosperms, a double fertilization event initiates the development of two distinct structures, the embryo and endosperm. The endosperm plays an important role in supporting embryonic growth by supplying nutrients, protecting the embryo and controlling embryo growth by acting as a mechanical barrier during seed development and germination. Its structure and function in the mature dry seed is divergent and specialized among different plant species. A subset of endospermic tissues are composed of living cells even after seed maturation, and play an active role in the regulation of seed germination. Transcriptome analysis has provided new insights into the regulatory functions of the endosperm during seed germination. It is well known that the embryo secretes signals to the endosperm to induce the degradation of the seed reserve and to promote endosperm weakening during germination. Recent advances in seed biology have shown that the endosperm is capable of sensing environmental signals, and can produce and secrete signals to regulate the growth of the embryo. Thus, germination is a systemic response that involves bidirectional interactions between the embryo and endosperm.

De Novo assembly of expressed transcripts and global transcriptomic analysis from seedlings of the paper mulberry (Broussonetia kazinoki x Broussonetia papyifera)

The paper mulberry is one of the multifunctional tree species in agroforestry systems and is also commonly utilized in traditional medicine in China and other Asian countries. However, little is known about its molecular genetics, which hinders research on and exploitation of this valuable resource. To discern the correlation between gene expression and the essential properties of the paper mulberry, we performed a transcriptomics analysis, assembling a total of 37,725 unigenes from 54,638,676 reads generated by RNA-seq. Among these, 22,692 unigenes showed greater than 60% similarity with genes from other species. The lengths of 13,566 annotated unigenes were longer than 1,000 bp. Functional clustering analysis with COG (Cluster of Orthologous Groups) revealed that 17,184 unigenes are primarily involved in transcription, translation, signal transduction, carbohydrate metabolism, secondary metabolism, and energy metabolism. GO (Gene Ontology) annotation suggests enrichment of genes encoding antioxidant activity, transporter activity, biosynthesis, metabolism and stress response, with a total of 30,659 unigenes falling in these categories. KEGG (Kyoto Encyclopedia of Genes and Genomes) metabolic pathway analysis showed that 7,199 unigenes are associated with 119 metabolic pathways. In addition to the basic metabolism, these genes are enriched for plant pathogen interaction, flavonoid metabolism and other secondary metabolic processes. Furthermore, differences in the transcriptomes of leaf, stem and root tissues were analyzed and 7,233 specifically expressed unigenes were identified. This global expression analysis provided novel insights about the molecular mechanisms of the biosynthesis of flavonoid, lignin and cellulose, as well as on the response to biotic and abiotic stresses including the remediation of contaminated soil by the paper mulberry.

Unlocking Triticeae genomics to sustainably feed the future

The tribe Triticeae includes the major crops wheat and barley. Within the last few years, the whole genomes of four Triticeae species-barley, wheat, Tausch's goatgrass (Aegilops tauschii) and wild einkorn wheat (Triticum urartu)-have been sequenced. The availability of these genomic resources for Triticeae plants and innovative analytical applications using next-generation sequencing technologies are helping to revitalize our approaches in genetic work and to accelerate improvement of the Triticeae crops. Comparative genomics and integration of genomic resources from Triticeae plants and the model grass Brachypodium distachyon are aiding the discovery of new genes and functional analyses of genes in Triticeae crops. Innovative approaches and tools such as analysis of next-generation populations, evolutionary genomics and systems approaches with mathematical modeling are new strategies that will help us discover alleles for adaptive traits to future agronomic environments. In this review, we provide an update on genomic tools for use with Triticeae plants and Brachypodium and describe emerging approaches toward crop improvements in Triticeae.

Transcriptomics approaches in the early Arabidopsis embryo

Early plant embryogenesis condenses the fundamental processes underlying plant development into a short sequence of predictable steps. The main tissues, as well as stem cells for their post-embryonic maintenance, are specified through genetic control networks. A key question is how cell fates are instructed by unique cellular transcriptomes, and important insights have recently been gained through cell type-specific transcriptomics during post-embryonic development. However, the poor accessibility and small size of Arabidopsis (Arabidopsis thaliana) embryos have obstructed similar progress during embryogenesis. Here, we review the current situation in plant embryo transcriptomics, and discuss how the recent development of novel cell-specific analysis technologies will enable the identification of cellular transcriptomes in the early Arabidopsis embryo.

Transcriptional dynamics of two seed compartments with opposing roles in Arabidopsis seed germination

Seed germination is a critical stage in the plant life cycle and the first step toward successful plant establishment. Therefore, understanding germination is of important ecological and agronomical relevance. Previous research revealed that different seed compartments (testa, endosperm, and embryo) control germination, but little is known about the underlying spatial and temporal transcriptome changes that lead to seed germination. We analyzed genome-wide expression in germinating Arabidopsis (Arabidopsis thaliana) seeds with both temporal and spatial detail and provide Web-accessible visualizations of the data reported (vseed.nottingham.ac.uk). We show the potential of this high-resolution data set for the construction of meaningful coexpression networks, which provide insight into the genetic control of germination. The data set reveals two transcriptional phases during germination that are separated by testa rupture. The first phase is marked by large transcriptome changes as the seed switches from a dry, quiescent state to a hydrated and active state. At the end of this first transcriptional phase, the number of differentially expressed genes between consecutive time points drops. This increases again at testa rupture, the start of the second transcriptional phase. Transcriptome data indicate a role for mechano-induced signaling at this stage and subsequently highlight the fates of the endosperm and radicle: senescence and growth, respectively. Finally, using a phylotranscriptomic approach, we show that expression levels of evolutionarily young genes drop during the first transcriptional phase and increase during the second phase. Evolutionarily old genes show an opposite pattern, suggesting a more conserved transcriptome prior to the completion of germination.

Mining genes involved in the stratification of Paris polyphylla seeds using high-throughput embryo transcriptome sequencing

Background

Paris polyphylla var. yunnanensis is an important medicinal plant. Seed dormancy is one of the main factors restricting artificial cultivation. The molecular mechanisms of seed dormancy remain unclear, and little genomic or transcriptome data are available for this plant.

Results

In this study, massive parallel pyrosequencing on the Roche 454-GS FLX Titanium platform was used to generate a substantial sequence dataset for the P. polyphylla embryo. 369,496 high quality reads were obtained, ranging from 50 to 1146 bp, with a mean of 219 bp. These reads were assembled into 47,768 unigenes, which included 16,069 contigs and 31,699 singletons. Using BLASTX searches of public databases, 15,757 (32.3%) unique transcripts were identified. Gene Ontology and Cluster of Orthologous Groups of proteins annotations revealed that these transcripts were broadly representative of the P. polyphylla embryo transcriptome. The Kyoto Encyclopedia of Genes and Genomes assigned 5961 of the unique sequences to specific metabolic pathways. Relative expression levels analysis showed that eleven phytohormone-related genes and five other genes have different expression patterns in the embryo and endosperm in the seed stratification process.

Conclusions

Gene annotation and quantitative RT-PCR expression analysis identified 464 transcripts that may be involved in phytohormone catabolism and biosynthesis, hormone signal, seed dormancy, seed maturation, cell wall growth and circadian rhythms. In particular, the relative expression analysis of sixteen genes (CYP707A, NCED, GA20ox2, GA20ox3, ABI2, PP2C, ARP3, ARP7, IAAH, IAAS, BRRK, DRM, ELF1, ELF2, SFR6, and SUS) in embryo and endosperm and at two temperatures indicated that these related genes may be candidates for clarifying the molecular basis of seed dormancy in P. polyphlla var. yunnanensis.

Visual analysis of transcriptome data in the context of anatomical structures and biological networks

The complexity and temporal as well as spatial resolution of transcriptome datasets is constantly increasing due to extensive technological developments. Here we present methods for advanced visualization and intuitive exploration of transcriptomics data as necessary prerequisites in order to facilitate the gain of biological knowledge. Color-coding of structural images based on the expression level enables a fast visual data analysis in the background of the examined biological system. The network-based exploration of these visualizations allows for comparative analysis of genes with specific transcript patterns and supports the extraction of functional relationships even from large datasets. In order to illustrate the presented methods, the tool HIVE was applied for visualization and exploration of database-retrieved expression data for master regulators of Arabidopsis thaliana flower and seed development in the context of corresponding tissue-specific regulatory networks.

Distinct cell wall architectures in seed endosperms in representatives of the Brassicaceae and Solanaceae

In some species, a crucial role has been demonstrated for the seed endosperm during germination. The endosperm has been shown to integrate environmental cues with hormonal networks that underpin dormancy and seed germination, a process that involves the action of cell wall remodeling enzymes (CWREs). Here, we examine the cell wall architectures of the endosperms of two related Brassicaceae, Arabidopsis (Arabidopsis thaliana) and the close relative Lepidium (Lepidium sativum), and that of the Solanaceous species, tobacco (Nicotiana tabacum). The Brassicaceae species have a similar cell wall architecture that is rich in pectic homogalacturonan, arabinan, and xyloglucan. Distinctive features of the tobacco endosperm that are absent in the Brassicaceae representatives are major tissue asymmetries in cell wall structural components that reflect the future site of radicle emergence and abundant heteromannan. Cell wall architecture of the micropylar endosperm of tobacco seeds has structural components similar to those seen in Arabidopsis and Lepidium endosperms. In situ and biomechanical analyses were used to study changes in endosperms during seed germination and suggest a role for mannan degradation in tobacco. In the case of the Brassicaceae representatives, the structurally homogeneous cell walls of the endosperm can be acted on by spatially regulated CWRE expression. Genetic manipulations of cell wall components present in the Arabidopsis seed endosperm demonstrate the impact of cell wall architectural changes on germination kinetics.

Molecular mechanisms of seed dormancy

Seed dormancy is an important component of plant fitness that causes a delay of germination until the arrival of a favourable growth season. Dormancy is a complex trait that is determined by genetic factors with a substantial environmental influence. Several of the tissues comprising a seed contribute to its final dormancy level. The roles of the plant hormones abscisic acid and gibberellin in the regulation of dormancy and germination have long been recognized. The last decade saw the identification of several additional factors that influence dormancy including dormancy-specific genes, chromatin factors and non-enzymatic processes. This review gives an overview of our present understanding of the mechanisms that control seed dormancy at the molecular level, with an emphasis on new insights. The various regulators that are involved in the induction and release of dormancy, the influence of environmental factors and the conservation of seed dormancy mechanisms between plant species are discussed. Finally, expected future directions in seed dormancy research are considered.