Integrated control of seed maturation and germination programs by activator and repressor functions of Viviparous-1 of maize

Current Insights into Weak Seed Dormancy and Pre-Harvest Sprouting in Crop Species

During the domestication of crops, seed dormancy has been reduced or eliminated to encourage faster and more consistent germination. This alteration makes cultivated crops particularly vulnerable to pre-harvest sprouting, which occurs when mature crops are subjected to adverse environmental conditions, such as excessive rainfall or high humidity. Consequently, some seeds may bypass the normal dormancy period and begin to germinate while still attached to the mother plant before harvest. Grains affected by pre-harvest sprouting are characterized by increased levels of α-amylase activity, resulting in poor processing quality and immediate grain downgrading. In the agriculture industry, pre-harvest sprouting causes annual economic losses exceeding USD 1 billion worldwide. This premature germination is influenced by a complex interplay of genetic, biochemical, and molecular factors closely linked to environmental conditions like rainfall. However, the exact mechanism behind this process is still unclear. Unlike pre-harvest sprouting, vivipary refers to the germination process and the activation of α-amylase during the soft dough stage, when the grains are still immature. Mature seeds with reduced levels of ABA or impaired ABA signaling (weak dormancy) are more susceptible to pre-harvest sprouting. While high seed dormancy can enhance resistance to pre-harvest sprouting, it can lead to undesirable outcomes for most crops, such as non-uniform seedling establishment after sowing. Thus, resistance to pre-harvest sprouting is crucial to ensuring productivity and sustainability and is an agronomically important trait affecting yield and grain quality. On the other hand, seed color is linked to sprouting resistance; however, the genetic relationship between both characteristics remains unresolved. The identification of mitogen-activated protein kinase kinase-3 (MKK3) as the gene responsible for pre-harvest sprouting-1 (Phs-1) represents a significant advancement in our understanding of how sprouting in wheat is controlled at the molecular and genetic levels. In seed maturation, Viviparous-1 (Vp-1) plays a crucial role in managing pre-harvest sprouting by regulating seed maturation and inhibiting germination through the suppression of α-amylase and proteases. Vp-1 is a key player in ABA signaling and is essential for the activation of the seed maturation program. Mutants of Vp-1 exhibit an unpigmented aleurone cell layer and exhibit precocious germination due to decreased sensitivity to ABA. Recent research has also revealed that TaSRO-1 interacts with TaVp-1, contributing to the regulation of seed dormancy and resistance to pre-harvest sprouting in wheat. The goal of this review is to emphasize the latest research on pre-harvest sprouting in crops and to suggest possible directions for future studies.

Advance Research on the Pre-Harvest Sprouting Trait in Vegetable Crop Seeds

Pre-harvest sprouting (PHS), the germination of seeds on the plant prior to harvest, poses significant challenges to agriculture. It not only reduces seed and grain yield, but also impairs the commodity quality of the fruit, ultimately affecting the success of the subsequent crop cycle. A deeper understanding of PHS is essential for guiding future breeding strategies, mitigating its impact on seed production rates and the commercial quality of fruits. PHS is a complex phenomenon influenced by genetic, physiological, and environmental factors. Many of these factors exert their influence on PHS through the intricate regulation of plant hormones responsible for seed germination. While numerous genes related to PHS have been identified in food crops, the study of PHS in vegetable crops is still in its early stages. This review delves into the regulatory elements, functional genes, and recent research developments related to PHS in vegetable crops. Meanwhile, this paper presents a novel understanding of PHS, aiming to serve as a reference for the study of this trait in vegetable crops.

Overexpression of ABI5 Binding Proteins Suppresses Inhibition of Germination Due to Overaccumulation of DELLA Proteins

Abscisic acid (ABA) and gibberellic acid (GA) antagonistically regulate many aspects of plant growth, including seed dormancy and germination. The effects of these hormones are mediated by a complex network of positive and negative regulators of transcription. The DELLA family of proteins repress GA response, and can promote an ABA response via interactions with numerous regulators, including the ABA-insensitive (ABI) transcription factors. The AFP family of ABI5 binding proteins are repressors of the ABA response. This study tested the hypothesis that the AFPs also interact antagonistically with DELLA proteins. Members of these protein families interacted weakly in yeast two-hybrid and bimolecular fluorescence complementation studies. Overexpression of AFPs in sleepy1, a mutant that over-accumulates DELLA proteins, suppressed DELLA-induced overaccumulation of storage proteins, hyperdormancy and hypersensitivity to ABA, but did not alter the dwarf phenotype of the mutant. The interaction appeared to reflect additive effects of the AFPs and DELLAs, consistent with action in convergent pathways.

Microautophagy Mediates Vacuolar Delivery of Storage Proteins in Maize Aleurone Cells

The molecular machinery orchestrating microautophagy, whereby eukaryotic cells sequester autophagic cargo by direct invagination of the vacuolar/lysosomal membrane, is still largely unknown, especially in plants. Here, we demonstrate microautophagy of storage proteins in the maize aleurone cells of the endosperm and analyzed proteins with potential regulatory roles in this process. Within the cereal endosperm, starchy endosperm cells accumulate storage proteins (mostly prolamins) and starch whereas the peripheral aleurone cells store oils, storage proteins, and specialized metabolites. Although both cell types synthesize prolamins, they employ different pathways for their subcellular trafficking. Starchy endosperm cells accumulate prolamins in protein bodies within the endoplasmic reticulum (ER), whereas aleurone cells deliver prolamins to vacuoles via an autophagic mechanism, which we show is by direct association of ER prolamin bodies with the tonoplast followed by engulfment via microautophagy. To identify candidate proteins regulating this process, we performed RNA-seq transcriptomic comparisons of aleurone and starchy endosperm tissues during seed development and proteomic analysis on tonoplast-enriched fractions of aleurone cells. From these datasets, we identified 10 candidate proteins with potential roles in membrane modification and/or microautophagy, including phospholipase-Dα5 and a possible EUL-like lectin. We found that both proteins increased the frequency of tonoplast invaginations when overexpressed in Arabidopsis leaf protoplasts and are highly enriched at the tonoplast surface surrounding ER protein bodies in maize aleurone cells, thus supporting their potential connections to microautophagy. Collectively, this candidate list now provides useful tools to study microautophagy in plants.

Molecular Aspects of Seed Development Controlled by Gibberellins and Abscisic Acids

Plants have evolved seeds to permit the survival and dispersion of their lineages by providing nutrition for embryo growth and resistance to unfavorable environmental conditions. Seed formation is a complicated process that can be roughly divided into embryogenesis and the maturation phase, characterized by accumulation of storage compound, acquisition of desiccation tolerance, arrest of growth, and acquisition of dormancy. Concerted regulation of several signaling pathways, including hormonal and metabolic signals and gene networks, is required to accomplish seed formation. Recent studies have identified the major network of genes and hormonal signals in seed development, mainly in maturation. Gibberellin (GA) and abscisic acids (ABA) are recognized as the main hormones that antagonistically regulate seed development and germination. Especially, knowledge of the molecular mechanism of ABA regulation of seed maturation, including regulation of dormancy, accumulation of storage compounds, and desiccation tolerance, has been accumulated. However, the function of ABA and GA during embryogenesis still remains elusive. In this review, we summarize the current understanding of the sophisticated molecular networks of genes and signaling of GA and ABA in the regulation of seed development from embryogenesis to maturation.

Maize endosperm development

Recent breakthroughs in transcriptome analysis and gene characterization have provided valuable resources and information about the maize endosperm developmental program. The high temporal-resolution transcriptome analysis has yielded unprecedented access to information about the genetic control of seed development. Detailed spatial transcriptome analysis using laser-capture microdissection has revealed the expression patterns of specific populations of genes in the four major endosperm compartments: the basal endosperm transfer layer (BETL), aleurone layer (AL), starchy endosperm (SE), and embryo-surrounding region (ESR). Although the overall picture of the transcriptional regulatory network of endosperm development remains fragmentary, there have been some exciting advances, such as the identification of OPAQUE11 (O11) as a central hub of the maize endosperm regulatory network connecting endosperm development, nutrient metabolism, and stress responses, and the discovery that the endosperm adjacent to scutellum (EAS) serves as a dynamic interface for endosperm-embryo crosstalk. In addition, several genes that function in BETL development, AL differentiation, and the endosperm cell cycle have been identified, such as ZmSWEET4c, Thk1, and Dek15, respectively. Here, we focus on current advances in understanding the molecular factors involved in BETL, AL, SE, ESR, and EAS development, including the specific transcriptional regulatory networks that function in each compartment during endosperm development.

Maize kernel development

Maize (Zea mays) is a leading cereal crop in the world. The maize kernel is the storage organ and the harvest portion of this crop and is closely related to its yield and quality. The development of maize kernel is initiated by the double fertilization event, leading to the formation of a diploid embryo and a triploid endosperm. The embryo and endosperm are then undergone independent developmental programs, resulting in a mature maize kernel which is comprised of a persistent endosperm, a large embryo, and a maternal pericarp. Due to the well-characterized morphogenesis and powerful genetics, maize kernel has long been an excellent model for the study of cereal kernel development. In recent years, with the release of the maize reference genome and the development of new genomic technologies, there has been an explosive expansion of new knowledge for maize kernel development. In this review, we overviewed recent progress in the study of maize kernel development, with an emphasis on genetic mapping of kernel traits, transcriptome analysis during kernel development, functional gene cloning of kernel mutants, and genetic engineering of kernel traits.

Transcription Factors Rc and OsVP1 Coordinately Regulate Preharvest Sprouting Tolerance in Red Pericarp Rice

Red pericarp associates with seed dormancy or preharvest sprouting (PHS) tolerance in crops. To identify this association's molecular mechanism, a PHS mutant Osviviparous1 (Osvp1) was characterized in rice and crossed with Kasalath, a red pericarp cultivar with Rc (red coleoptiles) genotype. Among the dehulled seeds of F₂ progenies, RcRcvp1vp1 seeds performed a lower PHS rate than rcrcvp1vp1 seeds and showed shallower pigmentation than RcRcVP1VP1 seeds. Kasalath and SL9 (an RcRcVP1VP1 substitution line with Nipponbare background) showed more ABA sensitivity than the Nipponbare (rcrcVP1VP1) by the germination assay, and the transcriptional abundance of ABA signal genes OsABI2, OsSnRK2, OsVP1, ABI5, and especially OsVP1 increased in the red pericarp line SL9. Moreover, OsVP1 can directly bind Rc (bHLH) promoter by yeast one-hybrid, which activates Rc and OsLAR expression in red pericarp rice. Furthermore, a luciferase complementation imaging assay showed that OsVP1 interacts with transcriptions factors Rc and OsC1. These results indicate that OsVP1 promotes proanthocyanidin accumulation through the interaction among OsVP1, Rc, and OsC1 and then increases the plant's ABA sensitivity and PHS resistance.

High temperature inhibits the accumulation of storage materials by inducing alternative splicing of OsbZIP58 during filling stage in rice

High temperature (HT) has an adverse effect on rice grain filling by inhibiting the accumulation of storage materials. However, the regulatory mechanism of this inhibition remains unknown. Here, we report that Opaque2 like transcription factor OsbZIP58 is a key factor regulating storage material accumulation under HT. The OsbZIP58 gene promotes expression of many seed storage protein genes and starch synthesis genes while inhibits expression of some starch hydrolyzing α-amylase genes under HT. The loss of OsbZIP58 function leads to floury and shrunken endosperms and dramatically reduced storage materials in the seeds under HT. HT is found to affect alternative splicing of OsbZIP58, promoting the formation of the truncated OsbZIP58β protein form over the full-length OsbZIP58α protein form. The OsbZIP58β form has a lower transcriptional activity than the OsbZIP58α form, especially under HT condition. Interestingly, rice varieties with less heat sensitivity have reduced alternative splicing of OsbZIP58. Therefore, OsbZIP58 is a crucial gene in regulating storage material accumulation under HT and lower alternative splicing of OsbZIP58 may contribute to heat tolerance during grain filling.

Identification of Quantitative Trait Loci Controlling Ethylene Production in Germinating Seeds in Maize (Zea mays L.)

Plant seed germination is a crucial developmental event that has significant effects on seedling establishment and yield production. This process is controlled by multiple intrinsic signals, particularly phytohormones. The gaseous hormone ethylene stimulates seed germination; however, the genetic basis of ethylene production in maize during seed germination remains poorly understood. In this study, we quantified the diversity of germination among 14 inbred lines representing the parental materials corresponding to multiple recombinant inbred line (RIL) mapping populations. Quantitative trait loci (QTLs) controlling ethylene production were then identified in germinating seeds from an RIL population constructed from two parental lines showing differences in both germination speed and ethylene production during germination. To explore the possible genetic correlations of ethylene production with other traits, seed germination and seed weight were evaluated using the same batch of samples. On the basis of high-density single nucleotide polymorphism-based genetic linkage maps, we detected three QTLs for ethylene production in germinating seeds, three QTLs for seed germination, and four QTLs for seed weight, with each QTL explaining 5.8%–13.2% of the phenotypic variation of the trait. No QTLs were observed to be co-localized, suggesting that the genetic bases underlying the three traits are largely different. Our findings reveal three chromosomal regions responsible for ethylene production during seed germination, and provide a valuable reference for the future investigation of the genetic mechanism underlying the role of the stress hormone ethylene in maize germination control under unfavourable external conditions.

Quantitative Trait Locus (QTLs) Mapping for Quality Traits of Wheat Based on High Density Genetic Map Combined With Bulked Segregant Analysis RNA-seq (BSR-Seq) Indicates That the Basic 7S Globulin Gene Is Related to Falling Number

Numerous quantitative trait loci (QTLs) have been identified for wheat quality; however, most are confined to low-density genetic maps. In this study, based on specific-locus amplified fragment sequencing (SLAF-seq), a high-density genetic map was constructed with 193 recombinant inbred lines derived from Chuanmai 42 and Chuanmai 39. In total, 30 QTLs with phenotypic variance explained (PVE) up to 47.99% were identified for falling number (FN), grain protein content (GPC), grain hardness (GH), and starch pasting properties across three environments. Five NAM genes closely adjacent to QGPC.cib-4A probably have effects on GPC. QGH.cib-5D was the only one detected for GH with high PVE of 33.31-47.99% across the three environments and was assumed to be related to the nearest pina-D1 and pinb-D1genes. Three QTLs were identified for FN in at least two environments, of which QFN.cib-3D had relatively higher PVE of 16.58-25.74%. The positive effect of QFN.cib-3D for high FN was verified in a double-haploid population derived from Chuanmai 42 × Kechengmai 4. The combination of these QTLs has a considerable effect on increasing FN. The transcript levels of Basic 7S globulin and Basic 7S globulin 2 in QFN.cib-3D were significantly different between low FN and high FN bulks, as observed through bulk segregant RNA-seq (BSR). These QTLs and candidate genes based on the high-density genetic map would be beneficial for further understanding of the genetic mechanism of quality traits and molecular breeding of wheat.

Control of rice pre-harvest sprouting by glutaredoxin-mediated abscisic acid signaling

Pre-harvest sprouting (PHS) is one of the major problems in cereal production worldwide, which causes significant losses of both yield and quality; however, the molecular mechanism underlying PHS remains largely unknown. Here, we identified a dominant PHS mutant phs9-D. The corresponding gene PHS9 encodes a higher plant unique CC-type glutaredoxin and is specifically expressed in the embryo at the late embryogenesis stage, implying that PHS9 plays some roles in the late stage of seed development. Yeast two-hybrid screening showed that PHS9 could interact with OsGAP, which is an interaction partner of the abscicic acid (ABA) receptor OsRCAR1. PHS9- or OsGAP overexpression plants showed reduced ABA sensitivity in seed germination, whereas PHS9 or OsGAP knock-out mutant plants showed increased ABA sensitivity in seed germination, suggesting that PHS9 and OsGAP acted as negative regulators in ABA signaling during seed germination. Interestingly, the germination of PHS9 and OsGAP overexpression or knock-out plant seeds was weakly promoted by H₂ O₂ , implying that PHS9 and OsGAP could affect reactive oxygen species (ROS) signaling during seed germination. These results indicate that PHS9 plays an important role in the regulation of rice PHS through the integration of ROS signaling and ABA signaling.

Maize VIVIPAROUS1 Interacts with ABA INSENSITIVE5 to Regulate GALACTINOL SYNTHASE2 Expression Controlling Seed Raffinose Accumulation

Raffinose, an oligosaccharide found in many seeds, plays an important role in seed vigor; however, the regulatory mechanism governing raffinose biosynthesis remains unclear. We report here that maize W22 wild type (WT) seeds, but not W22 viviparous1 ( zmvp1) mutant seeds, start accumulating galactinol and raffinose 28 days after pollination (DAP). Transcriptome analysis of the zmvp1 embryo showed that the expression of GALACTINOL SYNTHASE2 ( GOLS2) was down-regulated relative to WT. Further experiments showed that the expression of ZmGOLS2 was up-regulated by ZmABI5 but not by ZmVP1, and it was further increased by the coexpression of ZmABI5 and ZmVP1 in maize protoplasts. ZmABI5 interacted with ZmVP1, while ZmABI5, but not ZmVP1, directly binds to the ZmGOLS2 promoter. Together, all of the findings suggest that ZmVP1 interacts with ZmABI5 and regulates ZmGOLS2 expression and raffinose accumulation in maize seeds.

Maize opaque mutants are no longer so opaque

The endosperm of angiosperms is a zygotic seed organ that stores nutrient reserves to support embryogenesis and seed germination. Cereal endosperm is also a major source of human calories and an industrial feedstock. Maize opaque endosperm mutants commonly exhibit opaque, floury kernels, along with other abnormal seed and/or non-seed phenotypes. The opaque endosperm phenotype is sometimes accompanied by a soft kernel texture and increased nutritional quality, including a higher lysine content, which are valuable agronomic traits that have drawn attention of maize breeders. Recently, an increasing number of genes that underlie opaque mutants have been cloned, and their characterization has begun to shed light on the molecular basis of the opaque endosperm phenotype. These mutants are categorized by disruption of genes encoding zein or non-zein proteins localized to protein bodies, enzymes involved in endosperm metabolic processes, or transcriptional regulatory proteins associated with endosperm storage programs.

Endosperm sugar accumulation caused by mutation of PHS8/ISA1 leads to pre-harvest sprouting in rice

Pre-harvest sprouting (PHS) is an unfavorable trait in cereal crops that could seriously decrease grain yield and quality. Although some PHS-associated quantitative trait loci or genes in cereals have been reported, the molecular mechanism underlying PHS remains largely elusive. Here, we characterized a rice mutant, phs8, which exhibits PHS phenotype accompanied by sugary endosperm. Map-based cloning revealed that PHS8 encodes a starch debranching enzyme named isoamylase1. Mutation in PHS8 resulted in the phytoglycogen breakdown and sugar accumulation in the endosperm. Intriguingly, with increase of sugar contents, decreased expression of OsABI3 and OsABI5 as well as reduced sensitivity to abscisic acid (ABA) were found in the phs8 mutant. Using rice suspension cell system, we confirmed that exogenous sugar is sufficient to suppress the expression of both OsABI3 and OsABI5. Furthermore, overexpression of OsABI3 or OsABI5 could partially rescue the PHS phenotype of phs8. Therefore, our study presents important evidence supporting that endosperm sugar not only acts as an essential energy source for seed germination but also determines seed dormancy and germination by affecting ABA signaling.

Pre-harvest Sprouting and Grain Dormancy in Sorghum bicolor: What Have We Learned?

The possibility of obtaining sorghum grains with quality to match the standards for a diversity of end-uses is frequently hampered by the susceptibility to pre-harvest sprouting (PHS) displayed by many elite genotypes. For these reasons, obtaining resistance to PHS is considered in sorghum breeding programs, particularly when the crop is expected to approach harvest maturity under rainy or damp conditions prevalence. As in other cereals, the primary cause for sprouting susceptibility is a low dormancy prior to crop harvest; in consequence, most research has focused in understanding the mechanisms through which the duration of dormancy is differentially controlled in genotypes with contrasting sprouting behavior. With this aim two tannin-less, red-grained inbred lines were used as a model system: IS9530 (sprouting resistant) and Redland B2 (sprouting susceptible). Redland B2 grains are able to germinate well before reaching physiological maturity (PM) while IS9530 ones can start to germinate at 40-45 days after pollination, well after PM. Results show that the anticipated dormancy loss displayed by Redland B2 grains is related reduced embryo sensitivity to abscisic acid (ABA) and increased levels of GA upon imbibition. In turn, transcriptional data showed that ABA signal transduction is impaired in Redland B2, which appears to have an impact on GA catabolism, thus affecting the overall GA/ABA balance that regulates germination. QTL analyses were conducted to test whether previous candidate genes were located in a dormancy QTL, but also to identify new genes involved in dormancy. These analyses yielded several dormancy QTL and one of them located in chromosome 9 (qGI-9) was consistently detected even across environments. Fine mapping is already in progress to narrow down the number of candidate genes in qGI-9.

Identification and functional analysis of two alternatively spliced transcripts of ABSCISIC ACID INSENSITIVE3 (ABI3) in linseed flax (Linum usitatissimum L.)

Alternative splicing is a popular phenomenon in different types of plants. It can produce alternative spliced transcripts that encode proteins with altered functions. Previous studies have shown that one transcription factor, ABSCISIC ACID INSENSITIVE3 (ABI3), which encodes an important component in abscisic acid (ABA) signaling, is subjected to alternative splicing in both mono- and dicotyledons. In the current study, we identified two homologs of ABI3 in the genome of linseed flax. We screened two alternatively spliced flax LuABI3 transcripts, LuABI3-2 and LuABI3-3, and one normal flax LuABI3 transcript, LuABI3-1. Sequence analysis revealed that one of the alternatively spliced transcripts, LuABI3-3, retained a 6 bp intron. RNA accumulation analysis showed that all three transcripts were expressed during seed development, while subcellular localization and transgene experiments showed that LuABI3-3 had no biological function. The two normal transcripts, LuABI3-1 and LuABI3-2, are the important functional isoforms in flax and play significant roles in the ABA regulatory pathway during seed development, germination, and maturation.

Molecular Mechanisms Underlying Abscisic Acid/Gibberellin Balance in the Control of Seed Dormancy and Germination in Cereals

Seed dormancy is an adaptive trait that does not allow the germination of an intact viable seed under favorable environmental conditions. Non-dormant seeds or seeds with low level of dormancy can germinate readily under optimal environmental conditions, and such a trait leads to preharvest sprouting, germination of seeds on the mother plant prior to harvest, which significantly reduces the yield and quality of cereal crops. High level of dormancy, on the other hand, may lead to non-uniform germination and seedling establishment. Therefore, intermediate dormancy is considered to be a desirable trait as it prevents the problems of sprouting and allows uniformity of postharvest germination of seeds. Induction, maintenance, and release of seed dormancy are complex physiological processes that are influenced by a wide range of endogenous and environmental factors. Plant hormones, mainly abscisic acid (ABA) and gibberellin (GA), are the major endogenous factors that act antagonistically in the control of seed dormancy and germination; ABA positively regulates the induction and maintenance of dormancy, while GA enhances germination. Significant progress has been made in recent years in the elucidation of molecular mechanisms regulating ABA/GA balance and thereby dormancy and germination in cereal seeds, and this review summarizes the current state of knowledge on the topic.

Rich haplotypes of Viviparous-1 in Triticum aestivum subsp. spelta with different abscisic acid sensitivities

BACKGROUND

Viviparous-1 (Vp-1) is a major gene affecting pre-harvest sprouting (PHS) in common wheat, and improving PHS tolerance is a crucial factor for wheat breeding. Spelt wheat is always used as the donor parent to improve resistance and quality in wheat breeding: however, the roles of the Vp-1 genes in spelt wheat and their relationship to common wheat remain uncertain. The current study aimed to isolate and characterise Vp-1 haplotypes in spelt wheat (Triticum aestivum subsp. spelta).

RESULTS

In spelt wheat, a total of eight new Vp-1 haplotypes were identified: TaVp-1Ap, TaVp-1Aq and TaVp-1Ar in the A genome; TaVp-1Bj, TaVp-1Bh and TaVp-1Bi in the B genome; and TaVp-1Da and TaVp-1Db in the D genome. According to RT-PCR results, correctly spliced transcripts were more highly expressed in some haplotypes than in others, and their expression was highly associated with their distinct responsiveness to abscisic acid (ABA) exposure. The mis-splicing of Vp-1 transcripts and several indel variations detected in spelt wheat appear to have been retained through the hybridisation process.

CONCLUSION

Certain haplotypes detected in spelt wheat might be valuable in the breeding and selection of germplasm to improve PHS issues in wheat. © 2016 Society of Chemical Industry.

ABI3 mediates dehydration stress recovery response in Arabidopsis thaliana by regulating expression of downstream genes

ABI3, originally discovered as a seed-specific transcription factor is now implicated to act beyond seed physiology, especially during abiotic stress. In non-seed plants, ABI3 is known to act in desiccation stress signaling. Here we show that ABI3 plays a role in dehydration stress response in Arabidopsis. ABI3 gene was upregulated during dehydration stress and its expression was maintained during subsequent stress recovery phases. Comparative gene expression studies in response to dehydration stress and stress recovery were done with genes which had potential ABI3 binding sites in their upstream regulatory regions. Such studies showed that several genes including known seed-specific factors like CRUCIFERIN1, CRUCIFERIN3 and LEA-group of genes like LEA76, LEA6, DEHYDRIN LEA and LEA-LIKE got upregulated in an ABI3-dependent manner, especially during the stress recovery phase. ABI3 got recruited to regions upstream to the transcription start site of these genes during dehydration stress response through direct or indirect DNA binding. Interestingly, ABI3 also binds to its own promoter region during such stress signaling. Nucleosomes covering potential ABI3 binding sites in the upstream sequences of the above-mentioned genes alter positions, and show increased H3 K9 acetylation during stress-induced transcription. ABI3 thus mediates dehydration stress signaling in Arabidopsis through regulation of a group of genes that play a role primarily during stress recovery phase.

Serine carboxypeptidase 46 Regulates Grain Filling and Seed Germination in Rice (Oryza sativa L.)

Serine carboxypeptidase (SCP) is one of the largest groups of enzymes catalyzing proteolysis for functional protein maturation. To date, little is known about the function of SCPs in rice. In this study, we present a comprehensive analysis of the gene structure and expression profile of 59 rice SCPs. SCP46 is dominantly expressed in developing seeds, particularly in embryo, endosperm and aleurone layers, and could be induced by ABA. Functional characterization revealed that knock-down of SCP46 resulted in smaller grain size and enhanced seed germination. Furthermore, scp46 seed germination became less sensitive to the ABA inhibition than the Wild-type did; suggesting SCP46 is involved in ABA signaling. As indicated by RNA-seq and qRT-PCR analysis, numerous grain filling and seed dormancy related genes, such as SP, VP1 and AGPs were down-regulated in scp46. Yeast-two-hybrid assay also showed that SCP46 interacts with another ABA-inducible protein DI19-1. Taken together, we suggested that SCP46 is a master regulator of grain filling and seed germination, possibly via participating in the ABA signaling. The results of this study shed novel light into the roles of SCPs in rice.

Overexpression of a novel cold-responsive transcript factor LcFIN1 from sheepgrass enhances tolerance to low temperature stress in transgenic plants

As a perennial forage crop broadly distributed in eastern Eurasia, sheepgrass (Leymus chinensis (Trin.) Tzvel) is highly tolerant to low-temperature stress. Previous report indicates that sheepgrass is able to endure as low as -47.5 °C,allowing it to survive through the cold winter season. However, due to the lack of sufficient studies, the underlying mechanism towards the extraordinary low-temperature tolerance is unclear. Although the transcription profiling has provided insight into the transcriptome response to cold stress, more detailed studies are required to dissect the molecular mechanism regarding the excellent abiotic stress tolerance. In this work, we report a novel transcript factor LcFIN1 (L. chinensis freezing-induced 1) from sheepgrass. LcFIN1 showed no homology with other known genes and was rapidly and highly induced by cold stress, suggesting that LcFIN1 participates in the early response to cold stress. Consistently, ectopic expression of LcFIN1 significantly increased cold stress tolerance in the transgenic plants, as indicated by the higher survival rate, fresh weight and other stress-related indexes after a freezing treatment. Transcriptome analysis showed that numerous stress-related genes were differentially expressed in LcFIN1-overexpressing plants, suggesting that LcFIN1 may enhance plant abiotic stress tolerance by transcriptional regulation. Electrophoretic mobility shift assays and CHIP-qPCR showed that LcCBF1 can bind to the CRT/DRE cis-element located in the promoter region of LcFIN1, suggesting that LcFIN1 is directly regulated by LcCBF1. Taken together, our results suggest that LcFIN1 positively regulates plant adaptation response to cold stress and is a promising candidate gene to improve crop cold tolerance.

Dormancy and germination: How does the crop seed decide?

Whether seeds germinate or maintain dormancy is decided upon through very intricate physiological processes. Correct timing of these processes is most important for the plants life cycle. If moist conditions are encountered, a low dormancy level causes pre-harvest sprouting in various crop species, such as wheat, corn and rice, this decreases crop yield and negatively impacts downstream industrial processing. In contrast, a deep level of seed dormancy prevents normal germination even under favourable conditions, resulting in a low emergence rate during agricultural production. Therefore, an optimal seed dormancy level is valuable for modern mechanised agricultural systems. Over the past several years, numerous studies have demonstrated that diverse endogenous and environmental factors regulate the balance between dormancy and germination, such as light, temperature, water status and bacteria in soil, and phytohormones such as ABA (abscisic acid) and GA (gibberellic acid). In this updated review, we highlight recent advances regarding the molecular mechanisms underlying regulation of seed dormancy and germination processes, including the external environmental and internal hormonal cues, and primarily focusing on the staple crop species. Furthermore, future challenges and research directions for developing a full understanding of crop seed dormancy and germination are also discussed.

Histone Acetylation is Involved in Gibberellin-Regulated sodCp Gene Expression in Maize Aleurone Layers

The cereal aleurone layer plays an important role in seed germination, and reactive oxygen species (ROS) in aleurone layers act as crucial signal molecules in this progression. Recent studies have revealed that epigenetic modification is involved in plant development and seed germination. However, little is known about a possible relationship between histone modification and the ROS signaling pathway in cereal aleurone layers during seed germination. Here, we found that the expression of both histone acetyltransferases (HATs) and histone deacetylases (HDACs) was increased gradually during seed germination, accompanied by an increase in global acetylation levels of histones H3 and H4 in maize aleurone layers. The acetylation was found to be promoted by GA(3) and suppressed by ABA. However, when the HDAC inhibitor trichostatin A (TSA) was used, the increased H3K9ac and H4K5ac level correlated with an inhibition of the germination. These results indicated that the overall histone acetylation in the aleurone layers is not required for germination. Similarly these two hormones, GA(3) and ABA, exerted opposed effects on the expression of the ROS-related gene sodCp. Furthermore, chromatin immunoprecipitation experiments showed that the promoter region of the sodCp gene was hyperacetylated during germination, and this acetylation was promoted by GA(3) and inhibited by both ABA and TSA. These results suggested that GA(3)-mediated expression of the sodCp gene in aleurone layers is associated with histone hyperacetylation on the promoter and coding region of this gene, consequently leading to an accumulation of H(2)O(2) which regulated production of α-amylase during seed germination.

Transcriptome dynamics of developing maize leaves and genomewide prediction of cis elements and their cognate transcription factors

Maize is a major crop and a model plant for studying C4 photosynthesis and leaf development. However, a genomewide regulatory network of leaf development is not yet available. This knowledge is useful for developing C3 crops to perform C4 photosynthesis for enhanced yields. Here, using 22 transcriptomes of developing maize leaves from dry seeds to 192 h post imbibition, we studied gene up- and down-regulation and functional transition during leaf development and inferred sets of strongly coexpressed genes. More significantly, we developed a method to predict transcription factor binding sites (TFBSs) and their cognate transcription factors (TFs) using genomic sequence and transcriptomic data. The method requires not only evolutionary conservation of candidate TFBSs and sets of strongly coexpressed genes but also that the genes in a gene set share the same Gene Ontology term so that they are involved in the same biological function. In addition, we developed another method to predict maize TF-TFBS pairs using known TF-TFBS pairs in Arabidopsis or rice. From these efforts, we predicted 1,340 novel TFBSs and 253 new TF-TFBS pairs in the maize genome, far exceeding the 30 TF-TFBS pairs currently known in maize. In most cases studied by both methods, the two methods gave similar predictions. In vitro tests of 12 predicted TF-TFBS interactions showed that our methods perform well. Our study has significantly expanded our knowledge on the regulatory network involved in maize leaf development.

Genetic variation of transgenerational plasticity of offspring germination in response to salinity stress and the seed transcriptome of Medicago truncatula

BACKGROUND

Transgenerational plasticity provides phenotypic variation that contributes to adaptation. For plants, the timing of seed germination is critical for offspring survival in stressful environments, as germination timing can alter the environmental conditions a seedling experiences. Stored seed transcripts are important determinants of seed germination, but have not previously been linked with transgenerational plasticity of germination behavior. In this study we used RNAseq and growth chamber experiments of the model legume M. trucantula to test whether parental exposure to salinity stress influences the expression of stored seed transcripts and early offspring traits and test for genetic variation.

RESULTS

We detected genotype-dependent parental environmental effects (transgenerational plasticity) on the expression levels of stored seed transcripts, seed size, and germination behavior of four M. truncatula genotypes. More than 50% of the transcripts detected in the mature, ungerminated seed transcriptome were annotated as regulating seed germination, some of which are involved in abiotic stress response and post-embryonic development. Some genotypes showed increased seed size in response to parental exposure to salinity stress, but no parental environmental influence on germination timing. In contrast, other genotypes showed no seed size differences across contrasting parental conditions but displayed transgenerational plasticity for germimation timing, with significantly delayed germination in saline conditions when parental plants were exposed to salinity. In genotypes that show significant transgenerational plastic germination response, we found significant coexpression networks derived from salt responsive transcripts involved in post-transcriptional regulation of the germination pathway. Consistent with the delayed germination response to saline conditions in these genotypes, we found genes associated with dormancy and up-regulation of abscisic acid (ABA).

CONCLUSIONS

Our results demonstrate genetic variation in transgenerational plasticity within M. truncatula and show that parental exposure to salinity stress influences the expression of stored seed transcripts, seed weight, and germination behavior. Furthermore, we show that the parental environment influences gene expression to modulate biological pathways that are likely responsible for offspring germination responses to salinity stress.

Major alterations in transcript profiles between C3-C4 and C4 photosynthesis of an amphibious species Eleocharis baldwinii

Engineering C4 photosynthetic metabolism into C3 crops is regarded as a major strategy to increase crop productivity, and clarification of the evolutionary processes of C4 photosynthesis can help the better use of this strategy. Here, Eleocharis baldwinii, a species in which C4 photosynthesis can be induced from a C3-C4 state under either environmental or ABA treatments, was used to identify the major transcriptional modifications during the process from C3-C4 to C4. The transcriptomic comparison suggested that in addition to the major differences in C4 core pathway, the pathways of glycolysis, citrate acid metabolism and protein synthesis were dramatically modified during the inducement of C4 photosynthetic states. Transcripts of many transporters, including not only metabolite transporters but also ion transporters, were dramatically increased in C4 photosynthetic state. Many candidate regulatory genes with unidentified functions were differentially expressed in C3-C4 and C4 photosynthetic states. Finally, it was indicated that ABA, auxin signaling and DNA methylation play critical roles in the regulation of C4 photosynthesis. In summary, by studying the different photosynthetic states of the same species, this work provides the major transcriptional differences between C3-C4 and C4 photosynthesis, and many of the transcriptional differences are potentially related to C4 development and therefore are the potential targets for reverse genetics studies.

Distinct functions of COAR and B3 domains of maize VP1 in induction of ectopic gene expression and plant developmental phenotypes in Arabidopsis

Arabidopsis ABI3 and maize VP1 are orthologous transcription factors that regulate seed maturation. ABI3 and VP1 have a C-terminal B3 DNA binding domain and a conserved N-terminal co-activator/co-repressor (COAR) domain consisting of A1, B1, B2 sub-domains. The COAR domain mediates abscisic acid signaling via a physical interaction with ABI5-related bZIP proteins. In order to delineate the COAR and B3 domain dependent functions of VP1, we created site directed mutations in the B3 domain that disrupted DNA binding activity and characterized gene regulation by the mutant proteins in transgenic abi3 mutant Arabidopsis plants. In seeds, COAR domain function of VP1 mutants that lacked B3 DNA binding activity was sufficient for complementation of the desiccation intolerant seed phenotype of abi3. Similarly in seedlings, the B3 domain was dispensable for most VP1 induced gene expression and ectopic developmental phenotypes, except for a small subset of the genes that showed B3 dependent regulation. Unexpectedly, over-expression of the DNA-binding deficient VP1-K519R mutant protein caused quantitative changes in floral organ size including elongation of pistils and shortened stamen filaments that resulted in a self-incompatible longistyly flower morphology, a key component of heterostyly type self-incompatibility.

Structural basis of carotenoid cleavage: from bacteria to mammals

Carotenoids and their metabolic derivatives serve critical functions in both prokaryotic and eukaryotic cells, including pigmentation, photoprotection and photosynthesis as well as cell signaling. These organic compounds are also important for visual function in vertebrate and non-vertebrate organisms. Enzymatic transformations of carotenoids to various apocarotenoid products are catalyzed by a family of evolutionarily conserved, non-heme iron-containing enzymes named carotenoid cleavage oxygenases (CCOs). Studies have revealed that CCOs are critically involved in carotenoid homeostasis and essential for the health of organisms including humans. These enzymes typically display a high degree of regio- and stereo-selectivity, acting on specific positions of the polyene backbone located in their substrates. By oxidatively cleaving and/or isomerizing specific double bonds, CCOs generate a variety of apocarotenoid isomer products. Recent structural studies have helped illuminate the mechanisms by which CCOs mobilize their lipophilic substrates from biological membranes to perform their characteristic double bond cleavage and/or isomerization reactions. In this review, we aim to integrate structural and biochemical information about CCOs to provide insights into their catalytic mechanisms.

Distinct roles of LAFL network genes in promoting the embryonic seedling fate in the absence of VAL repression

The transition between seed and seedling phases of development is coordinated by an interaction between the closely related ABSCISIC ACID-INSENSITIVE3 (ABI3), FUSCA3 (FUS3), and LEAFY COTYLEDON2 (LEC2; AFL) and VIVIPAROUS1/ABI3-LIKE (VAL) clades of the B3 transcription factor family that respectively activate and repress the seed maturation program. In the val1 val2 double mutant, derepression of the LEC1, LEC1-LIKE (L1L), and AFL (LAFL) network is associated with misexpression of embryonic characteristics resulting in arrested seedling development. We show that while the frequency of the embryonic fate in val1 val2 seedlings depends on the developmental timing of seed rescue, VAL proteins repress LAFL genes during germination, but not during seed development. Quantitative analysis of LAFL mutants that suppress the val1 val2 seedling phenotype revealed distinct roles of LAFL genes in promoting activation of the LAFL network. LEC2 and FUS3 are both essential for coordinate activation of the network, whereas effects of LEC1, L1L, and ABI3 are additive. Suppression of the val1 val2 seedling phenotype by the B3 domain-deficient abi3-12 mutation indicates that ABI3 activation of the LAFL network requires the B3 DNA-binding domain. In the VAL-deficient background, coordinate regulation of the LAFL network is observed over a wide range of genetic and developmental conditions. Our findings highlight distinct functional roles and interactions of LAFL network genes that are uncovered in the absence of VAL repressors.

Functional characterization of two alternatively spliced transcripts of tomato ABSCISIC ACID INSENSITIVE3 (ABI3) gene

Alternative splicing can produce transcripts that encode proteins with altered functions. The transcripts of the ABSCISIC ACID INSENSITIVE3 (ABI3)/VIVIPAROUS1 (VP1) gene, which is an important component in abscisic acid (ABA) signaling, are subjected to alternative splicing in both monocotyledons and dicotyledons. We identified two alternatively spliced tomato (Solanum lycopersicum) SlABI3 transcripts, SlABI3-F and SlABI3-T, which encode the nucleus-localized full-length and truncated proteins, respectively. The tissue-specific accumulation of SlABI3-F and SlABI3-T was determined, particularly in seeds at different developmental stages and in response to phytohormonal and abiotic stress. Ectopic over-expression of SlABI3-F and SlABI3-T resulted in the induction of seed-specific genes SlSOM, SlEM1 and SlEM6 in vegetative tissues. However, over-expression of SlABI3-F, but not SlABI3-T, activated expression of the downstream gene SlABI5 and conferred hypersensitivity to exogenous ABA during seed germination and primary root growth. In addition, the SlABI3-F protein interacted with SlABI5 much stronger than SlABI3-T did in the yeast two-hybrid assay. These results suggest that SlABI3-F and SlABI3-T have similar and distinct functionality in the ABA signaling, dependent on which tissue/organ they accumulate in.

Abscisic acid perception and signaling transduction in strawberry: a model for non-climacteric fruit ripening

On basis of fruit differential respiration and ethylene effects, climacteric and non-climacteric fruits have been classically defined. Over the past decades, the molecular mechanisms of climacteric fruit ripening were abundantly described and found to focus on ethylene perception and signaling transduction. In contrast, until our most recent breakthroughs, much progress has been made toward understanding the signaling perception and transduction mechanisms for abscisic acid (ABA) in strawberry, a model for non-climacteric fruit ripening. Our reports not only have provided several lines of strong evidences for ABA-regulated ripening of strawberry fruit, but also have demonstrated that homology proteins of Arabidopsis ABA receptors, including PYR/PYL/RCAR and ABAR/CHLH, act as positive regulators of ripening in response to ABA. These receptors also trigger a set of ABA downstream signaling components, and determine significant changes in the expression levels of both sugar and pigment metabolism-related genes that are closely associated with ripening. Soluble sugars, especially sucrose, may act as a signal molecular to trigger ABA accumulation through an enzymatic action of 9-cis-epoxycarotenoid dioxygenase 1 (FaNCED1). This mini-review offers an overview of these processes and also outlines the possible, molecular mechanisms for ABA in the regulation of strawberry fruit ripening through the ABA receptors.

Misregulation of the LOB domain gene DDA1 suggests possible functions in auxin signalling and photomorphogenesis

The LATERAL ORGAN BOUNDARIES DOMAIN (LBD) gene family encodes plant-specific transcription factors. In this report, the LBD gene DOWN IN DARK AND AUXIN1 (DDA1), which is closely related to LATERAL ORGAN BOUNDARIES (LOB) and ASYMMETRIC LEAVES2 (AS2), was characterized. DDA1 is expressed primarily in vascular tissues and its transcript levels were reduced by exposure to exogenous indole-3-acetic acid (IAA or auxin) and in response to dark exposure. Analysis of a T-DNA insertion line, dda1-1, in which the insertion resulted in misregulation of DDA1 transcripts in the presence of IAA and in the dark revealed possible functions in auxin response and photomorphogenesis. dda1-1 plants exhibited reduced sensitivity to auxin, produced fewer lateral roots, and displayed aberrant hypocotyl elongation in the dark. Phenotypes resulting from fusion of a transcriptional repression domain to DDA1 suggest that DDA1 may act as both a transcriptional activator and a transcriptional repressor depending on the context. These results indicate that DDA1 may function in both the auxin signalling and photomorphogenesis pathways.

Functional characterization of the HD-ZIP IV transcription factor OCL1 from maize

OCL1 (OUTER CELL LAYER1) encodes a maize HD-ZIP class IV transcription factor (TF) characterized by the presence of a homeo DNA-binding domain (HD), a dimerization leucine zipper domain (ZIP), and a steroidogenic acute regulatory protein (StAR)-related lipid transfer domain (START) involved in lipid transport in animals but the function of which is still unknown in plants. By combining yeast and plant trans-activation assays, the transcriptional activation domain of OCL1 was localized to 85 amino acids in the N-terminal part of the START domain. Full-length OCL1 devoid of this activation domain is unable to trans-activate a reporter gene under the control of a minimal promoter fused to six repeats of the L1 box, a cis-element present in target genes of HD-ZIP IV TFs in Arabidopsis. In addition, ectopic expression of OCL1 leads to pleiotropic phenotypic aberrations in transgenic maize plants, the most conspicuous one being a strong delay in flowering time which is correlated with the misexpression of molecular markers for floral transition such as ZMM4 (Zea Mays MADS-box4) or DLF1 (DELAYED FLOWERING1). As suggested by the interaction in planta between OCL1 and SWI3C1, a bona fide subunit of the SWI/SNF complex, OCL1 may modulate transcriptional activity of its target genes by interaction with a chromatin remodelling complex.

Overexpression of Osta-siR2141 caused abnormal polarity establishment and retarded growth in rice

Small RNAs (smRNAs) including miRNAs and siRNAs are critical for gene regulation and plant development. Among the highly diverse siRNAs, trans-acting siRNAs (ta-siRNAs) have been shown to be plant-specific. In Arabidopsis, eight TAS loci belonging to four families (TAS1, TAS2, TAS3, and TAS4) have been identified, and bioinformatics analysis reveals that the sequence of TAS3 is highly conserved in plants. In this study, the function of TAS3 ta-siRNA (tasiR-ARF) has been revealed in rice (Oryza sativa L.) on polarity establishment and stage transition from vegetative to reproductive development by over-expressing Osta-siR2141. Osta-siR2141 replaced miR390 in the miR390 backbone for ectopic expression in rice, and overexpression of Osta-siR2141 caused disturbed vascular bundle development and adaxialization in polarity establishment. Transgenic lines also displayed abnormal shoot apical meristems (SAMs) and retarded growth at the vegetative stage. Molecular analysis revealed that overexpression of Osta-siR2141 resulted in the down-regulation of miR166 and the up-regulation of class III homeodomain-leucine zipper genes (HD-ZIPIIIs) in the vegetative stage but not in the reproductive stage. Moreover, overexpression of Osta-siR2141 in Arabidopsis disturbed polarity establishment and retarded stage transition, suggesting that tasiR-ARF was functionally conserved in rice and Arabidopsis.

Cross-species approaches to seed dormancy and germination: conservation and biodiversity of ABA-regulated mechanisms and the Brassicaceae DOG1 genes

Seed dormancy is genetically determined with substantial environmental influence mediated, at least in part, by the plant hormone abscisic acid (ABA). The ABA-related transcription factor ABI3/VP1 (ABA INSENSITIVE3/VIVIPAROUS1) is widespread among green plants. Alternative splicing of its transcripts appears to be involved in regulating seed dormancy, but the role of ABI3/VP1 goes beyond seeds and dormancy. In contrast, DOG1 (DELAY OF GERMINATION 1), a major quantitative trait gene more specifically involved in seed dormancy, was so far only known from Arabidopsis thaliana (AtDOG1) and whether it also has roles during the germination of non-dormant seeds was not known. Seed germination of Lepidium sativum ('garden cress') is controlled by ABA and its antagonists gibberellins and ethylene and involves the production of apoplastic hydroxyl radicals. We found orthologs of AtDOG1 in the Brassicaceae relatives L. sativum (LesaDOG1) and Brassica rapa (BrDOG1) and compared their gene structure and the sequences of their transcripts expressed in seeds. Tissue-specific analysis of LesaDOG1 transcript levels in L. sativum seeds showed that they are degraded upon imbibition in the radicle and the micropylar endosperm. ABA inhibits germination in that it delays radicle protrusion and endosperm weakening and it increased LesaDOG1 transcript levels during early germination due to enhanced transcription and/or inhibited degradation. A reduced decrease in LesaDOG1 transcript levels upon ABA treatment is evident in the late germination phase in both tissues. This temporal and ABA-related transcript expression pattern suggests a role for LesaDOG1 in the control of germination timing of non-dormant L. sativum seeds. The possible involvement of the ABA-related transcription factors ABI3 and ABI5 in the regulation of DOG1 transcript expression is discussed. Other species of the monophyletic genus Lepidium showed coat or embryo dormancy and are therefore highly suited for comparative seed biology.

Abscisic acid: emergence of a core signaling network

Abscisic acid (ABA) regulates numerous developmental processes and adaptive stress responses in plants. Many ABA signaling components have been identified, but their interconnections and a consensus on the structure of the ABA signaling network have eluded researchers. Recently, several advances have led to the identification of ABA receptors and their three-dimensional structures, and an understanding of how key regulatory phosphatase and kinase activities are controlled by ABA. A new model for ABA action has been proposed and validated, in which the soluble PYR/PYL/RCAR receptors function at the apex of a negative regulatory pathway to directly regulate PP2C phosphatases, which in turn directly regulate SnRK2 kinases. This model unifies many previously defined signaling components and highlights the importance of future work focused on defining the direct targets of SnRK2s and PP2Cs, dissecting the mechanisms of hormone interactions (i.e., cross talk) and defining connections between this new negative regulatory pathway and other factors implicated in ABA signaling.

The rice transcription factor IDEF1 is essential for the early response to iron deficiency, and induces vegetative expression of late embryogenesis abundant genes

Higher plants maintain iron homeostasis by regulating the expression of iron (Fe)-related genes in accordance with Fe availability. The transcription factor IDEF1 regulates the response to Fe deficiency in Oryza sativa (rice) by recognizing CATGC sequences within the Fe deficiency-responsive cis-acting element IDE1. To investigate the function of IDEF1 in detail, we analyzed the response to Fe deficiency in transgenic rice plants exhibiting induced or repressed IDEF1 expression. Fe-deficiency treatment in hydroponic culture revealed that IDEF1 knock-down plants are susceptible to early-stage Fe deficiency, in contrast to IDEF1-induced plants. Time-course expression analyses using quantitative reverse-transcriptase PCR revealed that the IDEF1 expression level was positively correlated with the level of induction of the Fe utilization-related genes OsIRO2, OsYSL15, OsIRT1, OsYSL2, OsNAS1, OsNAS2, OsNAS3 and OsDMAS1, just after the onset of Fe starvation. However, this overall transactivation mediated by IDEF1 became less evident in subsequent stages. Microarray and in-silico analyses revealed that genes positively regulated by IDEF1, especially at the early stage, exhibit over-representation of CATGC and IDE1-like elements within the proximal promoter regions. These results indicate the existence of early and subsequent responses to Fe deficiency, with the former requiring IDEF1 more specifically. Proximal regions of IDEF1-regulated gene promoters also showed enrichment of RY elements (CATGCA), which regulate gene expression during seed maturation. The expression of several genes encoding late embryogenesis abundant proteins, including Osem, was induced in Fe-deficient roots and/or leaves in an IDEF1-dependent manner, suggesting a possible function of seed maturation-related genes in Fe-deficient vegetative organs.

Temporal expression patterns of hormone metabolism genes during imbibition of Arabidopsis thaliana seeds: a comparative study on dormant and non-dormant accessions

Seed imbibition is a prerequisite for subsequent dormancy and germination control. Here, we investigated imbibition responses of Arabidopsis seeds by transcriptomic and hormone profile analyses using dormant [Cape Verde Islands (Cvi)] and non-dormant [Columbia (Col)] accessions. Once imbibed, seeds of both accessions swelled most up to 3 h, reflecting water uptake. Microarray analysis showed that in both accessions, seeds imbibed for 15 min, 30 min and 1 h were less active in gene expression than at 3 h. More than 2,000 genes were either up-regulated or down-regulated in seeds imbibed for 3 h. Some genes up-regulated at 3 h were already induced in seeds imbibed for 1 h, suggestive of genome reprogramming early after the onset of imbibition. Imbibition-induced genes in seeds imbibed for 3 h included those up-regulated in both Col and Cvi (common) or unique to either accession (accession specific). Up-regulated genes that were both common and Cvi-specific were over-represented for sugar metabolism and the pentose phosphate pathway, whereas Col-specific genes were over-represented for ribosomal protein genes. Quantification of plant hormones showed that ABA and salicylic acid (SA) contents were higher, but gibberellin A(4) (GA(4)), N(6)-(Delta(2)-isopentenyl)adenine (iP), jasmonic acid (JA), JA-isoleucine (JA-Ile) and IAA were lower in imbibed seeds of Cvi compared with Col. In addition, changes in IAA and JA were initiated before 1 h, whereas ABA and JA-Ile declined 3 h after the onset of imbibition. An increase in GA(4) and iP appeared to be correlated temporally with the initiation of secondary water uptake, which marks the completion of germination.

Functional analysis of the isoforms of an ABI3-like factor of Pisum sativum generated by alternative splicing

At least seven isoforms (PsABI3-1 to PsABI3-7) of a putative, pea ABI3-like factor, originated by alternative splicing, have been identified after cDNA cloning. A similar variability had previously only been described for monocot genes. The full-length isoform, PsABI3-1, contains the typical N-terminal acidic domains and C-terminal basic subdomains, B1 to B3. Reverse transcriptase-PCR analysis revealed that the gene is expressed just in seeds, starting at middle embryogenesis; no gene products are observed in embryo axes after 18 h post-imbibition although they are more persistent in cotyledons. The activity of the isoforms was studied by yeast one-hybrid assays. When yeast was transformed with the isoforms fused to the DNA binding domain of Gal4p, only the polypeptides PsABI3-2 and PsABI3-7 failed to complement the activity of Gal4p. Acidic domains A1 and A2 exhibit transactivating activity, but the former requires a small C-terminal extension to be active. Yeast two-hybrid analysis showed that PsABI3 is able to heterodimerize with Arabidopsis thaliana ABI5, thus proving that PsABI3 is functionally active. The minimum requirement for the interaction PsABI3-AtABI5 is the presence of the subdomain B1 with an extension, 81 amino acids long, at their C-terminal side. Finally, a transient onion transformation assay showed that both the active PsABI3-1 and the inactive PsABI3-2 isoforms are localized to nuclei. Considering that the major isoforms remain approximately constant in developing seeds although their relative proportion varied, the possible role of splicing in the regulatory network of ABA signalling is discussed.

Abscisic acid and the pre-harvest sprouting in cereals

Pre-harvest sprouting (PHS) leads to loss of grain weight and a reduction in the end use quality of kernels in cereals, especially in wheat, and PHS in rice also becomes a more and more serious problem recent years. Many factors are involved in the controlling this complex trait. Only recently, we have reported the large scale screening and charactersation of the rice phs mutants, providing insight into the molecular mechanism of pre-harvest sprouting in rice. It has been shown that mutations of genes in synthesis of the carotenoid precursors of ABA resulted in the pre-harvest sprouting, which is consequence of ABA deficiency, and photobleaching is likewise due to the absence of photoprotective carotenoids. The further study of all different rice phs mutants will help us to elucidate the complex phenomena and finally capture the target for improving PHS in rice or other cereals.

Structural and functional properties of Viviparous1 genes in dormant wheat

Viviparous 1 (Vp1) of maize is known to encode a transcription factor VP1 that controls seed germination. Hexaploid wheat possesses three Vp1 homoeologues (TaVp1): TaVp-A1, TaVp-B1 and TaVp-D1. In this study, we attempted to characterize the molecular properties of TaVp1 in a highly dormant wheat cultivar, Minamino-komugi (Minamino). The seeds of Minamino showed much higher sensitivity to the inhibitory effect of ABA on germination than those of non-dormant cultivars, Sanin-1 and Tozan-18. The sequence analyses of cDNAs also revealed that some of TaVp-A1 transcripts and TaVp-D1 transcripts were spliced incorrectly, presumably resulting in production of truncated or deleted proteins. Most TaVp-B1 transcripts were spliced correctly, but some had an additional 3-bp (AAG) insertion in the B3 domain, which may not affect their function. RT-PCR analyses showed that TaVp1 was highly expressed in Minamino embryos in maturing seeds but much less in roots and leaves of seedlings. The level of TaVp1 mRNA was high when the embryos were treated with ABA but markedly decreased in water-imbibed mature embryos whose dormancy had been broken. Expression analyses of the individual homoeologues showed that the level of TaVp-A1 transcripts was highest in embryos of DAP 20 but much lower in the matured embryos. TaVp-B1 was highly expressed in developing and maturing seed embryos, while TaVp-D1 mRNA existed at lower levels in developing embryos but increased as the seeds were matured. These results suggest that the majority of TaVp1, especially TaVp-B1, are properly spliced and may function as a transcription factor playing an important role on dormancy in Minamino. By employing an efficient transient expression system using diploid wheat seeds, we confirmed the dual function of TaVP-B1: the activation of Em expression and the repression of alpha-amylase expression.

Interaction between sugar and abscisic acid signalling during early seedling development in Arabidopsis

Sugars regulate important processes and affect the expression of many genes in plants. Characterization of Arabidopsis (Arabidopsis thaliana) mutants with altered sugar sensitivity revealed the function of abscisic acid (ABA) signalling in sugar responses. However, the exact interaction between sugar signalling and ABA is obscure. Therefore ABA deficient plants with constitutive ABI4 expression (aba2-1/35S::ABI4) were generated. Enhanced ABI4 expression did not rescue the glucose insensitive (gin) phenotype of aba2 seedlings indicating that other ABA regulated factors are essential as well. Interestingly, both glucose and ABA treatment of Arabidopsis seeds trigger a post-germination seedling developmental arrest. The glucose-arrested seedlings had a drought tolerant phenotype and showed glucose-induced expression of ABSCISIC ACID INSENSITIVE3 (ABI3), ABI5 and LATE EMBRYOGENESIS ABUNDANT (LEA) genes reminiscent of ABA signalling during early seedling development. ABI3 is a key regulator of the ABA-induced arrest and it is shown here that ABI3 functions in glucose signalling as well. Multiple abi3 alleles have a glucose insensitive (gin) phenotype comparable to that of other known gin mutants. Importantly, glucose-regulated gene expression is disturbed in the abi3 background. Moreover, abi3 was insensitive to sugars during germination and showed sugar insensitive (sis) and sucrose uncoupled (sun) phenotypes. Mutant analysis further identified the ABA response pathway genes ENHANCED RESPONSE TO ABA1 (ERA1) and ABI2 as intermediates in glucose signalling. Hence, three previously unidentified sugar signalling genes have been identified, showing that ABA and glucose signalling overlap to a larger extend than originally thought.

Transcriptome analysis of grain development in hexaploid wheat

Background

Hexaploid wheat is one of the most important cereal crops for human nutrition. Molecular understanding of the biology of the developing grain will assist the improvement of yield and quality traits for different environments. High quality transcriptomics is a powerful method to increase this understanding.

Results

The transcriptome of developing caryopses from hexaploid wheat (Triticum aestivum, cv. Hereward) was determined using Affymetrix wheat GeneChip^® oligonucleotide arrays which have probes for 55,052 transcripts. Of these, 14,550 showed significant differential regulation in the period between 6 and 42 days after anthesis (daa). Large changes in transcript abundance were observed which were categorised into distinct phases of differentiation (6–10 daa), grain fill (12–21 daa) and desiccation/maturation (28–42 daa) and were associated with specific tissues and processes. A similar experiment on developing caryopses grown with dry and/or hot environmental treatments was also analysed, using the profiles established in the first experiment to show that most environmental treatment effects on transcription were due to acceleration of development, but that a few transcripts were specifically affected. Transcript abundance profiles in both experiments for nine selected known and putative wheat transcription factors were independently confirmed by real time RT-PCR. These expression profiles confirm or extend our knowledge of the roles of the known transcription factors and suggest roles for the unknown ones.

Conclusion

This transcriptome data will provide a valuable resource for molecular studies on wheat grain. It has been demonstrated how it can be used to distinguish general developmental shifts from specific effects of treatments on gene expression and to diagnose the probable tissue specificity and role of transcription factors.