Molecular and epigenetic regulations and functions of the LAFL transcriptional regulators that control seed development

Analysis of Coix Seed Oil Biosynthesis Facilitates the Identification of Lysophosphatidic Acid Acyltransferase

Coix seed oil (CSO) is a natural substance with significant anticancer potential. However, the molecular mechanism and the gene regulatory network of lipid biosynthesis were not identified in Coix seed. Here, a comprehensive transcriptome analysis was conducted on two Coix varieties with different lipid contents. The results revealed that a total of 48,110 genes were generated by de novo assembly, of which 84.45% genes were successfully annotated by the database. Based on functional annotation and gene expression, the metabolic network of crucial genes for oil accumulation and fatty acid (FA) synthesis in Coix seed has been successfully established, and it has been confirmed that lysophosphatidic acid acyltransferase (LPAT) in triacylglycerol (TAG) biosynthesis contributed to the changes in the content of FAs and had a significant selective binding ability to unsaturated fatty acids (UFAs) by the heterologous expression of yeast. Our data provides valuable references for subsequent gene function characterization and biosynthesis pathway optimization.

Regulation of seed germination: ROS, epigenetic, and hormonal aspects

BACKGROUND

The whole life of a plant is regulated by complex environmental or hormonal signaling networks that control genomic stability, environmental signal transduction, and gene expression affecting plant development and viability. Seed germination, responsible for the transformation from seed to seedling, is a key initiation step in plant growth and is controlled by unique physiological and biochemical processes. It is continuously modulated by various factors including epigenetic modifications, hormone transport, ROS signaling, and interaction among them. ROS showed versatile crucial functions in seed germination including various physiological oxidations to nucleic acid, protein, lipid, or chromatin in the cytoplasm, cell wall, and nucleus.

AIM

of review: This review intends to provide novel insights into underlying mechanisms of seed germination especially associated with the ROS, and considers how these versatile regulatory mechanisms can be developed as useful tools for crop improvement.

KEY SCIENTIFIC CONCEPTS OF REVIEW

We have summarized the generation and elimination of ROS during seed germination, with a specific focus on uncovering and understanding the mechanisms of seed germination at the level of phytohormones, ROS, and epigenetic switches, as well as the close connections between them. The findings exhibit that ROS plays multiple roles in regulating the ethylene, ABA, and GA homeostasis as well as the Ca2+ signaling, NO signaling, and MAPK cascade in seed germination via either the signal trigger or the oxidative modifier agent. Further, ROS shows the potential in the nuclear genome remodeling and some epigenetic modifiers function, although the detailed mechanisms are unclear in seed germination. We propose that ROS functions as a hub in the complex network regulating seed germination.

Transcriptional regulation of transcription factor genes WRI1 and LAFL during Brassica napus seed development

The WRINKLED1 (WRI1) and LAFL (LEAFY COTYLEDON1 [LEC1], ABSCISIC ACID INSENSITIVE3 [ABI3], FUSCA3 [FUS3], and LEC2) transcription factors play essential roles in governing seed development and oil biosynthesis. To gain a comprehensive understanding of the transcriptional regulation of WRI1 and LAFL, we conducted genome-wide association studies for the expression profiles of WRI1 and LAFL in developing seeds at 20 and 40 days after flowering (DAF) using 302 rapeseed (Brassica napus) accessions. We identified a total of 237 expression quantitative trait nucleotides (eQTNs) and 51 expression QTN-by-environment interactions (eQEIs) associated with WRI1 and LAFL. Around these eQTNs and eQEIs, we pinpointed 41 and 8 candidate genes with known transcriptional regulations or protein interactions with their expression traits, respectively. Based on RNA-sequencing and assay for transposase-accessible chromatin with high-throughput sequencing data, we employed the Extreme Gradient Boosting and Basenji models which predicted 15 candidate genes potentially regulating the expression of WRI1 and LAFL. We further validated the predictions via tissue expression profile, haplotype analysis, and expression correlation analysis and verified the transcriptional activation activity of BnaC03.MYB56 (R2R3-MYB transcription factor 56) on the expression of BnaA09.LEC1 by dual-luciferase reporter and yeast 1-hybrid assays. BnaA10.AGL15 (AGAMOUS-LIKE 15), BnaC04.VAL1 (VIVIPAROUS1/ABSCISIC ACID INSENSITIVE3-LIKE 1), BnaC03.MYB56, and BnaA10.MYB56 were coexpressed with WRI1 and LAFL at 20 DAF in M35, a key module for seed development and oil biosynthesis. We further validated the positive regulation of MYB56 on seed oil accumulation using Arabidopsis (Arabidopsis thaliana) mutants. This study not only delivers a framework for future eQEI identification but also offers insights into the developmental regulation of seed oil accumulation.

Critical radicle length window governing loss of dehydration tolerance in germinated Perilla seeds: insights from physiological and transcriptomic analyses

BACKGROUND

Perilla (Perilla frutescens L. Britt.) is an important oilseed and medicinal crop that frequently faces seasonal drought stress during seed germination, leading to a loss of dehydration tolerance (DT), which affects seed emergence and significantly reduces yield. DT has been successfully re-established for many species seeds. However, the physiological mechanisms and gene networks that regulate Perilla's response to DT loss remain unclear.

RESULTS

Phenotypic analysis determined that the window for DT in Perilla seeds occurs at radicle lengths of 0-4 mm. Integrating physiological and transcriptomic analyses revealed that the loss of DT promotes the production of reactive oxygen species (ROS) and regulates oxidase activity and gene expression. This implies that DT may influence seed germination by modulating ROS activity. Four radicle length (i.e., 0, 1, 3, and 4 mm) stages were analyzed, and 262 differentially expressed genes (DEGs) were identified that responded to DT. The majority of these genes were associated with epigenetics, cell function, and transport mechanisms. Analysis of expression data shows that desiccation inhibits the signaling network of genes encoding small secreted peptides (SSPs) and receptor-like protein kinases (RLKs). Finally, a relevant network diagram of DT response was proposed. Based on this information, we have revealed the metabolism regulation maps of the four main pathways involving these DEGs (i.e., metabolic pathways, cell cycle, plant hormone signal transduction, and motor proteins).

CONCLUSIONS

In conclusion, these findings deepen our understanding of gene network responses to DT during Perilla seed germination and provide potential target genes for the genetic improvement of drought resistance in this crop.

Decoding histone 3 lysine methylation: Insights into seed germination and flowering

Histone lysine methylation is a highly conserved epigenetic modification across eukaryotes that contributes to creating different dynamic chromatin states, which may result in transcriptional changes. Over the years, an accumulated set of evidence has shown that histone methylation allows plants to align their development with their surroundings, enabling them to respond and memorize past events due to changes in the environment. In this review, we discuss the molecular mechanisms of histone methylation in plants. Writers, readers, and erasers of Arabidopsis histone methylation marks are described with an emphasis on their role in two of the most important developmental transition phases in plants, seed germination and flowering. Further, the crosstalk between different methylation marks is also discussed. An overview of the mechanisms of histone methylation modifications and their biological outcomes will shed light on existing research gaps and may provide novel perspectives to increase crop yield and resistance in the era of global climate change.

Voice from both sides: a molecular dialogue between transcriptional activators and repressors in seed-to-seedling transition and crop adaptation

High-quality seeds provide valuable nutrients to human society and ensure successful seedling establishment. During maturation, seeds accumulate storage compounds that are required to sustain seedling growth during germination. This review focuses on the epigenetic repression of the embryonic and seed maturation programs in seedlings. We begin with an extensive overview of mutants affecting these processes, illustrating the roles of core proteins and accessory components in the epigenetic machinery by comparing mutants at both phenotypic and molecular levels. We highlight how omics assays help uncover target-specific functional specialization and coordination among various epigenetic mechanisms. Furthermore, we provide an in-depth discussion on the Seed dormancy 4 (Sdr4) transcriptional corepressor family, comparing and contrasting their regulation of seed germination in the dicotyledonous species Arabidopsis and two monocotyledonous crops, rice and wheat. Finally, we compare the similarities in the activation and repression of the embryonic and seed maturation programs through a shared set of cis-regulatory elements and discuss the challenges in applying knowledge largely gained in model species to crops.

Appreciating animal induced pluripotent stem cells to shape plant cell reprogramming strategies

Animals and plants have developed resilience mechanisms to effectively endure and overcome physical damage and environmental challenges throughout their life span. To sustain their vitality, both animals and plants employ mechanisms to replenish damaged cells, either directly, involving the activity of adult stem cells, or indirectly, via dedifferentiation of somatic cells that are induced to revert to a stem cell state and subsequently redifferentiate. Stem cell research has been a rapidly advancing field in animal studies for many years, driven by its promising potential in human therapeutics, including tissue regeneration and drug development. A major breakthrough was the discovery of induced pluripotent stem cells (iPSCs), which are reprogrammed from somatic cells by expressing a limited set of transcription factors. This discovery enabled the generation of an unlimited supply of cells that can be differentiated into specific cell types and tissues. Equally, a keen interest in the connection between plant stem cells and regeneration has been developed in the last decade, driven by the demand to enhance plant traits such as yield, resistance to pathogens, and the opportunities provided by CRISPR/Cas-mediated gene editing. Here we discuss how knowledge of stem cell biology benefits regeneration technology, and we speculate on the creation of a universal genotype-independent iPSC system for plants to overcome regenerative recalcitrance.

LAFL Factors in Seed Development and Phase Transitions

Development is a chain reaction in which one event leads to another until the completion of a life cycle. Phase transitions are milestone events in the cycle of life. LEAFY COTYLEDON1 (LEC1), ABA INSENSITIVE3 (ABI3), FUSCA3 (FUS3), and LEC2 proteins, collectively known as LAFL, are master transcription factors (TFs) regulating seed and other developmental processes. Since the initial characterization of the LAFL genes, more than three decades of active research has generated tremendous amounts of knowledge about these TFs, whose roles in seed development and germination have been comprehensively reviewed. Recent advances in cell biology with genetic and genomic tools have allowed the characterization of the LAFL regulatory networks in previously challenging tissues at a higher throughput and resolution in reference species and crops. In this review, we provide a holistic perspective by integrating advances at the epigenetic, transcriptional, posttranscriptional, and protein levels to exemplify the spatiotemporal regulation of the LAFL networks in Arabidopsis seed development and phase transitions, and we briefly discuss the evolution of these TF networks.

Somatic embryogenesis from mature sorghum seeds: An underutilized genome editing recipient system

Somatic embryogenesis is a process of cell totipotency in vitro, whereby an embryogenic cell develops from vegetative tissues rather than from zygotes after double fertilization. Sorghum is a recalcitrant crop in genetic transformation; previous recipient systems have usually been from immature zygotic embryos, which needed more time and labors to prepare. Here, an efficient 2,4-dichlorophenoxyacetic acid (2,4-D)-induced somatic embryogenesis system from mature sorghum seeds was introduced. 2,4-D can induce two types of calli from a plumular axis section. Low-concentration 2,4-D (e.g., 2 mg/L) induces white and loose non-embryogenic calli (type 1), while high-concentration 2,4-D (e.g., 8 mg/L) induces yellow and compact embryogenic calli (type 2), which can be clearly distinguished by Sudan red staining. Germinating seeds have a long 2-day window for SE induction. Somatic embryogenesis can be enhanced by HDAC inhibitor, trichostatin A (TSA), a histone deacetylase treatment, which shows more SE productivity and a bigger size. Importantly, this easily prepared protocol does not show obvious genotype dependency in sorghum hybrids. In this study, a high-concentration 2,4-D-induced SE system was established from mature sorghum seeds. This finding provides a technical option for the genome editing recipient in sorghum.

Involvement of Abscisic Acid in Transition of Pea (Pisum sativum L.) Seeds from Germination to Post-Germination Stages

The transition from seed to seedling represents a critical developmental step in the life cycle of higher plants, dramatically affecting plant ontogenesis and stress tolerance. The release from dormancy to acquiring germination ability is defined by a balance of phytohormones, with the substantial contribution of abscisic acid (ABA), which inhibits germination. We studied the embryonic axis of Pisum sativum L. before and after radicle protrusion. Our previous work compared RNA sequencing-based transcriptomics in the embryonic axis isolated before and after radicle protrusion. The current study aims to analyze ABA-dependent gene regulation during the transition of the embryonic axis from the germination to post-germination stages. First, we determined the levels of abscisates (ABA, phaseic acid, dihydrophaseic acid, and neo-phaseic acid) using ultra-high-performance liquid chromatography-tandem mass spectrometry. Second, we made a detailed annotation of ABA-associated genes using RNA sequencing-based transcriptome profiling. Finally, we analyzed the DNA methylation patterns in the promoters of the PsABI3, PsABI4, and PsABI5 genes. We showed that changes in the abscisate profile are characterized by the accumulation of ABA catabolites, and the ABA-related gene profile is accompanied by the upregulation of genes controlling seedling development and the downregulation of genes controlling water deprivation. The expression of ABI3, ABI4, and ABI5, which encode crucial transcription factors during late maturation, was downregulated by more than 20-fold, and their promoters exhibited high levels of methylation already at the late germination stage. Thus, although ABA remains important, other regulators seems to be involved in the transition from seed to seedling.

Genetic, Epigenetic, and Environmental Control of Seed Dormancy and Germination

Seed germination is controlled by a combination of the seed dormancy level and environmental conditions such as light, temperature, moisture, and nitrate levels. Seed dormancy is programed genetically, but it is also sensitive to maternal environmental conditions before and after anthesis. Recent developments in molecular genetics and bioinformatics have greatly enhanced our understanding of the molecular mechanisms of seed dormancy and germination in model plants and economically important crop species. This chapter focuses on temperature as an environmental factor and discusses the genetic and epigenetic mechanisms of dormancy and germination.

Seed Longevity and Ageing: A Review on Physiological and Genetic Factors with an Emphasis on Hormonal Regulation

Upon storage, seeds inevitably age and lose their viability over time, which determines their longevity. Longevity correlates with successful seed germination and enhancing this trait is of fundamental importance for long-term seed storage (germplasm conservation) and crop improvement. Seed longevity is governed by a complex interplay between genetic factors and environmental conditions experienced during seed development and after-ripening that will shape seed physiology. Several factors have been associated with seed ageing such as oxidative stress responses, DNA repair enzymes, and composition of seed layers. Phytohormones, mainly abscisic acid, auxins, and gibberellins, have also emerged as prominent endogenous regulators of seed longevity, and their study has provided new regulators of longevity. Gaining a thorough understanding of how hormonal signalling genes and pathways are integrated with downstream mechanisms related to seed longevity is essential for formulating strategies aimed at preserving seed quality and viability. A relevant aspect related to research in seed longevity is the existence of significant differences between results depending on the seed equilibrium relative humidity conditions used to study seed ageing. Hence, this review delves into the genetic, environmental and experimental factors affecting seed ageing and longevity, with a particular focus on their hormonal regulation. We also provide gene network models underlying hormone signalling aimed to help visualize their integration into seed longevity and ageing. We believe that the format used to present the information bolsters its value as a resource to support seed longevity research for seed conservation and crop improvement.

Interaction between phenylpropane metabolism and oil accumulation in the developing seed of Brassica napus revealed by high temporal-resolution transcriptomes

BACKGROUND

Brassica napus is an important oilseed crop providing high-quality vegetable oils for human consumption and non-food applications. However, the regulation between embryo and seed coat for the synthesis of oil and phenylpropanoid compounds remains largely unclear.

RESULTS

Here, we analyzed the transcriptomes in developing seeds at 2-day intervals from 14 days after flowering (DAF) to 64 DAF. The 26 high-resolution time-course transcriptomes are clearly clustered into five distinct groups from stage I to stage V. A total of 2217 genes including 136 transcription factors, are specifically expressed in the seed and show high temporal specificity by being expressed only at certain stages of seed development. Furthermore, we analyzed the co-expression networks during seed development, which mainly included master regulatory transcription factors, lipid, and phenylpropane metabolism genes. The results show that the phenylpropane pathway is prominent during seed development, and the key enzymes in the phenylpropane metabolic pathway, including TT5, BAN, and the transporter TT19, were directly or indirectly related to many key enzymes and transcription factors involved in oil accumulation. We identified candidate genes that may regulate seed oil content based on the co-expression network analysis combined with correlation analysis of the gene expression with seed oil content and seed coat content.

CONCLUSIONS

Overall, these results reveal the transcriptional regulation between lipid and phenylpropane accumulation during B. napus seed development. The established co-expression networks and predicted key factors provide important resources for future studies to reveal the genetic control of oil accumulation in B. napus seeds.

Regulatory Effects of ABA and GA on the Expression of Conglutin Genes and LAFL Network Genes in Yellow Lupine (Lupinus luteus L.) Seeds

The maturation of seeds is a process of particular importance both for the plant itself by assuring the survival of the species and for the human population for nutritional and economic reasons. Controlling this process requires a strict coordination of many factors at different levels of the functioning of genetic and hormonal changes as well as cellular organization. One of the most important examples is the transcriptional activity of the LAFL gene regulatory network, which includes LEAFY COTYLEDON1 (LEC1) and LEC1-LIKE (L1L) and ABSCISIC ACID INSENSITIVE3 (ABI3), FUSCA3 (FUS3), and LEC2 (LEAFY COTYLEDON2), as well as hormonal homeostasis-of abscisic acid (ABA) and gibberellins (GA) in particular. From the nutritional point of view, the key to seed development is the ability of seeds to accumulate large amounts of proteins with different structures and properties. The world's food deficit is mainly related to shortages of protein, and taking into consideration the environmental changes occurring on Earth, it is becoming necessary to search for a way to obtain large amounts of plant-derived protein while maintaining the diversity of its origin. Yellow lupin, whose storage proteins are conglutins, is one of the plant species native to Europe that accumulates large amounts of this nutrient in its seeds. In this article we have shown the key changes occurring in the developing seeds of the yellow-lupin cultivar Taper by means of modern molecular biology techniques, including RNA-seq, chromatographic techniques and quantitative PCR analysis. We identified regulatory genes fundamental to the seed-filling process, as well as genes encoding conglutins. We also investigated how exogenous application of ABA and GA3 affects the expression of LlLEC2, LlABI3, LlFUS3, and genes encoding β- and δ-conglutins and whether it results in the amount of accumulated seed storage proteins. The research shows that for each species, even related plants, very specific changes can be identified. Thus the analysis and possibility of using such an approach to improve and stabilize yields requires even more detailed and extended research.

iTRAQ-Based Proteomic and Physiological Analyses Reveal the Mechanisms of Dehydration and Cryopreservation Tolerance of Sophora tonkinensis Gagnep. Seeds

Sophora tonkinensi is a shrub of the genus Sophora in the family Fabaceae with anti-inflammatory and pain-relieving effects. While the cultivation, chemical makeup, and medicinal properties of S. tonkinensis have been reported, the physiological mechanisms governing its dehydration and cryopreservation tolerance of seeds remain unclear. In this study, we investigated the morphological, physiological, biochemical, and protein expression characteristics of S. tonkinensis seeds subjected to dehydration and cryopreservation techniques via the observation of cell microstructure, determination of antioxidant enzyme activity, and iTRAQ-based proteomic analysis, respectively. The results of the study demonstrated that the seeds possessed a certain level of tolerance to dehydration. The highest germination percentage of 83.0% was observed after 2 h of dehydration (10.1% water content), which was identified as the optimal time point for cryopreservation. However, the germination percentage was reduced to only 30.5% when the water content reached 5.4%, indicating that S. tonkinensis seeds exhibit intermediate storage behavior. Further investigation revealed that during seed dehydration and cryopreservation treatment, liposomes were gradually and highly fused, whereas the activities of ROS scavenging and stress defense were significantly enhanced. During dehydration, the seed tissues formed a protective mechanism of stress resistance based on protein processing in the endoplasmic reticulum and antioxidant system, which was related to the dehydration tolerance. Moreover, only three differentially expressed LEA proteins were identified, and it is speculated that the strengthening of intracellular metabolism and the absence of specific LEA and dehydrins could be crucial factors for the reduced germination percentage after excessive dehydration and cryopreservation.

Recent progress in molecular genetics and omics-driven research in seed biology

Elucidating the mechanisms that control seed development, metabolism, and physiology is a fundamental issue in biology. Michel Caboche had long been a catalyst for seed biology research in France up until his untimely passing away last year. To honour his memory, we have updated a review written under his coordination in 2010 entitled "Arabidopsis seed secrets unravelled after a decade of genetic and omics-driven research". This review encompassed different molecular aspects of seed development, reserve accumulation, dormancy and germination, that are studied in the lab created by M. Caboche. We have extended the scope of this review to highlight original experimental approaches implemented in the field over the past decade such as omics approaches aimed at investigating the control of gene expression, protein modifications, primary and specialized metabolites at the tissue or even cellular level, as well as seed biodiversity and the impact of the environment on seed quality.

Transcriptomic comparison of seeds and silique walls from two rapeseed genotypes with contrasting seed oil content

Silique walls play pivotal roles in contributing photoassimilates and nutrients to fuel seed growth. However, the interaction between seeds and silique walls impacting oil biosynthesis is not clear during silique development. Changes in sugar, fatty acid and gene expression during Brassica napus silique development of L192 with high oil content and A260 with low oil content were investigated to identify key factors affecting difference of their seed oil content. During the silique development, silique walls contained more hexose and less sucrose than seeds, and glucose and fructose contents in seeds and silique walls of L192 were higher than that of A260 at 15 DAF, and sucrose content in the silique walls of L192 were lower than that of A260 at three time points. Genes related to fatty acid biosynthesis were activated over time, and differences on fatty acid content between the two genotypes occurred after 25 DAF. Genes related to photosynthesis expressed more highly in silique walls than in contemporaneous seeds, and were inhibited over time. Gene set enrichment analysis suggested photosynthesis were activated in L192 at 25 and 35 DAF in silique walls and at both 15 and 35 DAF in the seed. Expressions of sugar transporter genes in L192 was higher than that in A260, especially at 35 DAF. Expressions of genes related to fatty acid biosynthesis, such as BCCP2s, bZIP67 and LEC1s were higher in L192 than in A260, especially at 35 DAF. Meanwhile, genes related to oil body proteins were expressed at much lower levels in L192 than in A260. According to the WGCNA results, hub modules, such as ME.turquoise relative to photosynthesis, ME.green relative to embryo development and ME.yellow relative to lipid biosynthesis, were identified and synergistically regulated seed development and oil accumulation. Our results are helpful for understanding the mechanism of oil accumulation of seeds in oilseed rape for seed oil content improvement.

Regulation of seed storage protein synthesis in monocot and dicot plants: A comparative review

Seeds are a major source of nutrients for humans and animal livestock worldwide. With improved living standards, high nutritional quality has become one of the main targets for breeding. Storage protein content in seeds, which is highly variable depending on plant species, serves as a pivotal criterion of seed nutritional quality. In the last few decades, our understanding of the molecular genetics and regulatory mechanisms of storage protein synthesis has greatly advanced. Here, we systematically and comprehensively summarize breakthroughs on the conservation and divergence of storage protein synthesis in dicot and monocot plants. With regard to storage protein accumulation, we discuss evolutionary origins, developmental processes, characteristics of main storage protein fractions, regulatory networks, and genetic modifications. In addition, we discuss potential breeding strategies to improve storage protein accumulation and provide perspectives on some key unanswered problems that need to be addressed.

Genome-wide analysis of CtNF-YB and lipid synthesis regulation of CtNF-YB12 in Carthamus tinctorius L

The nuclear Factor YB of Carthamus tinctorius L. increased the content of unsaturated fatty acids by regulating the expression of genes involved in fatty acid synthesis and oil accumulation. Safflower (Carthamus tinctorius L.) seed oil is rich in linoleic acid and is widely used in food and medicine. Therefore, key genes regulating oil synthesis were mined through genetic engineering to provide genetic resources for improving oil content. Based on the conserved domain of the NF-YB, we screened and identified 14 CtNF-YB transcription factors in the safflower genome and divided them into three subfamilies through phylogenetic analysis. Regulatory motif analysis of the CtNF-YB promoter revealed specific cis-regulatory elements related to abiotic stress, growth, and development. Expression analysis of CtNF-YB family genes showed that non-Leafy Cotyledon 1(non-LEC1) genes were highly expressed in roots, leaves, and flowers; Leafy Cotyledon 1(LEC1) genes were highly expressed during early seed development; and Dr1-like genes were highly expressed in roots, stems, and leaves. CtNF-YB12 was identified as a LEC1 transcription factor based on phylogeny and BLAST alignment. Heterologous CtNF-YB12 expression in Arabidopsis thaliana increased seed pod length and seed size. Moreover, CtNF-YB12 overexpression increased the oil content of seeds, upregulated genes involved in fatty acid biosynthesis and glycolysis, and altered the content of unsaturated fatty acids, including oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3), as well as of sucrose, fructose, and glucose. CtNF-YB12 may increase the oil content by regulating key enzyme genes of oil synthesis, so it can be used as a reliable target.

Chromatin dynamics associated with seed desiccation tolerance/sensitivity at early germination in Medicago truncatula

Desiccation tolerance (DT) has contributed greatly to the adaptation of land plants to severe water-deficient conditions. DT is mostly observed in reproductive parts in flowering plants such as seeds. The seed DT is lost at early post germination stage but is temporally re-inducible in 1 mm radicles during the so-called DT window following a PEG treatment before being permanently silenced in 5 mm radicles of germinating seeds. The molecular mechanisms that activate/reactivate/silence DT in developing and germinating seeds have not yet been elucidated. Here, we analyzed chromatin dynamics related to re-inducibility of DT before and after the DT window at early germination in Medicago truncatula radicles to determine if DT-associated genes were transcriptionally regulated at the chromatin levels. Comparative transcriptome analysis of these radicles identified 948 genes as DT re-induction-related genes, positively correlated with DT re-induction. ATAC-Seq analyses revealed that the chromatin state of genomic regions containing these genes was clearly modulated by PEG treatment and affected by growth stages with opened chromatin in 1 mm radicles with PEG (R1P); intermediate openness in 1 mm radicles without PEG (R1); and condensed chromatin in 5 mm radicles without PEG (R5). In contrast, we also showed that the 103 genes negatively correlated with the re-induction of DT did not show any transcriptional regulation at the chromatin level. Additionally, ChIP-Seq analyses for repressive marks H2AK119ub and H3K27me3 detected a prominent signal of H3K27me3 on the DT re-induction-related gene sequences at R5 but not in R1 and R1P. Moreover, no clear H2AK119ub marks was observed on the DT re-induction-related gene sequences at both developmental radicle stages, suggesting that silencing of DT process after germination will be mainly due to H3K27me3 marks by the action of the PRC2 complex, without involvement of PRC1 complex. The dynamic of chromatin changes associated with H3K27me3 were also confirmed on seed-specific genes encoding potential DT-related proteins such as LEAs, oleosins and transcriptional factors. However, several transcriptional factors did not show a clear link between their decrease of chromatin openness and H3K27me3 levels, suggesting that their accessibility may also be regulated by additional factors, such as other histone modifications. Finally, in order to make these comprehensive genome-wide analyses of transcript and chromatin dynamics useful to the scientific community working on early germination and DT, we generated a dedicated genome browser containing all these data and publicly available at https://iris.angers.inrae.fr/mtseedepiatlas/jbrowse/?data=Mtruncatula.

Cell-penetrating peptide: A powerful delivery tool for DNA-free crop genome editing

Genome editing system based on the CRISPR/Cas (clustered regularly interspaced short palindromic repeats) technology is a milestone for biology. However, public concerns regarding genetically modified organisms (GMOs) and recalcitrance in the crop of choice for regeneration have limited its application. Cell-penetrating peptides (CPPs) are derived from protein transduction domains (PTDs) that can take on various cargoes across the plant wall, and membrane of target cells. Selected CPPs show mild cytotoxicity and are a suitable delivery tool for DNA-free genome editing. Moreover, CPPs may also be applied for the transient delivery of morphogenic transcription factors, also known as developmental regulators (DRs), to overcome the bottleneck of the crop of choice regeneration. In this review, we introduce a brief history of cell-penetrating peptides and discuss the practice of CPP-mediated DNA-free transfection and the prospects of this potential delivery tool for improving crop genome editing.

Seed dormancy 4 like1 of Arabidopsis is a key regulator of phase transition from embryo to vegetative development

Seed dormancy is an adaptive trait that enables plants to survive adverse conditions and restart growth in a season and location suitable for vegetative and reproductive growth. Control of seed dormancy is also important for crop production and food quality because it can help induce uniform germination and prevent preharvest sprouting. Rice preharvest sprouting quantitative trait locus analysis has identified Seed dormancy 4 (Sdr4) as a positive regulator of dormancy development. Here, we analyzed the loss-of-function mutant of the Arabidopsis ortholog, Sdr4 Like1 (SFL1), and found that the sfl1-1 seeds showed precocious germination at the mid- to late-maturation stage similar to rice sdr4 mutant, but converted to become more dormant than the wild type during maturation drying. Coordinated with the dormancy levels, expression levels of the seed maturation and dormancy master regulator genes, ABI3, FUS3, and DOG1 in sfl1-1 seeds were lower than in wild type at early- and mid-maturation stages, but higher at the late-maturation stage. In addition to the seed dormancy phenotype, sfl1-1 seedlings showed a growth arrest phenotype and heterochronic expression of LAFL (LEC1, ABI3, FUS3, LEC2) and DOG1 in the seedlings. These data suggest that SFL1 is a positive regulator of initiation and termination of the seed dormancy program. We also found genetic interaction between SFL1 and the SFL2, SFL3, and SFL4 paralogs of SFL1, which impacts on the timing of the phase transition from embryo maturation to seedling growth.

4D genetic networks reveal the genetic basis of metabolites and seed oil-related traits in 398 soybean RILs

Background

The yield and quality of soybean oil are determined by seed oil-related traits, and metabolites/lipids act as bridges between genes and traits. Although there are many studies on the mode of inheritance of metabolites or traits, studies on multi-dimensional genetic network (MDGN) are limited.

Results

In this study, six seed oil-related traits, 59 metabolites, and 107 lipids in 398 recombinant inbred lines, along with their candidate genes and miRNAs, were used to construct an MDGN in soybean. Around 175 quantitative trait loci (QTLs), 36 QTL-by-environment interactions, and 302 metabolic QTL clusters, 70 and 181 candidate genes, including 46 and 70 known homologs, were previously reported to be associated with the traits and metabolites, respectively. Gene regulatory networks were constructed using co-expression, protein–protein interaction, and transcription factor binding site and miRNA target predictions between candidate genes and 26 key miRNAs. Using modern statistical methods, 463 metabolite–lipid, 62 trait–metabolite, and 89 trait–lipid associations were found to be significant. Integrating these associations into the above networks, an MDGN was constructed, and 128 sub-networks were extracted. Among these sub-networks, the gene–trait or gene–metabolite relationships in 38 sub-networks were in agreement with previous studies, e.g., oleic acid (trait)–GmSEI–GmDGAT1a–triacylglycerol (16:0/18:2/18:3), gene and metabolite in each of 64 sub-networks were predicted to be in the same pathway, e.g., oleic acid (trait)–GmPHS–d-glucose, and others were new, e.g., triacylglycerol (16:0/18:1/18:2)–GmbZIP123–GmHD-ZIPIII-10–miR166s–oil content.

Conclusions

This study showed the advantages of MGDN in dissecting the genetic relationships between complex traits and metabolites. Using sub-networks in MGDN, 3D genetic sub-networks including pyruvate/threonine/citric acid revealed genetic relationships between carbohydrates, oil, and protein content, and 4D genetic sub-networks including PLDs revealed the relationships between oil-related traits and phospholipid metabolism likely influenced by the environment. This study will be helpful in soybean quality improvement and molecular biological research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02191-1.

An expanded role for the transcription factor WRINKLED1 in the biosynthesis of triacylglycerols during seed development

The transcription factor WRINKLED1 (WRI1) is known as a master regulator of fatty acid synthesis in developing oilseeds of Arabidopsis thaliana and other species. WRI1 is known to directly stimulate the expression of many fatty acid biosynthetic enzymes and a few targets in the lower part of the glycolytic pathway. However, it remains unclear to what extent and how the conversion of sugars into fatty acid biosynthetic precursors is controlled by WRI1. To shortlist possible gene targets for future in-planta experimental validation, here we present a strategy that combines phylogenetic foot printing of cis-regulatory elements with additional layers of evidence. Upstream regions of protein-encoding genes in A. thaliana were searched for the previously described DNA-binding consensus for WRI1, the ASML1/WRI1 (AW)-box. For about 900 genes, AW-box sites were found to be conserved across orthologous upstream regions in 11 related species of the crucifer family. For 145 select potential target genes identified this way, affinity of upstream AW-box sequences to WRI1 was assayed by Microscale Thermophoresis. This allowed definition of a refined WRI1 DNA-binding consensus. We find that known WRI1 gene targets are predictable with good confidence when upstream AW-sites are phylogenetically conserved, specifically binding WRI1 in the in vitro assay, positioned in proximity to the transcriptional start site, and if the gene is co-expressed with WRI1 during seed development. When targets predicted in this way are mapped to central metabolism, a conserved regulatory blueprint emerges that infers concerted control of contiguous pathway sections in glycolysis and fatty acid biosynthesis by WRI1. Several of the newly predicted targets are in the upper glycolysis pathway and the pentose phosphate pathway. Of these, plastidic isoforms of fructokinase (FRK3) and of phosphoglucose isomerase (PGI1) are particularly corroborated by previously reported seed phenotypes of respective null mutations.

Seed-to-Seedling Transition: Novel Aspects

Transition from seed to seedling represents a critical stage in plants’ life cycles. This process includes three significant events in the seeds: (i) tissue hydration, (ii) the mobilization of reserve nutrients, and (iii) the activation of metabolic activity. Global metabolic rearrangements lead to the initiation of radicle growth and the resumption of vegetative development. It requires massive reprogramming of the transcriptome, proteome, metabolome, and attendant signaling pathways, resulting in the silencing of seed-maturation genes and the activation of vegetative growth genes. This Special Issue discusses the mechanisms of genetic, epigenetic, and hormonal switches during seed-to-seedling transitions. Detailed information has also been covered regarding the influence of germination features on seedling establishment.

SK, Sheoran S, K. UB, K. BN, Kumar S, Singh AN, Singh HV. Seed Longevity in Legumes: Deeper Insights Into Mechanisms and Molecular Perspectives

Sustainable agricultural production largely depends upon the viability and longevity of high-quality seeds during storage. Legumes are considered as rich source of dietary protein that helps to ensure nutritional security, but associated with poor seed longevity that hinders their performance and productivity in farmer's fields. Seed longevity is the key determinant to assure proper seed plant value and crop yield. Thus, maintenance of seed longevity during storage is of prime concern and a pre-requisite for enhancing crop productivity of legumes. Seed longevity is significantly correlated with other seed quality parameters such as germination, vigor, viability and seed coat permeability that affect crop growth and development, consequently distressing crop yield. Therefore, information on genetic basis and regulatory networks associated with seed longevity, as well as molecular dissection of traits linked to longevity could help in developing crop varieties with good storability. Keeping this in view, the present review focuses towards highlighting the molecular basis of seed longevity, with special emphasis on candidate genes and proteins associated with seed longevity and their interplay with other quality parameters. Further, an attempt was made to provide information on 3D structures of various genetic loci (genes/proteins) associated to seed longevity that could facilitate in understanding the interactions taking place within the seed at molecular level. This review compiles and provides information on genetic and genomic approaches for the identification of molecular pathways and key players involved in the maintenance of seed longevity in legumes, in a holistic manner. Finally, a hypothetical fast-forward breeding pipeline has been provided, that could assist the breeders to successfully develop varieties with improved seed longevity in legumes.

A lineage-specific arginine in POS1 is required for fruit size control in Physaleae (Solanaceae) via gene co-option

Solanaceae have important economic value mainly due to their edible fruits. Physalis organ size 1/cytokinin response factor 3 (POS1/CRF3), a unique gene in Solanaceae, is involved in fruit size variation in Physalis but not in Solanum. However, the underlying mechanisms remain elusive. Here, we found that POS1/CRF3 was likely created via the fusion of CRF7 and CRF8 duplicates. Multiple genetic manipulations revealed that only POS1 and Capsicum POS1 (CaPOS1) functioned in fruit size control via the positive regulation of cell expansion. Comparative studies in a phylogenetic framework showed the directional enhancement of POS1-like expression in the flowers and fruits of Physaleae and the specific gain of certain interacting proteins associated with cell expansion by POS1 and CaPOS1. A lineage-specific single nucleotide polymorphism (SNP) caused the 68th amino acid histidine in the POS1 orthologs of non-Physaleae (Nicotiana and Solanum) to change to arginine in Physaleae (Physalis and Capsicum). Substituting the arginine in Physaleae POS1-like by histidine completely abolished their function in the fruits and the protein-protein interaction (PPI) with calreticulin-3. Transcriptomic comparison revealed the potential downstream pathways of POS1, including the brassinosteroid biosynthesis pathway. However, POS1-like may have functioned ancestrally in abiotic stress within Solanaceae. Our work demonstrated that heterometric expression and a SNP caused a single amino acid change to establish new PPIs, which contributed to the co-option of POS1 in multiple regulatory pathways to regulate cell expansion and thus fruit size in Physaleae. These results provide new insights into fruit morphological evolution and fruit yield control.

New Insights Into Tissue Culture Plant-Regeneration Mechanisms

Plant regeneration occurs when plants repair or replace damaged structures based on the totipotency and pluripotency of their cells. Tissue culture is one of the most widely used regenerative technologies. Recently, a series of breakthroughs were made in the study of plant regeneration. This review summarizes two regenerative pathways in tissue culture: somatic embryogenesis and de novo organogenesis. Furthermore, we review the environmental factors influencing plant regeneration from explant sources, basal culture medium, plant growth regulators, and light/dark treatment. Additionally, we analyse the molecular mechanisms underlying two pathways. This knowledge will promote an understanding of the fundamental principles of plant regeneration from precursor cells and lay a solid foundation for applying plant micropropagation and genetic modification.

Seed-to-Seedling Transition in Pisum sativum L.: A Transcriptomic Approach

The seed-to-seedling transition is a crucial step in the plant life cycle. The transition occurs at the end of seed germination and corresponds to the initiation of embryonic root growth. To improve our understanding of how a seed transforms into a seedling, we germinated the Pisum sativum L. seeds for 72 h and divided them into samples before and after radicle protrusion. Before radicle protrusion, seeds survived after drying and formed normally developed seedlings upon rehydration. Radicle protrusion increased the moisture content level in seed axes, and the accumulation of ROS first generated in the embryonic root and plumule. The water and oxidative status shift correlated with the desiccation tolerance loss. Then, we compared RNA sequencing-based transcriptomics in the embryonic axes isolated from pea seeds before and after radicle protrusion. We identified 24,184 differentially expressed genes during the transition to the post-germination stage. Among them, 2101 genes showed more prominent expression. They were related to primary and secondary metabolism, photosynthesis, biosynthesis of cell wall components, redox status, and responses to biotic stress. On the other hand, 415 genes showed significantly decreased expression, including the groups related to water deprivation (eight genes) and response to the ABA stimulus (fifteen genes). We assume that the water deprivation group, especially three genes also belonging to ABA stimulus (LTI65, LTP4, and HVA22E), may be crucial for the desiccation tolerance loss during a metabolic switch from seed to seedling. The latter is also accompanied by the suppression of ABA-related transcription factors ABI3, ABI4, and ABI5. Among them, HVA22E, ABI4, and ABI5 were highly conservative in functional domains and showed homologous sequences in different drought-tolerant species. These findings elaborate on the critical biochemical pathways and genes regulating seed-to-seedling transition.

Embryonic reactivation of FLOWERING LOCUS C by ABSCISIC ACID-INSENSITIVE 3 establishes the vernalization requirement in each Arabidopsis generation

Many over-wintering plants grown in temperate climate acquire competence to flower upon prolonged cold exposure in winter, through vernalization. In Arabidopsis thaliana, prolonged cold exposure induces the silencing of the potent floral repressor FLOWERING LOCUS C (FLC) through repressive chromatin modifications by Polycomb proteins. This repression is maintained to enable flowering after return to warmth, but is reset during seed development. Here, we show that embryonic FLC reactivation occurs in two phases: resetting of cold-induced FLC silencing during embryogenesis and further FLC activation during embryo maturation. We found that the B3 transcription factor (TF) ABSCISIC ACID-INSENSITIVE 3 (ABI3) mediates both FLC resetting in embryogenesis and further activation of FLC expression in embryo maturation. ABI3 binds to the cis-acting cold memory element at FLC and recruits a scaffold protein with active chromatin modifiers to reset FLC chromatin into an active state in late embryogenesis. Moreover, in response to abscisic acid (ABA) accumulation during embryo maturation, ABI3, together with the basic leucine zipper TF ABI5, binds to an ABA-responsive cis-element to further activate FLC expression to high level. Therefore, we have uncovered the molecular circuitries underlying embryonic FLC reactivation following parental vernalization, which ensures that each generation must experience winter cold prior to flowering.

Machine-Learning-Based Genome-Wide Association Studies for Uncovering QTL Underlying Soybean Yield and Its Components

A genome-wide association study (GWAS) is currently one of the most recommended approaches for discovering marker-trait associations (MTAs) for complex traits in plant species. Insufficient statistical power is a limiting factor, especially in narrow genetic basis species, that conventional GWAS methods are suffering from. Using sophisticated mathematical methods such as machine learning (ML) algorithms may address this issue and advance the implication of this valuable genetic method in applied plant-breeding programs. In this study, we evaluated the potential use of two ML algorithms, support-vector machine (SVR) and random forest (RF), in a GWAS and compared them with two conventional methods of mixed linear models (MLM) and fixed and random model circulating probability unification (FarmCPU), for identifying MTAs for soybean-yield components. In this study, important soybean-yield component traits, including the number of reproductive nodes (RNP), non-reproductive nodes (NRNP), total nodes (NP), and total pods (PP) per plant along with yield and maturity, were assessed using a panel of 227 soybean genotypes evaluated at two locations over two years (four environments). Using the SVR-mediated GWAS method, we were able to discover MTAs colocalized with previously reported quantitative trait loci (QTL) with potential causal effects on the target traits, supported by the functional annotation of candidate gene analyses. This study demonstrated the potential benefit of using sophisticated mathematical approaches, such as SVR, in a GWAS to complement conventional GWAS methods for identifying MTAs that can improve the efficiency of genomic-based soybean-breeding programs.

Rice LEAFY COTYLEDON1 Hinders Embryo Greening During the Seed Development

LEAFY COTYLEDON1 (LEC1) is the central regulator of seed development in Arabidopsis, while its function in monocots is largely elusive. We generated Oslec1 mutants using CRISPR/Cas9 technology. Oslec1 mutant seeds lost desiccation tolerance and triggered embryo greening at the early development stage. Transcriptome analysis demonstrated that Oslec1 mutation altered diverse hormonal pathways and stress response in seed maturation, and promoted a series of photosynthesis-related genes. Further, genome-wide identification of OsLEC1-binding sites demonstrated that OsLEC1 bound to genes involved in photosynthesis, photomorphogenesis, as well as abscisic acid (ABA) and gibberellin (GA) pathways, involved in seed maturation. We illustrated an OsLEC1-regulating gene network during seed development, including the interconnection between photosynthesis and ABA/GA biosynthesis/signaling. Our findings suggested that OsLEC1 acts as not only a central regulator of seed maturation but also an inhibitor of embryo greening during rice seed development. This study would provide new understanding for the OsLEC1 regulatory mechanisms on photosynthesis in the monocot seed development.

The Sunflower WRINKLED1 Transcription Factor Regulates Fatty Acid Biosynthesis Genes through an AW Box Binding Sequence with a Particular Base Bias

Sunflower is an important oilseed crop in which the biochemical pathways leading to seed oil synthesis and accumulation have been widely studied. However, how these pathways are regulated is less well understood. The WRINKLED1 (WRI1) transcription factor is considered a key regulator in the control of triacylglycerol biosynthesis, acting through the AW box binding element (CNTNG(N)₇CG). Here, we identified the sunflower WRI1 gene and characterized its activity in electrophoretic mobility shift assays. We studied its role as a co-regulator of sunflower genes involved in plastidial fatty acid synthesis. Sunflower WRI1-targets included genes encoding the pyruvate dehydrogenase complex, the α-CT and BCCP genes, genes encoding ACPs and the fatty acid synthase complex, together with the FATA1 gene. As such, sunflower WRI1 regulates genes involved in seed plastidial fatty acid biosynthesis in a coordinated manner, establishing a WRI1 push and pull strategy that drives oleic acid synthesis for its export into the cytosol. We also determined the base bias at the N positions in the active sunflower AW box motif. The sunflower AW box is sequence-sensitive at the non-conserved positions, enabling WRI1-binding. Moreover, sunflower WRI1 could bind to a non-canonical AW-box motif, opening the possibility of searching for new target genes.

Cold-induced secondary dormancy and its regulatory mechanisms in Beta vulgaris

The dynamic behaviour of seeds in soil seed banks depends on their ability to act as sophisticated environmental sensors to adjust their sensitivity thresholds for germination by dormancy mechanisms. Here we show that prolonged incubation of sugar beet fruits at low temperature (chilling at 5°C, generally known to release seed dormancy of many species) can induce secondary nondeep physiological dormancy of an apparently nondormant crop species. The physiological and biophysical mechanisms underpinning this cold-induced secondary dormancy include the chilling-induced accumulation of abscisic acid in the seeds, a reduction in the embryo growth potential and a block in weakening of the endosperm covering the embryonic root. Transcriptome analysis revealed distinct gene expression patterns in the different temperature regimes and upon secondary dormancy induction and maintenance. The chilling caused reduced expression of cell wall remodelling protein genes required for embryo cell elongation growth and endosperm weakening, as well as increased expression of seed maturation genes, such as for late embryogenesis abundant proteins. A model integrating the hormonal signalling and master regulator expression with the temperature-control of seed dormancy and maturation programmes is proposed. The revealed mechanisms of the cold-induced secondary dormancy are important for climate-smart agriculture and food security.

The Cycas genome and the early evolution of seed plants

Cycads represent one of the most ancient lineages of living seed plants. Identifying genomic features uniquely shared by cycads and other extant seed plants, but not non-seed-producing plants, may shed light on the origin of key innovations, as well as the early diversification of seed plants. Here, we report the 10.5-Gb reference genome of Cycas panzhihuaensis, complemented by the transcriptomes of 339 cycad species. Nuclear and plastid phylogenomic analyses strongly suggest that cycads and Ginkgo form a clade sister to all other living gymnosperms, in contrast to mitochondrial data, which place cycads alone in this position. We found evidence for an ancient whole-genome duplication in the common ancestor of extant gymnosperms. The Cycas genome contains four homologues of the fitD gene family that were likely acquired via horizontal gene transfer from fungi, and these genes confer herbivore resistance in cycads. The male-specific region of the Y chromosome of C. panzhihuaensis contains a MADS-box transcription factor expressed exclusively in male cones that is similar to a system reported in Ginkgo, suggesting that a sex determination mechanism controlled by MADS-box genes may have originated in the common ancestor of cycads and Ginkgo. The C. panzhihuaensis genome provides an important new resource of broad utility for biologists.

Transcriptional control of Arabidopsis seed development

The entire process of embryo development is under the tight control of various transcription factors. Together with other proteins, they act in a combinatorial manner and control distinct events during embryo development. Seed development is a complex process that proceeds through sequences of events regulated by the interplay of various genes, prominent among them being the transcription factors (TFs). The members of WOX, HD-ZIP III, ARF, and CUC families have a preferential role in embryonic patterning. While WOX TFs are required for initiating body axis, HD-ZIP III TFs and CUCs establish bilateral symmetry and SAM. And ARF5 performs a major role during embryonic root, ground tissue, and vasculature development. TFs such as LEC1, ABI3, FUS3, and LEC2 (LAFL) are considered the master regulators of seed maturation. Furthermore, several new TFs involved in seed storage reserves and dormancy have been identified in the last few years. Their association with those master regulators has been established in the model plant Arabidopsis. Also, using chromatin immunoprecipitation (ChIP) assay coupled with transcriptomics, genome-wide target genes of these master regulators have recently been proposed. Many seed-specific genes, including those encoding oleosins and albumins, have appeared as the direct target of LAFL. Also, several other TFs act downstream of LAFL TFs and perform their function during maturation. In this review, the function of different TFs in different phases of early embryogenesis and maturation is discussed in detail, including information about their genetic and molecular interactors and target genes. Such knowledge can further be leveraged to understand and manipulate the regulatory mechanisms involved in seed development. In addition, the genomics approaches and their utilization to identify TFs aiming to study embryo development are discussed.

Histone Modification and Chromatin Remodeling During the Seed Life Cycle

Seeds are essential for the reproduction and dispersion of spermatophytes. The seed life cycle from seed development to seedling establishment proceeds through a series of defined stages regulated by distinctive physiological and biochemical mechanisms. The role of histone modification and chromatin remodeling in seed behavior has been intensively studied in recent years. In this review, we summarize progress in elucidating the regulatory network of these two kinds of epigenetic regulation during the seed life cycle, especially in two model plants, rice and Arabidopsis. Particular emphasis is placed on epigenetic effects on primary tissue formation (e.g., the organized development of embryo and endosperm), pivotal downstream gene expression (e.g., transcription of DOG1 in seed dormancy and repression of seed maturation genes in seed-to-seedling transition), and environmental responses (e.g., seed germination in response to different environmental cues). Future prospects for understanding of intricate interplay of epigenetic pathways and the epigenetic mechanisms in other commercial species are also proposed.

Transcriptional regulation of plant seed development

Plant seeds, which are unique reproductive organs of gymnosperms and angiosperms, are used for edible, medicinal, and industrial purposes. Transcription factors (TFs) are master regulators of plant growth, development, and stress responses. This review describes, in detail, the functions of TFs in regulating seed development. Different TFs, or even different TF families, may have similar functions in seed development. For example, WUSCHEL-related homeobox, LEC2/FUS3/ABI3, and HEME ACTIVATOR PROTEIN3 families can control plant seed embryonic initiation and development. In contrast, some members of the same TF family may have completely opposite roles. For instance, AtMYB76 and AtMYB89 inhibit the accumulation of seed oil, whereas AtMYB96 promotes seed fatty acid accumulation in Arabidopsis thaliana. Compared with the number of studies that have addressed regulation by single TFs, only a few have focused on multiple-TF regulatory networks. This review should be useful as a reference for future studies on regulatory networks of TF complexes.

Crops for dry environments

Climate change necessitates increased stress resilience of food crops. We describe four potential solutions, with emphasis on a relatively novel approach aiming at true tolerance of drought rather than improved water-holding capacity of crops, which is a common approach in current breeding and genome editing efforts. Some Angiosperms are known to tolerate loss of 95% of their cellular water, without dying, not dissimilar to seeds. The molecular mechanisms and their regulation underlying this remarkable ability are potentially useful to design tolerant crops. Since most crops produce desiccation tolerant seeds, genomic information for this attribute is present but inactive in vegetative parts of the plant. Based on recent evidence from both seeds and desiccation tolerant Angiosperms we address possible routes to 'flipping the switch' to vegetative desiccation tolerance in major crops.

A reevaluation of the role of the ASIL trihelix transcription factors as repressors of the seed maturation program

Developmental transitions are typically tightly controlled at the transcriptional level. Two of these transitions involve the induction of the embryo maturation program midway through seed development and its repression during the vegetative phase of plant growth. Very little is known about the factors responsible for this regulation during early embryogenesis, and only a couple of transcription factors have been characterized as repressors during the postgerminative phase. Arabidopsis 6b-INTERACTING PROTEIN-LIKE1 (ASIL1), a trihelix transcription factor, has been proposed to repress maturation both embryonically and postembryonically. Preliminary data also suggested that its closest paralog, ASIL2, might play a role as well. We used a transcriptomic approach, coupled with phenotypical observations, to test the hypothesis that ASIL1 and ASIL2 redundantly turn off maturation during both phases of growth. Our results indicate that, contrary to what was previously published, neither of the ASIL genes plays a role in the regulation of maturation, at any point during plant development. Analyses of gene ontology (GO)-enriched terms and published transcriptomic datasets suggest that these genes might be involved in responses during the vegetative phase to certain biotic and abiotic stresses.

ABA signalling promotes cell totipotency in the shoot apex of germinating embryos

Somatic embryogenesis (SE) is a type of induced cell totipotency where embryos develop from vegetative tissues of the plant instead of from gamete fusion after fertilization. SE can be induced in vitro by exposing explants to growth regulators, such as the auxinic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The plant hormone abscisic acid (ABA) has been proposed to be a downstream signalling component at the intersection between 2,4-D- and stress-induced SE, but it is not known how these pathways interact to induce cell totipotency. Here we show that 2,4-D-induced SE from the shoot apex of germinating Arabidopsis thaliana seeds is characterized by transcriptional maintenance of an ABA-dependent seed maturation pathway. Molecular-genetic analysis of Arabidopsis mutants revealed a role for ABA in promoting SE at three different levels: ABA biosynthesis, ABA receptor complex signalling, and ABA-mediated transcription, with essential roles for the ABSCISIC ACID INSENSITIVE 3 (ABI3) and ABI4 transcription factors. Our data suggest that the ability of mature Arabidopsis embryos to maintain the ABA seed maturation environment is an important first step in establishing competence for auxin-induced cell totipotency. This finding provides further support for the role of ABA in directing processes other than abiotic stress response.

Transition from Seeds to Seedlings: Hormonal and Epigenetic Aspects

Transition from seed to seedling is one of the critical developmental steps, dramatically affecting plant growth and viability. Before plants enter the vegetative phase of their ontogenesis, massive rearrangements of signaling pathways and switching of gene expression programs are required. This results in suppression of the genes controlling seed maturation and activation of those involved in regulation of vegetative growth. At the level of hormonal regulation, these events are controlled by the balance of abscisic acid and gibberellins, although ethylene, auxins, brassinosteroids, cytokinins, and jasmonates are also involved. The key players include the members of the LAFL network-the transcription factors LEAFY COTYLEDON1 and 2 (LEC 1 and 2), ABSCISIC ACID INSENSITIVE3 (ABI3), and FUSCA3 (FUS3), as well as DELAY OF GERMINATION1 (DOG1). They are the negative regulators of seed germination and need to be suppressed before seedling development can be initiated. This repressive signal is mediated by chromatin remodeling complexes-POLYCOMB REPRESSIVE COMPLEX 1 and 2 (PRC1 and PRC2), as well as PICKLE (PKL) and PICKLE-RELATED2 (PKR2) proteins. Finally, epigenetic methylation of cytosine residues in DNA, histone post-translational modifications, and post-transcriptional downregulation of seed maturation genes with miRNA are discussed. Here, we summarize recent updates in the study of hormonal and epigenetic switches involved in regulation of the transition from seed germination to the post-germination stage.

Temporal Control of Seed Development in Dicots: Molecular Bases, Ecological Impact and Possible Evolutionary Ramifications

In flowering plants, seeds serve as organs of both propagation and dispersal. The developing seed passes through several consecutive stages, following a conserved general outline. The overall time needed for a seed to develop, however, may vary both within and between plant species, and these temporal developmental properties remain poorly understood. In the present paper, we summarize the existing data for seed development alterations in dicot plants. For genetic mutations, the reported cases were grouped in respect of the key processes distorted in the mutant specimens. Similar phenotypes arising from the environmental influence, either biotic or abiotic, were also considered. Based on these data, we suggest several general trends of timing alterations and how respective mechanisms might add to the ecological plasticity of the families considered. We also propose that the developmental timing alterations may be perceived as an evolutionary substrate for heterochronic events. Given the current lack of plausible models describing timing control in plant seeds, the presented suggestions might provide certain insights for future studies in this field.

The transcription factor CmLEC1 positively regulates the seed-setting rate in hybridization breeding of chrysanthemum

Distant hybridization is widely used to develop crop cultivars, whereas the hybridization process of embryo abortion often severely reduces the sought-after breeding effect. The LEAFY COTYLEDON1 (LEC1) gene has been extensively investigated as a central regulator of seed development, but it is far less studied in crop hybridization breeding. Here we investigated the function and regulation mechanism of CmLEC1 from Chrysanthemum morifolium during its seed development in chrysanthemum hybridization. CmLEC1 encodes a nucleic protein and is specifically expressed in embryos. CmLEC1's overexpression significantly promoted the seed-setting rate of the cross, while the rate was significantly decreased in the amiR-CmLEC1 transgenic chrysanthemum. The RNA-Seq analysis of the developing hybrid embryos revealed that regulatory genes involved in seed development, namely, CmLEA (late embryogenesis abundant protein), CmOLE (oleosin), CmSSP (seed storage protein), and CmEM (embryonic protein), were upregulated in the OE (overexpressing) lines but downregulated in the amiR lines vs. wild-type lines. Future analysis demonstrated that CmLEC1 directly activated CmLEA expression and interacted with CmC3H, and this CmLEC1-CmC3H interaction could enhance the transactivation ability of CmLEC1 for the expression of CmLEA. Further, CmLEC1 was able to induce several other key genes related to embryo development. Taken together, our results show that CmLEC1 plays a positive role in the hybrid embryo development of chrysanthemum plants, which might involve activating CmLEA's expression and interacting with CmC3H. This may be a new pathway in the LEC1 regulatory network to promote seed development, one perhaps leading to a novel strategy to not only overcome embryo abortion during crop breeding but also increase the seed yield.

Genetic and Molecular Control of Somatic Embryogenesis

Somatic embryogenesis is a method of asexual reproduction that can occur naturally in various plant species and is widely used for clonal propagation, transformation and regeneration of different crops. Somatic embryogenesis shares some developmental and physiological similarities with zygotic embryogenesis as it involves common actors of hormonal, transcriptional, developmental and epigenetic controls. Here, we provide an overview of the main signaling pathways involved in the induction and regulation of somatic embryogenesis with a focus on the master regulators of seed development, LEAFY COTYLEDON 1 and 2, ABSCISIC ACID INSENSITIVE 3 and FUSCA 3 transcription factors whose precise role during both zygotic and somatic embryogenesis remains to be fully elucidated.

Reproductive developmental transcriptome analysis of Tripidium ravennae (Poaceae)

BACKGROUND

Tripidium ravennae is a cold-hardy, diploid species in the sugarcane complex (Poaceae subtribe Saccharinae) with considerable potential as a genetic resource for developing improved bioenergy and ornamental grasses. An improved understanding of the genetic regulation of reproductive processes (e.g., floral induction, inflorescence development, and seed development) will enable future applications of precision breeding and gene editing of floral and seed development. In particular, the ability to silence reproductive processes would allow for developing seedless forms of valuable but potentially invasive plants. The objective of this research was to characterize the gene expression environment of reproductive development in T. ravennae.

RESULTS

During the early phases of inflorescence development, multiple key canonical floral integrators and pathways were identified. Annotations of type II subfamily of MADS-box transcription factors, in particular, were over-represented in the GO enrichment analyses and tests for differential expression (FDR p-value < 0.05). The differential expression of floral integrators observed in the early phases of inflorescence development diminished prior to inflorescence determinacy regulation. Differential expression analysis did not identify many unique genes at mid-inflorescence development stages, though typical biological processes involved in plant growth and development expressed abundantly. The increase in inflorescence determinacy regulatory elements and putative homeotic floral development unigenes at mid-inflorescence development coincided with the expression of multiple meiosis annotations and multicellular organism developmental processes. Analysis of seed development identified multiple unigenes involved in oxidative-reductive processes.

CONCLUSION

Reproduction in grasses is a dynamic system involving the sequential coordination of complex gene regulatory networks and developmental processes. This research identified differentially expressed transcripts associated with floral induction, inflorescence development, and seed development in T. ravennae. These results provide insights into the molecular regulation of reproductive development and provide a foundation for future investigations and analyses, including genome annotation, functional genomics characterization, gene family evolutionary studies, comparative genomics, and precision breeding.

ABA Metabolism and Homeostasis in Seed Dormancy and Germination

Abscisic acid (ABA) is a key hormone that promotes dormancy during seed development on the mother plant and after seed dispersal participates in the control of dormancy release and germination in response to environmental signals. The modulation of ABA endogenous levels is largely achieved by fine-tuning, in the different seed tissues, hormone synthesis by cleavage of carotenoid precursors and inactivation by 8'-hydroxylation. In this review, we provide an overview of the current knowledge on ABA metabolism in developing and germinating seeds; notably, how environmental signals such as light, temperature and nitrate control seed dormancy through the adjustment of hormone levels. A number of regulatory factors have been recently identified which functional relationships with major transcription factors, such as ABA INSENSITIVE3 (ABI3), ABI4 and ABI5, have an essential role in the control of seed ABA levels. The increasing importance of epigenetic mechanisms in the regulation of ABA metabolism gene expression is also described. In the last section, we give an overview of natural variations of ABA metabolism genes and their effects on seed germination, which could be useful both in future studies to better understand the regulation of ABA metabolism and to identify candidates as breeding materials for improving germination properties.

Genome-Wide Mapping of Histone H3 Lysine 4 Trimethylation (H3K4me3) and Its Involvement in Fatty Acid Biosynthesis in Sunflower Developing Seeds

Histone modifications are of paramount importance during plant development. Investigating chromatin remodeling in developing oilseeds sheds light on the molecular mechanisms controlling fatty acid metabolism and facilitates the identification of new functional regions in oil crop genomes. The present study characterizes the epigenetic modifications H3K4me3 in relationship with the expression of fatty acid-related genes and transcription factors in developing sunflower seeds. Two master transcriptional regulators identified in this analysis, VIV1 (homologous to Arabidopsis ABI3) and FUS3, cooperate in the regulation of WRINKLED 1, a transcriptional factor regulating glycolysis, and fatty acid synthesis in developing oilseeds.

Proteomics of developing pea seeds reveals a complex antioxidant network underlying the response to sulfur deficiency and water stress

Pea is a legume crop producing protein-rich seeds and is increasingly in demand for human consumption and animal feed. The aim of this study was to explore the proteome of developing pea seeds at three key stages covering embryogenesis, the transition to seed-filling, and the beginning of storage-protein synthesis, and to investigate how the proteome was influenced by S deficiency and water stress, applied either separately or combined. Of the 3184 proteins quantified by shotgun proteomics, 2473 accumulated at particular stages, thus providing insights into the proteome dynamics at these stages. Differential analyses in response to the stresses and inference of a protein network using the whole proteomics dataset identified a cluster of antioxidant proteins (including a glutathione S-transferase, a methionine sulfoxide reductase, and a thioredoxin) possibly involved in maintaining redox homeostasis during early seed development and preventing cellular damage under stress conditions. Integration of the proteomics data with previously obtained transcriptomics data at the transition to seed-filling revealed the transcriptional events associated with the accumulation of the stress-regulated antioxidant proteins. This transcriptional defense response involves genes of sulfate homeostasis and assimilation, thus providing candidates for targeted studies aimed at dissecting the signaling cascade linking S metabolism to antioxidant processes in developing seeds.

The B3 domain-containing transcription factor ZmABI19 coordinates expression of key factors required for maize seed development and grain filling

Grain filling in maize (Zea mays) is regulated by a group of spatiotemporally synchronized transcription factors (TFs), but the factors that coordinate their expression remain unknown. We used the promoter of the grain filling-specific TF gene Opaque2 (O2) to screen upstream regulatory factors and identified a B3 domain TF, ZmABI19, that directly binds to the O2 promoter for transactivation. zmabi19 mutants displayed developmental defects in the endosperm and embryo, and mature kernels were opaque and reduced in size. The accumulation of zeins, starch and lipids dramatically decreased in zmabi19 mutants. RNA sequencing revealed an alteration of the nutrient reservoir activity and starch and sucrose metabolism in zmabi19 endosperms, and plant phytohormone signal transduction and lipid metabolism in zmabi19 embryos. Chromatin immunoprecipitation followed by sequencing coupled with differential expression analysis identified 106 high-confidence direct ZmABI19 targets. ZmABI19 directly regulates multiple key grain filling TFs including O2, Prolamine-box binding factor 1, ZmbZIP22, NAC130, and Opaque11 in the endosperm and Viviparous1 in the embryo. A number of phytohormone-related genes were also bound and regulated by ZmABI19. Our results demonstrate that ZmABI19 functions as a grain filling initiation regulator. ZmABI19 roles in coupling early endosperm and embryo development are also discussed.