Early plant embryogenesis-dark ages or dark matter?

Cell wall-mediated maternal control of apical-basal patterning of the kelp Undaria pinnatifida

The role of maternal tissue in embryogenesis remains enigmatic in many complex organisms. Here, we investigate the contribution of maternal tissue to apical-basal patterning in the kelp embryo. Focussing on Undaria pinnatifida, we studied the effects of detachment from the maternal tissue using microsurgery, staining of cell wall modifications, morphometric measurements, flow cytometry, genotyping and a modified kelp fertilisation protocol synchronising kelp embryogenesis. Detached embryos are rounder and often show aberrant morphologies. When a part of the oogonial cell wall remains attached to the zygote, the apical-basal patterning is rescued. Furthermore, the absence of contact with maternal tissue increases parthenogenesis, highlighting the critical role of maternal signals in the initial stages of development. These results show a key role for the connection to the maternal oogonial cell wall in apical-basal patterning in kelps. This observation is reminiscent of another brown alga, Fucus, where the cell wall directs the cell fate. Our findings suggest a conserved mechanism across phylogenetically distant oogamous lineages, where localised secretion of sulphated F2 fucans mediates the establishment of the apical-basal polarity. In this model, the maternal oogonial cell wall mediates basal cell fate determination by providing an extrinsic patterning cue to the future kelp embryo.

The maternal embrace: the protection of plant embryos

All land plants-the embryophytes-produce multicellular embryos, as do other multicellular organisms, such as brown algae and animals. A unique characteristic of plant embryos is their immobile and confined nature. Their embedding in maternal tissues may offer protection from the environment, but also physically constrains development. Across the different land plants, a huge discrepancy is present between their reproductive structures whilst leading to similarly complex embryos. Therefore, we review the roles that maternal tissues play in the control of embryogenesis across land plants. These nurturing, constraining, and protective roles include both direct and indirect effects. In this review, we explore how the maternal surroundings affect embryogenesis and which chemical and mechanical barriers are in place. We regard these questions through the lens of evolution, and identify key questions for future research.

Integrated metabolomics and transcriptomics analysis highlight key pathways involved in the somatic embryogenesis of Darjeeling tea

BACKGROUND

Darjeeling tea is a globally renowned beverage, which faces numerous obstacles in sexual reproduction, such as self-incompatibility, poor seed germination, and viability, as well as issues with vegetative propagation. Somatic embryogenesis (SE) is a valuable method for rapid clonal propagation of Darjeeling tea. However, the metabolic regulatory mechanisms underlying SE in Darjeeling tea remain largely unknown. To address this, we conducted an integrated metabolomics and transcriptomics analysis of embryogenic callus (EC), globular embryo (GE), and heart-shaped embryo (HE).

RESULTS

The integrated analyses showed that various genes and metabolites involved in the phenylpropanoid pathway, auxin biosynthesis pathway, gibberellin, brassinosteroid and amino acids biosynthesis pathways were differentially enriched in EC, GE, and HE. Our results revealed that despite highly up-regulated auxin biosynthesis genes YUC1, TAR1 and AAO1 in EC, endogenous indole-3-acetic acid (IAA) was significantly lower in EC than GE and HE. However, bioactive Gibberellin A4 displayed higher accumulation in EC. We also found higher BABY BOOM (BBM) and Leafy cotyledon1 (LEC1) gene expression in GE along with high accumulation of castasterone, a brassinosteroid. Total flavonoids and phenolics levels were elevated in GE and HE compared to EC, especially the phenolic compound chlorogenic acid was highly accumulated in GE.

CONCLUSIONS

Integrated metabolome and transcriptome analysis revealed enriched metabolic pathways, including auxin biosynthesis and signal transduction, brassinosteroid, gibberellin, phenylpropanoid biosynthesis, amino acids metabolism, and transcription factors (TFs) during SE in Darjeeling tea. Notably, EC displayed lower endogenous IAA levels, conducive to maintaining differentiation, while higher IAA concentration in GE and HE was crucial for preserving embryo identity. Additionally, a negative correlation between bioactive gibberellin A4 (GA4) and IAA was observed, impacting callus growth in EC. The high accumulation of chlorogenic acid, a phenolic compound, might contribute to the low success rate in GE and HE formation in Darjeeling tea. TFs such as BBM1, LEC1, FUS3, LEA, WOX3, and WOX11 appeared to regulate gene expression, influencing SE in Darjeeling tea.

Evidence of a novel silencing effect on transgenes in the Arabidopsis thaliana sperm cell

We encountered unexpected transgene silencing in Arabidopsis thaliana sperm cells; transgenes encoding proteins with no specific intracellular localization (cytoplasmic proteins) were silenced transcriptionally or posttranscriptionally. The mRNA of cytoplasmic protein transgenes tagged with a fluorescent protein gene was significantly reduced, resulting in undetectable fluorescent protein signals in the sperm cell. Silencing of the cytoplasmic protein transgenes in the sperm cell did not affect the expression of either its endogenous homologous genes or cotransformed transgenes encoding a protein with targeted intracellular localization. This transgene silencing in the sperm cell persisted in mutants of the major gene silencing machinery including DNA methylation. The incomprehensible, yet real, transgene silencing phenotypes occurring in the sperm cell could mislead the interpretation of experimental results in plant reproduction, and this Commentary calls attention to that risk and highlights details of this novel cytoplasmic protein transgene silencing.

Quantitative Proteomic Analysis Deciphers the Molecular Mechanism for Endosperm Nuclear Division in Early Rice Seed Development

Understanding the molecular mechanisms underlying early seed development is important in improving the grain yield and quality of crop plants. We performed a comparative label-free quantitative proteomic analysis of developing rice seeds for the WT and osctps1-2 mutant, encoding a cytidine triphosphate synthase previously reported as the endospermless 2 (enl2) mutant in rice, harvested at 0 and 1 d after pollination (DAP) to understand the molecular mechanism of early seed development. In total, 5231 proteins were identified, of which 902 changed in abundance between 0 and 1 DAP seeds. Proteins that preferentially accumulated at 1 DAP were involved in DNA replication and pyrimidine biosynthetic pathways. Notably, an increased abundance of OsCTPS1 was observed at 1 DAP; however, no such changes were observed at the transcriptional level. We further observed that the inhibition of phosphorylation increased the stability of this protein. Furthermore, in osctps1-2, minichromosome maintenance (MCM) proteins were significantly reduced compared with those in the WT at 1 DAP, and mutations in OsMCM5 caused defects in seed development. These results highlight the molecular mechanisms underlying early seed development in rice at the post-transcriptional level.

The baseless mutant links protein phosphatase 2A with basal cell identity in the brown alga Ectocarpus

The first mitotic division of the initial cell is a key event in all multicellular organisms and is associated with the establishment of major developmental axes and cell fates. The brown alga Ectocarpus has a haploid-diploid life cycle that involves the development of two multicellular generations: the sporophyte and the gametophyte. Each generation deploys a distinct developmental programme autonomously from an initial cell, the first cell division of which sets up the future body pattern. Here, we show that mutations in the BASELESS (BAS) gene result in multiple cellular defects during the first cell division and subsequent failure to produce basal structures during both generations. BAS encodes a type B″ regulatory subunit of protein phosphatase 2A (PP2A), and transcriptomic analysis identified potential effector genes that may be involved in determining basal cell fate. The bas mutant phenotype is very similar to that observed in distag (dis) mutants, which lack a functional Tubulin-binding co-factor Cd1 (TBCCd1) protein, indicating that TBCCd1 and PP2A are two essential components of the cellular machinery that regulates the first cell division and mediates basal cell fate determination.

Transcriptome analysis and identification of abscisic acid and gibberellin-related genes during seed development of alfalfa (Medicago sativa L.)

BACKGROUND

Alfalfa (Medicago sativa) is a widely cultivated plant. Unlike many crops, the main goal of breeding alfalfa is to increase its aboveground biomass rather than the biomass of its seeds. However, the low yield of alfalfa seeds limits alfalfa production. Many studies have explored the factors affecting seed development, in which phytohormones, especially ABA and GAs, play an important role in seed development.

RESULTS

Here, we performed a transcriptome analysis of alfalfa seeds at five development stages. A total of 16,899 differentially expressed genes (DEGs) were identified and classified into 10 clusters, and the enriched Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were determined. The contents of ABA, GA₁, GA₃, GA₄ and GA₇ in alfalfa seeds at five development stages were determined. In addition, 14 ABA-related DEGs and 20 GA-related DEGs were identified and analysed. These DEGs are involved in plant hormone pathways and play an important role in seed development. Moreover, morphological and physiological analyses revealed the dynamic changes during the development of alfalfa seeds.

CONCLUSION

Overall, our study is the first to analyse the transcriptome across various stages of seed development in alfalfa. The results of our study could be used to improve alfalfa seed yield. The key ABA and GA related-genes are potential targets for improving alfalfa seed yield via genetic engineering in the future.

Integrated Proteomic and Metabolomic Analyses Provide Insights Into Acquisition of Embryogenic Ability in Agapanthus praecox

Somatic embryogenesis (SE) is an ideal model for plant cell totipotency. Transition from somatic cells to embryogenic cells is the key to SE. The poor frequency of embryogenic callus (EC) induction has limited the application of SE in many plants, such as Agapanthus praecox. We performed large-scale, quantitative proteomic and metabolomic analyses with different callus differentiation directions (SE and organogenesis) and stages (initial SE and repetitive SE) to better understand the morphological, physiological, and molecular characteristics of the acquisition of embryogenic ability in A. praecox. Integrated proteomic and metabolomic analyses suggested that callus differentiation direction was potentially regulated by pathways related to carbohydrate and energy metabolism (fatty acid metabolism, pyruvate metabolism, glycolysis/gluconeogenesis, pentose and glucuronate interconversions, starch and sucrose metabolism, galactose metabolism, carbon fixation pathways in prokaryotes, carbohydrate digestion and absorption, and fructose and mannose metabolism), chromatin accessibility and DNA methylation, reactive oxygen species responses and resistance (ascorbate and aldarate metabolism), and plant hormonal signaling. As a validation, we found that carbon source combination and plant hormone regulation in the culture medium significantly affected the acquisition of embryogenic ability, thereby inducing EC. Interestingly, plant hormonal signaling-related genes showed different expression patterns significantly when callus cultured with different carbon sources. Thus, our results suggested that energy supply and hormone signal transduction seemed to cooperatively contribute to the activation of embryogenic ability. Altogether, this study revealed valuable information regarding the molecular and biochemical changes that occurred during EC induction and provided valuable foundation for comprehensive understanding of the mechanisms associated with SE and organogenesis in A. praecox.

New insights into cell-cell communications during seed development in flowering plants

The evolution of seeds is a major reason why flowering plants are a dominant life form on Earth. The developing seed is composed of two fertilization products, the embryo and endosperm, which are surrounded by a maternally derived seed coat. Accumulating evidence indicates that efficient communication among all three seed components is required to ensure coordinated seed development. Cell communication within plant seeds has drawn much attention in recent years. In this study, we review current knowledge of cross-talk among the endosperm, embryo, and seed coat during seed development, and highlight recent advances in this field.

Current Proteomic and Metabolomic Knowledge of Zygotic and Somatic Embryogenesis in Plants

Embryogenesis is the primary developmental program in plants. The mechanisms that underlie the regulation of embryogenesis are an essential research subject given its potential contribution to mass in vitro propagation of profitable plant species. Somatic embryogenesis (SE) refers to the use of in vitro techniques to mimic the sexual reproduction program known as zygotic embryogenesis (ZE). In this review, we synthesize the current state of research on proteomic and metabolomic studies of SE and ZE in angiosperms (monocots and dicots) and gymnosperms. The most striking finding was the small number of studies addressing ZE. Meanwhile, the research effort focused on SE has been substantial but disjointed. Together, these research gaps may explain why the embryogenic induction stage and the maturation of the somatic embryo continue to be bottlenecks for efficient and large-scale regeneration of plants. Comprehensive and integrative studies of both SE and ZE are needed to provide the molecular foundation of plant embryogenesis, information which is needed to rationally guide experimental strategies to solve SE drawbacks in each species.

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.

Plastid-Localized EMB2726 Is Involved in Chloroplast Biogenesis and Early Embryo Development in Arabidopsis

Embryogenesis is a critical developmental process that establishes the body organization of higher plants. During this process, the biogenesis of chloroplasts from proplastids is essential. A failure in chloroplast development during embryogenesis can cause morphologically abnormal embryos or embryonic lethality. In this study, we isolated a T-DNA insertion mutant of the Arabidopsis gene EMBRYO DEFECTIVE 2726 (EMB2726). Heterozygous emb2726 seedlings produced about 25% albino seeds with embryos that displayed defects at the 32-cell stage and that arrested development at the late globular stage. EMB2726 protein was localized in chloroplasts and was expressed at all stages of development, such as embryogenesis. Moreover, the two translation elongation factor Ts domains within the protein were critical for its function. Transmission electron microscopy revealed that the cells in emb2726 embryos contained undifferentiated proplastids and that the expression of plastid genome-encoded photosynthesis-related genes was dramatically reduced. Expression studies of DR5:GFP, pDRN:DRN-GFP, and pPIN1:PIN1-GFP reporter lines indicated normal auxin biosynthesis but altered polar auxin transport. The expression of pSHR:SHR-GFP and pSCR:SCR-GFP confirmed that procambium and ground tissue precursors were lacking in emb2726 embryos. The results suggest that EMB2726 plays a critical role during Arabidopsis embryogenesis by affecting chloroplast development, possibly by affecting the translation process in plastids.

Comparative Embryogenesis in Angiosperms: Activation and Patterning of Embryonic Cell Lineages

Following fertilization in flowering plants (angiosperms), egg and sperm cells unite to form the zygote, which generates an entire new organism through a process called embryogenesis. In this review, we provide a comparative perspective on early zygotic embryogenesis in flowering plants by using the Poaceae maize and rice as monocot grass and crop models as well as Arabidopsis as a eudicot model of the Brassicaceae family. Beginning with the activation of the egg cell, we summarize and discuss the process of maternal-to-zygotic transition in plants, also taking recent work on parthenogenesis and haploid induction into consideration. Aspects like imprinting, which is mainly associated with endosperm development and somatic embryogenesis, are not considered. Controversial findings about the timing of zygotic genome activation as well as maternal versus paternal contribution to zygote and early embryo development are highlighted. The establishment of zygotic polarity, asymmetric division, and apical and basal cell lineages represents another chapter in which we also examine and compare the role of major signaling pathways, cell fate genes, and hormones in early embryogenesis. Except for the model Arabidopsis, little is known about embryopatterning and the establishment of the basic body plan in angiosperms. Using available in situ hybridization, RNA-sequencing, and marker data, we try to compare how and when stem cell niches are established. Finally, evolutionary aspects of plant embryo development are discussed.

A ClearSee-Based Clearing Protocol for 3D Visualization of Arabidopsis thaliana Embryos

Tissue clearing methods combined with confocal microscopy have been widely used for studying developmental biology. In plants, ClearSee is a reliable clearing method that is applicable to a wide range of tissues and is suitable for gene expression analysis using fluorescent reporters, but its application to the Arabidopsis thaliana embryo, a model system to study morphogenesis and pattern formation, has not been described in the original literature. Here, we describe a ClearSee-based clearing protocol which is suitable for obtaining 3D images of Arabidopsis thaliana embryos. The method consists of embryo dissection, fixation, washing, clearing, and cell wall staining and enables high-quality 3D imaging of embryo morphology and expression of fluorescent reporters with the cellular resolution. Our protocol provides a reliable method that is applicable to the analysis of morphogenesis and gene expression patterns in Arabidopsis thaliana embryos.

Zygotic Embryogenesis in Flowering Plants

In the context of plant regeneration, in vitro systems to produce embryos are frequently used. In many of these protocols, nonzygotic embryos are initiated that will produce shoot-like structures but may lack a primary root. By increasing the auxin-to-cytokinin ratio in the growth medium, roots are then regenerated in a second step. Therefore, in vitro systems might not or only partially execute a similar developmental program as employed during zygotic embryogenesis. There are, however, in vitro systems that can remarkably mimic zygotic embryogenesis such as Brassica microspore-derived embryos. In this case, the patterning process of these haploid embryos closely follows zygotic embryogenesis and all fundamental tissue types are generated in a rather similar manner. In this review, we discuss the most fundamental molecular events during early zygotic embryogenesis and hope that this brief summary can serve as a reference for studying and developing in vitro embryogenesis systems in the context of doubled haploid production.

Plasmogamic Paternal Contributions to Early Zygotic Development in Flowering Plants

Flowering plant zygotes possess complete developmental potency, and the mixture of male and female genetic and cytosolic materials in the zygote is a trigger to initiate embryo development. Plasmogamy, the fusion of the gamete cytoplasms, facilitates the cellular dynamics of the zygote. In the last decade, mutant analyses, live cell imaging-based observations, and direct observations of fertilized egg cells by in vitro fusion of isolated gametes have accelerated our understanding of the post-plasmogamic events in flowering plants including cell wall formation, gamete nuclear migration and fusion, and zygotic cell elongation and asymmetric division. Especially, it has become more evident that paternal parent-of-origin effects, via sperm cytoplasm contents, not only control canonical early zygotic development, but also activate a biparental signaling pathway critical for cell fate determination after the first cell division. Here, we summarize the plasmogamic paternal contributions via the entry of sperm contents during/after fertilization in flowering plants.

Transcriptomic Insights into Mechanisms of Early Seed Maturation in the Garden Pea (Pisum sativum L.)

The garden pea (Pisum sativum L.) is a legume crop of immense economic value. Extensive breeding has led to the emergence of numerous pea varieties, of which some are distinguished by accelerated development in various stages of ontogenesis. One such trait is rapid seed maturation, which, despite novel insights into the genetic control of seed development in legumes, remains poorly studied. This article presents an attempt to dissect mechanisms of early maturation in the pea line Sprint-2 by means of whole transcriptome RNA sequencing in two developmental stages. By using a de novo assembly approach, we have obtained a reference transcriptome of 25,756 non-redundant entries expressed in pea seeds at either 10 or 20 days after pollination. Differential expression in Sprint-2 seeds has affected 13,056 transcripts. A comparison of the two pea lines with a common maturation rate demonstrates that while at 10 days after pollination, Sprint-2 seeds show development retardation linked to intensive photosynthesis, morphogenesis, and cell division, and those at 20 days show a rapid onset of desiccation marked by the cessation of translation and cell anabolism and accumulation of dehydration-protective and -storage moieties. Further inspection of certain transcript functional categories, including the chromatin constituent, transcription regulation, protein turnover, and hormonal regulation, has revealed transcriptomic trends unique to specific stages and cultivars. Among other remarkable features, Sprint-2 demonstrated an enhanced expression of transposable element-associated open reading frames and an altered expression of major maturation regulators and DNA methyltransferase genes. To the best of our knowledge, this is the first comparative transcriptomic study in which the issue of the seed maturation rate is addressed.

The Winner Takes It All: Auxin-The Main Player during Plant Embryogenesis

In plants, the first asymmetrical division of a zygote leads to the formation of two cells with different developmental fates. The establishment of various patterns relies on spatial and temporal gene expression, however the precise mechanism responsible for embryonic patterning still needs elucidation. Auxin seems to be the main player which regulates embryo development and controls expression of various genes in a dose-dependent manner. Thus, local auxin maxima and minima which are provided by polar auxin transport underlie cell fate specification. Diverse auxin concentrations in various regions of an embryo would easily explain distinct cell identities, however the question about the mechanism of cellular patterning in cells exposed to similar auxin concentrations still remains open. Thus, specification of cell fate might result not only from the cell position within an embryo but also from events occurring before and during mitosis. This review presents the impact of auxin on the orientation of the cell division plane and discusses the mechanism of auxin-dependent cytoskeleton alignment. Furthermore, close attention is paid to auxin-induced calcium fluxes, which regulate the activity of MAPKs during postembryonic development and which possibly might also underlie cellular patterning during embryogenesis.

Square one: zygote polarity and early embryogenesis in flowering plants

In the last two decades, work on auxin signaling has helped to understand many aspects of the fundamental process underlying the specification of tissue types in the plant embryo. However, the immediate steps after fertilization including the polarization of the zygote and the initial body axis formation remained poorly understood. Valuable insight into these enigmatic processes has been gained by studying fertilization in grasses. Recent technical advances in transcriptomics of developing embryos with high spatial and temporal resolution give an emerging picture of the rapid changes of the zygotic developmental program. Together with the use of live imaging of novel fluorescent marker lines, these data are now the basis of unraveling the very first steps of the embryonic patterning process.

Division Plane Establishment and Cytokinesis

Plant cells divide their cytoplasmic content by forming a new membrane compartment, the cell plate, via a rerouting of the secretory pathway toward the division plane aided by a dynamic cytoskeletal apparatus known as the phragmoplast. The phragmoplast expands centrifugally and directs the cell plate to the preselected division site at the plasma membrane to fuse with the parental wall. The division site is transiently decorated by the cytoskeletal preprophase band in preprophase and prophase, whereas a number of proteins discovered over the last decade reside continuously at the division site and provide a lasting spatial reference for phragmoplast guidance. Recent studies of membrane fusion at the cell plate have revealed the contribution of functionally conserved eukaryotic proteins to distinct stages of cell plate biogenesis and emphasize the coupling of cell plate formation with phragmoplast expansion. Together with novel findings concerning preprophase band function and the setup of the division site, cytokinesis and its spatial control remain an open-ended field with outstanding and challenging questions to resolve.

Constitutive signaling activity of a receptor-associated protein links fertilization with embryonic patterning in Arabidopsis thaliana

In flowering plants, the asymmetrical division of the zygote is the first hallmark of apical-basal polarity of the embryo and is controlled by a MAP kinase pathway that includes the MAPKKK YODA (YDA). In Arabidopsis, YDA is activated by the membrane-associated pseudokinase SHORT SUSPENSOR (SSP) through an unusual parent-of-origin effect: SSP transcripts accumulate specifically in sperm cells but are translationally silent. Only after fertilization is SSP protein transiently produced in the zygote, presumably from paternally inherited transcripts. SSP is a recently diverged, Brassicaceae-specific member of the BRASSINOSTEROID SIGNALING KINASE (BSK) family. BSK proteins typically play broadly overlapping roles as receptor-associated signaling partners in various receptor kinase pathways involved in growth and innate immunity. This raises two questions: How did a protein with generic function involved in signal relay acquire the property of a signal-like patterning cue, and how is the early patterning process activated in plants outside the Brassicaceae family, where SSP orthologs are absent? Here, we show that Arabidopsis BSK1 and BSK2, two close paralogs of SSP that are conserved in flowering plants, are involved in several YDA-dependent signaling events, including embryogenesis. However, the contribution of SSP to YDA activation in the early embryo does not overlap with the contributions of BSK1 and BSK2. The loss of an intramolecular regulatory interaction enables SSP to constitutively activate the YDA signaling pathway, and thus initiates apical-basal patterning as soon as SSP protein is translated after fertilization and without the necessity of invoking canonical receptor activation.

Advances in the study of egg activation of higher plants

Fertilization in higher plants induces many structural and physiological changes in the fertilized egg, and represents the transition from the haploid female gamete to the diploid zygote, the first cell of a sporophyte. Some changes are induced extremely rapidly following fusion with sperm cells and are the preclusions of egg activation. This review focuses on the early changes that occur in the egg after fusion with sperm cells, but before nuclear fusion. Reported changes include cell shrinkage, cell wall formation, polarity change, oscillation in Ca2+ concentration, and DNA synthesis. In addition, the current understanding of egg activation is summarized and the possible functional relevance of the changes is explored.

Identification of Two Auxin-Regulated Potassium Transporters Involved in Seed Maturation

The seed is the most important plant reproductive unit responsible for the evolutionary success of flowering plants. Aside from its essential function in the sexual reproduction of plants, the seed also represents the most economically important agricultural product worldwide, providing energy, nutrients, and raw materials for human nutrition, livestock feed, and countless manufactured goods. Hence, improvements in seed quality or size are highly valuable, due to their economic potential in agriculture. Recently, the importance of indolic compounds in regulating these traits has been reported for Arabidopsis thaliana. The transcriptional and physiological mechanisms involved, however, remain largely undisclosed. Potassium transporters have been suggested as possible mediators of embryo cell size, controlling turgor pressure during seed maturation. In addition, it has been demonstrated that the expression of K⁺ transporters is effectively regulated by auxin. Here, we provide evidence for the identification of two Arabidopsis K⁺ transporters, HAK/KT12 (At1g60160) and KUP4 (At4g23640), that are likely to be implicated in determining seed size during seed maturation and, at the same time, show a differential regulation by indole-3-acetic acid and indole-3-acetamide.

Maternal control of embryogenesis by MPK6 and its upstream MKK4/MKK5 in Arabidopsis

In flowering plants, developing embryos reside in maternal sporophytes. It is known that maternal generation influences the development of next-generation embryos; however, little is known about the signaling components in the process. Previously, we demonstrated that Arabidopsis mitogen-activated protein kinase 6 (MPK6) and MPK3 play critical roles in plant reproduction. In addition, we noticed that a large fraction of seeds from mpk6 single-mutant plants showed a wrinkled seed coat or a burst-out embryo phenotype. Here, we report that these seed phenotypes can be traced back to defective embryogenesis. The defective embryos have shorter suspensors and reduced growth along the longitudinal axis. Furthermore, the cotyledons fail to bend over to progress to the bent-cotyledon stage. As a result of the uneven circumference along the axis, the seed coat wrinkles to develop raisin-like morphology after dehydration. In more severe cases, the embryo can be pushed out from the micropylar end, resulting in the burst-out embryo seed phenotype. Genetic analyses demonstrated that the defective embryogenesis of the mpk6 mutant is a maternal effect. Heterozygous or homozygous mpk6 embryos have defects only in mpk6 homozygous maternal plants, but not in wild-type or heterozygous maternal plants. The loss of function of MKK4/MKK5 also results in the same phenotypes, suggesting that MKK4/MKK5 might act upstream of MPK6 in this pathway. The maternal-mediated embryo defects are associated with changes in auxin activity maxima and PIN localization. In summary, this research demonstrates that the Arabidopsis MKK4/MKK5-MPK6 cascade is an important player in the maternal control of embryogenesis.

Transcriptional integration of paternal and maternal factors in the Arabidopsis zygote

In this study, Ueda et al. show that paternal SSP/YDA signaling directly phosphorylates WRKY2 that in turn up-regulates transcription of the major patterning gene WOX8 in the plant zygote. Their results reveal a framework of how maternal and paternal factors are integrated in the zygote to regulate embryo patterning in plants.

In many plants, the asymmetric division of the zygote sets up the apical–basal axis of the embryo. Unlike animals, plant zygotes are transcriptionally active, implying that plants have evolved specific mechanisms to control transcriptional activation of patterning genes in the zygote. In Arabidopsis, two pathways have been found to regulate zygote asymmetry: YODA (YDA) mitogen-activated protein kinase (MAPK) signaling, which is potentiated by sperm-delivered mRNA of the SHORT SUSPENSOR (SSP) membrane protein, and up-regulation of the patterning gene WOX8 by the WRKY2 transcription factor. How SSP/YDA signaling is transduced into the nucleus and how these pathways are integrated have remained elusive. Here we show that paternal SSP/YDA signaling directly phosphorylates WRKY2, which in turn leads to the up-regulation of WOX8 transcription in the zygote. We further discovered the transcription factors HOMEODOMAIN GLABROUS11/12 (HDG11/12) as maternal regulators of zygote asymmetry that also directly regulate WOX8 transcription. Our results reveal a framework of how maternal and paternal factors are integrated in the zygote to regulate embryo patterning.