Maternal control of suspensor programmed cell death via gibberellin signaling

Cell fate determination during sexual plant reproduction

The flowering plant life cycle is completed by an alternation of diploid and haploid generations. The diploid sporophytes produce initial cells that undergo meiosis and produce spores. From haploid spores, male or female gametophytes, which produce gametes, develop. The union of gametes at fertilization restores diploidy in the zygote that initiates a new cycle of diploid sporophyte development. During this complex process, cell fate determination occurs at each of the critical stages and necessarily underpins successful plant reproduction. Here, we summarize available knowledge on the regulatory mechanism of cell fate determination at these critical stages of sexual reproduction, including sporogenesis, gametogenesis, and early embryogenesis, with particular emphasis on regulatory pathways of both male and female gametes before fertilization, and both apical and basal cell lineages of a proembryo after fertilization. Investigations reveal that cell fate determination involves multiple regulatory factors, such as positional information, differential distribution of cell fate determinants, cell-to-cell communication, and cell type-specific transcription factors. These factors temporally and spatially act for different cell type differentiation to ensure successful sexual reproduction. These new insights into regulatory mechanisms underlying sexual cell fate determination not only updates our knowledge on sexual plant reproduction, but also provides new ideas and tools for crop breeding.

The regulatory network and critical factors promoting programmed cell death during embryogenesis

Programmed cell death (PCD) is essential for animal and plant development. However, the knowledge of the mechanism regulating PCD in plants remains limited, largely due to technical limitations. Previously, we determined that the protease NtCP14 could trigger PCD in the embryonic suspensor of tobacco (Nicotiana tabacum), providing a unique opportunity to overcome the limitations by creating synchronous two-celled proembryos with ongoing PCD for transcriptome analysis and regulatory factor screening. Here, we performed comparative transcriptome analysis using isolated two-celled proembryos and explored the potential regulatory network underlying NtCP14-triggered PCD. Multiple phytohormones, calcium, microtubule organization, the immunity system, soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins, long non-coding RNAs and alternative splicing are addressed as critical factors involved in the early stage of suspensor PCD. Genes thought to play crucial roles in suspensor PCD are highlighted. Notably, decreased antioxidant gene expression and increased reactive oxygen species (ROS) levels during suspensor PCD suggest a critical role for ROS signaling in the initiation of NtCP14-triggered PCD. Furthermore, five genes in the regulatory network are recommended as immediate downstream elements of NtCP14. Together, our analysis outlines an overall molecular network underlying protease-triggered PCD and provides a reliable database and valuable clues for targeting elements immediately downstream of NtCP14 to overcome technical bottlenecks and gain deep insight into the molecular mechanism regulating plant PCD.

Stress-Induced Autophagy Is Essential for Microspore Cell Fate Transition to the Initial Cell of Androgenesis

The isolated microspores can be reprogrammed towards embryogenesis via stress treatment during in vitro culture, and produce (doubled) haploid plants as a breeding source of new genetic variability. However, the mechanism underlying the cell fate transition from gametogenesis to embryogenesis remains largely unknown. Here, we report that autophagy plays a key role in cell fate transition for microspore embryogenesis (referred to as androgenesis) in Nicotiana tabacum. Immunofluorescence and transmission electronic microscopy detection unveiled that autophagy was triggered in microspores following exposure to inductive stress, and a transient wave of the numerous autophagy-related genes (ATGs) expression occurred before the initiation of microspore embryogenesis. Suppression or promotion of the original autophagy levels could inhibit microspore embryogenesis, indicating that stress-induced autophagic homeostasis is essential for cell fate transition. Furthermore, quantitative proteomics analysis revealed that autophagy might be involved in lignin biosynthesis and chromatin decondensation for promoting reprogramming for androgenesis initiation. Altogether, we reveal an essential role of autophagy in the microspore cell fate transition and androgenesis initiation, providing novel insight for understanding this critical developmental process.

Comparison of the Ways in Which Nitidine Chloride and Bufalin Induce Programmed Cell Death in Hematological Tumor Cells

The objective of this work to study the programmed cell death (PCD) in hematological tumor cells induced by nitidine chloride (NC) and bufalin (BF). Hematological tumor cells were exposed to various doses of NC and BF to measure the level of growth inhibition. While inverted microscope is used to observe cell morphology, western blot technique is used to detect apoptosis-related protein expression levels. The effects of NC and BF on hematological tumor cells were different. Although abnormal cell morphology could be seen under the inverted microscope, the western blot results showed that the two medicines induced PCD through different pathways. Drug resistance varied in intensity across distinct cells. THP-1, Jurkat, and RPMI-8226 each had half maximum inhibitory concentrations (IC50) of 36.23 nM, 26.71 nM, and 40.46 nM in BF, and 9.24 µM, 4.33 µM, and 28.18 µM in NC, respectively. Different hematopoietic malignancy cells exhibit varying degrees of drug resistance, and the mechanisms by which apoptosis of hematologic tumor cells is triggered by NC and BF are also distinct.

Transcriptomic Profiling of Embryogenic and Non-Embryogenic Callus Provides New Insight into the Nature of Recalcitrance in Cannabis

Differential gene expression profiles of various cannabis calli including non-embryogenic and embryogenic (i.e., rooty and embryonic callus) were examined in this study to enhance our understanding of callus development in cannabis and facilitate the development of improved strategies for plant regeneration and biotechnological applications in this economically valuable crop. A total of 6118 genes displayed significant differential expression, with 1850 genes downregulated and 1873 genes upregulated in embryogenic callus compared to non-embryogenic callus. Notably, 196 phytohormone-related genes exhibited distinctly different expression patterns in the calli types, highlighting the crucial role of plant growth regulator (PGRs) signaling in callus development. Furthermore, 42 classes of transcription factors demonstrated differential expressions among the callus types, suggesting their involvement in the regulation of callus development. The evaluation of epigenetic-related genes revealed the differential expression of 247 genes in all callus types. Notably, histone deacetylases, chromatin remodeling factors, and EMBRYONIC FLOWER 2 emerged as key epigenetic-related genes, displaying upregulation in embryogenic calli compared to non-embryogenic calli. Their upregulation correlated with the repression of embryogenesis-related genes, including LEC2, AGL15, and BBM, presumably inhibiting the transition from embryogenic callus to somatic embryogenesis. These findings underscore the significance of epigenetic regulation in determining the developmental fate of cannabis callus. Generally, our results provide comprehensive insights into gene expression dynamics and molecular mechanisms underlying the development of diverse cannabis calli. The observed repression of auxin-dependent pathway-related genes may contribute to the recalcitrant nature of cannabis, shedding light on the challenges associated with efficient cannabis tissue culture and regeneration protocols.

The female germ unit is essential for pollen tube funicular guidance in Arabidopsis thaliana

In angiosperm, two immotile sperm cells are delivered to the female gametes for fertilization by a pollen tube, which perceives guidance cues from ovules at least at two critical sites, micropyle for short-distance guidance and funiculus for comparably longer distance guidance. Compared with the great progress in understanding pollen tube micropylar guidance, little is known about the signaling for funicular guidance. Here, we show that funiculus plays an important role in pollen tube guidance and report that female gametophyte (FG) plays a critical role in funicular guidance by analysis of a 3-dehydroquinate synthase (DHQS) mutant. Loss function of DHQS in FG interrupts pollen tube funicular guidance, suggesting that the guiding signal is generated from FG. We show the evidence that the capacity of funicular guidance is established during FG functional specification after the establishment of cell identity. Specific expression of DHQS in the synergid cells, central cells, or egg cells can rescue funicular guidance defect in dhqs/+, indicating all the female germ unit cells are involved in the funicular guidance. The finding reveals that the attracting signal of pollen tube funicular guidance was generated at a site and stage manner and provides novel clue to locate and search for the signal.

Phospholipase C is a novel regulator at the early stages of microspore embryogenesis in Nicotiana tabacum

Microspore transfers the developmental fate into embryogenesis in vitro regulated by determinant factors of stress-induced. However, the key regulators of microspore embryogenesis (ME) are still largely undiscovered to reveal the mechanism of cell fate transition. Here, we report that Phospholipase C (PLC) is involved at the early stages of ME in Nicotiana tabacum. NtPLC2/3/4 are expressed at the initial stages of ME. The expression levels of NtPLC2/3 are transient activated after 3 days in culture, while the expression level of NtPLC4 maintains relatively stable. Inhibition of PLCs induces the decrease in NtPLC2/3/4 expression level and decline of ME yield. We confirm that lipids in microspore are degraded and then re-accumulate at first embryonic division stage. Inhibition of PLCs suppresses the lipids metabolism at the early stages of ME. Thus, we propose that PLCs-mediated lipid metabolism is a novel regulator at the early stages of ME.

Molecular Aspects of Seed Development Controlled by Gibberellins and Abscisic Acids

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

The parental contributions to early plant embryogenesis and the concept of maternal-to-zygotic transition in plants

The maternal-to-zygotic transition (MZT) is a major developmental transition in the life cycles of animals. It consists of two associated processes: maternal transcript clearance and zygotic genome activation (ZGA). The concept of MZT has been controversially discussed in plants. In this short review, we summarize recent advances in understanding the timing of ZGA and the similarities and differences between ZGA in eudicots and monocots. We discuss the parental contributions to the transcriptome of the proembryo and parental control of early embryogenesis, and we examine distinct differences in the ZGA between animals and plants, update relevant concepts on MZT, and highlight outstanding questions in this field.

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.

Autophagy in sexual plant reproduction: new insights

Autophagy is a mechanism by which damaged or unwanted cells are degraded and their constituents recycled. Over the past decades, research focused on autophagy has expanded from yeast to mammals and plants, and the core machinery regulating autophagy appears to be conserved. In plants, autophagy has essential roles in responses to stressful conditions and also contributes to normal development, especially in the context of reproduction. Here, based on recent efforts to understand the roles and molecular mechanisms underlying autophagy, we highlight the specific roles of autophagy in plant reproduction and provide new insights for further studies.

Plant development: Suspensors as a battlefield for parental tug-of-war?

Parental contributions to zygotes can influence early embryogenesis and may regulate the distribution of maternal resources to progeny. A new study in Arabidopsis thaliana has demonstrated that signaling components from maternal sporophytic tissues and paternal gametes converge in zygotes to promote elongation of the extraembryonic suspensor, which supports the developing embryo proper.

Gene expression variation in Arabidopsis embryos at single-nucleus resolution

Soon after fertilization of egg and sperm, plant genomes become transcriptionally activated and drive a series of coordinated cell divisions to form the basic body plan during embryogenesis. Early embryonic cells rapidly diversify from each other, and investigation of the corresponding gene expression dynamics can help elucidate underlying cellular differentiation programs. However, current plant embryonic transcriptome datasets either lack cell-specific information or have RNA contamination from surrounding non-embryonic tissues. We have coupled fluorescence-activated nuclei sorting together with single-nucleus mRNA-sequencing to construct a gene expression atlas of Arabidopsis thaliana early embryos at single-cell resolution. In addition to characterizing cell-specific transcriptomes, we found evidence that distinct epigenetic and transcriptional regulatory mechanisms operate across emerging embryonic cell types. These datasets and analyses, as well as the approach we devised, are expected to facilitate the discovery of molecular mechanisms underlying pattern formation in plant embryos. This article has an associated 'The people behind the papers' interview.

Endosperm development is an autonomously programmed process independent of embryogenesis

The seeds of flowering plants contain three genetically distinct structures: the embryo, endosperm, and seed coat. The embryo and endosperm need to interact and exchange signals to ensure coordinated growth. Accumulating evidence has confirmed that embryo growth is supported by the nourishing endosperm and regulated by signals originating from the endosperm. Available data also support that endosperm development requires communication with the embryo. Here, using single-fertilization mutants, Arabidopsis thaliana dmp8 dmp9 and gex2, we demonstrate that in the absence of a zygote and embryo, endosperm initiation, syncytium formation, free nuclear cellularization, and endosperm degeneration occur as in the wild type in terms of the cytological process and time course. Although rapid embryo expansion accelerates endosperm breakdown, our findings strongly suggest that endosperm development is an autonomously organized process, independent of egg cell fertilization and embryo-endosperm communication. This work confirms both the altruistic and self-directed nature of the endosperm during coordinated embryo-endosperm development. Our findings provide insights into the intricate interaction between the two fertilization products and will help to distinguish the physiological roles of the signaling between endosperm and embryo. These findings also open new avenues in agro-biotechnology for crop improvement.

Equal parental contribution to the transcriptome is not equal control of embryogenesis

In animals, early embryogenesis is maternally controlled, whereas in plants, parents contribute equally to the proembryo transcriptome. Thus, the question remains whether equivalent parental contribution to the transcriptome of the early proembryo means equal control of early embryogenesis. Here, on the basis of cell-lineage-specific and allele-specific transcriptome analysis, we reveal that paternal and maternal genomes contribute equally to the transcriptomes of both the apical cell lineage and the basal cell lineage of early proembryos. However, a strong maternal effect on basal cell lineage development was found, indicating that equal parental contribution to the transcriptome is not necessarily coupled with equivalent parental control of proembryonic development. Parental contributions to embryogenesis therefore cannot be concluded solely on the basis of the ratio of paternal/maternal transcripts. Furthermore, we demonstrate that parent-of-origin genes display developmental-stage-dependent and cell-lineage-dependent allelic expression patterns. These findings will facilitate the investigation of specific parental roles in specific processes of early embryogenesis.

Three STIGMA AND STYLE STYLISTs Pattern the Fine Architectures of Apical Gynoecium and Are Critical for Male Gametophyte-Pistil Interaction

The gynoecium is derived from the fusion of carpels and is considered to have evolved from a simple setup followed by adaptive adjustment in cell type and tissue distribution to facilitate efficient sexual reproduction [1, 2]. As a sequence of the adjustment, the apical gynoecium differentiates into a stigma and a style. Both the structural patterning and functional specification of the apical gynoecium are critical for plant fertility [3, 4]. However, how the fine structures of the apical gynoecium are established at the interface interacting with pollen and pollen tubes remain to be elucidated. Here, we report a novel angiosperm-specific gene family, STIGMA AND STYLE STYLIST 1-3 (SSS1, SSS2, and SSS3). The SSS1 expresses predominately in the transmitting tract tissue of style, SSS2 expresses intensively in stigma, and SSS3 expresses mainly in stylar peripheral region round the transmitting tract. SSSs coregulate the patterning of the apical gynoecium via controlling cell expansion or elongation. Both the architecture and function of apical gynoecium can be affected by the alteration of SSS expression, indicating their critical roles in the establishment of a proper female interface for communication with pollen tubes. The NGATHA3 (NGA3) transcription factor [5, 6] can directly bind to SSSs promoter and control SSSs expression. Overexpression of SSSs could rescue the stylar defect of nga1nga3 double mutant, indicating their context in the same regulatory pathway. Our findings reveal a novel molecular mechanism responsible for patterning the fine architecture of apical gynoecium and establishing a proper interface for pollen tube growth, which is therefore crucial for plant sexual reproduction.

Detection of Embryonic Suspensor Cell Death by Whole-Mount TUNEL Assay in Tobacco

Embryonic suspensor in angiosperms is a short-lived structure that connects the embryo to surrounding maternal tissues, which is necessary for early embryogenesis. Timely degeneration via programed cell death is the most distinct feature of the suspensor during embryogenesis. Therefore, the molecular mechanism regulating suspensor cell death is worth in-depth study for embryonic development. However, this process can hardly be detected using conventional methods since early embryos are deeply embedded in the seed coats and inaccessible through traditional tissue section. Hence, it is necessary to develop a reliable protocol for terminal deoxynucleotidyl transferase (TdT) dUTP Nick-End Labeling (TUNEL) analysis using limited living early embryos. Here, we provide a detailed protocol for the whole-mount detection of suspensor cell death using a TUNEL system in tobacco. This method is especially useful for the direct and rapid detection of the spatial-temporal characters of programed cell death during embryogenesis, as well as for the diminishment of the artifacts during material treatment by traditional methods.

The short and intricate life of the suspensor

The suspensor is a short-lived tissue critical for proper embryonic development in many higher plants. While the tissue was initially thought to simply suspend the embryo in the endosperm, it has been found through decades of research that it serves multiple important purposes. The suspensor has been found to be vital for proper embryo patterning and numerous studies have been undertaken into the complex transcriptional cross-talk between the suspensor and the embryo proper. Indeed, many suspensor mutants also display abnormalities in the embryo. The suspensor's role as a nutrient conduit has been shown using ultrastructural and histochemical techniques. Biochemical approaches have found that the suspensor is a centre of early embryonic hormone production in several species. The suspensor has also been frequently used as a model for programmed cell death as it shows signs of termination almost immediately upon developing. This review covers the essential functions of the suspensor throughout its short existence from multiple disciplines including structural, genetic and biochemical perspectives.