Plant Reproduction最新文献

Key message: Anther development and dehiscence is considered from an evolutionary perspective to identify drivers for differentiation, functional conservation and to identify key questions for future male reproduction research. Development of viable pollen and its timely release from the anther are essential for fertilisation of angiosperm flowers. The formation and subsequent dehiscence of the anther are under tight regulatory control, and these processes are remarkably conserved throughout the diverse families of the angiosperm clade. Anther development is a complex process, which requires timely formation and communication between the multiple somatic anther cell layers (the epidermis, endothecium, middle layer and tapetum) and the developing pollen. These layers go through regulated development and selective degeneration to facilitate the formation and ultimate release of the pollen grains. Insight into the evolution and divergence of anther development and dehiscence, especially between monocots and dicots, is driving greater understanding of the male reproductive process and increased, resilient crop yields. This review focuses on anther structure from an evolutionary perspective by highlighting their diversity across plant species. We summarise new findings that illustrate the complexities of anther development and evaluate how they challenge established models of anther form and function, and how they may help to deliver future sustainable crop yields.

The vegetative cell of the angiosperm male gametophyte (pollen) functions as a free-living, single-celled organism that both produces and transports sperm to egg. Whole-genome duplication (WGD) should have strong effects on pollen because of the haploid to diploid transition and because of both genetic and epigenetic effects on cell-level phenotypes. To disentangle historical effects of WGD on pollen performance, studies can compare 1n pollen from diploids to neo-2n pollen from diploids and synthetic autotetraploids to older 2n pollen from established neo-autotetraploids. WGD doubles both gene number and bulk nuclear DNA mass, and a substantial proportion of diploid and autotetraploid heterozygosity can be transmitted to 2n pollen. Relative to 1n pollen, 2n pollen can exhibit heterosis due to higher gene dosage, higher heterozygosity and new allelic interactions. Doubled genome size also has consequences for gene regulation and expression as well as epigenetic effects on cell architecture. Pollen volume doubling is a universal effect of WGD, whereas an increase in aperture number is common among taxa with simultaneous microsporogenesis and pored apertures, mostly eudicots. WGD instantly affects numerous evolved compromises among mature pollen functional traits and these are rapidly shaped by highly diverse tissue interactions and pollen competitive environments in the early post-WGD generations. 2n pollen phenotypes generally incur higher performance costs, and the degree to which these are met or evolve by scaling up provisioning and metabolic vigor needs further study.

Key message: Comparative genetics and genomics among green plants, including algae, provide deep insights into the evolution of land plant sexual reproduction. Land plants have evolved successive changes during their conquest of the land and innovations in sexual reproduction have played a major role in their terrestrialization. Recent years have seen many revealing dissections of the molecular mechanisms of sexual reproduction and much new genomics data from the land plant lineage, including early diverging land plants, as well as algae. This new knowledge is being integrated to further understand how sexual reproduction in land plants evolved, identifying highly conserved factors and pathways, but also molecular changes that underpinned the emergence of new modes of sexual reproduction. Here, we review recent advances in the knowledge of land plant sexual reproduction from an evolutionary perspective and also revisit the evolution of angiosperm double fertilization.

In plants, small RNAs have been recognized as key genetic and epigenetic regulators of development. Small RNAs are usually 20 to 30 nucleotides in length and they control, in a sequence specific manner, the transcriptional or post-transcriptional expression of genes. In this review, we present a comprehensive overview of the most recent findings about the function of small RNAs in ovule development, including megasporogenesis and megagametogenesis, both in sexual and apomictic plants. We discuss recent studies on the role of miRNAs, siRNAs and trans-acting RNAs (ta-siRNAs) in early female germline differentiation. The mechanistic complexity and unique regulatory features are reviewed, and possible directions for future research are provided.

Key message: Biolistic delivery into pollen. In recent years, genome editing techniques, such as the CRISPR/Cas9 system, have been highlighted as a new approach to plant breeding. Agrobacterium-mediated transformation has been widely utilized to generate transgenic plants by introducing plasmid DNA containing CRISPR/Cas9 into plant cells. However, this method has general limitations, such as the limited host range of Agrobacterium and difficulties in tissue culture, including callus induction and regeneration. To avoid these issues, we developed a method to genetically modify germ cells without the need for Agrobacterium-mediated transfection and tissue culture using tobacco as a model. In this study, plasmid DNA containing sequences of Cas9, guide RNA, and fluorescent reporter was introduced into pollen using a biolistic delivery system. Based on the transient expression of fluorescent reporters, the Arabidopsis UBQ10 promoter was found to be the most suitable promoter for driving the expression of the delivered gene in pollen tubes. We also evaluated the delivery efficiency in male germ cells in the pollen by expression of the introduced fluorescent marker. Mutations were detected in the target gene in the genomic DNA extracted from CRISPR/Cas9-introduced pollen tubes, but were not detected in the negative control. Bombarded pollen germinated pollen tubes and delivered their contents into the ovules in vivo. Although it is necessary to improve biolistic delivery efficiency and establish a method for the screening of genome-modified seeds, our findings provide important insights for the detection and production of genome-modified seeds by pollen biolistic delivery.

The two-sex model makes the assumption that there are only two sexual reproductive states: male and female. However, in land plants (embryophytes) the application of this model to the alternation of generations life cycle requires the subtle redefinition of several common terms related to sexual reproduction, which seems to obscure aspects of one or the other plant generation: For instance, the homosporous sporophytic plant is treated as being asexual, and the gametophytes of angiosperms treated like mere gametes. In contrast, the proposal is made that the sporophytes of homosporous plants are indeed sexual reproductive organisms, as are the gametophytes of heterosporous plants. This view requires the expansion of the number of sexual reproductive states we accept for these plant species; therefore, a three-sex model for homosporous plants and a four-sex model for heterosporous plants are described and then contrasted with the current two-sex model. These new models allow the use of sexual reproductive terms in a manner largely similar to that seen in animals, and may better accommodate the plant alternation of generations life cycle than does the current plant two-sex model. These new models may also help stimulate new lines of research, and examples of how they might alter our view of events in the flower, and may lead to new questions about sexual determination and differentiation, are presented. Thus it is suggested that land plant species have more than merely two sexual reproductive states and that recognition of this may promote our study and understanding of them.

Key message: Developmental and transcriptomic analysis of Brachypodium embryogenesis and comparison with Arabidopsis identifies conserved and divergent phases of embryogenesis and reveals widespread heterochrony of developmental gene expression. Embryogenesis, transforming the zygote into the mature embryo, represents a fundamental process for all flowering plants. Current knowledge of cell specification and differentiation during plant embryogenesis is largely based on studies of the dicot model plant Arabidopsis thaliana. However, the major crops are monocots and the transcriptional programs associated with the differentiation processes during embryogenesis in this clade were largely unknown. Here, we combined analysis of cell division patterns with development of a temporal transcriptomic resource during embryogenesis of the monocot model plant Brachypodium distachyon. We found that early divisions of the Brachypodium embryo were highly regular, while later stages were marked by less stereotypic patterns. Comparative transcriptomic analysis between Brachypodium and Arabidopsis revealed that early and late embryogenesis shared a common transcriptional program, whereas mid-embryogenesis was divergent between species. Analysis of orthology groups revealed widespread heterochronic expression of potential developmental regulators between the species. Interestingly, Brachypodium genes tend to be expressed at earlier stages than Arabidopsis counterparts, which suggests that embryo patterning may occur early during Brachypodium embryogenesis. Detailed investigation of auxin-related genes shows that the capacity to synthesize, transport and respond to auxin is established early in the embryo. However, while early PIN1 polarity could be confirmed, it is unclear if an active response is mounted. This study presents a resource for studying Brachypodium and grass embryogenesis and shows that divergent angiosperms share a conserved genetic program that is marked by heterochronic gene expression.

Key message: Short-term heat stress during male meiosis causes defects in crossover formation, meiotic progression and cell wall formation in the monocot barley, ultimately leading to pollen abortion. High temperature conditions cause a reduction of fertility due to alterations in meiotic processes and gametogenesis. The male gametophyte development has been shown to be particularly sensitive to heat stress, and even short-term and modest temperature shifts cause alterations in crossover formation. In line with previous reports, we observed that male meiosis in the monocot barley exposed for 24-45 h to heat stress (32-42 °C) partially or completely eliminates obligate crossover formation and causes unbalanced chromosome segregation and meiotic abortion. Depending on the severity of heat stress, the structure and organization of the chromosomes were altered. In addition to alterations in chromosome structure and dynamics, heat treatment abolished or reduced the formation of a callose wall surrounding the meiocytes and interrupted the cell cycle progression leading to cytokinesis defects and microspore cell death.

Key message: Intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during early stages of pollen development. To improve our understanding of the gene regulatory mechanisms that drive developmental processes, we performed a genome-wide study of alternative splicing and isoform switching during five key stages of pollen development in field mustard, Brassica rapa. Surprisingly, for several hundred genes (12.3% of the genes analysed), isoform switching results in stage-specific expression of intron-retaining transcripts at the meiotic stage of pollen development. In such cases, we report temporally regulated switching between expression of a canonical, translatable isoform and an intron-retaining transcript that is predicted to produce a truncated and presumably inactive protein. The results suggest a new pervasive mechanism underlying modulation of protein levels in a plant developmental program. The effect is not based on gene expression induction but on the type of transcript produced. We conclude that intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during meiosis, especially genes related to ribosome biogenesis, mRNA transport and nuclear envelope architecture. We also propose that stage-specific expression of a non-functional isoform of Brassica rapa BrSDG8, a non-redundant member of histone methyltransferase gene family, linked to alternative splicing regulation, may contribute to the intron retention observed.

Key message: Characterization of hybrid seed failure in Prunus provides insight into conserved or lineage-specific hybrid incompatibility mechanisms in plant species. Postzygotic hybrid incompatibility resulting from a cross between different species involves complex mechanisms occurring at various developmental stages. Embryo arrest, followed by seed abortion, is the first stage of such incompatibility reactions and inhibits hybrid seed development. In Prunus, a rosaceous woody species, some interspecific crosses result in fruit drop during the early stage of fruit development, in which inferior seed development may be accounted for the observed hybrid incompatibility. In this study, we investigated ovule development and the transcriptomes of developing ovules in inter-subgeneric crosses of Prunus. We conducted a cross of Prunus mume (subgenus Prunus), pollinated by P. persica (subgenus Amygdalus), and found that ovule and seed coat degeneration occurs before fruit drop. Transcriptome analysis identified differentially expressed genes enriched in several GO pathways, including organelle development, stimulus response, and signaling. Among these pathways, the organelle-related genes were actively regulated during ovule development, as they showed higher expression in the early stage of interspecific crosses and declined in the later stage, suggesting that the differential regulation of organelle function may induce the degeneration of hybrid ovules. Additionally, genes related to ovule and seed coat development, such as genes encoding AGL-like and auxin response, were differentially regulated in Prunus interspecific crosses. Our results provide histological and molecular information on hybrid seed abortion in Prunus that could be utilized to develop new hybrid crops. Additionally, we compared and discussed transcriptome responses to hybrid seed failure in Prunus and other plant species, which provides insight into conserved or lineage-specific hybrid incompatibility mechanisms in some plant species.