Plant Reproduction最新文献

Self-incompatibility systems based on self-recognition evolved in hermaphroditic plants to maintain genetic variation of offspring and mitigate inbreeding depression. Despite these benefits in diploid plants, for polyploids who often face a scarcity of mating partners, self-incompatibility can thwart reproduction. In contrast, self-compatibility provides an immediate advantage: a route to reproductive viability. Thus, diploid selfing lineages may facilitate the formation of new allopolyploid species. Here, we describe the mechanism of establishment of at least four allopolyploid species in Brassicaceae (Arabidopsis suecica, Arabidopsis kamchatica, Capsella bursa-pastoris, and Brassica napus), in a manner dependent on the prior loss of the self-incompatibility mechanism in one of the ancestors. In each case, the degraded S-locus from one parental lineage was dominant over the functional S-locus of the outcrossing parental lineage. Such dominant loss-of-function mutations promote an immediate transition to selfing in allopolyploids and may facilitate their establishment.

At the heart of meiosis is crossover recombination, i.e., reciprocal exchange of chromosome fragments between parental genomes. Surprisingly, in most eukaryotes, including plants, several recombination pathways that can result in crossover event operate in parallel during meiosis. These pathways emerged independently in the course of evolution and perform separate functions, which directly translate into their roles in meiosis. The formation of one crossover per chromosome pair is required for proper chromosome segregation. This "obligate" crossover is ensured by the major crossover pathway in plants, and in many other eukaryotes, known as the ZMM pathway. The secondary pathways play important roles also in somatic cells and function mainly as repair mechanisms for DNA double-strand breaks (DSBs) not used for crossover formation. One of the consequences of the functional differences between ZMM and other DSB repair pathways is their distinct sensitivities to polymorphisms between homologous chromosomes. From a population genetics perspective, these differences may affect the maintenance of genetic variability. This might be of special importance when considering that a significant portion of plants uses inbreeding as a predominant reproductive strategy, which results in loss of interhomolog polymorphism. While we are still far from fully understanding the relationship between meiotic recombination pathways and genetic variation in populations, recent studies of crossovers in plants offer a new perspective.

Meiosis is a highly conserved specialised cell division in sexual life cycles of eukaryotes, forming the base of gene reshuffling, biological diversity and evolution. Understanding meiotic machinery across different plant lineages is inevitable to understand the lineage-specific evolution of meiosis. Functional and cytogenetic studies of meiotic proteins from all plant lineage representatives are nearly impossible. So, we took advantage of the genomics revolution to search for core meiotic proteins in accumulating plant genomes by the highly sensitive homology search approaches, PSI-BLAST, HMMER and CLANS. We could find that most of the meiotic proteins are conserved in most of the lineages. Exceptionally, Arabidopsis thaliana ASY4, PHS1, PRD2, PRD3 orthologs were mostly not detected in some distant algal lineages suggesting their minimal conservation. Remarkably, an ancestral duplication of SPO11 to all eukaryotes could be confirmed. Loss of SPO11-1 in Chlorophyta and Charophyta is likely to have occurred, suggesting that SPO11-1 and SPO11-2 heterodimerisation may be a unique feature in land plants of Viridiplantae. The possible origin of the meiotic proteins described only in plants till now, DFO and HEIP1, could be traced and seems to occur in the ancestor of vascular plants and Streptophyta, respectively. Our comprehensive approach is an attempt to provide insights about meiotic core proteins and thus the conservation of meiotic pathways across plant kingdom. We hope that this will serve the meiotic community a basis for further characterisation of interesting candidates in future.

Key message: In barley (Hordeum vulgare), MTOPVIB is critical for meiotic DSB and accompanied SC and CO formation while dispensable for meiotic bipolar spindle formation. Homologous recombination during meiosis assures genetic variation in offspring. Programmed meiotic DNA double-strand breaks (DSBs) are repaired as crossover (CO) or non-crossover (NCO) during meiotic recombination. The meiotic topoisomerase VI (TopoVI) B subunit (MTOPVIB) plays an essential role in meiotic DSB formation critical for CO-recombination. More recently MTOPVIB has been also shown to play a role in meiotic bipolar spindle formation in rice and maize. Here, we describe a meiotic DSB-defective mutant in barley (Hordeum vulgare L.). CRISPR-associated 9 (Cas9) endonuclease-generated mtopVIB plants show complete sterility due to the absence of meiotic DSB, synaptonemal complex (SC), and CO formation leading to the occurrence of univalents and their unbalanced segregation into aneuploid gametes. In HvmtopVIB plants, we also frequently found the bi-orientation of sister kinetochores in univalents during metaphase I and the precocious separation of sister chromatids during anaphase I. Moreover, the near absence of polyads after meiosis II, suggests that despite being critical for meiotic DSB formation in barley, MTOPVIB seems not to be strictly required for meiotic bipolar spindle formation.

Key message: Vegetative-to-reproductive phase transition in female cannabis seedlings occurs autonomously with the de novo development of single flowers. To ensure successful sexual reproduction, many plant species originating from seedlings undergo juvenile-to-adult transition. This phase transition precedes and enables the vegetative-to-reproductive shift in plants, upon perception of internal and/or external signals such as temperature, photoperiod, metabolite levels, and phytohormones. This study demonstrates that the juvenile seedlings of cannabis gradually shift to the adult vegetative stage, as confirmed by the formation of lobed leaves, and upregulation of the phase-transition genes. In the tested cultivar, the switch to the reproductive stage occurs with the development of a pair of single flowers in the 7th node. Histological analysis indicated that transition to the reproductive stage is accomplished by the de novo establishment of new flower meristems which are not present in a vegetative stage, or as dormant meristems at nodes 4 and 6. Moreover, there were dramatic changes in the transcriptomic profile of flowering-related genes among nodes 4, 6, and 7. Downregulation of flowering repressors and an intense increase in the transcription of phase transition-related genes occur in parallel with an increase in the transcription of flowering integrators and meristem identity genes. These results support and provide molecular evidence for previous findings that cannabis possesses an autonomous flowering mechanism and the transition to reproductive phase is controlled in this plant mainly by internal signals.

Key message: Differential spatial and temporal expression patterns due to regulatory cis-elements and two different isoforms are detected among CpMDAR4 alleles in papaya. The aim of this research was to study the effects of cis-element differences between the X, Y and Y^h alleles on the expression of CpMDAR4, a potential candidate gene for sex differentiation in papaya, using a transcriptional reporter system in a model species Arabidopsis thaliana. Possible effects of a retrotransposon insertion in the Y and Y^h alleles on the transcription and expression of CpMDAR4 alleles in papaya flowers were also examined. When comparing promoters and cis-regulatory elements among genes in the non-recombining region of the sex chromosomes, paired genes exhibited differences. Our results showed that differences in the promoter sequences of the CpMDAR4 alleles drove the expression of a reporter gene to different flower tissues in Arabidopsis. β-glucuronidase staining analysis of T₂ and T₃ lines for constructs containing 5' deletions of native Y and Y^h allele promoters showed the loss of specific expression of the reporter gene in the anthers, confirming the existence and location of cis-regulatory element POLLEN1LELAT52. The expression analysis of CpMDAR4 alleles in papaya flowers also showed that all alleles are actively expressed in different flower tissues, with the existence of a shorter truncated isoform, with unknown function, for the Y and Y^h alleles due to an LTR-RT insertion in the Y and Y^h chromosomes. The observed expression patterns in Arabidopsis thaliana flowers and the expression patterns of CpMDAR4 alleles in papaya flowers suggest that MDAR4 might have a role on development of reproductive organs in papaya, and that it constitutes an important candidate for sex differentiation.

Key message: ClearSee alpha and FAST9 were optimized for imaging Arabidopsis seeds up to the torpedo stages. The methods preserve the fluorescence of reporter proteins and seed shape, allowing phenotyping embryos in intact seeds. Tissue clearing methods eliminate the need for sectioning, thereby helping better understand the 3D organization of tissues and organs. In the past fifteen years, clearing methods have been developed to preserve endogenous fluorescent protein tags. Some of these methods (ClearSee, TDE, PEA-Clarity, etc.) were adapted to clear various plant species, with the focus on roots, leaves, shoot apical meristems, and floral parts. However, these methods have not been used in developing seeds beyond the early globular stage. Tissue clearing is problematic in post-globular seeds due to various apoplastic barriers and secondary metabolites. In this study, we compared six methods for their efficiency in clearing Arabidopsis thaliana seeds at post-globular embryonic stages. Three methods (TDE, ClearSee, and ClearSee alpha) have already been reported in plants, whereas the others (fsDISCO, FAST9, and CHAPS clear) are used in this context for the first time. These methods were assessed for seed morphological changes, clearing capacity, removal of tannins, and spectral properties. We tested each method in seeds from globular to mature stages. The pros and cons of each method are listed herein. ClearSee alpha appears to be the method of choice as it preserves seed morphology and prevents tannin oxidation. However, FAST9 with 60% iohexol as a mounting medium is faster, clears better, and appears suitable for embryonic shape imaging. Our results may guide plant researchers to choose a suitable method for imaging fluorescent protein-labeled embryos in intact Arabidopsis seeds.

Key message: The first record of gene expression during seed development within the Nymphaeales provides evidence for a variety of biological processes, including dynamic epigenetic patterning during sexual reproduction in the water lily Nymphaea thermarum. Studies of gene expression during seed development have been performed for a growing collection of species from a phylogenetically broad sampling of flowering plants (angiosperms). However, angiosperm lineages whose origins predate the divergence of monocots and eudicots have been largely overlooked. In order to provide a new resource for understanding the early evolution of seed development in flowering plants, we sequenced transcriptomes of whole ovules and seeds from three key stages of reproductive development in the waterlily Nymphaea thermarum, an experimentally tractable member of the Nymphaeales. We first explore patterns of gene expression, beginning with mature ovules and continuing through fertilization into early- and mid-stages of seed development. We find patterns of gene expression that corroborate histological/morphological observations of seed development in this species, such as expression of genes involved in starch synthesis and transcription factors that have been associated with embryo and endosperm development in other species. We also find evidence for processes that were previously not known to be occurring during seed development in this species, such as epigenetic modification. We then examine the expression of genes associated with patterning DNA and histone methylation-processes that are essential for seed development in distantly related and structurally diverse monocots and eudicots. Around 89% of transcripts putatively homologous to DNA and histone methylation modifiers are expressed during seed development in N. thermarum, including homologs of genes known to pattern imprinting-related epigenetic modifications. Our results suggest that dynamic epigenetic patterning is a deeply conserved aspect of angiosperm seed development.

It is widely known that an optimal nucleotide sequence context immediately upstream of the AUG start codon greatly improves the efficiency of translation initiation of mRNA in mammalian and plant somatic cells, which in turn increases protein levels. However, it is still unclear whether a similar regulatory mechanism is also present in highly differentiated cells. Here, we surveyed this issue in Arabidopsis thaliana sperm cells and found that the sequence context-mediated regulation of translation initiation in sperm cells is generally similar to that in somatic cells. A simple motif of four adenine nucleotides at positions - 1 to - 4 greatly improved the efficiency of translation initiation, and when the motif was present there, translation was even initiated at some non-AUG codons in sperm cells. However, unlike that in mammalian cells, a mainly effective nucleotide site to regulate the efficiency of translation initiation was not present at positions - 1 to - 4 in sperm cells. Meanwhile, different from somatic cells, sperm cells did not use eukaryotic translation initiation factor 1 to regulate the efficiency in a poor context consisting of the lowest frequency nucleotides. All these results contribute to our understanding of the cytoplasmic event of translation initiation in highly differentiated sperm cells.