Central Cell in Flowering Plants: Specification, Signaling, and Evolution

Conserved CKI1-mediated signaling is required for female germline specification in Marchantia polymorpha

In land plants, gametes derive from a small number of dedicated haploid cells.1 In angiosperms, one central cell and one egg cell are differentiated in the embryo sac as female gametes for double fertilization, while in non-flowering plants, only one egg cell is generated in the female sexual organ, called the archegonium.2,3 The central cell specification of Arabidopsis thaliana is controlled by the histidine kinase CYTOKININ-INDEPENDENT 1 (CKI1), which is a two-component signaling (TCS) activator sharing downstream regulatory components with the cytokinin signaling pathway.4,5,6,7 Our phylogenetic analysis suggested that CKI1 orthologs broadly exist in land plants. However, the role of CKI1 in non-flowering plants remains unclear. Here, we found that the sole CKI1 ortholog in the liverwort Marchantia polymorpha, MpCKI1, which functions through conserved downstream TCS components, regulates the female germline specification for egg cell development in the archegonium. In M. polymorpha, the archegonium develops three-dimensionally from a single cell accumulating MpBONOBO (MpBNB), a master regulator for germline initiation and differentiation.8 We visualized female germline specification by capturing the distribution pattern of MpBNB in discrete stages of early archegonium development, and found that MpBNB accumulation is restricted to female germline cells. MpCKI1 is required for the proper MpBNB accumulation in the female germline, and is critical for the asymmetric cell divisions that specify the female germline cells. These results suggest that CKI1-mediated TCS originated during early land plant evolution and participates in female germ cell specification in deeply diverged plant lineages.

Sexual reproduction: Is the genetic pathway for female germ cell specification conserved in land plants?

Land plants share several core factors responsible for female gametophyte development, despite their differing structures and developmental programs. New work providing molecular dissection of reproductive phases in non-angiosperm plants is a powerful tool for elucidating the underlying genetic network.

Oat species and interspecific amphiploids show predominance of diploid nuclei in the syncytial endosperm

Apart from apomictic types, the Polygonum-type eight-nuclear embryo sac is considered to be dominant in grasses. A triploid endosperm is formed as a result of double fertilisation. This study showed, for the first time, the dominance of diploid nuclei in the syncytial stage of the central cell of embryo sac in oat species and amphiploids. The dominance of diploid nuclei, which were the basis for the formation of polyploid nuclei, was weaker in amphiploids due to aneuploid events. The genomic in situ hybridisation method applied in the study did not distinguish the maternal and paternal haploid nuclei of embryo sac. However, this method demonstrated the lack of a set of genomes of one haploid nucleus. Embryological analyses of the initial stages of oat endosperm development revealed a fertilised egg cell, and two polar nuclei differing in size. It can be assumed that the formation of diploid oat endosperm occurred after the fusion of one polar nucleus and the nucleus of a male gamete, while the second polar nucleus gave rise to 1n nuclei. The levels of ploidy of syncytial nuclei were not influenced by both aneuploid events and correlated with pollen developmental anomalies. The differences in the analysed cytogenetic events distinguished amphiploids and their parental species in the ordination space.

Re-imagining Reproduction: The Queer Possibilities of Plants

How did plant sexuality come to so hauntingly resemble human sexual formations? How did plant biology come to theorize plant sexuality with binary formulations of male/female, sex/gender, sperm/egg, active males and passive females-all of which resemble western categories of sex, gender, and sexuality? Tracing the extant language of sex and sexuality in plant reproductive biology, we examine the histories of science to explore how plant reproductive biology emerged historically from formations of colonial racial and sexual politics and how evolutionary biology was premised on the imaginations of racialized heterosexual romance. Drawing on key examples, the paper aims to (un)read plant sexuality and sexual anatomy and bodies to imagine new possibilities for plant sex, sexualities, and their relationalities. In short, plant sex and sexuality are not two different objects of inquiry but are intimately related-it is their inter-relation that is the focus of this essay. One of the key impulses from the humanities that we bring to this essay is a careful consideration of how terms and terminologies are related to each other historically and culturally. In anthropomorphizing plants, if plant sexuality were modeled on human sexual formations, might a re-imagination of plant sexuality open new vistas for the biological sciences? While our definitions of plant sexuality will always be informed by contemporary society and culture, interrogating the histories of our theories and terminologies can help us reimagine a biology that allows for new and more accurate understandings of plants, plant biology, and the evolution of reproduction.

The phenomenon of autonomous endosperm in sexual and apomictic plants

Endosperm is a key nutritive tissue that supports the developing embryo or seedling, and serves as a major nutritional source for human and livestock feed. In sexually-reproducing flowering plants, it generally develops after fertilization. However, autonomous endosperm (AE) formation (i.e. independent of fertilization) is also possible. Recent findings of AE loci/ genes and aberrant imprinting in native apomicts, together with a successful initiation of parthenogenesis in rice and lettuce, have enhanced our understanding of the mechanisms bridging sexual and apomictic seed formation. However, the mechanisms driving AE development are not well understood. This review presents novel aspects related to AE development in sexual and asexual plants underlying stress conditions as the primary trigger for AE. Both application of hormones to unfertilized ovules and mutations that impair epigenetic regulation lead to AE development in sexual Arabidopsis thaliana, which may point to a common pathway for both phenomena. Apomictic-like AE development under experimental conditions can take place due to auxin-dependent gene expression and/or DNA methylation.

Central-cell-produced attractants control fertilization recovery

Animal fertilization relies on hundreds of sperm racing toward the egg, whereas, in angiosperms, only two sperm cells are delivered by a pollen tube to the female gametes (egg cell and central cell) for double fertilization. However, unsuccessful fertilization under this one-pollen-tube design can be detrimental to seed production and plant survival. To mitigate this risk, unfertilized-gamete-controlled extra pollen tube entry has been evolved to bring more sperm cells and salvage fertilization. Despite its importance, the underlying molecular mechanism of this phenomenon remains unclear. In this study, we report that, in Arabidopsis, the central cell secretes peptides SALVAGER1 and SALVAGER2 in a directional manner to attract pollen tubes when the synergid-dependent attraction fails or is terminated by pollen tubes carrying infertile sperm cells. Moreover, loss of SALs impairs the fertilization recovery capacity of the ovules. Therefore, this research uncovers a female gamete-attraction system that salvages seed production for reproductive assurance.

Seed nutripriming with zinc is an apt tool to alleviate malnutrition

More than 2 billion people worldwide suffer from micronutrient malnutrition, sometimes known as hidden hunger. Zn malnutrition affects around a third of the world's population. The physicochemical features of soil, which limit the availability of Zn to plants, cause Zn deficiency. The eating habits of certain populations are more depended on Zn-deficient staple foods. Due to the high expense and certain interventions such as diet diversification, zinc supplementation and food fortification cannot be achieved in disadvantaged populations. Biofortification is the most practical technique for alleviating Zn malnutrition. Seed priming with nutrients is a promising biofortification approach for edible crops. Seed nutripriming with zinc is a cost-effective and environmentally benign approach of biofortification. Seeds can be nutriprimed with Zn using a variety of methods such as Zn fertilisers, Zn chelated compounds and Zn nanoparticles. Nutripriming with nanoparticles is gaining popularity these days due to its numerous advantages and vast biofortification potential. Seeds enriched with Zn also aid plant performance in Zn-deficient soil. Zn an essential trace element can regulate physiological, biochemical and molecular processes of plant cells and thus can enhance germination, growth, yield and bioavailable Zn in edible crops. Moreover, zinc emerges as an important element of choice for the management of COVID-19 symptoms.

Nucleolar histone deacetylases HDT1, HDT2 and HDT3 regulate plant reproductive development

Nucleolus is a membrane-less organelle where ribosomes are assembled and rRNAs transcribed and processed. The assembled ribosomes composed of ribosomal proteins and rRNAs synthesize proteins for cell survival. In plants, the loss of nucleolar ribosomal proteins often causes gametophytically or embryonically lethality. The amount of rRNAs are under stringent regulation according to demand and partially switched off by epigenetic modifications. However, the molecular mechanism for the selective activation or silencing is still unclear, and the transcriptional coordination of rRNAs and ribosomal proteins is also unknown. Here we report the critical role of three Arabidopsis nucleolar protein HDT1, HDT2 and HDT3 in fertility and transcription of rDNAs and rRNA processing-related genes through histone acetylation. This study highlights the important roles of transcriptional repression of ribosome biogenesis-related genes for plant reproductive development.

Polyspermy Block in the Central Cell During Double Fertilization of Arabidopsis thaliana

During double fertilization in angiosperms, two male gametes (sperm cells), are released from a pollen tube into the receptive region between two female gametes; the egg cell and the central cell of the ovule. The sperm cells fertilize the egg cell and the central cell in a one-to-one manner to yield a zygote and an endosperm, respectively. The one-to-one distribution of the sperm cells to the two female gametes is strictly regulated, possibly via communication among the four gametes. Polyspermy block is the mechanism by which fertilized female gametes prevent fertilization by a secondary sperm cell, and has been suggested to operate in the egg cell rather than the central cell. However, whether the central cell also has the ability to avoid polyspermy during double fertilization remains unclear. Here, we assessed the one-to-one fertilization mechanism of the central cell by laser irradiation of the female gametes and live cell imaging of the fertilization process in Arabidopsis thaliana. We successfully disrupted an egg cell within the ovules by irradiation using a femtosecond pulse laser. In the egg-disrupted ovules, the central cell predominantly showed single fertilization by one sperm cell, suggesting that neither the egg cell nor its fusion with one sperm cell is necessary for one-to-one fertilization (i.e., monospermy) of the central cell. In addition, using tetraspore mutants possessing multiple sperm cell pairs in one pollen, we demonstrated that normal double fertilization was observed even when excess sperm cells were released into the receptive region between the female gametes. In ovules accepting four sperm cells, the egg cell never fused with more than one sperm cell, whereas half of the central cells fused with more than one sperm cell (i.e., polyspermy) even 1 h later. Our results suggest that the central cell can block polyspermy during double fertilization, although the central cell is more permissive to polyspermy than the egg cell. The potential contribution of polyspermy block by the central cell is discussed in terms of how it is involved in the one-to-one distribution of the sperm cells to two distinct female gametes.