Sexual reproduction in higher plants I: fertilization and the initiation of zygotic program

PbRbohH/J mediates ROS generation to regulate the growth of pollen tube in pear

Respiratory burst oxidase homolog (Rboh) family genes play crucial functions in development and growth. However, comprehensive and systematic investigation of Rboh family members in Rosaceae and their specific functions during pear pollen development are still limited. In the study, 63 Rboh genes were identified from eight Rosaceae genomes (Malus domestica, Pyrus bretschneideri, Pyrus communis, Prunus persica, Rubus occidentalis, Fragaria vesca, Prunus mume and Prunus avium) and divided into seven main subfamilies (I-VII) according to phylogenetic and structural features. Different modes of gene duplication led to the expansion of Rboh family, with purifying selection playing a vital role in the evolution of Rboh genes. In addition, RNA sequencing and qRT-PCR results indicated that PbRbohH and PbRbohJ were specifically high-expressed in pear pollen. Subsequently, subcellular localization revealed that PbRbohH/J distributed at the plasma membrane. Furthermore, by pharmacological analysis and antisense oligodeoxynucleotide assay, PbRbohH/J were demonstrated to mediate the formation of reactive oxygen species (ROS) to manage pollen tube growth. In conclusion, our results provide useful insights into the functions, expression patterns, evolutionary history of the Rboh genes in pear and other Rosaceae species.

Molecular bases of rice grain size and quality for optimized productivity

The accomplishment of further optimization of crop productivity in grain yield and quality is a great challenge. Grain size is one of the crucial determinants of rice yield and quality; all of these traits are typical quantitative traits controlled by multiple genes. Research advances have revealed several molecular and developmental pathways that govern these traits of agronomical importance. This review provides a comprehensive summary of these pathways, including those mediated by G-protein, the ubiquitin-proteasome system, mitogen-activated protein kinase, phytohormone, transcriptional regulators, and storage product biosynthesis and accumulation. We also generalize the excellent precedents for rice variety improvement of grain size and quality, which utilize newly developed gene editing and conventional gene pyramiding capabilities. In addition, we discuss the rational and accurate breeding strategies, with the aim of better applying molecular design to breed high-yield and superior-quality varieties.

Plant cell totipotency: Insights into cellular reprogramming

Plant cells have a powerful capacity in their propagation to adapt to environmental change, given that a single plant cell can give rise to a whole plant via somatic embryogenesis without the need for fertilization. The reprogramming of somatic cells into totipotent cells is a critical step in somatic embryogenesis. This process can be induced by stimuli such as plant hormones, transcriptional regulators and stress. Here, we review current knowledge on how the identity of totipotent cells is determined and the stimuli required for reprogramming of somatic cells into totipotent cells. We highlight key molecular regulators and associated networks that control cell fate transition from somatic to totipotent cells. Finally, we pose several outstanding questions that should be addressed to enhance our understanding of the mechanisms underlying plant cell totipotency.

A one-step rectification of sperm cell targeting ensures the success of double fertilization

Successful fertilization in animals depends on competition among millions of sperm cells, whereas double fertilization in flowering plants usually involves just one pollen tube releasing two immobile sperm cells. It is largely a mystery how the plant sperm cells fuse efficiently with their female targets within an embryo sac. We show that the initial positioning of sperm cells upon discharge from the pollen tube is usually inopportune for gamete fusions and that adjustment of sperm cell targeting occurs through release and re-adhesion of one sperm cell, while the other connected sperm cell remains in stagnation. This enables proper adhesion of each sperm cell to a female gamete and coordinates the gamete fusions. Our findings reveal inner embryo sac dynamics that ensure the reproductive success of flowering plants and suggest a requirement for sperm cell differentiation as the basis of double fertilization.

Embryo sac formation and early embryo development in Agave tequilana (Asparagaceae)

Agave tequilana is an angiosperm species that belongs to the family Asparagaceae (formerly Agavaceae). Even though there is information regarding to some aspects related to the megagametogenesis of A. tequilana, this is the first report describing the complete process of megasporogenesis, megagametogenesis, the early embryo and endosperm development process in detail. The objective of this work was to study and characterize all the above processes and the distinctive morphological changes of the micropylar and chalazal extremes after fertilization in this species. The agave plant material for the present study was collected from commercial plantations in the state of Jalisco, Mexico. Ovules and immature seeds, previously fixed in FAA and kept in ethanol 70%, were stained based on a tissue clarification technique by using a Mayer’s-Hematoxylin solution. The tissue clarification technique was successfully used for the characterization of the megasporogenesis, megagametogenesis, mature embryo sac formation, the early embryo and endosperm development processes by studying intact cells. The embryo sac of A. tequilana was confirmed to be of the monosporic Polygonum-type and an helobial endosperm formation. Also, the time-lapse of the developmental processes studied was recorded.

A tobacco homolog of DCN1 is involved in pollen development and embryogenesis

KEY MESSAGE

We show that DCN1 binds ubiquitin and RUB/NEDD8, associates with cullin, and is functionally conserved. DCN1 activity is required for pollen development transitions and embryogenesis, and for pollen tube growth. Plant proteomes show remarkable plasticity in reaction to environmental challenges and during developmental transitions. Some of this adaptability comes from ubiquitin-mediated protein degradation regulated by cullin-RING E3 ubiquitin ligases (CRLs). CRLs are activated through modification of the cullin subunit with the ubiquitin-like protein RUB/NEDD8 by an E3 ligase called defective in cullin neddylation 1 (DCN1). Here we show that tobacco DCN1 binds ubiquitin and RUB/NEDD8 and associates with cullin. When knocked down by RNAi, tobacco pollen formation was affected and zygotic embryogenesis was blocked around the globular stage. Additionally, we found that RNAi of DCN1 inhibited the stress-triggered reprogramming of cultured microspores from their intrinsic gametophytic mode of development to an embryogenic state. This stress-induced developmental switch is a known feature in many important crops and leads ultimately to the formation of haploid embryos and plants. Compensating the RNAi effect by re-transformation with a promoter-silencing construct restored pollen development and zygotic embryogenesis, as well as the ability for stress-induced formation of embryogenic microspores. Overexpression of DCN1 accelerated pollen tube growth and increased the potential for microspore reprogramming. These results demonstrate that the biochemical function of DCN1 is conserved in plants and that its activity is involved in transitions during pollen development and embryogenesis, and for pollen tube growth.

The maternal-to-zygotic transition in higher plants

During early embryogenesis in mammals and higher plants, the maternal-to-zygotic transition (MZT) marks the turnover of developmental control from maternal products to de novo zygotic genome transcripts. Intensive studies in animals indicate that early embryonic development is largely maternally controlled. In recent years, the MZT has drawn the attention of botanists, as it is important for understanding the mechanism of embryogenesis and hybrid vigor. In this study, we present a brief overview of some aspects of the MZT in flowering plants. Based on what we have learned from Nicotiana tabacum, we hypothesize that the MZT occurs before zygotic cell division and that the development of the fertilized egg cell in flowering plants can be divided into two phases: the zygote stage, which is mainly controlled maternally, and the one-celled proembryo stage, in which zygotic genome activation (ZGA) occurs and is required for zygote division.

EMBRYONIC FACTOR 19 encodes a pentatricopeptide repeat protein that is essential for the initiation of zygotic embryogenesis in Arabidopsis

Early embryogenesis is the most fundamental developmental process in biology. Screening of ethyl methanesulfonate (EMS)-mutagenized populations of Arabidopsis thaliana led to the identification of a zygote-lethal mutant embryonic factor 19 (fac19) in which embryo development was arrested at the elongated zygote to octant stage. The number of endosperm nuclei decreased significantly in fac19 embryos. Genetic analysis showed fac19 was caused by a single recessive mutation with typical mendelian segregation, suggesting equal maternal and paternal contributions of FAC19 towards zygotic embryogenesis. Positional cloning showed that FAC19 encodes a putative mitochondrial protein with 16 conserved pentatricopeptide repeat (PPR) motifs. The fac19 mutation caused a conversion from hydrophilic serine located in a previously unknown domain to hydrophobic leucine. Crosses between FAC19/fac19 and the T-DNA insertion mutants in the same gene failed to complement the fac19 defects, confirming the identity of the gene. This study revealed the critical importance of a PPR protein-mediated mitochondrial function in early embryogenesis.

The Arabidopsis acbp1acbp2 double mutant lacking acyl-CoA-binding proteins ACBP1 and ACBP2 is embryo lethal

*In Arabidopsis thaliana, the amino acid sequences of membrane-associated acyl-CoA-binding proteins ACBP1 and ACBP2 are highly conserved. We have shown previously that, in developing seeds, ACBP1 accumulates in the cotyledonary cells of embryos and ACBP1 is proposed to be involved in lipid transfer. We show here by immunolocalization, using ACBP2-specific antibodies, that ACBP2 is also expressed in the embryos at various stages of seed development in Arabidopsis. *Phenotypic analyses of acbp1 and acbp2 single mutants revealed that knockout of either ACBP1 or ACBP2 alone did not affect their life cycle as both single mutants exhibited normal growth and development similar to the wild-type. However, the acbp1acbp2 double mutant was embryo lethal and was also defective in callus induction. *On lipid and acyl-CoA analyses, the siliques, but not the leaves, of the acbp1 mutant accumulated galactolipid monogalactosyldiacylglycerol and 18:0-CoA, but the levels of most polyunsaturated species of phospholipid, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine, declined. *As recombinant ACBP1 and ACBP2 bind unsaturated phosphatidylcholine and acyl-CoA esters in vitro, we propose that ACBP1 and ACBP2 are essential in lipid transfer during early embryogenesis in Arabidopsis.