Male gametophyte development: a molecular perspective

Functional redundancy of transcription factors SlNOR and SlNOR-like1 is required for pollen development in tomato

In tomato, SlNOR and SlNOR-like1, members of the NAC family of transcription factors (TFs), are known to play critical roles in regulating fruit ripening and are highly expressed in floral organs. However, their role in flower development remains unclear. In this study, we generated and functionally characterized a double knockout mutant, nor/nor-like1. Our findings reveal that the pollen abortion of the nor/nor-like1 impedes ovarian enlargement, resulting in fruit formation failure. Histological analyses demonstrate that the pollen wall collapse occurs during the mature pollen stage and leads to the abnormal pollen wall component deposition at the microspore stage, resulting in the male sterility in the double knockout mutant lines. Kyoto Encyclopedia of Genes and Genomes enrichment pathway analyses further suggest that the loss of SlNOR and SlNOR-like1 function affects several metabolic pathways related to pollen development, including 'ABC transporters', 'lipid metabolism', 'phenylpropanoid biosynthesis', 'hormone signal transduction', 'starch and sucrose metabolism', and 'cutin, suberine, and wax biosynthesis'. Furthermore, our results demonstrate that SlNOR and SlNOR-like1 could directly bind to the promoters of key genes associated with pollen wall formation and activate their expression, including ATP-binding cassette transporters of the G family (SlABCG8/9/23), ECERIFERUM (SlCER1), and glycine-rich protein (SlGRP92). These findings suggest that SlNOR and SlNOR-like1 may play a redundant role in the biosynthesis and transport of sporopollenin precursors, cuticular wax biosynthesis, and exine formation. In summary, our study highlights a previously uncharacterized role of SlNOR and SlNOR-like1 in tomato pollen wall formation and male fertility.

Modern Comprehensive Metabolomic Profiling of Pollen Using Various Analytical Techniques

Pollen is a cornerstone of life for plants. Its durability, adaptability, and complex design are the key factors to successful plant reproduction, genetic diversity, and the maintenance of ecosystems. A detailed study of its chemical composition is important to understand the mechanism of pollen-pollinator interactions, pollination processes, and allergic reactions. In this study, a multimodal approach involving Fourier transform infrared spectrometry (FTIR), direct mass spectrometry with an atmospheric solids analysis probe (ASAP), matrix-assisted laser desorption/ionization (MALDI) and ultra-high-performance liquid chromatography-mass spectrometry (UHPLC-MS) was applied for metabolite profiling. ATR-FTIR provided an initial overview of the present metabolite classes. Phenylpropanoid, lipidic, and carbohydrate structures were revealed. The hydrophobic outer layer of pollen was characterized in detail by ASAP-MS profiling, and esters, phytosterols, and terpenoids were observed. Diacyl- and triacylglycerols and carbohydrate structures were identified in MALDI-MS spectra. The MALDI-MS imaging of lipids proved to be helpful during the microscopic characterization of pollen species in their mixture. Polyphenol profiling and the quantification of important secondary metabolites were performed by UHPLC-MS in context with pollen coloration and their antioxidant and antimicrobial properties. The obtained results revealed significant chemical differences among Magnoliophyta and Pinophyta pollen. Additionally, some variations within Magnoliophyta species were observed. The obtained metabolomics data were utilized for pollen differentiation at the taxonomic scale and provided valuable information in relation to pollen interactions during reproduction and its related applications.

Genetic Analysis and Fine Mapping of Spontaneously Mutated Male Sterility Gene in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Chinese cabbage (Brassica rapa L. ssp. pekinensis), an important traditional vegetable indigenous to China, is a typical cross-pollinated Brassica crop exhibiting pronounced heterosis. However, its small flower organs make artificial pollination for hybrid seed production highly challenging. The use of male-sterile lines has emerged as a crucial approach in hybrid seed production. Therefore, understanding the genetic and molecular mechanisms underlying male sterility in Chinese cabbage holds profound theoretical and economic importance and is pivotal for advancing Chinese cabbage crossbreeding. Here, cytological comparative analysis of anthers from sterile line 366-2S and fertile line 366-2F revealed abnormalities in 366-2S during the late tetrad stage, including delayed tapetum degradation and the aggregation of tetrad microspores without separation, which prevented pollen production and caused male sterility. Construction of the F2 segregating population, with 366-2S as the female parent and genetically diverse fertile material Y636-9 as the male parent, indicated that male sterility in 366-2S is controlled by a single recessive gene. Using bulked segregant analysis sequencing and kompetitive allele-specific polymerase chain reaction (KASP) technology, the sterile gene was mapped to 65 kb between the PA11 and PA13 markers, with 11 genes in the candidate region. Functional annotation, expression, and sequence variation analyses identified BraA09g012710.3C, encoding acyl-CoA synthetase 5, as a candidate gene for 366-2S male sterility. Quantitative real-time polymerase chain reaction analysis revealed minimal expression of BraA09g012710.3C in 366-2S but high expression in the flower buds of 366-2F. Further analysis of candidate gene DNA sequences identified a large deletion encompassing BraA09g012710.3C, BraA09g012720.3C, BraA09g012730.3C, and BraA09g012740.3C in sterile line 366-2S (A09: 7452347-7479709). Cloning and verification of the other three deleted genes in the F2 population via agarose gel electrophoresis confirmed their presence in F2 sterile individuals, indicating that their deletion was not associated with male sterility, underscoring BraA09g012710.3C as the key gene driving male sterility in 366-2S.

The atypical proteome of mitochondria from mature pollen grains

To propagate their genetic material, flowering plants rely on the production of large amounts of pollen grains that are capable of germinating on a compatible stigma. Pollen germination and pollen tube growth are thought to be extremely energy-demanding processes. This raises the question of whether mitochondria from pollen grains are specifically tuned to support this developmental process. To address this question, we isolated mitochondria from both mature pollen and floral buds using the isolation of mitochondria tagged in specific cell-type (IMTACT) strategy and examined their respective proteomes. Strikingly, mitochondria from mature pollen grains have lost many proteins required for genome maintenance, gene expression, and translation. Conversely, a significant accumulation of proteins associated with the tricarboxylic acid (TCA) cycle, the electron transport chain (ETC), and Ca2+ homeostasis was observed. This supports the current model in which pollen requires large quantities of ATP for tube growth but also identifies an unexpected depletion of the gene expression machinery, aligned with the fact that the mitochondrial genome is actively degraded during pollen maturation. Altogether, our results uncover that mitochondria from mature pollen grains are strategically prepared for action by increasing their respiratory capacity and dismantling their gene expression machinery, which raises new questions about the assembly of respiratory complexes in pollen mitochondria, as they rely on the integration of proteins coded by the nuclear and mitochondrial genomes. In addition, the approach described here opens a new range of possibilities for studying mitochondria during pollen development and in pollen-specific mitochondrial events.

Bioimaging insights into structural pathways of cell-to-cell communication within the male (MGU) and female (FGU) germ units of Arabidopsis thaliana

KEY MESSAGE

Cytoplasmic connections are present between cells within male and female germ units (MGU, FGU), suggesting potential structural pathways for communication. Cell-to-cell communication within the male germ unit (MGU), which consists of two sperm cells and the vegetative cell nucleus, and the female germ unit (FGU), comprising the synergids, the egg cell, and the central cell, is crucial for gamete maturation, fertilization, and early embryogenesis in angiosperms. The MGU facilitates the transport and delivery of immotile sperm cells via the elongating pollen tube to the FGU/embryo sac, which is deeply embedded within the ovule and the ovary. Through applying various bioimaging techniques at both electron and light microscopy levels, we examine the structure and the function of these units in the model plant Arabidopsis thaliana, with a particular focus on potential structural pathways for communication. In the MGU, this communication is facilitated by a cytoplasmic projection that connects the sperm cells to the lobed vegetative nucleus. In the FGU, the extracellular matrix adjacent to the egg cell, central cell, and synergids plays a similar role. We discuss our findings in the context of previous studies on Hyacinthus orientalis, where, in contrast to Arabidopsis-which possesses a tricellular pollen structure-sperm cells are formed within the growing pollen tube.

A review on the function of arabinogalactan-proteins during pollen grain development

KEY MESSAGE

Overview of the current understanding of PG development, PT growth and the role of AGPs in these processes. The pollen grain (PG) is a complex structure composed of three cells: the vegetative cell which develops into a pollen tube (PT) and two sperm cells that will fuse with the egg cell and central cell, giving rise to the embryo and endosperm, respectively. This resilient gametophyte is constantly subjected to selective pressures, leading to a diverse range of characteristics, with one of its defining features being the pollen cell wall. In this review, we closely examine the developmental stages of PG formation and PT growth, with a specific focus on the dynamic roles of arabinogalactan-proteins (AGPs) throughout these processes. AGPs are initially present in pollen mother cells and persist throughout PT growth. In the early stages, AGPs play a crucial role in primexine anchoring, followed by nexine and intine formation as well as cellulose deposition, thereby providing essential structural support to the PG. As PGs mature, AGPs continue to be essential, as their absence often leads to the collapse of PGs before they reach full maturity. Moreover, the absence of AGPs during PT growth leads to abnormal growth patterns, likely due to disruptions of cellulose, callose, and F-actin deposition, as well as perturbations in calcium ion (Ca2+) signalling. Understanding the intricate interplay between AGPs and PG development sheds light on the underlying mechanisms that drive reproductive success and highlights the indispensable role of AGPs in ensuring the integrity and functionality of PGs.

Heat Stress Inhibits Pollen Development by Degrading mRNA Capping Enzyme ARCP1 and ARCP2

Pollen development and germination are critical for successful generation of offspring in plants, yet they are highly susceptible to heat stress (HS). However, the molecular mechanism underlying this process has not been fully elucidated. In this study, we highlight the essential roles of two mRNA capping enzymes, named Arabidopsis mRNA capping phosphatase (ARCP) 1 and 2, in regulating male and female gamete development. The transmission efficiencies of gametes carrying arcp1 arcp2 from arcp1+/- arcp2-/- and arcp1-/- arcp2+/- mutants are 30% and zero, respectively. These mutants exhibited a significant increase in misshaped pollen, with germination rates approximately half of those in wild type. ARCP1/2 exhibit RNA triphosphatase and RNA guanylyltransferase activities, which are required for proper pollen development. Through RNA-seq analysis, genes involved in pollen development/germination and HS response were identified as downregulated genes in pollen from arcp1+/- arcp2-/- mutant. Furthermore, ARCP2 protein is degraded under HS condition, and inducing the expression of ARCP2 can increase the pollen germination rate under elevated temperature. We propose that HS triggers the degradation of mRNA capping enzymes, which in turn disrupts the transcriptome that required for pollen development and pollen germination and ultimately leads to male sterility.

SlTDF1: A key regulator of tapetum degradation and pollen development in tomato

Pollen formation and development during the life cycle of flowering plant are crucial for maintaining reproductive and genetic diversity. In this study, an R2R3MYB family transcription factor, SlTDF1 (SlMYB35), was predominantly expressed in stamens. Repressed expression of SlTDF1 results in a delay in the degradation of the anther tapetum in tomatoes, which in turn leads to the formation of abnormal pollen, including a reduction in the number of single-fruit seeds and fertility when compared to wild-type plants. Analysis of paraffin sections demonstrated that SlTDF1 is a crucial factor in the maturation of tomato pollen. Further analysis of the transcriptomic data revealed that downregulation of the SlTDF1 gene significantly suppressed the expression of genes related to sugar metabolism and anther development. The findings of this study indicated that SlTDF1 plays a pivotal role in regulating tomato pollen development. Moreover, these findings provide a genetic resource for male sterility in tomato plants.

Regulation of cell cycle in plant gametes: when is the right time to divide?

Cell division is a fundamental process shared across diverse life forms, from yeast to humans and plants. Multicellular organisms reproduce through the formation of specialized types of cells, the gametes, which at maturity enter a quiescent state that can last decades. At the point of fertilization, signalling lifts the quiescent state and triggers cell cycle reactivation. Studying how the cell cycle is regulated during plant gamete development and fertilization is challenging, and decades of research have provided valuable, yet sometimes contradictory, insights. This Review summarizes the current understanding of plant cell cycle regulation, gamete development, quiescence, and fertilization-triggered reactivation.

DUO1 Activated Zinc Finger (AtDAZ) protein role in the generative cell body morphogenesis

Arabidopsis MYB transcription factor, AtDUO1 regulates generative cell body (GC) morphogenesis from round to semi and fully elongated forms before pollen mitosis-II (PM II). It was hypothesised that DUO1 might regulate morphogenesis through any of its direct target genes or components of the DUO1-DAZ1 network. The developmental analysis of plants harbouring T-DNA insertions in some DUO1 target genes using light and fluorescence microscopy revealed abnormal GC morphogenesis only in daz1 and daz2, but gcs1, trm16, mapkkk10, mapkkk20, tet11, and tip1 all undergo normal elongation indicating that these target genes have no important roles in morphogenesis or may be redundant. The important regulatory role of DUO1 was confirmed through the observed incomplete rescue of morphogenesis of mutant duo1-1 GCs by DAZ1 and independently by a C-terminally deleted version of DUO1 (DUO1∆C3) lacking activation sequences. The evidence supports the important role of DAZ1 in GC shape partial morphogenesis. The C-terminus of DUO1 may regulate some target genes that affect GC body elongation. Furthermore, an intact DUO1 is shown to be indispensable for GC shape and nucleus elongation and subsequently for timely division and sperm cell morphogenesis. The development of the GC cytoplasmic projection is regulated independently of DUO1, and all its target genes were able to form it.

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.

Selfing Shapes Fixation of a Mutant Allele Under Flux Equilibrium

Sexual reproduction with alternative generations in a life cycle is an important feature in eukaryotic evolution. Partial selfing can regulate the efficacy of purging deleterious alleles in the gametophyte phase and the masking effect in heterozygotes in the sporophyte phase. Here, we develop a new theory to analyze how selfing shapes fixation of a mutant allele that is expressed in the gametophyte or the sporophyte phase only or in two phases. In an infinitely large population, we analyze a critical selfing rate beyond which the mutant allele tends to be fixed under equilibrium between irreversible mutation and selection effects. The critical selfing rate varies with genes expressed in alternative phases. In a finite population with partial self-fertilization, we apply Wright's method to calculate the fixation probability of the mutant allele under flux equilibrium among irreversible mutation, selection, and drift effects and compare it with the fixation probability derived from diffusion model under equilibrium between selection and drift effects. Selfing facilitates fixation of the deleterious allele expressed in the gametophyte phase only but impedes fixation of the deleterious allele expressed in the sporophyte phase only. Selfing facilitates or impedes fixation of the deleterious allele expressed in two phases, depending upon how phase variation in selection occurs in a life cycle. The overall results help to understand the adaptive strategy that sexual reproductive plant species evolve through the joint effects of partial selfing and alternative generations in a life cycle.

Transcription factors WRKY2 and WRKY34 control LATERAL ORGAN BOUNDARIES DOMAIN10 expression in pollen vegetative cell nuclei

The intricate regulation of gene expression determining cell fate during male gametogenesis involves a complex interplay of multiple transcriptional regulators. In Arabidopsis (Arabidopsis thaliana), the LATERAL ORGAN BOUNDARIES DOMAIN 10 (LBD10) transcription factor is prominent in early microspores and both the germ and vegetative cells of bicellular pollen, playing an important role in pollen development. However, in mature pollen, LBD10 exclusively localizes in the vegetative cell nucleus (VCN). Here, we identify cis-acting elements and trans-acting factors responsible for the specific expression of LBD10 in the VCN during pollen maturation. Using a series of LBD10 promoter deletion constructs fused with GUS or GFP reporters, we pinpoint two crucial core promoter sequences. These sequences are situated within two 200 bp regions upstream of the start codon and independently govern LBD10 expression in the VCN. We demonstrate that a W-box motif (AGTCAC) at -770 bp is essential for activating the expression of LBD10 in vegetative cells during pollen maturation. Our transient gene expression assays using Arabidopsis protoplasts and chromatin immunoprecipitation assays show that the transcription factors WRKY2 and WRKY34 recognize the LBD10 promoter region containing W-box motifs. Collectively, our findings suggest that WRKY2 and WRKY34 binding to the W-box motifs plays a role in the VCN-specific expression of LBD10 in pollen. This interaction may contribute to male gametophyte development, shedding light on the intricate regulatory network governing this critical biological process.

The BNB-GLID module regulates germline fate determination in Marchantia polymorpha

Germline fate determination is a critical event in sexual reproduction. Unlike animals, plants specify the germline by reprogramming somatic cells at the late stages of their development. However, the genetic basis of germline fate determination and how it evolved during the land plant evolution are still poorly understood. Here, we report that the plant homeodomain finger protein GERMLINE IDENTITY DETERMINANT (GLID) is a key regulator of the germline specification in liverwort, Marchantia polymorpha. Loss of the MpGLID function causes failure of germline initiation, leading to the absence of sperm and egg cells. Remarkably, the overexpression of MpGLID in M. polymorpha induces the ectopic formation of cells with male germline cell features exclusively in male thalli. We further show that MpBONOBO (BNB), with an evolutionarily conserved function, can induce the formation of male germ cell-like cells through the activation of MpGLID by directly binding to its promoter. The Arabidopsis (Arabidopsis thaliana) MpGLID ortholog, MALE STERILITY1 (AtMS1), fails to replace the germline specification function of MpGLID in M. polymorpha, demonstrating that a derived function of MpGLID orthologs has been restricted to tapetum development in flowering plants. Collectively, our findings suggest the presence of the BNB-GLID module in complex ancestral land plants that has been retained in bryophytes, but rewired in flowering plants for male germline fate determination.

Ectopic expression of a thaumatin-like protein impairs the timely deposition and dissolution of callose during microsporogenesis, leading to microspore death and male sterility in Arabidopsis

KEY MESSAGE

Two pollen-preferential thaumatin-like proteins show both common and distinctive expression profiles. Precocious expression of one of them drastically disturbs timely deposition and dissolution of callose during microsporogenesis, leading to microspore death. Thaumatin-like proteins (TLPs), members of the pathogenesis-related protein family 5 (PR-5), are involved in plant defenses against biotic and abiotic stresses through antifungal activity and enhanced tolerance. Accordingly, studies on TLPs have focused on their responses to various pathogens and stresses and on engineering agronomically valuable crops that can be cultivated in suboptimal environments. On the other hand, the role of TLP members in plant development and their genetic regulation remains largely unexplored. Recently, we reported that the generative cell internalization after pollen mitosis I, an essential pollen patterning step for the nonmotile sperm cell delivery through a pollen tube, depends on STICKY GENERATIVE CELL which suppresses callose deposition in the nascent generative cell and interacts with a germline cell preferential GCTLP1 in Arabidopsis. Here, we additionally identified GCTLP2 which is similarly expressed in the germline cells. We generated various transgenic lines and examined their expressions and phenotypes to elucidate GCTLP functions during pollen development. Expression profiles suggest two GCTLP proteins may have common but also distinctive roles during pollen development. Importantly, ectopic expression analyses show that precocious expression of GCTLP2 severely disturbs the timely deposition and degradation of callose during microsporogenesis which is essential to produce viable microspores. Therefore, our study broadens the knowledge of TLP function and callose regulation for successful pollen development in Arabidopsis.

ICE1 interacts with IDD14 to transcriptionally activate QQS to increase pollen germination and viability

In flowering plants, sexual reproductive success depends on the production of viable pollen grains. However, the mechanisms by which QUA QUINE STARCH (QQS) regulates pollen development and how transcriptional activators facilitate the transcription of QQS in this process remain poorly understood. Here, we demonstrate that INDUCER OF CBF EXPRESSION 1 (ICE1), a basic helix-loop-helix (bHLH) transcription factor, acts as a key transcriptional activator and positively regulates QQS expression to increase pollen germination and viability in Arabidopsis thaliana by interacting with INDETERMINATE DOMAIN14 (IDD14). In our genetic and biochemical experiments, overexpression of ICE1 greatly promoted both the activation of QQS and high pollen viability mediated by QQS. IDD14 additively enhanced ICE1 function by promoting the binding of ICE1 to the QQS promoter. In addition, mutation of ICE1 significantly repressed QQS expression; the impaired function of QQS and the abnormal anther dehiscence jointly affected pollen development of the ice1-2 mutant. Our results also showed that the enhancement of pollen activity by ICE1 depends on QQS. Furthermore, QQS interacted with CUT1, the key enzyme for long-chain lipid biosynthesis. This interaction both promoted CUT1 activity and regulated pollen lipid metabolism, ultimately determining pollen hydration and fertility. Our results not only provide new insights into the key function of QQS in promoting pollen development by regulating pollen lipid metabolism, but also elucidate the mechanism that facilitates the transcription of QQS in this vital developmental process.

Heat stress at the bicellular stage inhibits sperm cell development and transport into pollen tubes

For successful double fertilization in flowering plants (angiosperms), pollen tubes deliver 2 nonmotile sperm cells toward female gametes (egg and central cell, respectively). Heatwaves, especially during the reproduction period, threaten male gametophyte (pollen) development, resulting in severe yield losses. Using maize (Zea mays) as a crop and grass model system, we found strong seed set reduction when moderate heat stress was applied for 2 d during the uni- and bicellular stages of pollen development. We show that heat stress accelerates pollen development and impairs pollen germination capabilities when applied at the unicellular stage. Heat stress at the bicellular stage impairs sperm cell development and transport into pollen tubes. To understand the course of the latter defects, we used marker lines and analyzed the transcriptomes of isolated sperm cells. Heat stress affected the expression of genes associated with transcription, RNA processing and translation, DNA replication, and the cell cycle. This included the genes encoding centromeric histone 3 (CENH3) and α-tubulin. Most genes that were misregulated encode proteins involved in the transition from metaphase to anaphase during pollen mitosis II. Heat stress also activated spindle assembly check point and meta- to anaphase transition genes in sperm cells. In summary, misregulation of the identified genes during heat stress at the bicellular stage results in sperm cell development and transport defects ultimately leading to sterility.

A wave of specific transcript and protein accumulation accompanies pollen dehydration

In flowering plants, male gametes are immotile and carried by dry pollen grains to the female organ. Dehydrated pollen is thought to withstand abiotic stress when grains are dispersed from the anther to the pistil, after which sperm cells are delivered via pollen tube growth for fertilization and seed set. Yet, the underlying molecular changes accompanying dehydration and the impact on pollen development are poorly understood. To gain a systems perspective, we analyzed published transcriptomes and proteomes of developing Arabidopsis thaliana pollen. Waves of transcripts are evident as microspores develop to bicellular, tricellular, and mature pollen. Between the "early"- and "late"-pollen-expressed genes, an unrecognized cluster of transcripts accumulated, including those encoding late-embryogenesis abundant (LEA), desiccation-related protein, transporters, lipid-droplet associated proteins, pectin modifiers, cysteine-rich proteins, and mRNA-binding proteins. Results suggest dehydration onset initiates after bicellular pollen is formed. Proteins accumulating in mature pollen like ribosomal proteins, initiation factors, and chaperones are likely components of mRNA-protein condensates resembling "stress" granules. Our analysis has revealed many new transcripts and proteins that accompany dehydration in developing pollen. Together with published functional studies, our results point to multiple processes, including (1) protect developing pollen from hyperosmotic stress, (2) remodel the endomembrane system and walls, (3) maintain energy metabolism, (4) stabilize presynthesized mRNA and proteins in condensates of dry pollen, and (5) equip pollen for compatibility determination at the stigma and for recovery at rehydration. These findings offer novel models and molecular candidates to further determine the mechanistic basis of dehydration and desiccation tolerance in plants.

Molecular regulatory mechanisms of staminate strobilus development and dehiscence in Torreya grandis

Gymnosperms are mostly dioecious, and their staminate strobili undergo a longer developmental period than those of angiosperms. However, the underlying molecular mechanisms remain unclear. This study aimed to identify key genes and pathways involved in staminate strobilus development and dehiscence in Torreya grandis. Through weighted gene co-expression network analysis (WGCNA), we identified fast elongation-related genes enriched in carbon metabolism and auxin signal transduction, whereas dehiscence-related genes were abundant in alpha-linolenic acid metabolism and the phenylpropanoid pathway. Based on WGCNA, we also identified PHYTOCHROME-INTERACTING FACTOR4 (TgPIF4) as a potential regulator for fast elongation of staminate strobilus and 2 WRKY proteins (TgWRKY3 and TgWRKY31) as potential regulators for staminate strobilus dehiscence. Multiple protein-DNA interaction analyses showed that TgPIF4 directly activates the expression of TRANSPORT INHIBITOR RESPONSE2 (TgTIR2) and NADP-MALIC ENZYME (TgNADP-ME). Overexpression of TgPIF4 significantly promoted staminate strobilus elongation by elevating auxin signal transduction and pyruvate content. TgWRKY3 and TgWRKY31 bind to the promoters of the lignin biosynthesis gene PHENYLALANINE AMMONIA-LYASE (TgPAL) and jasmonic acid metabolism gene JASMONATE O-METHYLTRANSFERASE (TgJMT), respectively, and directly activate their transcription. Overexpression of TgWRKY3 and TgWRKY31 in the staminate strobilus led to early dehiscence, accompanied by increased lignin and methyl jasmonate levels, respectively. Collectively, our findings offer a perspective for understanding the growth of staminate strobili in gymnosperms.

The Mechanism of Ovule Abortion in Self-Pollinated 'Hanfu' Apple Fruits and Related Gene Screening

Apples exhibit S-RNase-mediated self-incompatibility and typically require cross-pollination in nature. 'Hanfu' is a cultivar that produces abundant fruit after self-pollination, although it also shows a high rate of seed abortion afterwards, which greatly reduces fruit quality. In this study, we investigated the ovule development process and the mechanism of ovule abortion in apples after self-pollination. Using a DIC microscope and biomicroscope, we found that the abortion of apple ovules occurs before embryo formation and results from the failure of sperm-egg fusion. Further, we used laser-assisted microdissection (LAM) cutting and sperm and egg cell sequencing at different periods after pollination to obtain the genes related to ovule abortion. The top 40 differentially expressed genes (DEGs) were further verified, and the results were consistent with switching the mechanism at the 5' end of the RNA transcript (SMART-seq). Through this study, we can preliminarily clarify the mechanism of ovule abortion in self-pollinated apple fruits and provide a gene reserve for further study and improvement of 'Hanfu' apple fruit quality.

Gametic selection favours polyandry and selfing

Competition among pollen or sperm (gametic selection) can cause evolution. Mating systems shape the intensity of gametic selection by determining the competitors involved, which can in turn cause the mating system itself to evolve. We model the bidirectional relationship between gametic selection and mating systems, focusing on variation in female mating frequency (monandry-polyandry) and self-fertilisation (selfing-outcrossing). First, we find that monandry and selfing both reduce the efficiency of gametic selection in removing deleterious alleles. This means that selfing can increase mutation load, in contrast to cases without gametic selection where selfing purges deleterious mutations and decreases mutation load. Second, we explore how mating systems evolve via their effect on gametic selection. By manipulating gametic selection, polyandry can evolve to increase the fitness of the offspring produced. However, this indirect advantage of post-copulatory sexual selection is weak and is likely to be overwhelmed by any direct fitness effects of mating systems. Nevertheless, gametic selection can be potentially decisive for selfing evolution because it significantly reduces inbreeding depression, which favours selfing. Thus, the presence of gametic selection could be a key factor driving selfing evolution.

Redundant function of the Arabidopsis phosphatidylinositol 4-phosphate 5-kinase genes PIP5K4-6 is essential for pollen germination

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K) is a key enzyme producing the signaling lipid phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2 ] in eukaryotes. Although PIP5K genes are reported to be involved in pollen tube germination and growth, the essential roles of PIP5K in these processes remain unclear. Here, we performed a comprehensive genetic analysis of the Arabidopsis thaliana PIP5K4, PIP5K5, and PIP5K6 genes and revealed that their redundant function is essential for pollen germination. Pollen with the pip5k4pip5k5pip5k6 triple mutation was sterile, while pollen germination efficiency and pollen tube growth were reduced in the pip5k6 single mutant and further reduced in the pip5k4pip5k6 and pip5k5pip5k6 double mutants. YFP-fusion proteins, PIP5K4-YFP, PIP5K5-YFP, and PIP5K6-YFP, which could rescue the sterility of the triple mutant pollen, preferentially localized to the tricolpate aperture area and the future germination site on the plasma membrane prior to germination. Triple mutant pollen grains under the germination condition, in which spatiotemporal localization of the PtdIns(4,5)P2 fluorescent marker protein 2xmCHERRY-2xPHPLC as seen in the wild type was abolished, exhibited swelling and rupture of the pollen wall, but neither the conspicuous protruding site nor site-specific deposition of cell wall materials for germination. These data indicate that PIP5K4-6 and their product PtdIns(4,5)P2 are essential for pollen germination, possibly through the establishment of the germination polarity in a pollen grain.

MBD2 couples DNA methylation to transposable element silencing during male gametogenesis

DNA methylation is an essential component of transposable element (TE) silencing, yet the mechanism by which methylation causes transcriptional repression remains poorly understood1-5. Here we study the Arabidopsis thaliana Methyl-CpG Binding Domain (MBD) proteins MBD1, MBD2 and MBD4 and show that MBD2 acts as a TE repressor during male gametogenesis. MBD2 bound chromatin regions containing high levels of CG methylation, and MBD2 was capable of silencing the FWA gene when tethered to its promoter. MBD2 loss caused activation at a small subset of TEs in the vegetative cell of mature pollen without affecting DNA methylation levels, demonstrating that MBD2-mediated silencing acts strictly downstream of DNA methylation. TE activation in mbd2 became more significant in the mbd5 mbd6 and adcp1 mutant backgrounds, suggesting that MBD2 acts redundantly with other silencing pathways to repress TEs. Overall, our study identifies MBD2 as a methyl reader that acts downstream of DNA methylation to silence TEs during male gametogenesis.

Cytological and transcriptomic analyses provide insights into the pollen fertility of synthetic allodiploid Brassica juncea hybrids

KEY MESSAGE

Imbalanced chromosomes and cell cycle arrest, along with down-regulated genes in DNA damage repair and sperm cell differentiation, caused pollen abortion in synthetic allodiploid Brassica juncea hybrids. Interspecific hybridization is considered to be a major pathway for species formation and evolution in angiosperms, but the occurrence of pollen abortion in the hybrids is common, prompting us to recheck male gamete development in allodiploid hybrids after the initial combination of different genomes. Here, we investigated the several key meiotic and mitotic events during pollen development using the newly synthesised allodiploid B. juncea hybrids (AB, 2n = 2× = 18) as a model system. Our results demonstrated the partial synapsis and pairing of non-homologous chromosomes concurrent with chaotic spindle assembly, affected chromosome assortment and distribution during meiosis, which finally caused difference in genetic constitution amongst the final tetrads. The mitotic cell cycle arrest during microspore development resulted in the production of anucleate pollen cells. Transcription analysis showed that sets of key genes regulating cyclin (CYCA1;2 and CYCA2;3), DNA damage repair (DMC1, NBS1 and MMD1), and ubiquitin-proteasome pathway (SINAT4 and UBC) were largely downregulated at the early pollen meiosis stages, and those genes involved in sperm cell differentiation (DUO1, PIRL1, PIRL9 and LBD27) and pollen wall synthesis (PME48, VGDH11 and COBL10) were mostly repressed at the late pollen mitosis stages in the synthetic allodiploid B. juncea hybrids (AB). In conclusion, this study elucidated the related mechanisms affecting pollen fertility during male gametophyte development at the cytological and transcriptomic levels in the synthetic allodiploid B. juncea hybrids.

The impact of heat stress in plant reproduction

The increment in global temperature reduces crop productivity, which in turn threatens food security. Currently, most of our food supply is produced by plants and the human population is estimated to reach 9 billion by 2050. Gaining insights into how plants navigate heat stress in their reproductive phase is essential for effectively overseeing the future of agricultural productivity. The reproductive success of numerous plant species can be jeopardized by just one exceptionally hot day. While the effects of heat stress on seedlings germination and root development have been extensively investigated, studies on reproduction are limited. The intricate processes of gamete development and fertilization unfold within a brief timeframe, largely concealed within the flower. Nonetheless, heat stress is known to have important effects on reproduction. Considering that heat stress typically affects both male and female reproductive structures concurrently, it remains crucial to identify cultivars with thermotolerance. In such cultivars, ovules and pollen can successfully undergo development despite the challenges posed by heat stress, enabling the completion of the fertilization process and resulting in a robust seed yield. Hereby, we review the current understanding of the molecular mechanisms underlying plant resistance to abiotic heat stress, focusing on the reproductive process in the model systems of Arabidopsis and Oryza sativa.

MYB transcription factors and their roles in the male reproductive development of flowering plants

As one of the largest transcription factor families with complex functional differentiation in plants, the MYB transcription factors (MYB TFs) play important roles in the physiological and biochemical processes of plant growth and development. Male reproductive development, an essential part of sexual reproduction in flowering plants, is undoubtedly regulated by MYB TFs. In this review, we summarize the roles of the MYB TFs involved in the three stages of male reproductive development: pollen grains formation and maturation, filament elongation and anther dehiscence, and fertilization. Also, the potential downstream target genes and upstream regulators of these MYB TFs are discussed. Furthermore, we propose the underlying regulatory mechanisms of these MYB TFs: (1) A complex network of MYB TFs regulates various aspects of male reproductive development; (2) MYB homologous genes in different species may be functionally conserved or differentiated; (3) MYB TFs often form regulatory complexes with bHLH TFs.

The Ka /Ks and πa /πs Ratios under Different Models of Gametophytic and Sporophytic Selection

Alternation of generations in plant life cycle provides a biological basis for natural selection occurring in either the gametophyte or the sporophyte phase or in both. Divergent biphasic selection could yield distinct evolutionary rates for phase-specific or pleiotropic genes. Here, we analyze models that deal with antagonistic and synergistic selection between alternative generations in terms of the ratio of nonsynonymous to synonymous divergence (Ka/Ks). Effects of biphasic selection are opposite under antagonistic selection but cumulative under synergistic selection for pleiotropic genes. Under the additive and comparable strengths of biphasic allelic selection, the absolute Ka/Ks for the gametophyte gene is equal to in outcrossing but smaller than, in a mixed mating system, that for the sporophyte gene under antagonistic selection. The same pattern is predicted for Ka/Ks under synergistic selection. Selfing reduces efficacy of gametophytic selection. Other processes, including pollen and seed flow and genetic drift, reduce selection efficacy. The polymorphism (πa) at a nonsynonymous site is affected by the joint effects of selfing with gametophytic or sporophytic selection. Likewise, the ratio of nonsynonymous to synonymous polymorphism (πa/πs) is also affected by the same joint effects. Gene flow and genetic drift have opposite effects on πa or πa/πs in interacting with gametophytic and sporophytic selection. We discuss implications of this theory for detecting natural selection in terms of Ka/Ks and for interpreting the evolutionary divergence among gametophyte-specific, sporophyte-specific, and pleiotropic genes.

Genomic survey of high-throughput RNA-Seq data implicates involvement of long intergenic non-coding RNAs (lincRNAs) in cytoplasmic male-sterility and fertility restoration in pigeon pea

BACKGROUND

Long-intergenic non-coding RNAs (lincRNAs) originate from intergenic regions and have no coding potential. LincRNAs have emerged as key players in the regulation of various biological processes in plant development. Cytoplasmic male-sterility (CMS) in association with restorer-of-fertility (Rf) systems makes it a highly reliable tool for exploring heterosis for producing commercial hybrid seeds. To date, there have been no reports of lincRNAs during pollen development in CMS and fertility restorer lines in pigeon pea.

OBJECTIVE

Identification of lincRNAs in the floral buds of cytoplasmic male-sterile (AKCMS11) and fertility restorer (AKPR303) pigeon pea lines.

METHODS

We employed a computational approach to identify lincRNAs in the floral buds of cytoplasmic male-sterile (AKCMS11) and fertility restorer (AKPR303) pigeon pea lines using RNA-Seq data.

RESULTS

We predicted a total of 2145 potential lincRNAs of which 966 were observed to be differentially expressed between the sterile and fertile pollen. We identified, 927 cis-regulated and 383 trans-regulated target genes of the lincRNAs. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of the target genes revealed that these genes were specifically enriched in pathways like pollen and pollen tube development, oxidative phosphorylation, etc. We detected 23 lincRNAs that were co-expressed with 17 pollen-related genes with known functions. Fifty-nine lincRNAs were predicted to be endogenous target mimics (eTMs) for 25 miRNAs, and found to be associated with pollen development. The, lincRNA regulatory networks revealed that different lincRNA-miRNA-mRNA networks might be associated with CMS and fertility restoration.

CONCLUSION

Thus, this study provides valuable information by highlighting the functions of lincRNAs as regulators during pollen development in pigeon pea and utilization in hybrid seed production.

Thermosensitivity of pollen: a molecular perspective

A current trend in climate comprises adverse weather anomalies with more frequent and intense temperature events. Heatwaves are a serious threat to global food security because of the susceptibility of crop plants to high temperatures. Among various developmental stages of plants, even a slight rise in temperature during reproductive development proves detrimental, thus making sexual reproduction heat vulnerable. In this context, male gametophyte or pollen development stages are the most sensitive ones. High-temperature exposure induces pollen abortion, reducing pollen viability and germination rate with a concomitant effect on seed yield. This review summarizes the ultrastructural, morphological, biochemical, and molecular changes underpinning high temperature-induced aberrations in male gametophytes. Specifically, we highlight the temperature sensing cascade operating in pollen, involving reactive oxygen species (ROS), heat shock factors (HSFs), a hormones and transcriptional regulatory network. We also emphasize integrating various omics approaches to decipher the molecular events triggered by heat stress in pollen. The knowledge of genes, proteins, and metabolites conferring thermotolerance in reproductive tissues can be utilized to breed/engineer thermotolerant crops to ensure food security.

Polyphenol oxidases regulate pollen development through modulating flavonoids homeostasis in tobacco

Pollen development is critical in plant reproduction. Polyphenol oxidases (PPOs) genes encode defense-related enzymes, but the role of PPOs in pollen development remains largely unexplored. Here, we characterized NtPPO genes, and then investigated their function in pollen via creating NtPPO9/10 double knockout mutant (cas-1), overexpression 35S::NtPPO10 (cosp) line and RNAi lines against all NtPPOs in Nicotiana tabacum. NtPPOs were abundantly expressed in the anther and pollen (especially NtPPO9/10). The pollen germination, polarity ratio and fruit weights were significantly reduced in the NtPPO-RNAi and cosp lines, while they were normal in cas-1 likely due to compensation by other NtPPO isoforms. Comparisons of metabolites and transcripts between the pollen of WT and NtPPO-RNAi, or cosp showed that decreased enzymatic activity of NtPPOs led to hyper-accumulation of flavonoids. This accumulation might reduce the content of ROS. Ca²⁺ and actin levels also decreased in pollen of the transgenic lines.Thus, the NtPPOs regulate pollen germination through the flavonoid homeostasis and ROS signal pathway. This finding provides novel insights into the native physiological functions of PPOs in pollen during reproduction.

The effects of heading time on yield performance and HvGAMYB expression in spring barley subjected to drought

In the lifetime of a plant, flowering is not only an essential part of the reproductive process but also a critical developmental stage that can be vulnerable to environmental stresses. To ensure survival during drought, plants accelerate the flowering process, and this response is known as "drought escape." HvGAMYB-transcription factor associated, among others, with flowering process and anther development in barley-has also an important role in developmental modification and yield performance in plants subjected to stressed conditions. Due to the fact that information about the mechanisms associated both with the flowering acceleration and the anther or pollen disruption is limited, the exploration of the potential HvGAMYB role in flower development may shed light on pollen and spike morphology formations in plants grown under unfavorable water conditions. The aim of this study was to characterize differences in responses to drought among early- and late-heading barley genotypes. These two subgroups of plants-differentiated in terms of phenology-were analyzed, and traits linked to plant phenotype, physiology, and yield were investigated. In our study, the drought stress reactions of two barley subgroups showed a wide range of diversity in terms of yield performance, anther morphology, chlorophyll fluorescence kinetics, and pollen viability. The studied plants exhibited different yield performances under control and drought conditions. Moreover, the random distribution of genotypes on the biplot showing variability of OJIP parameters in the second developmental point of our investigation revealed that prolonged drought stress caused that among early- and late-heading plants, the studied genotypes exhibited different responses to applied stress conditions. The results of this study also showed that the HvGAMYB expression level was correlated positively with traits associated with lateral spike morphology in the second developmental point of this investigation, which showed that this association occurred only under prolonged drought and highlighted the drought stress duration effect on the HvGAMYB expression level.

Molecular regulation of tomato male reproductive development

The reproductive success of flowering plants, which directly affects crop yield, is sensitive to environmental changes. A thorough understanding of how crop reproductive development adapts to climate changes is vital for ensuring global food security. In addition to being a high-value vegetable crop, tomato is also a model plant used for research on plant reproductive development. Tomato crops are cultivated under highly diverse climatic conditions worldwide. Targeted crosses of hybrid varieties have resulted in increased yields and abiotic stress resistance; however, tomato reproduction, especially male reproductive development, is sensitive to temperature fluctuations, which can lead to aborted male gametophytes, with detrimental effects on fruit set. We herein review the cytological features as well as genetic and molecular pathways influencing tomato male reproductive organ development and responses to abiotic stress. We also compare the shared features among the associated regulatory mechanisms of tomato and other plants. Collectively, this review highlights the opportunities and challenges related to characterizing and exploiting genic male sterility in tomato hybrid breeding programs.

Genome-wide analysis reveals the crucial role of lncRNAs in regulating the expression of genes controlling pollen development

The genomic location and stage-specific expression pattern of many long non-coding RNAs reveal their critical role in regulating protein-coding genes crucial in pollen developmental progression and male germ line specification. Long non-coding RNAs (lncRNAs) are transcripts longer than 200 bp with no apparent protein-coding potential. Multiple investigations have revealed high expression of lncRNAs in plant reproductive organs in a cell and tissue-specific manner. However, their potential role as essential regulators of molecular processes involved in sexual reproduction remains largely unexplored. We have used developing field mustard (Brassica rapa) pollen as a model system for investigating the potential role of lncRNAs in reproductive development. Reference-based transcriptome assembly performed to update the existing genome annotation identified novel expressed protein-coding genes and long non-coding RNAs (lncRNAs), including 4347 long intergenic non-coding RNAs (lincRNAs, 1058 expressed) and 2,045 lncRNAs overlapping protein-coding genes on the opposite strand (lncNATs, 780 expressed). The analysis of expression profiles reveals that lncRNAs are significant and stage-specific contributors to the gene expression profile of developing pollen. Gene co-expression networks accompanied by genome location analysis identified 38 cis-acting lincRNA, 31 cis-acting lncNAT, 7 trans-acting lincRNA and 14 trans-acting lncNAT to be substantially co-expressed with target protein-coding genes involved in biological processes regulating pollen development and male lineage specification. These findings provide a foundation for future research aiming at developing strategies to employ lncRNAs as regulatory tools for gene expression control during reproductive development.

The DC1 Domain Protein BINUCLEATE POLLEN is Required for POLLEN Development in Arabidopsis thaliana

The development of the male gametophyte is a tightly regulated process that requires the precise control of cell division and gene expression. A relevant aspect to understand the events underlying pollen development regulation constitutes the identification and characterization of the genes required for this process. In this work, we showed that the DC1 domain protein BINUCLEATE POLLEN (BNP) is essential for pollen development and germination. Pollen grains carrying a defective BNP alleles failed to complete mitosis II and exhibited impaired pollen germination. By yeast two-hybrid analysis and bimolecular fluorescence complementation assays, we identified a set of BNP-interacting proteins. Among confirmed interactors, we found the NAC family transcriptional regulators Vascular Plant One-Zinc Finger 1 (VOZ1) and VOZ2. VOZ1 localization changes during pollen development, moving to the vegetative nucleus at the tricellular stage. We observed that this relocalization requires BNP; in the absence of BNP in pollen from bnp/BNP plants, VOZ1 nuclear localization is impaired. As the voz1voz2 double mutants showed the same developmental defect observed in bnp pollen grains, we propose that BNP requirement to complete microgametogenesis could be linked to its interaction with VOZ1/2 proteins. BNP could have the role of a scaffold protein, recruiting VOZ1/2 to the endosomal system into assemblies that are required for their further translocation to the nucleus, where they act as transcriptional regulators.

RNA-seq analysis of synchronized developing pollen isolated from a single anther

Pollen development, from unicellular microspores to anthesis, is a complex process involving the coordinated specification, differentiation and functions of different cell types. Key to understanding this development is identifying the genes expressed at precise stages of development. However, transcriptomic studies on pollen prior to anthesis are complicated by the inaccessible nature of pollen developing in the anther and the resistant pollen wall. To assist with understanding gene expression during pollen development we have developed a protocol to perform RNA-Seq on pollen isolated from a single anther (SA RNA-Seq). The protocol involves removing pollen from a single anther for analysis and viewing the remaining pollen to determine the developmental stage. The isolated pollen is chemically lysed and mRNA isolated from the lysate using an oligo-dT column before library preparation. Here, we report on the development and testing of our method and the generation of a transcriptome for three stages of pollen development from Arabidopsis (Arabidopsis thaliana) and two stages from male kiwifruit (Actinidia chinensis). This protocol enables the transcriptome of precise developmental stages of pollen to be analyzed, and uses a small number of plants, potentially facilitating studies that require a range of treatments or the analysis of the first generation of transgenic plants.

Histone variants and modifications during abiotic stress response

Plants have developed multiple mechanisms as an adaptive response to abiotic stresses, such as salinity, drought, heat, cold, and oxidative stress. Understanding these regulatory networks is critical for coping with the negative impact of abiotic stress on crop productivity worldwide and, eventually, for the rational design of strategies to improve plant performance. Plant alterations upon stress are driven by changes in transcriptional regulation, which rely on locus-specific changes in chromatin accessibility. This process encompasses post-translational modifications of histone proteins that alter the DNA-histones binding, the exchange of canonical histones by variants that modify chromatin conformation, and DNA methylation, which has an implication in the silencing and activation of hypervariable genes. Here, we review the current understanding of the role of the major epigenetic modifications during the abiotic stress response and discuss the intricate relationship among them.

The SlHB8 acts as a negative regulator in tapetum development and pollen wall formation in Tomato

Pollen development is crucial for the fruit setting process of tomatoes, but the underlying regulatory mechanism remains to be elucidated. Here, we report the isolation of one HD-Zip III family transcription factor, SlHB8, whose expression levels decreased as pollen development progressed. SlHB8 knockout using CRISPR/Cas9 increased pollen activity, subsequently inducing fruit setting, whereas overexpression displayed opposite phenotypes. Overexpression lines under control of the 35 s and p2A11 promoters revealed that SlHB8 reduced pollen activity by affecting early pollen development. Transmission electron microscopy and TUNEL analyses showed that SlHB8 accelerated tapetum degradation, leading to collapsed and infertile pollen without an intine and an abnormal exine. RNA-seq analysis of tomato anthers at the tetrad stage showed that SlHB8 positively regulates SPL/NZZ expression and the tapetum programmed cell death conserved genetic pathway DYT1-TDF1-AMS-MYB80 as well as other genes related to tapetum and pollen wall development. In addition, DNA affinity purification sequencing, electrophoretic mobility shift assay, yeast one-hybrid assay and dual-luciferase assay revealed SlHB8 directly activated the expression of genes related to pollen wall development. The study findings demonstrate that SlHB8 is involved in tapetum development and degradation and plays an important role in anther development.

AtZAT4, a C2H2-Type Zinc Finger Transcription Factor from Arabidopsis thaliana, Is Involved in Pollen and Seed Development

Pollen plays an essential role in plant fertility by delivering the male gametes to the embryo sac before double fertilization. In several plant species, including Arabidopsis, C2H2-type zinc-finger transcription factors (TFs) have been involved in different stages of pollen development and maturation. ZINC FINGER of Arabidopsis thaliana 4 (AtZAT4) is homologous to such TFs and subcellular localization analysis has revealed that AtZAT4 is located in the nucleus. Moreover, analysis of AtZAT4 expression revealed strong levels of it in flowers and siliques, suggesting a role of the encoded protein in the regulation of genes that are associated with reproductive development. We characterized a T-DNA insertional heterozygous mutant Atzat4 (+/−). The relative gene expression analysis of Atzat4 (+/−) showed significant transcript reductions in flowers and siliques. Furthermore, the Atzat4 (+/−) phenotypic characterization revealed defects in the male germline, showing a reduction in pollen tube germination and elongation. Atzat4 (+/−) presented reduced fertility, characterized by a smaller silique size compared to the wild type (WT), and a lower number of seeds per silique. Additionally, seeds displayed lower viability and germination. Altogether, our data suggest a role for AtZAT4 in fertilization and seed viability, through the regulation of gene expression associated with reproductive development.

Phylogenetically Distant BABY BOOM Genes From Setaria italica Induce Parthenogenesis in Rice

The combination of apomixis and hybrid production is hailed as the holy grail of agriculture for the ability of apomixis to fix heterosis of F1 hybrids in succeeding generations, thereby eliminating the need for repeated crosses to produce F1 hybrids. Apomixis, asexual reproduction through seed, achieves this feat by circumventing two processes that are fundamental to sexual reproduction (meiosis and fertilization) and replacing them with apomeiosis and parthenogenesis, resulting in seeds that are clonal to the maternal parent. Parthenogenesis, embryo development without fertilization, has been genetically engineered in rice, maize, and pearl millet using PsASGR-BABY BOOM-like (PsASGR-BBML) transgenes and in rice using the OsBABY BOOM1 (OsBBM1) cDNA sequence when expressed under the control of egg cell-specific promoters. A phylogenetic analysis revealed that BABY BOOM (BBM)/BBML genes from monocots cluster within three different clades. The BBM/BBML genes shown to induce parthenogenesis cluster within clade 1 (the ASGR-BBML clade) along with orthologs from other monocot species, such as Setaria italica. For this study, we tested the parthenogenetic potential of three BBM transgenes from S. italica, each a member of a different phylogenetic BBM clade. All transgenes were genomic constructs under the control of the AtDD45 egg cell-specific promoter. All SiBBM transgenes induced various levels of parthenogenetic embryo development, resulting in viable haploid T1 seedlings. Poor seed set and lower haploid seed production were characteristics of multiple transgenic lines. The results presented in this study illustrate that further functional characterization of BBMs in zygote/embryo development is warranted.

Hydroxyproline-O-Galactosyltransferases Synthesizing Type II Arabinogalactans Are Essential for Male Gametophytic Development in Arabidopsis

In flowering plants, male reproductive function is determined by successful development and performance of stamens, pollen grains, and pollen tubes. Despite the crucial role of highly glycosylated arabinogalactan-proteins (AGPs) in male gamete formation, pollen grain, and pollen tube cell walls, the underlying mechanisms defining these functions of AGPs have remained elusive. Eight partially redundant Hyp-galactosyltransferases (named GALT2-GALT9) genes/enzymes are known to initiate Hyp-O-galactosylation for Hyp-arabinogalactan (AG) production in Arabidopsis thaliana. To assess the contributions of these Hyp-AGs to male reproductive function, we used a galt2galt5galt7galt8galt9 quintuple Hyp-GALT mutant for this study. Both anther size and pollen viability were compromised in the quintuple mutants. Defects in male gametogenesis were observed in later stages of maturing microspores after meiosis, accompanied by membrane blebbing and numerous lytic vacuoles. Cytological and ultramicroscopic observations revealed that pollen exine reticulate architecture and intine layer development were affected such that non-viable collapsed mature pollen grains were produced, which were devoid of cell content and nuclei, with virtually no intine. AGP immunolabeling demonstrated alterations in cell wall architecture of the anther, pollen grains, and pollen tube. Specifically, the LM2 monoclonal antibody (which recognized β-GlcA epitopes on AGPs) showed a weak signal for the endothecium, microspores, and pollen tube apex. Pollen tube tips also displayed excessive callose deposition. Interestingly, expression patterns of pollen-specific AGPs, namely AGP6, AGP11, AGP23, and AGP40, were determined to be higher in the quintuple mutants. Taken together, our data illustrate the importance of type-II AGs in male reproductive function for successful fertilization.

The poly(A) polymerase PAPS1 mediates pollen maturation by regulating sperm cell differentiation in plants

In flowering plants, a haploid microspore undergoes an asymmetric division to produce the male germline that encounters a mitotic division to produce two germ cells. The resulting germ cells undergo a series of specialization events to produce the two sperm cells required for double fertilization. These events include to upregulate male germline-specific while downregulating male germline-nonspecific regulon, but how these specializations events are regulated, are still unresolved. To know how plant sperm cell is specialized, we mutagenized Arabidopsis double homozygous transgenic line (MGH3p-MGH3::eGFP and ACTIN11p-H2B::mRFP) by an ethyl methane sulfonate (EMS) treatment and isolated a mutant with sperms identity loss, resulting in a completely male defective plant. Second-generation sequencing identified a point mutation G/A causing premature stop codon TGG/TGA in the poly(A) polymerase PAPS1 that is linked with phenotype. Further, we found that paps1 mutant fails to upregulate male germline-specific regulon and to downregulate male germline-nonspecific factors required for sperm cell differentiation and attaining pollen maturation. Previously, polyadenylation of pre-mRNAs by PAPS1 has been found crucial for both RNA-based silencing processes and the processing of pre-mRNAs into mature mRNAs ready for translation. This study concludes that PAPS1 mediates sperm cell differentiation through upregulating specific while silencing the nonspecific factors of male germlines.

The non-Mendelian behavior of plant B chromosomes

B chromosomes, also known as supernumerary chromosomes, are dispensable elements in the genome of many plants, animals, and fungi. Many B chromosomes have evolved one or more drive mechanisms to transmit themselves at a higher frequency than predicted by Mendelian genetics, and these mechanisms counteract the tendency of non-essential genetic elements to be lost over time. The frequency of Bs in a population results from a balance between their effect on host fitness and their transmission rate. Here, we will summarize the findings of the drive process of plant B chromosomes, focusing on maize and rye.

Phytochrome interacting factor 3 regulates pollen mitotic division through auxin signalling and sugar metabolism pathways in tomato

The development of viable pollen determines male fertility, and is crucial for reproduction in flowering plants. Phytochrome interacting factor 3 (PIF3) acts as a central regulator of plant growth and development, but its relationship with pollen development has not been determined. Through genetic, histological and transcriptomic analyses, we identified an essential role for SlPIF3 in regulating tomato (Solanum lycopersicum) pollen development. Knocking out SlPIF3 using clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 resulted in pollen mitosis I arrest, and a failure to form viable pollen. We further demonstrated that both glutamate synthase 1 (SlGLT1) and cell wall invertase 9 (SlCWIN9), involved in auxin and sugar homeostasis, respectively, colocalised with SlPIF3 in the anthers and were directly regulated by SlPIF3. Knockout of either SlGLT1 or SlCWIN9 phenocopied the pollen phenotype of SlPIF3 knockout (Slpif3) lines. Slpif3 fertility was partially restored by exogenous auxin indole-3-acetic acid in a dose-dependent manner. This study reveals a mechanism by which SlPIF3 regulates pollen development and highlights a new strategy for creating hormone-regulated genic male sterile lines for tomato hybrid seed production.

OsSPLs Regulate Male Fertility in Response to Different Temperatures by Flavonoid Biosynthesis and Tapetum PCD in PTGMS Rice

Photoperiod and thermo-sensitive genic male sterile (PTGMS) rice is an important resource for two line hybrid rice production. The SQUAMOSA–promoter binding, such as the (SPL) gene family, encode the plant specific transcription factors that regulate development and defense responses in plants. However, the reports about SPLs participating in male fertility regulation are limited. Here, we identified 19 OsSPL family members and investigated their involvement in the fertility regulation of the PTGMS rice lines, PA2364S and PA2864S, with different fertility transition temperatures. The results demonstrated that OsSPL2, OsSPL4, OsSPL16 and OsSPL17 affect male fertility in response to temperature changes through the MiR156-SPL module. WGCNA (weighted gene co-expression network analysis) revealed that CHI and APX1 were co-expressed with OsSPL17. Targeted metabolite and flavonoid biosynthetic gene expression analysis revealed that OsSPL17 regulates the expression of flavonoid biosynthesis genes CHI, and the up regulation of flavanones (eriodictvol and naringenin) and flavones (apigenin and luteolin) content contributed to plant fertility. Meanwhile, OsSPL17 negatively regulates APX1 to affect APX (ascorbate peroxidase) activity, thereby regulating ROS (reactive oxygen species) content in the tapetum, controlling the PCD (programmed cell death) process and regulating male fertility in rice. Overall, this report highlights the potential role of OsSPL for the regulation of male fertility in rice and provides a new insight for the further understanding of fertility molecular mechanisms in PTGMS rice.

Patterns of gene expression in pollen of cotton (Gossypium hirsutum) indicate downregulation as a feature of thermotolerance

Reproductive performance in plants is impaired as maximum temperatures consistently approach 40°C. However, the timing of heatwaves critically affects their impact. We studied the molecular responses during pollen maturation in cotton to investigate the vulnerability to high temperature. Tetrads (TEs), uninucleate and binucleate microspores, and mature pollen were subjected to SWATH-MS and RNA-seq analyses after exposure to 38/28°C (day/night) for 5 days. The results indicated that molecular signatures were downregulated progressively in response to heat during pollen development. This was even more evident in leaves, where three-quarters of differentially changed proteins decreased in abundance during heat. Functional analysis showed that translation of genes increased in TEs after exposure to heat; however, the reverse pattern was observed in mature pollen and leaves. For example, proteins involved in transport were highly abundant in TEs whereas in later stages of pollen formation and leaves, heat suppressed synthesis of proteins involved in cell-to-cell communication. Moreover, a large number of heat shock proteins were identified in heat-affected TEs, but these proteins were less abundant in mature pollen and leaves. We speculate that the sensitivity of TE cells to heat is related to high rates of translation targeted to pathways that might not be essential for thermotolerance. Molecular signatures during stages of pollen development after heatwaves could provide markers for future genetic improvement.

Effects of Ionizing Radiation on Flora Ten Years after the Fukushima Dai-ichi Disaster

The aim of this work is to analyze the effects of ionizing radiation and radionuclides (like 137Cs) in several higher plants located around the Fukushima Dai-ichi Nuclear Power Plant (FNPP), evaluating both their adaptive processes and evolution. After the FNPP accident in March 2011 much attention was focused to the biological consequences of ionizing radiation and radionuclides released in the area surrounding the nuclear plant. This unexpected mishap led to the emission of radionuclides in aerosol and gaseous forms from the power plant, which contaminated a large area, including wild forest, cities, farmlands, mountains, and the sea, causing serious problems. Large quantities of 131I, 137Cs, and 134Cs were detected in the fallout. People were evacuated but the flora continued to be affected by the radiation exposure and by the radioactive dusts' fallout. The response of biota to FNPP irradiation was a complex interaction among radiation dose, dose rate, temporal and spatial variation, varying radiation sensitivities of the different plants' species, and indirect effects from other events. The repeated ionizing radiations, acute or chronic, guarantee an adaptation of the plant species, demonstrating a radio-resistance. Consequently, ionizing radiation affects the genetic structure, especially during chronic irradiation, reducing genetic variability. This reduction is associated with the different susceptibility of plant species to chronic stress. This would confirm the adaptive theory associated with this phenomenon. The effects that ionizing radiation has on different life forms are examined in this review using the FNPP disaster as a case study focusing the attention ten years after the accident.