A novel class of MYB factors controls sperm-cell formation in plants

Transcriptome dynamics in the Arabidopsis male germline during pollen-pistil interactions

When pollen lands on a receptive stigma, it germinates and extends a tube inside the transmitting tissue of the pistil to deliver the sperm cells for double fertilization. The growth of the pollen tube triggers significant alterations in its gene expression. The extent to which these changes occur in the vegetative cell or extend to the sperm cells transported by the tube is unclear but important to determine since sperm cells are believed to acquire a competency for fertilization during pollen-pistil interactions. To address these questions, we compared the transcriptomes of Arabidopsis thaliana sperm cells and vegetative nuclei isolated from mature pollen grains with those isolated from in vitro-grown pollen tubes. Importantly, we also compared transcriptomes of sperm cells obtained from pollen tubes grown under semi-in vivo conditions where tubes passed through a pistil section. Our data show that extensive transcriptomic changes occur in sperm cells during pollen tube growth, some of which are elicited only as sperms are carried through the pistil. Their analysis reveals a host of previously unidentified transcripts that may facilitate sperm maturation and gamete fusion. The vegetative cell undergoes even more extensive transcriptomic reprogramming during pollen tube growth, mainly through the upregulation of genes associated with pollen tube growth and vesicle-mediated transport. Interestingly, ATAC-seq data show that the promoters of genes upregulated in sperm during pollen tube growth are already accessible in sperm chromatin of mature pollen grains, suggesting pre-configured promoter accessibility. This study's expression data can be further explored here: https://bar.utoronto.ca/eFP-Seq_Browser/.

Cell proliferation suppressor RBR1 interacts with ARID1 to promote pollen mitosis via stabilizing DUO1 in Arabidopsis

In plants, sperm cell formation involves two rounds of pollen mitoses, in which the microspore initiates the first pollen mitosis (PMI) to produce a vegetative cell and a generative cell, then the generative cell continues the second mitosis (PMII) to produce two sperm cells. DUO1, a R2R3 Myb transcription factor, is activated in the generative cell to promote S-G2/M transition during PMII. Loss-of-function of DUO1 caused a complete arrest of PMII. Despite the importance of DUO1, how DUO1 is regulated is largely unexplored. We previously demonstrated that ARID1, an ARID transcription factor, stimulates DUO1 transcription. Here, we show that cell proliferation suppressor RBR1 interacts with ARID1 to stabilize DUO1. While the C-terminus of RBR1 is dispensable for vegetative growth, it plays a crucial role in reproductive development and facilitates interaction with ARID1. Moreover, DUO1 is a short-lived protein, ARID1 promotes the RBR1-DUO1 interaction, and RBR1 stabilizes DUO1 in a proteasome-dependent manner. Thus, RBR1 promotes DUO1-dependent PMII progression via antagonizing its repressive role in the cell cycle factors CDKA;1 and CYCB1;1. Collectively, we uncover that ARID1 and RBR1 act in concert to regulate DUO1 at both the transcriptional and posttranscriptional levels, balancing cell specification and cell division.

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.

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 Role of Female and Male Genes in Regulating Pollen Tube Guidance in Flowering Plants

In flowering plants, fertilization is a complex process governed by precise communication between the male and female gametophytes. This review focuses on the roles of various female gametophyte cells-synergid, central, and egg cells-in facilitating pollen tube guidance and ensuring successful fertilization. Synergid cells play a crucial role in attracting the pollen tube, while the central cell influences the direction of pollen tube growth, and the egg cell is responsible for preventing polyspermy, ensuring correct fertilization. The review also examines the role of the pollen tube in this communication, highlighting the mechanisms involved in its growth regulation, including the importance of pollen tube receptors, signal transduction pathways, cell wall dynamics, and ion homeostasis. The Ca2+ concentration gradient is identified as a key factor in guiding pollen tube growth toward the ovule. Moreover, the review briefly compares these communication processes in angiosperms with those in non-flowering plants, such as mosses, ferns, and early gymnosperms, providing evolutionary insights into gametophytic signaling. Overall, this review synthesizes the current understanding of male-female gametophyte interactions and outlines future directions for research in plant reproductive biology.

ARID1 is required to regulate and reinforce H3K9me2 in sperm cells in Arabidopsis

Heterochromatin de-condensation in companion gametic cells is conserved in both plants and animals. In plants, microspore undergoes asymmetric pollen mitosis (PMI) to produce a vegetative cell (VC) and a generative cell (GC). Subsequently, the GC undergoes pollen mitosis (PMII) to produce two sperm cells (SC). Consistent with heterochromatin de-condensation in the VC, H3K9me2, a heterochromatin mark, is barely detected in VC. However, how H3K9me2 is differentially regulated during pollen mitosis remains unclear. Here, we show that H3K9me2 is gradually evicted from the VC since PMI but remain unchanged in the GC and SC. ARID1, a pollen-specific transcription factor that facilitates PMII, promotes H3K9me2 maintenance in the GC/SC but slows down its eviction in the VC. The genomic targets of ARID1 mostly overlaps with H3K9me2 loci, and ARID1 recruits H3K9 methyltransferase SUVH6. Our results uncover that differential pattern of H3K9me2 between two cell types is regulated by ARID1 during pollen mitosis.

The miR159a-DUO1 module regulates pollen development by modulating auxin biosynthesis and starch metabolism in citrus

Achieving seedlessness in citrus varieties is one of the important objectives of citrus breeding. Male sterility associated with abnormal pollen development is an important factor in seedlessness. However, our understanding of the regulatory mechanism underlying the seedlessness phenotype in citrus is still limited. Here, we determined that the miR159a-DUO1 module played an important role in regulating pollen development in citrus, which further indirectly modulated seed development and fruit size. Both the overexpression of csi-miR159a and the knocking out of DUO1 in Hong Kong kumquat (Fortunella hindsii) resulted in small and seedless fruit phenotypes. Moreover, pollen was severely aborted in both transgenic lines, with arrested pollen mitotic I and abnormal pollen starch metabolism. Through additional cross-pollination experiments, DUO1 was proven to be the key target gene for miR159a to regulate male sterility in citrus. Based on DNA affinity purification sequencing (DAP-seq), RNA-seq, and verified interaction assays, YUC2/YUC6, SS4 and STP8 were identified as downstream target genes of DUO1, those were all positively regulated by DUO1. In transgenic F. hindsii lines, the miR159a-DUO1 module down-regulated the expression of YUC2/YUC6, which decreased indoleacetic acid (IAA) levels and modulated auxin signaling to repress pollen mitotic I. The miR159a-DUO1 module reduced the expression of the starch synthesis gene SS4 and sugar transport gene STP8 to disrupt starch metabolism in pollen. Overall, this work reveals a new mechanism by which the miR159a-DUO1 module regulates pollen development and elucidates the molecular regulatory network underlying male sterility in citrus.

The male germ unit association is independently regulated of GUM in Arabidopsis thaliana

Cytoplasmic projections (CPs) formed by the generative and sperm cells link the male gametes with the vegetative cell (VC) nucleus, which are required to build the male germ unit (MGU) assemblage in the angiosperm pollen grain. As molecular and genetic controls underlying CP development and formation of the MGU are unknown, it was hypothesized that physical association between germ cells and the VC nucleus might be lost in germ unit malformed (gum) mutants or in those which either block generative cell (GC) division or that additionally prevent gamete differentiation. In vivo, analysis of marked cellular components demonstrated a linkage of sperm cells (SCs) and the VC nucleus in gum mutant alleles despite their increased physical separation. Similarly, for several independent classes of bicellular pollen mutants, undivided GCs were associated with the VC nucleus like GCs in wild-type pollen. We conclude that the early formation of GC CPs to establish the MGU is regulated independently of DUO1-DAZ1 and DUO3 transcription factors as well as cyclin-dependent kinase function (CDKA;1). As the absence of cytoplasmic protrusion was expected in the gum mutants in Arabidopsis, early histological studies reported temporal disappearance of cytoplasmic protrusion in several organisms. Our findings demonstrated the striking importance of live imaging to verify the broad conservation of the persistent MGU contact in all the angiosperms and its important role in successful double fertilization.

Differential expression and evolutionary diversification of RNA helicases in Boechera sexual and apomictic reproduction

In higher plants, sexual reproduction is characterized by meiosis of the first cells of the germlines, and double fertilization of the egg and central cell after gametogenesis. In contrast, in apomicts of the genus Boechera, meiosis is omitted or altered and only the central cell requires fertilization, while the embryo forms parthenogenetically from the egg cell. To deepen the understanding of the transcriptional basis underlying these differences, we applied RNA-seq to compare expression in reproductive tissues of different Boechera accessions. This confirmed previous evidence of an enrichment of RNA helicases in plant germlines. Furthermore, few RNA helicases were differentially expressed in female reproductive ovule tissues harboring mature gametophytes from apomictic and sexual accessions. For some of these genes, we further found evidence for a complex recent evolutionary history. This included a homolog of Arabidopsis thaliana FASCIATED STEM4 (FAS4). In contrast to AtFAS4, which is a single-copy gene, FAS4 is represented by three homologs in Boechera, suggesting a potential for subfunctionalization to modulate reproductive development. To gain first insights into functional roles of FAS4, we studied Arabidopsis lines carrying mutant alleles. This identified the crucial importance of AtFAS4 for reproduction, as we observed developmental defects and arrest during male and female gametogenesis.

The transcription factors and pathways underpinning male reproductive development in Arabidopsis

As Arabidopsis flowers mature, specialized cells within the anthers undergo meiosis, leading to the production of haploid microspores that differentiate into mature pollen grains, each containing two sperm cells for double fertilization. During pollination, the pollen grains are dispersed from the anthers to the stigma for subsequent fertilization. Transcriptomic studies have identified a large number of genes expressed over the course of male reproductive development and subsequent functional characterization of some have revealed their involvement in floral meristem establishment, floral organ growth, sporogenesis, meiosis, microsporogenesis, and pollen maturation. These genes encode a plethora of proteins, ranging from transcriptional regulators to enzymes. This review will focus on the regulatory networks that control male reproductive development, starting from flower development and ending with anther dehiscence, with a focus on transcription factors and some of their notable target genes.

The R2R3 MYB gene TaMYB305 positively regulates anther and pollen development in thermo-sensitive male-sterility wheat with Aegilops kotschyi cytoplasm

MAIN CONCLUSION

The loss of TaMYB305 function down-regulated the expression of jasmonic acid synthesis pathway genes, which may disturb the jasmonic acid synthesis, resulting in abnormal pollen development and reduced fertility. The MYB family, as one of the largest transcription factor families found in plants, regulates plant development, especially the development of anthers. Therefore, it is important to identify potential MYB transcription factors associated with pollen development and to study its role in pollen development. Here, the transcripts of an R2R3 MYB gene TaMYB305 from KTM3315A, a thermo-sensitive cytoplasmic male-sterility line with Aegilops kotschyi cytoplasm (K-TCMS) wheat, was isolated. Quantitative real-time PCR (qRT-PCR) and promoter activity analysis revealed that TaMYB305 was primarily expressed in anthers. The TaMYB305 protein was localized in the nucleus, as determined by subcellular localization analysis. Our data demonstrated that silencing of TaMYB305 was related to abnormal development of stamen, including anther indehiscence and pollen abortion in KAM3315A plants. In addition, TaMYB305-silenced plants exhibited alterations in the transcriptional levels of genes involved in the synthesis of jasmonic acid (JA), indicating that TaMYB305 may regulate the expression of genes related to JA synthesis and play an important role during anther and pollen development of KTM3315A. These results provide novel insight into the function and molecular mechanism of R2R3-MYB genes in pollen development.

EGG CELL 1 contributes to egg-cell-dependent preferential fertilization in Arabidopsis

During double fertilization in angiosperms, the pollen tube delivers two sperm cells into an embryo sac; one sperm cell fuses with an egg cell, and the other sperm cell fuses with the central cell. It has long been proposed that the preference for fusion with one or another female gamete cell depends on the sperm cells and occurs during gamete recognition. However, up to now, sperm-dependent preferential fertilization has not been demonstrated, and results on preferred fusion with either female gamete have remained conflicting. To investigate this topic, we generated Arabidopsis thaliana mutants that produce single sperm-like cells or whose egg cells are eliminated; we found that although the three different types of sperm-like cell are functionally equivalent in their ability to fertilize the egg and the central cell, each type of sperm-like cell fuses predominantly with the egg cell. This indicates that it is the egg cell that controls its preferential fertilization. We also found that sperm-activating small secreted EGG CELL 1 proteins are involved in the regulation of egg-cell-dependent preferential fertilization, revealing another important role for this protein family during double fertilization.

Histone Variant H3.3 Controls Arabidopsis Fertility by Regulating Male Gamete Development

Reprograming of chromatin structures and changes in gene expression are critical for plant male gamete development, and epigenetic marks play an important role in these processes. Histone variant H3.3 is abundant in euchromatin and is largely associated with transcriptional activation. The precise function of H3.3 in gamete development remains unclear in plants. Here, we report that H3.3 is abundantly expressed in Arabidopsis anthers and its knockout mutant h3.3-1 is sterile due to male sterility. Transcriptome analysis of young inflorescence has identified 2348 genes downregulated in h3.3-1 mutant, among which 1087 target genes are directly bound by H3.3, especially at their 3' ends. As a group, this set of H3.3 targets is enriched in the reproduction-associated processes including male gamete generation, pollen sperm cell differentiation and pollen tube growth. The function of H3.3 in male gamete development is dependent on the Anti-Silencing Factor 1A/1B (ASF1A/1B)-Histone regulator A (HIRA)-mediated pathway. Our results suggest that ASF1A/1B-HIRA-mediated H3.3 deposition at its direct targets for transcription activation forms the regulatory networks responsible for male gamete development.

Cell cycle status of male and female gametes during Arabidopsis reproduction

Fertilization in Arabidopsis (Arabidopsis thaliana) is a highly coordinated process that begins with a pollen tube delivering the 2 sperm cells into the embryo sac. Each sperm cell can then fertilize either the egg or the central cell to initiate embryo or endosperm development, respectively. The success of this double fertilization process requires a tight cell cycle synchrony between the male and female gametes to allow karyogamy (nuclei fusion). However, the cell cycle status of the male and female gametes during fertilization remains elusive as DNA quantification and DNA replication assays have given conflicting results. Here, to reconcile these results, we quantified the DNA replication state by DNA sequencing and performed microscopic analyses of fluorescent markers covering all phases of the cell cycle. We show that male and female Arabidopsis gametes are both arrested prior to DNA replication at maturity and initiate their DNA replication only during fertilization.

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.

Cell-type-specific alternative splicing in the Arabidopsis germline

During sexual reproduction in flowering plants, the two haploid sperm cells (SCs) embedded within the cytoplasm of a growing pollen tube are carried to the embryo sac for double fertilization. Pollen development in flowering plants is a dynamic process that encompasses changes at transcriptome and epigenome levels. While the transcriptome of pollen and SCs in Arabidopsis (Arabidopsis thaliana) is well documented, previous analyses have mostly been based on gene-level expression. In-depth transcriptome analysis, particularly the extent of alternative splicing (AS) at the resolution of SC and vegetative nucleus (VN), is still lacking. Therefore, we performed RNA-seq analysis to generate a spliceome map of Arabidopsis SCs and VN isolated from mature pollen grains. Based on our de novo transcriptome assembly, we identified 58,039 transcripts, including 9,681 novel transcripts, of which 2,091 were expressed in SCs and 3,600 in VN. Four hundred and sixty-eight genes were regulated both at gene and splicing levels, with many having functions in mRNA splicing, chromatin modification, and protein localization. Moreover, a comparison with egg cell RNA-seq data uncovered sex-specific regulation of transcription and splicing factors. Our study provides insights into a gamete-specific AS landscape at unprecedented resolution.

Genetic control of generative cell shape by DUO1 in Arabidopsis

The main features of generative cell morphogenesis, formation of a cytoplasmic projection and elongation of the GC body, operate through independent genetic pathways. Male gametogenesis in developing angiosperm pollen involves distinctive changes in cell morphogenesis. Re-shaping and elongation of the generative cell (GC) are linked to the formation of a GC cytoplasmic projection connected to the vegetative cell nucleus. Although genetic control of GC morphogenesis is unknown, we suspected the involvement of the germline-specific MYB transcription factor DUO POLLEN1 (DUO1). We used light and fluorescence microscopy to examine male germline development in pollen of wild-type Arabidopsis and in four allelic duo1 mutants expressing introduced cell markers. Our analysis shows that the undivided GC in duo1 pollen forms a cytoplasmic projection, but the cell body fails to elongate. In contrast GCs of cyclin-dependent kinase function mutants, which fail to divide like duo1 mutants, achieve normal morphogenesis. We conclude that DUO1 has an essential role in the elongation of the GC, but DUO1-independent pathways control the development of the GC cytoplasmic projection. The two main features of GC morphogenesis therefore operate through independently regulated genetic pathways.

Cytoskeleton Remodeling in Arabidopsis Stigmatic Cells Following Pollination

In plants, the first interaction that occurs between the male gametophytes (pollen grains) and the stigmatic epidermis of the female organ is crucial for successful reproduction. The stigma consists of a dome of flask-shaped cells specialized in pollen capture. In these stigmatic cells, the cytoskeleton network (cortical microtubules and actin microfilaments) actively responds to pollen contact and undergoes dynamic remodeling required for successful pollen acceptance to occur. Here, we have designed several microscopy mountings to monitor stigmatic cytoskeleton dynamics. These designs are based on the constraints linked to the tightly regulated pollen-stigma interaction and depend upon the experimental goal, either a static view or live-cell imaging.

The type-B response regulators ARR10, ARR12, and ARR18 specify the central cell in Arabidopsis

In Arabidopsis thaliana, the female gametophyte consists of two synergid cells, an egg cell, a diploid central cell, and three antipodal cells. CYTOKININ INDEPENDENT 1 (CKI1), a histidine kinase constitutively activating the cytokinin signaling pathway, specifies the central cell and restricts the egg cell. However, the mechanism regulating CKI1-dependent central cell specification is largely unknown. Here, we showed that the type-B ARABIDOPSIS RESPONSE REGULATORS10, 12, and 18 (ARR10/12/18) localize at the chalazal pole of the female gametophyte. Phenotypic analysis showed that the arr10 12 18 triple mutant is female sterile. We examined the expression patterns of embryo sac marker genes and found that the embryo sac of arr10 12 18 plants had lost central cell identity, a phenotype similar to that of the Arabidopsis cki1 mutant. Genetic analyses demonstrated that ARR10/12/18, CKI1, and ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN2, 3, and 5 (AHP2/3/5) function in a common pathway to regulate female gametophyte development. In addition, constitutively activated ARR10/12/18 in the cki1 embryo sac partially restored the fertility of cki1. Results of transcriptomic analysis supported the conclusion that ARR10/12/18 and CKI1 function together to regulate the identity of the central cell. Our results demonstrated that ARR10/12/18 function downstream of CKI1-AHP2/3/5 as core factors to determine cell fate of the female gametophyte.

The egg cell is preferentially fertilized in Arabidopsis double fertilization

In flowering plants (angiosperms), fertilization of the egg cell by one sperm cell produces an embryo, whereas fusion of a second sperm cell with the central cell generates the endosperm. In most angiosperms like Arabidopsis, a pollen grain contains two isomorphic sperm cells required for this double fertilization process. A long-standing unsolved question is whether the two fertilization events have any preference. A tool to address this question is the usage of the cyclin-dependent kinase a1 (cdka;1) mutant pollen, which produces a single sperm-like cell (SLC). Here, we first adopt a complementation-based fluorescence-labeling method to successfully separate and collect cdka;1 mutant pollen containing a single SLC. Single-cell RNA-sequencing analysis revealed that cdka;1 SLCs show a gene expression profile highly similar to that of sperm cells and not to the generative cell, precursor of the two sperm cells. Pollination assays using a limited number of cdka;1 mutant pollen revealed that in 98.2% of the ovules, single fertilization of the egg cell occurred. Pollination of pistils with excessive cdka;1 mutant pollen allowed the delivery of a second SLC via fertilization recovery, which fertilized the central cell, resulting in 20.7% double-fertilized ovules. This indicates that cdka;1 SLCs are able to fertilize both the egg and the central cell. Taken together, our findings have answered a long-standing question and support that preferential fertilization of the egg cell is evident in Arabidopsis.

Global transcriptome analysis reveals potential genes associated with genic male sterility of rapeseed (Brassica napus L.)

Rapeseed is the third leading source of edible oil in the world. Genic male sterility (GMS) lines provide crucial material for harnessing heterosis for rapeseed. GMS lines have been widely used successfully for rapeseed hybrid production. The physiological and molecular mechanism of pollen development in GMS lines of rapeseed (Brassica napus L.) need to be determined for the creation of hybrids and cultivation of new varieties. However, limited studies have focused on systematically mining genes that regulate the pollen development of GMS lines in B. napus. In the present study, to determine the stage at which pollen development begins to show abnormality in the GMS lines, we performed semi-thin section analysis of the anthers with five pollen development stages. The results indicated that the abnormal pollen development in DGMS lines might start at the meiotic stage, and abnormal pollen development in RGMS lines probably occurred before the tetrad stage. To investigate the critical genes and pathways involved in pollen development in GMS lines, we constructed and sequenced 24 transcriptome libraries for the flower buds from the fertile and sterile lines of two recessive GMS (RGMS) lines (6251AB and 6284AB) and two dominant GMS (DGMS) lines (4001AB and 4006AB). A total of 23,554 redundant DEGs with over two-fold change between sterile and fertile lines were obtained. A total of 346 DEGs were specifically related to DGMS, while 1,553 DEGs were specifically related to RGMS. A total of 1,545 DEGs were shared between DGMS and RGMS. And 253 transcription factors were found to be differentially expressed between the sterile and fertile lines of GMS. In addition, 6,099 DEGs possibly related to anther, pollen, and microspore development processes were identified. Many of these genes have been reported to be involved in anther and microspore developmental processes. Several DEGs were speculated to be key genes involved in the regulation of fertility. Three differentially expressed genes were randomly selected and their expression levels were verified by quantitative PCR (qRT-PCR). The results of qRT-PCR largely agreed with the transcriptome sequencing results. Our findings provide a global view of genes that are potentially involved in GMS occurrence. The expression profiles and function analysis of these DEGs were provided to expand our understanding of the complex molecular mechanism in pollen and sterility development in B. napus.

H3K27 methylation regulates the fate of two cell lineages in male gametophytes

During angiosperm male gametogenesis, microspores divide to produce a vegetative cell (VC) and a male germline (MG), each with distinct cell fates. The mechanism underlying determination of the MG cell/VC fate remains an important area of research, with many unanswered questions. Here, we report that H3K27me3 is essential for VC fate commitment in male Arabidopsis thaliana gametophytes; H3K27me3 erasure contributes to MG cell fate initiation. VC-targeted H3K27me3 erasure disturbed VC development and shifted the VC fate toward a gamete destination, which suggests that MG cells require H3K27me3 erasure to trigger gamete cell fate. Multi-omics and cytological analyses confirmed the occurrence of extensive cell identity transition due to H3K27me3 erasure. Therefore, we experimentally confirmed that MG cell/VC fate is epigenetically regulated. H3K27 methylation plays a critical role in guiding MG cell/VC fate determination for pollen fertility in Arabidopsis. Our work also provides evidence for two previous hypotheses: the germline cell fate is specified by the differential distribution of unknown determinants and VC maintains the default microspore program (i.e. the H3K27me3 setting) while MG requires reprogramming.

The Physcomitrium patens egg cell expresses several distinct epigenetic components and utilizes homologues of BONOBO genes for cell specification

Although land plant germ cells have received much attention, knowledge about their specification is still limited. We thus identified transcripts enriched in egg cells of the bryophyte model species Physcomitrium patens, compared the results with angiosperm egg cells, and selected important candidate genes for functional analysis. We used laser-assisted microdissection to perform a cell-type-specific transcriptome analysis on egg cells for comparison with available expression profiles of vegetative tissues and male reproductive organs. We made reporter lines and knockout mutants of the two BONOBO (PbBNB) genes and studied their role in reproduction. We observed an overlap in gene activity between bryophyte and angiosperm egg cells, but also clear differences. Strikingly, several processes that are male-germline specific in Arabidopsis are active in the P. patens egg cell. Among those were the moss PbBNB genes, which control proliferation and identity of both female and male germlines. Pathways shared between male and female germlines were most likely present in the common ancestors of land plants, besides sex-specifying factors. A set of genes may also be involved in the switches between the diploid and haploid moss generations. Nonangiosperm gene networks also contribute to the specification of the P. patens egg cell.

Cycling in a crowd: Coordination of plant cell division, growth, and cell fate

The reiterative organogenesis that drives plant growth relies on the constant production of new cells, which remain encased by interconnected cell walls. For these reasons, plant morphogenesis strictly depends on the rate and orientation of both cell division and cell growth. Important progress has been made in recent years in understanding how cell cycle progression and the orientation of cell divisions are coordinated with cell and organ growth and with the acquisition of specialized cell fates. We review basic concepts and players in plant cell cycle and division, and then focus on their links to growth-related cues, such as metabolic state, cell size, cell geometry, and cell mechanics, and on how cell cycle progression and cell division are linked to specific cell fates. The retinoblastoma pathway has emerged as a major player in the coordination of the cell cycle with both growth and cell identity, while microtubule dynamics are central in the coordination of oriented cell divisions. Future challenges include clarifying feedbacks between growth and cell cycle progression, revealing the molecular basis of cell division orientation in response to mechanical and chemical signals, and probing the links between cell fate changes and chromatin dynamics during the cell cycle.

Deciphering the synergistic and redundant roles of CG and non-CG DNA methylation in plant development and transposable element silencing

DNA methylation plays key roles in transposable element (TE) silencing and gene expression regulation. DNA methylation occurs at CG, CHG and CHH sequence contexts in plants. However, the synergistic and redundant roles of CG and non-CG methylation are poorly understood. By introducing CRISPR/Cas9-induced met1 mutation into the ddcc (drm1 drm2 cmt2 cmt3) mutant, we attempted to knock out all five DNA methyltransferases in Arabidopsis and then investigate the synergistic and redundant roles of CG and non-CG DNA methylation. We found that the homozygous ddcc met1 quintuple mutants are embryonically lethal, although met1 and ddcc mutants only display some developmental abnormalities. Unexpectedly, the ddcc met1 quintuple mutations only reduce transmission through the male gametophytes. The ddcc met1^+/- mutants show apparent size divergence, which is not associated with difference in DNA methylation patterns, but associated with the difference in the levels of DNA damage. Finally, we show that a group of TEs are specifically activated in the ddcc met1^+/- mutants. This work reveals that CG and non-CG DNA methylation synergistically and redundantly regulate plant reproductive development, vegetative development and TE silencing in Arabidopsis. Our findings provide insights into the roles of DNA methylation in plant development.

Insights into the molecular evolution of fertilization mechanism in land plants

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

From birth to function: Male gametophyte development in flowering plants

Male germline development in flowering plants involves two distinct and successive phases, microsporogenesis and microgametogenesis, which involve one meiosis followed by two rounds of mitosis. Many aspects of distinctions after mitosis between the vegetative cell and the male germ cells are seen, from morphology to structure, and the differential functions of the two cell types in the male gametophyte are differentially needed and required for double fertilization. The two sperm cells, carriers of the hereditary substances, depend on the vegetative cell/pollen tube to be delivered to the female gametophyte for double fertilization. Thus, the intercellular communication and coordinated activity within the male gametophyte probably represent the most subtle regulation in flowering plants to guarantee the success of reproduction. This review will focus on what we have known about the differentiation process and the functional diversification of the vegetative cell and the male germ cell, the most crucial cell types for plant fertility and crop production.

RH17 restricts reproductive fate and represses autonomous seed coat development in sexual Arabidopsis

Plant sexual and asexual reproduction through seeds (apomixis) is tightly controlled by complex gene regulatory programs, which are not yet fully understood. Recent findings suggest that RNA helicases are required for plant germline development. This resembles their crucial roles in animals, where they are involved in controlling gene activity and the maintenance of genome integrity. Here, we identified previously unknown roles of Arabidopsis RH17 during reproductive development. Interestingly, RH17 is involved in repression of reproductive fate and of elements of seed development in the absence of fertilization. In lines carrying a mutant rh17 allele, development of supernumerary reproductive cell lineages in the female flower tissues (ovules) was observed, occasionally leading to formation of two embryos per seed. Furthermore, seed coat, and putatively also endosperm development, frequently initiated autonomously. Such induction of several features phenocopying distinct elements of apomixis by a single mutation is unusual and suggests that RH17 acts in regulatory control of plant reproductive development. Furthermore, an in-depth understanding of its action might be of use for agricultural applications.

Genome-Wide Characterization of R2R3-MYB Transcription Factors in Pitaya Reveals a R2R3-MYB Repressor HuMYB1 Involved in Fruit Ripening through Regulation of Betalain Biosynthesis by Repressing Betalain Biosynthesis-Related Genes

The MYB (myeloblastosis) superfamily constitutes one of the most abundant transcription factors (TFs) regulating various biological processes in plants. However, the molecular characteristics and functions of MYB TFs in pitaya remain unclear. To date, no genome-wide characterization analysis of this gene family has been conducted in the Cactaceae species. In this study, 105 R2R3-MYB members were identified from the genome data of Hylocereus undatus and their conserved motifs, physiological and biochemical characteristics, chromosome locations, synteny relationship, gene structure and phylogeny were further analyzed. Expression analyses suggested that three up-regulated HuMYBs and twenty-two down-regulated HuMYBs were probably involved in fruit ripening of pitaya. Phylogenetic analyses of R2R3-MYB repressors showed that seven HuMYBs (HuMYB1, HuMYB21, HuMYB48, HuMYB49, HuMYB72, HuMYB78 and HuMYB101) were in clades containing R2R3-MYB repressors. HuMYB1 and HuMYB21 were significantly down-regulated with the betalain accumulation during fruit ripening of ‘Guanhuahong’ pitaya (H. monacanthus). However, only HuMYB1 had R2 and R3 repeats with C1, C2, C3 and C4 motifs. HuMYB1 was localized exclusively to the nucleus and exhibited transcriptional inhibition capacities. Dual luciferase reporter assay demonstrated that HuMYB1 inhibited the expression of betalain-related genes: HuADH1, HuCYP76AD1-1 and HuDODA1. These results suggested that HuMYB1 is a potential repressor of betalain biosynthesis during pitaya fruit ripening. Our results provide the first genome-wide analyses of the R2R3-MYB subfamily involved in pitaya betalain biosynthesis and will facilitate functional analysis of this gene family in the future.

Development and Molecular Genetics of Marchantia polymorpha

Bryophytes occupy a basal position in the monophyletic evolution of land plants and have a life cycle in which the gametophyte generation dominates over the sporophyte generation, offering a significant advantage in conducting genetics. Owing to its low genetic redundancy and the availability of an array of versatile molecular tools, including efficient genome editing, the liverwort Marchantia polymorpha has become a model organism of choice that provides clues to the mechanisms underlying eco-evo-devo biology in plants. Recent analyses of developmental mutants have revealed that key genes in developmental processes are functionally well conserved in plants, despite their morphological differences, and that lineage-specific evolution occurred by neo/subfunctionalization of common ancestral genes. We suggest that M. polymorpha is an excellent platform to uncover the conserved and diversified mechanisms underlying land plant development.

Reproduction Multitasking: The Male Gametophyte

The gametophyte represents the sexual phase in the alternation of generations in plants; the other, nonsexual phase is the sporophyte. Here, we review the evolutionary origins of the male gametophyte among land plants and, in particular, its ontogenesis in flowering plants. The highly reduced male gametophyte of angiosperm plants is a two- or three-celled pollen grain. Its task is the production of two male gametes and their transport to the female gametophyte, the embryo sac, where double fertilization takes place. We describe two phases of pollen ontogenesis-a developmental phase leading to the differentiation of the male germline and the formation of a mature pollen grain and a functional phase representing the pollen tube growth, beginning with the landing of the pollen grain on the stigma and ending with double fertilization. We highlight recent advances in the complex regulatory mechanisms involved, including posttranscriptional regulation and transcript storage, intracellular metabolic signaling, pollen cell wall structure and synthesis, protein secretion, and phased cell-cell communication within the reproductive tissues.

Proteomic Analysis of Generative and Vegetative Nuclei Reveals Molecular Characteristics of Pollen Cell Differentiation in Lily

In plants, the cell fates of a vegetative cell (VC) and generative cell (GC) are determined after the asymmetric division of the haploid microspore. The VC exits the cell cycle and grows a pollen tube, while the GC undergoes further mitosis to produce two sperm cells for double fertilization. However, our understanding of the mechanisms underlying their fate differentiation remains limited. One major advantage of the nuclear proteome analysis is that it is the only method currently able to uncover the systemic differences between VC and GC due to GC being engulfed within the cytoplasm of VC, limiting the use of transcriptome. Here, we obtained pure preparations of the vegetative cell nuclei (VNs) and generative cell nuclei (GNs) from germinating lily pollens. Utilizing these high-purity VNs and GNs, we compared the differential nucleoproteins between them using state-of-the-art quantitative proteomic techniques. We identified 720 different amount proteins (DAPs) and grouped the results in 11 fate differentiation categories. Among them, we identified 29 transcription factors (TFs) and 10 cell fate determinants. Significant differences were found in the molecular activities of vegetative and reproductive nuclei. The TFs in VN mainly participate in pollen tube development. In comparison, the TFs in GN are mainly involved in cell differentiation and male gametogenesis. The identified novel TFs may play an important role in cell fate differentiation. Our data also indicate differences in nuclear pore complexes and epigenetic modifications: more nucleoporins synthesized in VN; more histone variants and chaperones; and structural maintenance of chromosome (SMC) proteins, chromatin remodelers, and DNA methylation-related proteins expressed in GN. The VC has active macromolecular metabolism and mRNA processing, while GC has active nucleic acid metabolism and translation. Moreover, the members of unfolded protein response (UPR) and programmed cell death accumulate in VN, and DNA damage repair is active in GN. Differences in the stress response of DAPs in VN vs. GN were also found. This study provides a further understanding of pollen cell differentiation mechanisms and also a sound basis for future studies of the molecular mechanisms behind cell fate differentiation.

Identification of QTL for barley grain size

Background

Barley grain size is one of the key factors determining storage capacity during grain filling. Large, well-filled grains also have a high malt extract potential. Grain size is a complex quantitative trait and can be easily affected by environmental factors thus the identification of genes controlling the trait and the use of molecular markers linked to the genes in breeding program is the most effective way of improving grain size.

Methods

Grain sizes of 188 doubled-haploid (DH) lines derived from the cross of a Japanese malting barley variety (Naso Nijo) and a Chinese feed barley variety (TX9425) were obtained from three different sites in two consecutive years. The average data were used for identifying QTL for grain size.

Results

A total of four significant QTL were identified for grain length (GL) and three for grain width (GW). The two major GL QTL are located at similar positions to the QTL for malt extract on 2H and uzu gene on 3H, respectively. However, the GL QTL on 2H is more likely a different one from the malt extract QTL as most of the candidate genes are located outside the fine mapped QTL region for malt extract. The GL QTL on 3H is closely linked with uzu gene but not due to a pleiotropic effect of uzu. The three QTL for grain width on 1H, 2H and 5H, respectively, were located at same position to those for GL.

Analysis of sticky generative cell mutants reveals that suppression of callose deposition in the generative cell is necessary for generative cell internalization and differentiation in Arabidopsis

In flowering plants, double fertilization between male and female gametophytes, which are separated by distance, largely depends on the unique pattern of the male gametophyte (pollen): two non-motile sperm cells suspended within a tube-producing vegetative cell. A morphological screen to elucidate the genetic control governing the strategic patterning of pollen has led to the isolation of a sticky generative cell (sgc) mutant that dehisces abnormal pollen with the generative cell immobilized at the pollen wall. Analyses revealed that the sgc mutation is specifically detrimental to pollen development, causing ectopic callose deposition that impedes the timely internalization and differentiation of the generative cell. We found that the SGC gene encodes the highly conserved domain of unknown function 707 (DUF707) gene that is broadly expressed but is germline specific during pollen development. Additionally, transgenic plants co-expressing fluorescently fused SGC protein and known organelle markers showed that SGC localizes in the endoplasmic reticulum, Golgi apparatus and vacuoles in pollen. A yeast two-hybrid screen with an SGC bait identified a thaumatin-like protein that we named GCTLP1, some homologs of which bind and/or digest β-1,3-glucans, the main constituent of callose. GCTLP1 is expressed in a germline-specific manner and colocalizes with SGC during pollen development, indicating that GCTLP1 is a putative SGC interactor. Collectively, our results show that SGC suppresses callose deposition in the nascent generative cell, thereby allowing the generative cell to fully internalize into the vegetative cell and correctly differentiate as the germline progenitor, with the potential involvement of the GCTLP1 protein, during pollen development in Arabidopsis.

Heterochromatic silencing is reinforced by ARID1-mediated small RNA movement in Arabidopsis pollen

In the plant male germline, transposable elements (TEs) are reactivated in the companion vegetative nucleus, resulting in siRNA production and the intercellular movement of these siRNAs to reinforce TE silencing in sperm. However, the mechanism by which siRNA movement is regulated remains unexplored. Here we show that ARID1, a transcription factor which is constitutively expressed in the vegetative nucleus but dynamically accumulates in the generative cell (the progenitor of sperm) to promote the second pollen mitosis, mediates siRNA movement to reinforce heterochromatic silencing in the male germline. We looked for regulators involved in the accumulation of ARID1 in the generative cell, and found that AGO9, a germline-specific AGO in Arabidopsis, is required for the accumulation of ARID1 in the generative cell. Mutations in either ARID1 or AGO9 lead to the interruption of not only the second pollen mitosis but also the movement of siRNA from the vegetative nucleus to the male germline, resulting in the release of heterochromatic silencing in the male germline. Moreover, conditional knockdown of ARID1 in the generative cell causes reduced heterochromatic silencing in both bicellular and mature pollen. This study provides insights into how a spatiotemporal transcription factor coordinates heterochromatic silencing and male germline maturation.

R2R3-MYB transcription factors, StmiR858 and sucrose mediate potato flavonol biosynthesis

Flavonols and other phenylpropanoids protect plants from biotic and abiotic stress and are dietarily desirable because of their health-promoting properties. The ability to develop new potatoes (Solanum tuberosum) with optimal types and amounts of phenylpropanoids is limited by lack of knowledge about the regulatory mechanisms. Exogenous sucrose increased flavonols, whereas overexpression of the MYB StAN1 induced sucrolytic gene expression. Heterologous StAN1 protein bound promoter fragments from sucrolytic genes (SUSY1 and INV1). Two additional MYBs and one microRNA were identified that regulated potato flavonols. Overexpression analysis showed MYB12A and C increased amounts of flavonols and other phenylpropanoids. Endogenous flavonol amounts in light-exposed organs were much higher those in the dark. Expression levels of StMYB12A and C were high in flowers but low in tubers. Transient overexpression of miR858 altered potato flavonol metabolism. Endogenous StmiR858 expression was much lower in flowers than leaves and correlated with flavonol amounts in these organs. Collectively, these findings support the hypothesis that sucrose, MYBs, and miRNA control potato phenylpropanoid metabolism in a finely tuned manner that includes a feedback loop between sucrose and StAN1. These findings will aid in the development of potatoes with phenylpropanoid profiles optimized for crop performance and human health.

Epigenetic reprogramming rewires transcription during the alternation of generations in Arabidopsis

Alternation between morphologically distinct haploid and diploid life forms is a defining feature of most plant and algal life cycles, yet the underlying molecular mechanisms that govern these transitions remain unclear. Here, we explore the dynamic relationship between chromatin accessibility and epigenetic modifications during life form transitions in Arabidopsis. The diploid-to-haploid life form transition is governed by the loss of H3K9me2 and DNA demethylation of transposon-associated cis-regulatory elements. This event is associated with dramatic changes in chromatin accessibility and transcriptional reprogramming. In contrast, the global loss of H3K27me3 in the haploid form shapes a chromatin accessibility landscape that is poised to re-initiate the transition back to diploid life after fertilisation. Hence, distinct epigenetic reprogramming events rewire transcription through major reorganisation of the regulatory epigenome to guide the alternation of generations in flowering plants.

Resetting FLOWERING LOCUS C Expression After Vernalization Is Just Activation in the Early Embryo by a Different Name

The reproductive success of many plants depends on their capacity to respond appropriately to their environment. One environmental cue that triggers flowering is the extended cold of winter, which promotes the transition from vegetative to reproductive growth in a response known as vernalization. In annual plants of the Brassicaceae, the floral repressor, FLOWERING LOCUS C (FLC), is downregulated by exposure to low temperatures. Repression is initiated during winter cold and then maintained as the temperature rises, allowing plants to complete their life cycle during spring and summer. The two stages of FLC repression, initiation and maintenance, are distinguished by different chromatin states at the FLC locus. Initiation involves the removal of active chromatin marks and the deposition of the repressive mark H3K27me3 over a few nucleosomes in the initiation zone, also known as the nucleation region. H3K27me3 then spreads to cover the entire locus, in a replication dependent manner, to maintain FLC repression. FLC is released from repression in the next generation, allowing progeny of a vernalized plant to respond to winter. Activation of FLC in this generation has been termed resetting to denote the restoration of the pre-vernalized state in the progeny of a vernalized plant. It has been assumed that resetting must differ from the activation of FLC expression in progeny of plants that have not experienced winter cold. Considering that there is now strong evidence indicating that chromatin undergoes major modifications during both male and female gametogenesis, it is time to challenge this assumption.

Flow Cytometry and Sorting in Arabidopsis

Flow cytometry and sorting represents a valuable and mature experimental platform for the analysis of cellular populations. Applications involving higher plants started to emerge around 40 years ago and are now widely employed both to provide unique information regarding basic and applied questions in the biosciences and to advance agricultural productivity in practical ways. Further development of this platform is being actively pursued, and this promises additional progress in our understanding of the interactions of cells within complex tissues and organs. Higher plants offer unique challenges in terms of flow cytometric analysis, first since their organs and tissues are, almost without exception, three-dimensional assemblies of different cell types held together by tough cell walls, and, second, because individual plant cells are generally larger than those of mammals.This chapter, which updates work last reviewed in 2014 [Galbraith DW (2014) Flow cytometry and sorting in Arabidopsis. In: Sanchez Serrano JJ, Salinas J (eds) Arabidopsis Protocols, 3rd ed. Methods in molecular biology, vol 1062. Humana Press, Totowa, pp 509-537], describes the application of techniques of flow cytometry and sorting to the model plant species Arabidopsis thaliana, in particular emphasizing (a) fluorescence labeling in vivo of specific cell types and of subcellular components, (b) analysis using both conventional cytometers and spectral analyzers, (c) fluorescence-activated sorting of protoplasts and nuclei, and (d) transcriptome analyses using sorted protoplasts and nuclei, focusing on population analyses at the level of single protoplasts and nuclei. Since this is an update, details of new experimental methods are emphasized.

Overexpression of ThMYB8 mediates salt stress tolerance by directly activating stress-responsive gene expression

MYB transcription factors are important in abiotic stress responses; however, the detailed mechanisms are unclear. Tamarix hispida contains multiple MYB genes. The present study characterized T. hispida MYB8 (ThMYB8) during salt stress using transgenic T. hispida and Arabidopsis assays. ThMYB8 overexpression and ThMYB8 RNAi analysis demonstrated that ThMYB8 enhanced the salt stress tolerance. Transgenic Arabidopsis ectopic expression of ThMYB8 significantly increased root growth, fresh weight, and seed germination rate compared with that of the wild-type under salt stress. Physiological parameters analysis in T. hispida and Arabidopsis showed that ThMYB8 overexpressing plants had the lowest levels of O², H₂O₂, cell death, malondialdehyde, and electrolyte leakage. Overexpression of ThMYB8 regulated Na⁺ and K⁺ concentrations in plant tissues while maintaining K⁺/Na⁺ homeostasis. Analysis using qRT-PCR and ChIP-PCR identified possible downstream ThMYB8-regulated genes. ThMYB8 regulated the expression of ThCYP450-2 (cytochrome p450-2), Thltk (leucine-rich repeat transmembrane protein kinase), and ThTIP (aquaporin TIP) by binding to the MBSI motif ('CAACTG') in their promoters. The results indicated that ThMYB8 enhanced salt stress tolerance in T. hispida by regulating gene expression related to the activation of stress-associated physiological changes, such as enhanced reactive oxygen species scavenging capability, maintaining K⁺/Na⁺ homeostasis, and decreasing the malondialdehyde content and lipid peroxidation cell membranes.

KATANIN-dependent mechanical properties of the stigmatic cell wall mediate the pollen tube path in Arabidopsis

Successful fertilization in angiosperms depends on the proper trajectory of pollen tubes through the pistil tissues to reach the ovules. Pollen tubes first grow within the cell wall of the papilla cells, applying pressure to the cell. Mechanical forces are known to play a major role in plant cell shape by controlling the orientation of cortical microtubules (CMTs), which in turn mediate deposition of cellulose microfibrils (CMFs). Here, by combining imaging, genetic and chemical approaches, we show that isotropic reorientation of CMTs and CMFs in aged Col-0 and katanin1-5 (ktn1-5) papilla cells is accompanied by a tendency of pollen tubes to coil around the papillae. We show that this coiled phenotype is associated with specific mechanical properties of the cell walls that provide less resistance to pollen tube growth. Our results reveal an unexpected role for KTN1 in pollen tube guidance on the stigma by ensuring mechanical anisotropy of the papilla cell wall.

Flowering plants produce small particles known as pollen that – with the help of the wind, bees and other animals – carry male sex cells (sperm) to female sex cells (eggs) contained within flowers. When a grain of pollen lands on the female organ of a flower, called the pistil, it gives rise to a tube that grows through the pistil towards the egg cells at the base.

The surface of the pistil is covered in a layer of long cells named papillae. Like most plant cells, the papillae are surrounded by a rigid structure known as the cell wall, which is mainly composed of strands known as microfibrils. The pollen tube exerts pressure on a papilla to allow it to grow through the cell wall towards the base of the pistil. Previous studies have shown that the pistil produces signals that guide pollen tubes to the eggs. However, it remains unclear how pollen tubes orient themselves on the surface of papillae to grow in the right direction through the pistil.

Riglet et al. combined microscopy, genetic and chemical approaches to study how pollen tubes grow through the surface of the pistils of a small weed known as Arabidopsis thaliana. The experiments showed that an enzyme called KATANIN conferred mechanical properties to the cell walls of papillae that allowed pollen tubes to grow towards the egg cells, and also altered the orientation of the microfibrils in these cell walls. In A. thaliana plants that were genetically modified to lack KATANIN the pollen tubes coiled around the papillae and sometimes grew in the opposite direction to where the eggs were.

KATANIN is known to cut structural filaments inside the cells of plants, animals and most other living things. By revealing an additional role for KATANIN in regulating the mechanical properties of the papilla cell wall, these findings indicate this enzyme may also regulate the mechanical properties of cells involved in other biological processes.

AtNOT1 is required for gametophyte development in Arabidopsis

In flowering plants, pollen development is under a dynamic and well-orchestrated transcriptional control, characterized by an early phase with high transcript diversity and a late post-mitotic phase skewed to a cell-type-specific transcriptome. Such transcriptional changes require a balance between synthesis and degradation of mRNA transcripts, the latter being initiated by deadenylation. The CCR4-NOT complex is the main evolutionary conserved deadenylase complex in eukaryotes, and its function is essential during germline specification in animals. We hypothesized that the CCR4-NOT complex might play a central role in mRNA turnover during microgametogenesis in Arabidopsis. Disruption of NOT1 gene, which encodes the scaffold protein of the CCR4-NOT complex, showed abnormal seed set. Genetic analysis failed to recover homozygous progeny, and reciprocal crosses confirmed reduced transmission through the male and female gametophytes. Concordantly, not1 embryo sacs showed delayed development and defects in embryogenesis. not1 pollen grains exhibited abnormal male germ unit configurations and failed to germinate. Transcriptome analysis of pollen from not1/+ mutants revealed that lack of NOT1 leads to an extensive transcriptional deregulation during microgametogenesis. Therefore, our work establishes NOT1 as an important player during gametophyte development in Arabidopsis.

Gametophyte-specific DEAD-box RNA helicase 29 is required for functional maturation of male and female gametophytes in Arabidopsis

The maturation of male and female gametophytes together with its impact on plant sexual reproduction has not received much attention, and the molecular mechanisms underlying the process are largely unknown. Here, we show that Arabidopsis DEAD-box RNA helicase 29 (RH29) is critical for the functional maturation of both male and female gametophytes. Homozygous rh29 mutants could not be obtained, and heterozygous mutant plants were semi-sterile. Progression of the cell cycle in rh29 female gametophytes was delayed. Delayed pollination experiments showed that rh29 female gametophytes underwent cell-fate specification but were unable to develop into functional gametophytes. Functional specification but not morphogenesis was also disrupted in rh29 male gametophytes, causing defective pollen tube growth in the pistil. RH29 was highly and specifically expressed in gametophytic cells. RH29 shares high amino acid sequence identity with yeast Dbp10p, which partially rescues the aborted-ovules phenotype of rh29/RH29 plants. RH29 is essential for the synthesis of REGULATORY PARTICLE TRIPLE A ATPase 5a (RPT5a), a subunit of the regulatory particle of the 26S proteasome. Our results suggest that gametophyte functional maturation is a necessary process for successful fertilization and that RH29 is essential for the functional maturation of both male and female gametophytes.

Live-cell imaging of early events following pollen perception in self-incompatible Arabidopsis thaliana

Early events occurring at the surface of the female organ are critical for plant reproduction, especially in species with a dry stigma. After landing on the stigmatic papilla cells, the pollen hydrates and germinates a tube, which penetrates the cell wall and grows towards the ovules to convey the male gametes to the embryo sac. In self-incompatible species within the Brassicaceae, these processes are blocked when the stigma encounters an incompatible pollen. Based on the generation of self-incompatible Arabidopsis lines and by setting up a live imaging system, we showed that control of pollen hydration has a central role in pollen selectivity. The faster the pollen pumps water from the papilla during an initial period of 10 min, the faster it germinates. Furthermore, we found that the self-incompatibility barriers act to block the proper hydration of incompatible pollen and, when hydration is promoted by high humidity, an additional control prevents pollen tube penetration into the stigmatic wall. In papilla cells, actin bundles focalize at the contact site with the compatible pollen but not with the incompatible pollen, raising the possibility that stigmatic cells react to the mechanical pressure applied by the invading growing tube.

MYB81, a microspore-specific GAMYB transcription factor, promotes pollen mitosis I and cell lineage formation in Arabidopsis

Sexual reproduction in flowering plants relies on the production of haploid gametophytes that consist of germline and supporting cells. During male gametophyte development, the asymmetric mitotic division of an undetermined unicellular microspore segregates these two cell lineages. To explore genetic regulation underlying this process, we screened for pollen cell patterning mutants and isolated the heterozygous myb81-1 mutant that sheds ~50% abnormal pollen. Typically, myb81-1 microspores fail to undergo pollen mitosis I (PMI) and arrest at polarized stage with a single central vacuole. Although most myb81-1 microspores degenerate without division, a small fraction divides at later stages and fails to acquire correct cell fates. The myb81-1 allele is transmitted normally through the female, but rarely through pollen. We show that myb81-1 phenotypes result from impaired function of the GAMYB transcription factor MYB81. The MYB81 promoter shows microspore-specific activity and a MYB81-RFP fusion protein is only expressed in a narrow window prior to PMI. Ectopic expression of MYB81 driven by various promoters can severely impair vegetative or reproductive development, reflecting the strict microspore-specific control of MYB81. Our data demonstrate that MYB81 has a key role in the developmental progression of microspores, enabling formation of the two male cell lineages that are essential for sexual reproduction in Arabidopsis.

Intercellular delivery of small RNAs in plant gametes

Small RNAs are 20-24 nucleotides in length. In plants, small RNAs are classified into microRNAs (miRNAs) and small interfering RNAs (siRNAs), based on their biogenesis and molecular features. In contrast to the extensive knowledge of the roles of small RNAs in sporophytic tissues, the distribution and function of small RNAs in gametophytic cells have been less well studied. However, with the improvement of single-cell sorting and RNA sequencing technologies, the distribution of small RNAs, especially siRNAs, between sperm cells and the vegetative cell, as well as the function of sperm-delivered small RNAs during early seed development have been elucidated. This review summarizes work from the past 5 years regarding small RNAs in male gametes, emphasizing the intercellular communication and biological significance of small RNAs in Arabidopsis.

Building new insights in plant gametogenesis from an evolutionary perspective

Extant bryophytes are thought to preserve characteristics of ancestral land plants, with a life cycle dominated by the haploid gametophyte. The gametophyte produces gametes in specialized organs that differentiate after an extensive phase of vegetative development. During land plant evolution, these organs became extremely reduced. As a result, in flowers of angiosperms the haploid phase of the life cycle is reduced to few-celled gametophytes, namely the embryo sac (female) and pollen (male). Although many factors contributing to gametogenesis have been identified in flowering plants, the extreme reduction of the gametophytes has prevented a clear molecular dissection of key processes of gametogenesis. Recent studies in the model bryophyte Marchantia polymorpha have identified conserved transcription factors regulating the equivalent steps in the sexual reproduction of land plants. These include FEMALE GAMETOPHYTE MYB for female gametophyte development, BONOBO for gamete progenitor cell specification, DUO POLLEN1 for sperm differentiation and members of the RWP-RK domain family for female gamete formation. These studies demonstrate that M. polymorpha is a powerful model to untangle the core processes of gametogenesis in land plants. We anticipate that a deeper understanding of gametogenesis in bryophytes will circumscribe the origin of plant germ cells and define the differentiation programmes of sperm and eggs.