Meiocyte size is a determining factor for unreduced gamete formation in Arabidopsis thaliana.

Polyploidy, the presence of more than two sets of chromosomes within a cell, is a widespread phenomenon in plants. The main route to polyploidy is considered through the production of unreduced gametes that are formed as a consequence of meiotic defects. Nevertheless, for reasons poorly understood, the frequency of unreduced gamete formation differs substantially among different plant species. The previously identified meiotic mutant jason (jas) in Arabidopsis thaliana forms about 60% diploid (2n) pollen. JAS is required to maintain an organelle band as a physical barrier between the two meiotic spindles, preventing previously separated chromosome groups from uniting into a single cell. In this study, we characterized the jas suppressor mutant telamon (tel) that restored the production of haploid pollen in the jas background. The tel mutant did not restore the organelle band, but enlarged the size of male jas tel meiocytes, suggesting that enlarged meiocytes can bypass the requirement of the organelle band. Consistently, enlarged meiocytes generated by a tetraploid jas mutant formed reduced gametes. The results reveal that meiocyte size impacts chromosome segregation in meiosis II, suggesting an alternative way to maintain the ploidy stability in meiosis during evolution.

The meiotic regulator JASON utilizes alternative translation initiation sites to produce differentially localized forms.

The JASON (JAS) protein plays an important role in maintaining an organelle band across the equator of male meiotic cells during the second division, with its loss leading to unreduced pollen in Arabidopsis. In roots cells, JAS localizes to the Golgi, tonoplast and plasma membrane. Here we explore the mechanism underlying the localization of JAS. Overall, our data show that leaky ribosom scanning and alternative translation initiation sites (TISs) likely leads to the formation of two forms of JAS: a long version with an N-terminal Golgi localization signal and a short version with a different N-terminal signal targeting the protein to the plasma membrane. The ratio of the long and short forms of JAS is developmentally regulated, with both being produced in roots but the short form being predominant and functional during meiosis. This regulation of TISs in meiocytes ensures that the short version of JAS is formed during meiosis to ensure separation of chromosome groups and the production of reduced pollen. We hypothesize that increased occurrence of unreduced pollen under stress conditions may be a consequence of altered usage of JAS TISs during stress.

Analysis of Fluorescent Reporter Activity in the Male Germline During Pollen Development by Confocal Microscopy.

The male germline of flowering plants develops within the vegetative cell of the male gametophyte (pollen). The germline is established by asymmetric division of the microspore to form the generative cell. Mitotic division of the generative cell then produces the two sperm cells required for double fertilization. These differentiate to produce the proteins required for gamete attachment and fusion. An important aspect of understanding germline development is the characterization of germline gene expression. Here, we describe the use of a fluorescent reporter to study germline gene expression in developing pollen to assess the timing and specificity of expression.

A Conserved cis-Regulatory Module Determines Germline Fate through Activation of the Transcription Factor DUO1 Promoter.

The development of the male germline within pollen relies upon the activation of numerous target genes by the transcription factor DUO POLLEN1 (DUO1). The expression of DUO1 is restricted to the male germline and is first detected shortly after the asymmetric division that segregates the germ cell lineage. Transcriptional regulation is critical in controlling DUO1 expression, since transcriptional and translational fusions show similar expression patterns. Here, we identify key promoter sequences required for the germline-specific regulation of DUO1 transcription. Combining promoter deletion analyses with phylogenetic footprinting in eudicots and in Arabidopsis accessions, we identify a cis-regulatory module, Regulatory region of DUO1 (ROD1), which replicates the expression pattern of DUO1 in Arabidopsis (Arabidopsis thaliana). We show that ROD1 from the legume Medicago truncatula directs male germline-specific expression in Arabidopsis, demonstrating conservation of DUO1 regulation among eudicots. ROD1 contains several short conserved cis-regulatory elements, including three copies of the motif DNGTGGV, required for germline expression and tandem repeats of the motif YAACYGY, which enhance DUO1 transcription in a positive feedback loop. We conclude that a cis-regulatory module conserved in eudicots directs the spatial and temporal expression of the transcription factor DUO1 to specify male germline fate and sperm cell differentiation.

The R2R3 MYB transcription factor DUO1 activates a male germline-specific regulon essential for sperm cell differentiation in Arabidopsis.

The male germline in flowering plants arises through asymmetric division of a haploid microspore. The resulting germ cell undergoes mitotic division and specialization to produce the two sperm cells required for double fertilization. The male germline-specific R2R3 MYB transcription factor DUO1 POLLEN1 (DUO1) plays an essential role in sperm cell specification by activating a germline-specific differentiation program. Here, we show that ectopic expression of DUO1 upregulates a significant number (~63) of germline-specific or enriched genes, including those required for fertilization. We validated 14 previously unknown DUO1 target genes by demonstrating DUO1-dependent promoter activity in the male germline. DUO1 is shown to directly regulate its target promoters through binding to canonical MYB sites, suggesting that the DUO1 target genes validated thus far are likely to be direct targets. This work advances knowledge of the DUO1 regulon that encompasses genes with a range of cellular functions, including transcription, protein fate, signaling, and transport. Thus, the DUO1 regulon has a major role in shaping the germline transcriptome and functions to commit progenitor germ cells to sperm cell differentiation.

Imprinting of the polycomb group gene MEDEA serves as a ploidy sensor in Arabidopsis.

Balanced maternal and paternal genome contributions are a requirement for successful seed development. Unbalanced contributions often cause seed abortion, a phenomenon that has been termed "triploid block." Misregulation of imprinted regulatory genes has been proposed to be the underlying cause for abnormalities in growth and structure of the endosperm in seeds with deviating parental contributions. We identified a mutant forming unreduced pollen that enabled us to investigate direct effects of unbalanced parental genome contributions on seed development and to reveal the underlying molecular mechanism of dosage sensitivity. We provide evidence that parent-of-origin-specific expression of the Polycomb group (PcG) gene MEDEA is causally responsible for seed developmental aberrations in Arabidopsis seeds with increased paternal genome contributions. We propose that imprinted expression of PcG genes is an evolutionary conserved mechanism to balance parental genome contributions in embryo nourishing tissues.

Arabidopsis DUO POLLEN3 is a key regulator of male germline development and embryogenesis.

Male germline development in angiosperms produces the pair of sperm cells required for double fertilization. A key regulator of this process in Arabidopsis thaliana is the male germline-specific transcription factor DUO POLLEN1 (DUO1) that coordinates germ cell division and gamete specification. Here, we uncover the role of DUO3, a nuclear protein that has a distinct, but overlapping role with DUO1 in male germline development. DUO3 is a conserved protein in land plants and is related to GON-4, a cell lineage regulator of gonadogenesis in Caenorhabditis elegans. Mutant duo3-1 germ cells either fail to divide or show a delay in division, and we show that, unlike DUO1, DUO3 promotes entry into mitosis independent of the G2/M regulator CYCB1;1. We also show that DUO3 is required for the expression of a subset of germline genes under DUO1 control and that like DUO1, DUO3 is essential for sperm cell specification and fertilization. Furthermore, we demonstrate an essential sporophytic role for DUO3 in cell division and embryo patterning. Our findings demonstrate essential developmental roles for DUO3 in cell cycle progression and cell specification in both gametophytic and sporophytic tissues.

Male gametophyte development: a molecular perspective.

Pollen grains represent the highly reduced haploid male gametophyte generation in flowering plants, consisting of just two or three cells when released from the anthers. Their role is to deliver twin sperm cells to the embryo sac to undergo fusion with the egg and central cell. This double fertilization event along with the functional specialization of the male gametophyte, are considered to be key innovations in the evolutionary success of flowering plants. This review encompasses important recent advances in our understanding of the molecular mechanisms controlling male gametophyte development. A brief overview of pollen development is presented, followed by a discussion of genome-wide transcriptomic studies of haploid gene expression. The progress achieved through genetic analysis of landmark events of male gametogenesis is discussed, with a focus on sperm cell production, and an emerging model of the regulatory network governing male germline development is presented. The review concludes with a perspective of the impact these data will have on future research strategies to further develop our understanding of the gametophytic control of pollen development.

Control of plant germline proliferation by SCF(FBL17) degradation of cell cycle inhibitors.

Flowering plants possess a unique reproductive strategy, involving double fertilization by twin sperm cells. Unlike animal germ lines, the male germ cell lineage in plants only forms after meiosis and involves asymmetric division of haploid microspores, to produce a large, non-germline vegetative cell and a germ cell that undergoes one further division to produce the twin sperm cells. Although this switch in cell cycle control is critical for sperm cell production and delivery, the underlying molecular mechanisms are unknown. Here we identify a novel F-box protein of Arabidopsis thaliana, designated FBL17 (F-box-like 17), that enables this switch by targeting the degradation of cyclin-dependent kinase A;1 inhibitors specifically in male germ cells. We show that FBL17 is transiently expressed in the male germ line after asymmetric division and forms an SKP1-Cullin1-F-box protein (SCF) E3 ubiquitin ligase complex (SCF(FBL17)) that targets the cyclin-dependent kinase inhibitors KRP6 and KRP7 for proteasome-dependent degradation. Accordingly, the loss of FBL17 function leads to the stabilization of KRP6 and inhibition of germ cell cycle progression. Our results identify SCF(FBL17) as an essential male germ cell proliferation complex that promotes twin sperm cell production and double fertilization in flowering plants.

Dual function of Arabidopsis glucan synthase-like genes GSL8 and GSL10 in male gametophyte development and plant growth.

Members of the glucan synthase-like (GSL) family are believed to be involved in synthesis of the cell-wall component callose in specialized locations throughout the plant. We identified two members of the Arabidopsis GSL gene family, GSL8 and GSL10, that are independently required for male gametophyte development and plant growth. Analysis of gsl8 and gsl10 mutant pollen during development revealed specific malfunctions associated with asymmetric microspore division. GSL8 and GSL10 are not essential for normal microspore growth and polarity, but play a role in entry of microspores into mitosis. Impaired function of GSL10 also leads to perturbation of microspore division symmetry, irregular callose deposition and failure of generative-cell engulfment by the cytoplasm of the vegetative cell. Silencing of GSL8 or GSL10 in transgenic lines expressing gene-specific dsRNAi constructs resulted in a dwarfed growth habit, thereby revealing additional and independent wild-type gene functions for normal plant growth.

Proteomic and biochemical evidence links the callose synthase in Nicotiana alata pollen tubes to the product of the NaGSL1 gene.

The NaGSL1 gene has been proposed to encode the callose synthase (CalS) enzyme from Nicotiana alata pollen tubes based on its similarity to fungal 1,3-beta-glucan synthases and its high expression in pollen and pollen tubes. We have used a biochemical approach to link the NaGSL1 protein with CalS enzymic activity. The CalS enzyme from N. alata pollen tubes was enriched over 100-fold using membrane fractionation and product entrapment. A 220 kDa polypeptide, the correct molecular weight to be NaGSL1, was specifically detected by anti-GSL antibodies, was specifically enriched with CalS activity, and was the most abundant polypeptide in the CalS-enriched fraction. This polypeptide was positively identified as NaGSL1 using both MALDI-TOF MS and LC-ESI-MS/MS analysis of tryptic peptides. Other low-abundance polypeptides in the CalS-enriched fractions were identified by MALDI-TOF MS as deriving from a 103 kDa plasma membrane H+-ATPase and a 60 kDa beta-subunit of mitochondrial ATPase, both of which were deduced to be contaminants in the product-entrapped material. These analyses thus suggest that NaGSL1 is required for CalS activity, although other smaller (<30 kDa) or low-abundance proteins could also be involved.