Morphological development of anthers induced by the dimorphic smut fungus Microbotryum violaceum in female flowers of the dioecious plant Silene latifolia

The genome of Salmacisia buchloëana, the parasitic puppet master pulling strings of sexual phenotypic monstrosities in buffalograss

To complete its parasitic lifecycle, Salmacisia buchloëana, a biotrophic fungus, manipulates reproductive organ development, meristem determinacy, and resource allocation in its dioecious plant host, buffalograss (Bouteloua dactyloides; Poaceae). To gain insight into S. buchloëana's ability to manipulate its host, we sequenced and assembled the 20.1 Mb genome of S. buchloëana into 22 chromosome-level pseudomolecules. Phylogenetic analysis suggests that S. buchloëana is nested within the genus Tilletia and diverged from Tilletia caries and Tilletia walkeri ∼40 MYA. We find that S. buchloëana contains a novel chromosome arm with no syntenic relationship to other publicly available Tilletia genomes, and that genes on the novel arm are upregulated upon infection, suggesting that this unique chromosomal segment may have played a critical role in S. buchloëana's evolution and host specificity. Salmacisia buchloëana has one of the largest fractions of serine peptidases (1.53% of the proteome) and one of the highest GC contents (62.3%) in all classified fungi. Analysis of codon base composition indicated that GC content is controlled more by selective constraints than directional mutation, and that S. buchloëana has a unique bias for the serine codon UCG. Finally, we identify 3 inteins within the S. buchloëana genome, 2 of which are located in a gene often used in fungal taxonomy. The genomic and transcriptomic resources generated here will aid plant pathologists and breeders by providing insight into the extracellular components contributing to sex determination in dioecious grasses.

Agricultural soil legacy influences multitrophic interactions between crops, their pathogens and pollinators

Soil legacy influences plant interactions with antagonists and below-ground mutualists. Plant-antagonist interactions can jeopardize plant-pollinator interactions, while soil mutualists can enhance plant-pollinator interactions. This suggests that soil legacy, either directly or mediated through plant symbionts, affects pollinators. Despite the importance of pollinators to natural and managed ecosystems, information on how soil legacy affects plant-pollinator interactions is limited. We assessed effects of soil management legacy (organic versus conventional) on floral rewards and plant interactions with wild pollinators, herbivores, beneficial fungi and pathogens. We used an observational dataset and structural equation models to evaluate hypothesized relationships between soil and pollinators, then tested observed correlations in a manipulative experiment. Organic legacy increased mycorrhizal fungal colonization and improved resistance to powdery mildew, which promoted pollinator visitation. Further, soil legacy and powdery mildew independently and interactively impacted floral traits and floral reward nutrients, which are important to pollinators. Our results indicate that pollination could be an overlooked consequence of soil legacy and suggests opportunity to develop long-term soil management plans that benefit pollinators and pollination.

Chemical genetics in Silene latifolia elucidate regulatory pathways involved in gynoecium development

Dioecious plants possess diverse sex determination systems and unique mechanisms of reproductive organ development; however, little is known about how sex-linked genes shape the expression of regulatory cascades that lead to developmental differences between sexes. In Silene latifolia, a dioecious plant with stable dimorphism in floral traits, early experiments suggested that female-regulator genes act on the factors that determine the boundaries of the flower whorls. To identify these regulators, we sequenced the transcriptome of male flowers with fully developed gynoecia, induced by rapid demethylation in the parental generation. Eight candidates were found to have a positive role in gynoecium promotion, floral organ size, and whorl boundary, and affect the expression of class B MADS-box flower genes. To complement our transcriptome analysis, we closely examined the floral organs in their native state using field emission environmental scanning electron microscopy, and examined the differences between females and androhermaphrodites in their placenta and ovule organization. Our results reveal the regulatory pathways potentially involved in sex-specific flower development in the classical model of dioecy, S. latifolia. These pathways include previously hypothesized and unknown female-regulator genes that act on the factors that determine the flower boundaries, and a negative regulator of anther development, SUPERMAN-like (SlSUP).

Vector preference and heterogeneity in host sex ratio can affect pathogen spread in natural plant populations

Vector-borne diseases threaten human and agricultural health and are a critical component of the ecology of plants and animals. While previous studies have shown that pathogen spread can be affected by vector preferences for host infection status, less attention has been paid to vector preference for host sex, despite abundant evidence of sex-specific variation in disease burden. We investigated vector preference for host infection status and sex in the sterilizing "anther-smut" pathogen (Microbotryum) of the alpine carnation, Dianthus pavonius. The pathogen is transferred among hosts by pollinators that visit infected flowers and become contaminated with spores produced by infected anthers. The host plant has a mixed breeding system with hermaphrodites and females. In experimental floral arrays, pollinators strongly preferred healthy hermaphrodites over both females and diseased plants, consistently across different guilds of pollinators and over multiple years. Using an agent-based model, we showed that pollinator preferences for sex can affect pathogen spread in populations with variable sex ratios, even if there is no preference for infection status. Our results demonstrate that vector preferences for host traits other than infection status can play a critical role in pathogen transmission dynamics when there is heterogeneity for those traits in the host population.

How Do Smut Fungi Use Plant Signals to Spatiotemporally Orientate on and In Planta?

Smut fungi represent a large group of biotrophic plant pathogens that cause extensive yield loss and are also model organisms for studying plant-pathogen interactions. In recent years, they have become biotechnological tools. After initial penetration of the plant epidermis, smut fungi grow intra-and intercellularly without disrupting the plant-plasma membrane. Following the colonialization step, teliospores are formed and later released. While some smuts only invade the tissues around the initial penetration site, others colonize in multiple plant organs resulting in spore formation distal from the original infection site. The intimate contact zone between fungal hyphae and the host is termed the biotrophic interaction zone and enables exchange of signals and nutrient uptake. Obviously, all steps of on and in planta growth require fine sensing of host conditions as well as reprogramming of the host by the smut fungus. In this review, we highlight selected examples of smut fungal colonization styles, directional growth in planta, induction of spore formation, and the signals required, pointing to excellent reviews for details, to draw attention to some of the open questions in this important research field.

An asexual flower of Silene latifolia and Microbotryum lychnidis-dioicae promotes sex-organ development

Silene latifolia is a dioecious flowering plant with sex chromosomes in the family Caryophyllaceae. Development of a gynoecium and stamens are suppressed in the male and female flowers of S. latifolia, respectively. Microbotryum lychnidis-dioicae promotes stamen development when it infects the female flower. If suppression of the stamen and gynoecium development is regulated by the same mechanism, suppression of gynoecium and stamen development is released simultaneously with the infection by M. lychnidis-dioicae. To assess this hypothesis, an asexual mutant without a gynoecium or stamen was infected with M. lychnidis-dioicae. A filament of the stamen in the infected asexual mutant was elongated at stages 11 and 12 of flower bud development as well as in the male, but the gynoecium did not form. Instead of the gynoecium, a filamentous structure was suppressed as in the male flower. Developmental suppression of the stamen was released by M. lychnidis-dioicae, but that of gynoecium development was not released. M. lychnidis-dioicae would have a function similar to stamen-promoting factor (SPF), since the elongation of the stamen that is not observed in the healthy asexual mutant was observed after stage 8 of flower bud development. An infection experiment also revealed that a deletion on the Y chromosome of the asexual mutant eliminated genes for maturation of tapetal cells because the tapetal cells did not mature in the asexual mutant infected with M. lychnidis-dioicae.

Development of the VIGS System in the Dioecious Plant Silene latifolia

(1) Background: Silene latifolia is a dioecious plant, whose sex is determined by XY-type sex chromosomes. Microbotryum lychnidis-dioicae is a smut fungus that infects S. latifolia plants and causes masculinization in female flowers, as if Microbotryum were acting as a sex-determining gene. Recent large-scale sequencing efforts have promised to provide candidate genes that are involved in the sex determination machinery in plants. These candidate genes are to be analyzed for functional characterization. A virus vector can be a tool for functional gene analyses; (2) Methods: To develop a viral vector system in S. latifolia plants, we selected Apple latent spherical virus (ALSV) as an appropriate virus vector that has a wide host range; (3) Results: Following the optimization of the ALSV inoculation method, S. latifolia plants were infected with ALSV at high rates in the upper leaves. In situ hybridization analysis revealed that ALSV can migrate into the flower meristems in S. latifolia plants. Successful VIGS (virus-induced gene silencing) in S. latifolia plants was demonstrated with knockdown of the phytoene desaturase gene. Finally, the developed method was applied to floral organ genes to evaluate its usability in flowers; (4) Conclusion: The developed system enables functional gene analyses in S. latifolia plants, which can unveil gene functions and networks of S. latifolia plants, such as the mechanisms of sex determination and fungal-induced masculinization.

Sex and the flower - developmental aspects of sex chromosome evolution

Background

The evolution of dioecious plants is occasionally accompanied by the establishment of sex chromosomes: both XY and ZW systems have been found in plants. Structural studies of sex chromosomes are now being followed up by functional studies that are gradually shedding light on the specific genetic and epigenetic processes that shape the development of separate sexes in plants.

Scope

This review describes sex determination diversity in plants and the genetic background of dioecy, summarizes recent progress in the investigation of both classical and emerging model dioecious plants and discusses novel findings. The advantages of interspecies hybrids in studies focused on sex determination and the role of epigenetic processes in sexual development are also overviewed.

Conclusions

We integrate the genic, genomic and epigenetic levels of sex determination and stress the impact of sex chromosome evolution on structural and functional aspects of plant sexual development. We also discuss the impact of dioecy and sex chromosomes on genome structure and expression.

The role of infectious disease in the evolution of females: Evidence from anther-smut disease on a gynodioecious alpine carnation

In flowering plants, the evolution of females is widely hypothesized to be the first step in the evolutionary pathway to separate male and female sexes, or dioecy. Natural enemies have the potential to drive this evolution if they preferentially attack hermaphrodites over females. We studied sex-based differences in exposure to anther-smut (Microbotryum), a sterilizing pollinator-transmitted disease, in Dianthus pavonius, a gynodioecious perennial herb. We found that within a heavily diseased population, females consistently had lower levels of Microbotryum spore deposition relative to hermaphrodites and that this difference was driven by rapid floral closing in females following successful pollination. We further show that this protective closing behavior is frequency dependent; females close faster when they are rare. These results indicate that anther-smut disease is an important source of selection for females, especially since we found in a common garden experiment no evidence that females have any inherent fecundity advantages over hermaphrodites. Finally, we show that among populations, those where anther-smut is present have a significantly higher frequency of females than those where the disease is absent. Taken together our results indicate that anther-smut disease is likely an important biotic factor driving the evolution and maintenance of females in this gynodioecious species.

Pas de deux: An Intricate Dance of Anther Smut and Its Host

The successful interaction between pathogen/parasite and host requires a delicate balance between fitness of the former and survival of the latter. To optimize fitness a parasite/pathogen must effectively create an environment conducive to reproductive success, while simultaneously avoiding or minimizing detrimental host defense response. The association between Microbotryum lychnidis-dioicae and its host Silene latifolia serves as an excellent model to examine such interactions. This fungus is part of a species complex that infects species of the Caryophyllaceae, replacing pollen with the fungal spores. In the current study, transcriptome analyses of the fungus and its host were conducted during discrete stages of bud development so as to identify changes in fungal gene expression that lead to spore development and to identify changes associated with infection in the host plant. In contrast to early biotrophic phase stages of infection for the fungus, the latter stages involve tissue necrosis and in the case of infected female flowers, further changes in the developmental program in which the ovary aborts and a pseudoanther is produced. Transcriptome analysis via Illumina RNA sequencing revealed enrichment of fungal genes encoding small secreted proteins, with hallmarks of effectors and genes found to be relatively unique to the Microbotryum species complex. Host gene expression analyses also identified interesting sets of genes up-regulated, including those involving stress response, host defense response, and several agamous-like MADS-box genes (AGL61 and AGL80), predicted to interact and be involved in male gametophyte development.

Identification and Initial Characterization of the Effectors of an Anther Smut Fungus and Potential Host Target Proteins

(1) Background: Plant pathogenic fungi often display high levels of host specificity and biotrophic fungi; in particular, they must manipulate their hosts to avoid detection and to complete their obligate pathogenic lifecycles. One important strategy of such fungi is the secretion of small proteins that serve as effectors in this process. Microbotryum violaceum is a species complex whose members infect members of the Caryophyllaceae; M. lychnidis-dioicae, a parasite on Silene latifolia, is one of the best studied interactions. We are interested in identifying and characterizing effectors of the fungus and possible corresponding host targets; (2) Methods: In silico analysis of the M. lychnidis-dioicae genome and transcriptomes allowed us to predict a pool of small secreted proteins (SSPs) with the hallmarks of effectors, including a lack of conserved protein family (PFAM) domains and also localized regions of disorder. Putative SSPs were tested for secretion using a yeast secretion trap method. We then used yeast two-hybrid analyses for candidate-secreted effectors to probe a cDNA library from a range of growth conditions of the fungus, including infected plants; (3) Results: Roughly 50 SSPs were identified by in silico analysis. Of these, 4 were studied further and shown to be secreted, as well as examined for potential host interactors. One of the putative effectors, MVLG_01732, was found to interact with Arabidopsis thaliana calcium-dependent lipid binding protein (AtCLB) and with cellulose synthase interactive protein 1 orthologues; and (4) Conclusions: The identification of a pool of putative effectors provides a resource for functional characterization of fungal proteins that mediate the delicate interaction between pathogen and host. The candidate targets of effectors, e.g., AtCLB, involved in pollen germination suggest tantalizing insights that could drive future studies.

Three-dimensional ultrastructural study of the anther of Silene latifolia infected with Microbotryum lychnidis-dioicae

When Microbotryum lychnidis-dioicae infects a male Silene latifolia, M. lychnidis-dioicae smut spores develop in the pollen sac instead of pollen. In contrast, when M. lychnidis-dioicae infects a female S. latifolia, the female flowers become male-like, promoting stamen formation. However, it is unclear when and how M. lychnidis-dioicae invades the anther. It is important to investigate not only whether hyphae exist when the apical meristem tissue differentiates into flowers and anthers, but also whether hyphae exist when stamen filaments form. We used Grocott’s methenamine silver stain and lectin stain, which stain chitin in the fungal cell wall, to search for M. lychnidis-dioicae in flower tissues. A few M. lychnidis-dioicae hyphae were observed intercellularly in the center of the connective of vascular bundles at the early anther developmental stage. Subsequently, large numbers of deeply stained M. lychnidis-dioicae hyphae were observed intercellularly in the cells surrounding the pollen sac, as well as in the center of the pollen sac. Hyphae stained with lectin were observed intercellularly in all of the stamen filaments at flower development stages. Hyphae were observed in the peduncle connecting the flower and stem. It is thought that M. lychnidis-dioicae invaded the anther via the stamen filament over a long period. Additionally, in total, 163 sections of connective were obtained, and the cell structure of each anther was colored and subjected to three-dimensional reconstruction. The M. lychnidis-dioicae hyphae observed in the connective were mainly old hyphae with large vacuoles or dead hyphae (S1 Fig). These hyphae branched out, towards the pollen sac, while growing between the cells. We also observed that the host cells that collapsed near the hyphae had thick cell walls and teliospores. Cell wall collapse and cell degeneration were observed only around hyphae with thick cell walls.

Cell Death and Cell Cycle Arrest of Silene latifolia Stamens and Pistils After Microbotryum lychnidis-dioicae Infection

Mechanisms of suppression of pistil primordia in male flowers and of stamen primordia in female flowers differ in diclinous plants. In this study, we investigated how cell death and cell cycle arrest are related to flower organ formation in Silene latifolia. Using in situ hybridization and a TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay, we detected both cell cycle arrest and cell death in suppressed stamens of female flowers and suppressed pistils of male flowers in S. latifolia. In female flowers infected with Microbotryum lychnidis-dioicae, developmental suppression of stamens is released, and cell cycle arrest and cell death do not occur. Smut spores are formed in S. latifolia anthers infected with M. lychnidis-dioicae, followed by cell death in the endothelium, middle layer, tapetal cells and pollen mother cells. Cell death is difficult to detect using a fluorescein isothiocyanate-labeled TUNEL assay due to strong autofluorescence in the anther. We therefore combined a TUNEL assay in an infrared region with transmission electron microscopy to detect cell death in anthers. We show that following infection by M. lychnidis-dioicae, a TUNEL signal was not detected in the endothelium, middle layer or pollen mother cells, and cell death with outflow of cell contents, including the nucleoplast, was observed in tapetal cells.

Resurgence of Less-Studied Smut Fungi as Models of Phytopathogenesis in the Omics Age

The smut fungi form a large, diverse, and nonmonophyletic group of plant pathogens that have long served as both important pests of human agriculture and, also, as fertile organisms of scientific investigation. As modern techniques of molecular genetic analysis became available, many previously studied species that proved refractive to these techniques fell by the wayside and were neglected. Now, as the advent of rapid and affordable next-generation sequencing provides genomic and transcriptomic resources for even these "forgotten" fungi, several species are making a comeback and retaking prominent places in phytopathogenic research. In this review, we highlight several of these smut fungi, with special emphasis on Microbotryum lychnidis-dioicae, an anther smut whose molecular genetic tools have finally begun to catch up with its historical importance in classical genetics and now provide mechanistic insights for ecological studies, evolution of host-pathogen interaction, and investigations of emerging infectious disease.

Reliable transformation system for Microbotryum lychnidis-dioicae informed by genome and transcriptome project

Microbotryum lychnidis-dioicae is a member of a species complex infecting host plants in the Caryophyllaceae. It is used as a model system in many areas of research, but attempts to make this organism tractable for reverse genetic approaches have not been fruitful. Here, we exploited the recently obtained genome sequence and transcriptome analysis to inform our design of constructs for use in Agrobacterium-mediated transformation techniques currently available for other fungi. Reproducible transformation was demonstrated at the genomic, transcriptional and functional levels. Moreover, these initial proof-of-principle experiments provide evidence that supports the findings from initial global transcriptome analysis regarding expression from the respective promoters under different growth conditions of the fungus. The technique thus provides for the first time the ability to stably introduce transgenes and over-express target M. lychnidis-dioicae genes.

Fungal Infection Induces Sex-Specific Transcriptional Changes and Alters Sexual Dimorphism in the Dioecious Plant Silene latifolia

Sexual dimorphism, including differences in morphology, behavior and physiology between females and males, is widespread in animals and plants and is shaped by gene expression differences between the sexes. Such expression differences may also underlie sex-specific responses of hosts to pathogen infections, most notably when pathogens induce partial sex reversal in infected hosts. The genetic changes associated with sex-specific responses to pathogen infections on the one hand, and sexual dimorphism on the other hand, remain poorly understood. The dioecious White Campion (Silene latifolia) displays sexual dimorphism in floral traits and infection with the smut fungus Micobrotryum lychnidis-dioicae induces a partial sex reversal in females. We find strong sex-specific responses to pathogen infection and reduced sexual dimorphism in infected S. latifolia. This provides a direct link between pathogen-mediated changes in sex-biased gene expression and altered sexual dimorphism in the host. Expression changes following infection affected mainly genes with male-biased expression in healthy plants. In females, these genes were up-regulated, leading to a masculinization of the transcriptome. In contrast, infection in males was associated with down-regulation of these genes, leading to a demasculinization of the transcriptome. To a lesser extent, genes with female-biased expression in healthy plants were also affected in opposite directions in the two sexes. These genes were overall down-regulated in females and up-regulated in males, causing, respectively, a defeminization in infected females and a feminization of the transcriptome in infected males. Our results reveal strong sex-specific responses to pathogen infection in a dioecious plant and provide a link between pathogen-induced changes in sex-biased gene expression and sexual dimorphism.

Females and males differ from each other in many traits, including morphology, behavior and physiology. Differences in gene expression between the sexes, known as sex-biased gene expression, contribute to such sexual dimorphism. Here we characterize the responses of females and males of the dioecious plant Silene latifolia to infection with the anther smut fungus Micobrotryum lychnidis-dioicae. This fungus sterilizes the plant and induces a partial sex reversal in female hosts that form rudimentary stamens, thus allowing the fungus to transmit its spores via pollinators. Our comparisons of gene expression in healthy and infected plants reveal strong sex-specific responses to anther smut infection. Expression changes in females and males are in opposite directions and are associated with reduced sexual dimorphism between infected females and males. Our study reveals that infection with the anther smut fungus alters the extent of sex-biased gene expression in S. latifolia in a sex-specific manner and highlights how transcriptomic changes in females and males shape sexual dimorphism.

Modulation of host cell biology by plant pathogenic microbes

Plant-pathogen interactions can result in dramatic visual changes in the host, such as galls, phyllody, pseudoflowers, and altered root-system architecture, indicating that the invading microbe has perturbed normal plant growth and development. These effects occur on a cellular level but range up to the organ scale, and they commonly involve attenuation of hormone homeostasis and deployment of effector proteins with varying activities to modify host cell processes. This review focuses on the cellular-reprogramming mechanisms of filamentous and bacterial plant pathogens that exhibit a biotrophic lifestyle for part, if not all, of their lifecycle in association with the host. We also highlight strategies for exploiting our growing knowledge of microbial host reprogramming to study plant processes other than immunity and to explore alternative strategies for durable plant resistance.

Size Does Matter: Staging of Silene latifolia Floral Buds for Transcriptome Studies

Dioecious plants in the Caryophyllaceae family are susceptible to infection by members of the anthericolous smut fungi. In our studies of the Silene latifolia/Microbotryum lychnidis-dioicae pathosystem, we were interested in characterizing the plant-pathogen interaction at the molecular level before and during teliosporogenesis. This takes place during floral bud development, and we hoped to capture the interaction by Illumina Next-Gen RNA-Sequencing. Using previous literature that documented the stages of the floral buds for S. latifolia, we examined the floral buds from plants grown and infected under growth chamber conditions, using the disserting microscope to determine the stage of floral buds based on the morphology. We compiled the information and determined the size of floral buds that correspond to the desired stages of development for tissue collection, for the purpose of RNA-sequencing. This offers a practical approach for researchers who require a large number of floral buds/tissue categorized by stages of development, ascertaining whether infected/uninfected buds are at comparable stages of development and whether this also holds true for male vs. female buds. We also document our experience in infecting the plants and some of the unusual morphologies we observed after infection.

Structure and development of 'witches' broom' galls in reproductive organs of Byrsonima sericea (Malpighiaceae) and their effects on host plants

Galls are anomalies in plant development of parasitic origin that affect the cellular differentiation or growth and represent a remarkable plant-parasite interaction. Byrsonima sericea DC. (Malpighiaceae) is a super host of several different types of gall in both vegetative and reproductive organs. The existence of galls in reproductive organs and their effects on the host plant are seldom described in the literature. In this paper, we present a novel study of galls in plants of the Neotropical region: the 'witches' broom' galls developed in floral structures of B. sericea. The unaffected inflorescences are characterised by a single indeterminate main axis with spirally arranged flower buds. The flower buds developed five unaffected brownish hairy sepals and five pairs of elliptical yellow elaiophores, five yellow fringed petals, 10 stamens and a pistil with superior tricarpellar and trilocular ovary. The affected inflorescences showed changes in architecture, with branches arising from the main axis and flower buds. The flower buds exhibited several morphological and anatomical changes. The sepals, petals and carpels converted into leaf-like structures after differentiation. Stamens exhibited degeneration of the sporogenous tissue and structures containing hyphae and spores. The gynoecium did not develop, forming a central meristematic region, from which emerges the new inflorescence. In this work, we discuss the several changes in development of reproductive structures caused by witches' broom galls and their effects on reproductive success of the host plants.

Pistil Smut Infection Increases Ovary Production, Seed Yield Components, and Pseudosexual Reproductive Allocation in Buffalograss

Sex expression of dioecious buffalograss [Bouteloua dactyloides Columbus (syn. Buchloë dactyloides (Nutt.) Engelm.)] is known to be environmentally stable with approximate 1:1, male to female, sex ratios. Here we show that infection by the pistil smut fungus [Salmacisiabuchloëana Huff & Chandra (syn. Tilletia buchloëana Kellerman and Swingle)] shifts sex ratios of buffalograss to be nearly 100% phenotypically hermaphroditic. In addition, pistil smut infection decreased vegetative reproductive allocation, increased most seed yield components, and increased pseudosexual reproductive allocation in both sex forms compared to uninfected clones. In female sex forms, pistil smut infection resulted in a 26 fold increase in ovary production and a 35 fold increase in potential harvest index. In male sex forms, pistil smut infection resulted in 2.37 fold increase in floret number and over 95% of these florets contained a well-developed pistil. Although all ovaries of infected plants are filled with fungal teliospores and hence reproductively sterile, an average male-female pair of infected plants exhibited an 87 fold increase in potential harvest index compared to their uninfected clones. Acquiring an ability to mimic the effects of pistil smut infection would enhance our understanding of the flowering process in grasses and our efforts to increase seed yield of buffalograss and perhaps other grasses.

Flower development, pollen fertility and sex expression analyses of three sexual phenotypes of Coccinia grandis

BACKGROUND

Coccinia grandis is a dioecious species of Cucurbitaceae having heteromorphic sex chromosomes. The chromosome constitution of male and female plants is 22 + XY and 22 + XX respectively. Y chromosome of male sex is conspicuously large and plays a decisive role in determining maleness. Sex modification has been studied in hypogynous Silene latifolia (Caryophyllaceae) but there is no such report in epigynous Coccinia grandis. Moreover, the role of organ identity genes during sex expression in Coccinia has not been evaluated earlier. Investigations on sexual phenotypes of C. grandis including a rare gynomonoecious (GyM) form and AgNO3 mediated sex modification have added a new dimension to the understanding of sex expression in dioecious flowering plants.

RESULTS

Morphometric analysis showed the presence of staminodes in pistillate flowers and histological study revealed the absence of carpel initials in male flowers. Though GyM plant had XX sex chromosomes, the development of stamens occurred in hermaphrodite flowers but the pollens were not fertile. Silver nitrate (AgNO3) application enhanced stamen growth in wild type female flowers like that of GyM plant but here also the pollens were sterile. Differential expression of CgPI could be involved in the development of different floral phenotypes.

CONCLUSIONS

The three principle factors, Gynoecium Suppression (SuF), Stamen Promoting Factor (SPF) and Male Fertility (mF) that control sex expression in dioecious C. grandis assumed to be located on Y chromosome, play a decisive role in determining maleness. However, the characteristic development of stamens in hermaphrodite flowers of GyM plant having XX sex chromosomes indicates that Y-linked SPF regulatory pathway is somehow bypassed. Our experimental findings together with all other previous chromosomal and molecular cytogenetical data strongly support the view that C. grandis could be used as a potential model system to study sex expression in dioecious flowering plant.

Structure of floral galls of Byrsonima sericea (Malpighiaceae) induced by Bruggmanniella byrsonimae (Cecidomyiidae, Diptera) and their effects on host plants

Galls are anomalies in plant development from parasitic origin, and affect cellular differentiation or growth of plants. This parasite-plant interaction occurs in many environments and typically in vegetative organs of plants. The existence of galls in reproductive organs and their effects on the host plant are seldom described in the literature. In this paper, we present a novel study of galls in plants of the neotropical region. Galls of Bruggmmaniella byrsonimae develop in the flower buds of Byrsonima sericea DC. (Malpighiaceae) and affect development of the reproductive organs and the reproductive effort of these plants. The sepals and petals show hypertrophy of parenchyma tissues after differentiation, and the stamens exhibit degeneration of the sporogenic tissue. The gynoecium is not entirely developed; ovary and ovules are often absent. Changes in vascular tissues are also frequent, which may indicate high demand for nutrient resources by the new tissues initiated by the larva. We compared the amount of inflorescences, galls and fruits to evaluate possible effects on host reproduction. The results suggest that the Cecidomyiidae galls in flower organs affect fruit set and the reproductive success of B. sericea.

A fungal parasite regulates a putative female-suppressor gene homologous to maize tasselseed2 and causes induced hermaphroditism in male buffalograss

Parasitically induced hermaphroditism is a fascinating illustration of floral sex organ modification; however, knowledge of how parasites induce hermaphroditism in plants is limited. Here, we show the fungal parasite pistil smut induces development of female sex organs (pistils) in flowers of male buffalograss, potentially by downregulating a putative female-suppressor gene, BdTs2, homologous to maize Tasselseed2 (ZmTs2). Full-length BdTs2, isolated using rapid amplification of cDNA ends, exhibits 89% nucleotide sequence similarity with ZmTs2 and 85% amino acid sequence homology with ZmTs2 protein. Scanning electron micrographs demonstrate that unisexual buffalograss flowers develop through a process of selective abortion of opposite sex organs within hermaphroditic floral primordia. Quantitative real-time polymerase chain reaction showed that high expression levels of BdTs2 within male inflorescences correlate with the selective abortion of gynoecium, leading to the development of unisexual male flowers. RNA in situ hybridization confirmed the expression of BdTs2 precisely within vestigial gynoeciums of male flowers and not in other floral organs of the inflorescence. Furthermore, we show that BdTs2 expression is downregulated by pistil smut infection, which corresponds to the presence of pistils in flowers otherwise destined to become unisexual male. This study provides a potential molecular basis for pistil smut-induced hermaphroditism in male buffalograss.

A SUPERMAN-like gene is exclusively expressed in female flowers of the dioecious plant Silene latifolia

To elucidate the mechanism(s) underlying dioecious flower development, the present study analyzed a SUPERMAN (SUP) homolog, SlSUP, which was identified in Silene latifolia. The sex of this plant is determined by heteromorphic X and Y sex chromosomes. It was revealed that SlSUP is a single-copy autosomal gene expressed exclusively in female flowers. Introduction of a genomic copy of SlSUP into the Arabidopsis thaliana sup (sup-2) mutant complemented the excess-stamen and infertile phenotypes of sup-2, and the overexpression of SlSUP in transgenic Arabidopsis plants resulted in reduced stamen numbers as well as the suppression of petal elongation. During the development of the female flower in S. latifolia, the expression of SlSUP is first detectable in whorls 2 and 3 when the normal expression pattern of the B-class flowering genes was already established and persisted in the stamen primordia until the ovule had matured completely. In addition, significant expression of SlSUP was detected in the ovules, suggestive of the involvement of this gene in ovule development. Furthermore, it was revealed that the de-suppression of stamen development by infection of the S. latifolia female flower with Microbotryum violaceum was accompanied by a significant reduction in SlSUP transcript levels in the induced organs. Taken together, these results demonstrate that SlSUP is a female flower-specific gene and suggest that SlSUP has a positive role in the female flower developmental pathways of S. latifolia.

Silene as a model system in ecology and evolution

The genus Silene, studied by Darwin, Mendel and other early scientists, is re-emerging as a system for studying interrelated questions in ecology, evolution and developmental biology. These questions include sex chromosome evolution, epigenetic control of sex expression, genomic conflict and speciation. Its well-studied interactions with the pathogen Microbotryum has made Silene a model for the evolution and dynamics of disease in natural systems, and its interactions with herbivores have increased our understanding of multi-trophic ecological processes and the evolution of invasiveness. Molecular tools are now providing new approaches to many of these classical yet unresolved problems, and new progress is being made through combining phylogenetic, genomic and molecular evolutionary studies with ecological and phenotypic data.

Male-fertility genes expressed in male flower buds of Silene latifolia include homologs of anther-specific genes

When the female plant of Silene latifolia is infected with the smut fungus Microbotryum violaceum, its rudimentary stamens develop into anthers which contain fungus teliospores instead of pollen. To identify genes required for maturation of anthers in S. latifolia, we performed a cDNA subtraction approach with healthy male buds and female buds infected with M. violaceum. We isolated five cDNA clones, which were preferentially expressed in healthy male buds during stages associated with a burst in tapetal activity. These five cDNAs are predicted to encode a mandelonitrile lyase protein (SlMDL1), a strictosidine synthase protein (SlSs), a glycosyl hydrolase 17 protein (SlGh17), a proline-rich protein APG precursor (SlAPG), and a chalcone-synthase-like protein (SlChs). All five genes showed expression in both healthy and fungus-infected male buds, but not expressed in either healthy or infected female buds. The first three genes were highly expressed in both tapetum and pollen grains while the last two genes were expressed only inside the tapetum of male flower buds. Phylogenetic analysis results showed that SlChs and SlGh17 belong to anther-specific subgroups of chalcone-synthase-like genes and glycosyl hydrolase 17 family genes, respectively. Our results suggest that the isolated five genes are related to the fertility of the anther leading to the development of fertile pollen. The smut fungus was not able to induce the expression of the five genes in the infected female buds. This raises the possibility that these genes are under the control of master gene(s) on the Y chromosome.

Different transmission strategies of a parasite in male and female hosts

We investigated whether a parasite with two routes of transmission responds to the different transmission opportunities offered by male and female hosts by using different transmission strategies in the two sexes. The parasite Ascogregarina culicis, which infects the mosquito Aedes aegypti, can be transmitted as its host's pupa transforms into an adult or when a female lays its eggs. As the latter transmission route is missing in males, we expected, and found, that the parasite releases a greater proportion of its infectious forms during emergence when it is within a male than when it infects a female. The transmission route, which influences the parasite's dispersal and the evolution of its virulence, was also affected by the dose of infection and the parasite's previous transmission route. Our results emphasize the complexity underlying the development of parasites and show their ability to tune their strategy to their environment.

Floral development of an asexual and female-like mutant carrying two deletions in gynoecium-suppressing and stamen-promoting functional regions on the Y chromosome of the dioecious plant Silene latifolia

Sexual dimorphism is controlled by genes on the Y chromosome in the dioecious plant Silene latifolia. K034 is the first mutant with female flowers and asexual flowers in one individual. Its stamens are suppressed completely, and its gynoecium exhibits two suppression patterns. One gynoecium resembles a thin rod, as in wild-type males (asexual flower); the other is imperfectly suppressed, having 1-3 carpels (female-like flower). The ratio of these patterns was 9 : 1. To exclude the possibility of chimerism in K034, we crossed a female-like flower of K034 with a wild-type male. Progeny obtained from this crossing had asexual and female-like flowers in one individual. This two-flower-type phenotype was inherited without separating. To examine the identity of flower organs in K034, we analyzed the development of asexual and female-like flowers using scanning electron microscopy and in situ hybridization with SLM1 and SLM2 (orthologs of AGAMOUS and PISTILLATA, respectively) as probes. Mitotic spreads of root tip chromosomes from hairy root cultures showed that K034 had 25 chromosomes. Fluorescent in situ hybridization analysis, using a subtelomeric repetitive sequence (KpnI subfamily) as a probe, indicated that K034 possessed two X chromosomes and one Y chromosome (Y(d)), of which Y(d) had been rearranged to lose the pseudoautosomal region (PAR). PCR analysis using Y-specific sequence-tagged site (STS) markers clarified that Y(d) of K034 had two other deletions in gynoecium-suppressing and stamen-promoting regions. It is reasonable to suggest that these sex chromosomal abnormalities resulted in two abnormal sexual phenotypes: the asexual and imperfect female (female-like) flowers in K034.

Sex chromosome-linked genes in plants

Recent studies of plant sex chromosome-linked genes have revealed many interesting characteristics, although there are limited reports about heteromorphic sex chromosomes in flowering plants. Sex chromosome-linked genes in angiosperms have been characterized mainly in the dioecious plant Silene latifolia. Although all such genes were isolated from transcripts of male flower buds of S. latifolia, most seem to be housekeeping genes except for the petal- and stamen-specific MADS box gene on the Y chromosome (SlAP3Y) and the male reproductive organ-specific gene on the X chromosome (MROS3X). Recent evolutionary studies have revealed at least three evolutionary strata on the X chromosome that are related to stepwise loss of recombination between the sex chromosomes. Moreover, genetic maps showed conservation of gene organization on the X chromosome in the genus Silene and substantial pericentric inversion between the X and Y chromosomes of S. latifolia during evolution. A comparison between paralogs on the sex chromosomes revealed that introns of the Y-linked genes are longer than those of X-linked paralogs. Although analyses of sex chromosome-linked genes suggest that degeneration of the Y chromosome has occurred, the Y chromosome in flowering plants remains the largest in the male genome, unlike that of mammals. Accumulation of repetitive sequences and the entire chloroplast genome on the Y chromosome appear to have contributed to this large size. However, more detailed studies will be required to help explain the basis for the fact that heteromorphic sex chromosomes in angiosperms are large.

Expression of ASK1-like genes in arrested stamens of female Silene latifolia plants

Silene latifolia is a dioecious plant in which sex is determined by heteromorphic sex chromosomes. In female plants, stamen development is arrested before microspore mother cells are formed. In this study, we isolated four cDNAs (SlSKP1-1 to 4) encoding ASK1-like protein as expression markers to reveal when expression levels are reduced in arrested stamens of female flowers. Expression patterns of the SlSKP1 genes were analyzed by in-situ hybridization. We use the flower development classification of Grant et al. (in Plant J 6:471-480, 1994). SlSKP1 genes were highly expressed in primary parietal cells and primary sporogenous cells that develop into microspore mother cells in male flowers. Expression levels started to be reduced in the external stamens of the female flowers when stamen development was arrested at stage 7. Although microspore mother cells could not be developed in female flowers and SlSKP1 expression may be unnecessary in arrested stamens, SlSKP1 genes were still expressed in sporogenous cells of degenerated stamens at stage 8. Parietal cells stopped differentiating earlier than sporogenous cells in arrested stamens. These results suggest that not all types of cell are arrested simultaneously at a particular stage of stamen development during stamen suppression of S. latifolia.

Ultrastructural analysis of the behavior of the dimorphic fungus Microbotryum violaceum in fungus-induced anthers of female Silene latifolia flowers

The development of male organs is induced in female flowers of the dioecious plant Silene latifolia by infection with the fungus Microbotryum violaceum. Stamens in a healthy female flower grow only to stage 6, whereas those in an infected female flower develop to the mature stage (stage 12), at which the stamens are filled with fungal teliospores instead of pollen grains. To investigate these host-parasite interactions, young floral buds and fungus-induced anthers of infected female flowers were examined by electron microscopy following fixation by a high-pressure freezing method. Using this approach, we found that parasitic hyphae of this fungus contain several extracellular vesicles and have a consistent appearance up to stage 8. At that stage, parasitic hyphae are observed adjacent to dying sporogenous cells in the infected female anther. At stage 9, an increased number of dead and dying sporogenous cells is observed, among which the sporogenous hyphae of the fungus develop and form initial teliospores. Several types of electron-dense material are present in proximity to some fungi at this stage. The initial teliospores contain two types of vacuoles, and the fungus cell wall contains abundant carbohydrate, as revealed by silver protein staining. The sporogenous cell is probably sensitive to infection by the fungus, resulting in disruption. In addition, the fungus accelerates cell death in the anther and utilizes constituents of the dead host cell to form the mature teliospore.

The anther smut disease on Gypsophila repens: a case of parasite sub-optimal performance following a recent host shift?

The study of how parasites adapt to new hosts is of great importance for understanding the emergence of new diseases. Here, we report a study of the anther smut disease on Gypsophila repens (Caryophyllaceae). In contrast to what is usually found on other host species, infected natural populations of G. repens are extremely rare. Moreover, symptoms of diseased plants are incomplete and highly variable over the time. These results suggest that the fungus infecting G. repens is a case of a parasite not capable of exploiting its host optimally. Molecular analyses of Microbotryum violaceum strains infecting this and other Caryophyllaceae revealed that this sub-optimal behaviour probably resulted from a recent host shift from the morphologically similar plant Petrorhagia saxifraga. With its exceptionally low virulence and prevalence, but apparent self-sustainability, the disease on G. repens may thus represent an interesting case study for investigating the conditions leading to adaptation of parasites on new hosts.

An anther- and petal-specific gene SlMF1 is a multicopy gene with homologous sequences on sex chromosomes

A male flower-specific gene SlMF1 was isolated from male flower buds of the dioecious plant Silene latifolia. SlMF1 is expressed in all the floral meristems at the very early stage of development in both male and female flower buds. At the mature stage of development in male flower buds, SlMF1 transcripts were specifically accumulated in pollen mother cells, tapetal cells, and the developing tips of petals. Genomic Southern hybridization revealed that SlMF1 was a multicopy gene with a Y chromosome-linked homologous sequence. PCR analyses with flow-sorted chromosomes showed that SlMF1 was localized on both autosomes and the X chromosome.

Sex-Specific Cell Division during Development of Unisexual Flowers in the Dioecious Plant Silene latifolia

We analyzed cell division patterns during the differentiation of unisexual flowers of the dioecious plant Silene latifolia using in situ hybridization with histone H4 and cyclin A1 genes. The gene expression patterns indicated that the activation of cell divisions in whorls 3 and 4 was reversed in young male and female flower buds. During maturation of flower buds, a remarkable reduction in cell division activity occurred in the male gynoecium primordium and female stamen primordia. Our analyses showed that differential activation and reduction of cell division strongly correlated with sex-specific promotion and cessation in the sex differentiation of unisexual flowers.