Evolution of unisexual flowers entails one of the most extreme changes in plant development. Cultivated spinach, Spinacia oleracea L., is uniquely suited for the study of unisexual flower development as it is dioecious and it achieves unisexually by the absence of organ development, rather than by organ abortion or suppression. Male staminate flowers lack fourth whorl primordia and female pistillate flowers lack third whorl primordia. Based on theoretical considerations, early inflorescence or floral organ identity genes would likely be directly involved in sex-determination in those species in which organ initiation rather than organ maturation is regulated. In this study, we tested the hypothesis that sexual dimorphism occurs through the regulation of B class floral organ gene expression by experimentally knocking down gene expression by viral induced gene silencing.
Suppression of B class genes in spinach resulted in the expected homeotic transformation of stamens into carpels but also affected the number of perianth parts and the presence of fourth whorl. Phenotypically normal female flowers developed on SpPI-silenced male plants. Suppression of the spinach C class floral organ identity gene, SpAG, resulted in loss of reproductive organ identity, and indeterminate flowers, but did not result in additional sex-specific characteristics or structures. Analysis of the genomic sequences of both SpAP3 and SpPI did not reveal any allelic differences between males and females.
Sexual dimorphism in spinach is not the result of homeotic transformation of established organs, but rather is the result of differential initiation and development of the third and fourth whorl primordia. SpAG is inferred to have organ identity and meristem termination functions similar to other angiosperm C class genes. In contrast, while SpPI and SpAP3 resemble other angiosperms in their essential functions in establishing stamen identity, they also appear to have an additional function in regulating organ number and identity outside of the third whorl. We present a model for the evolution of dioecy in spinach based on the regulation of B class expression.
Background and Aims
Phoenix dactylifera (date palm) is a dioecious species displaying strong dimorphism between pistillate and staminate flowers. The mechanisms involved in the development of unisexual flowers are as yet unknown.
This paper describes the results of inflorescence and flower development studies using different histological and molecular cytological approaches. Nuclear integrity and cell division patterns in reproductive organs were investigated through DAPI staining and in situ hybridization using a histone H4 gene probe.
The earliest sex-related difference in flower buds is observed at an otherwise ‘bisexual’ stage, at which the number of cells in the gynoecium of pistillate flowers is higher than in their staminate counterparts. In the pistillate flower, staminodes (sterile stamens) display precocious arrest of development followed by cell differentiation. In the staminate flower, pistillodes (sterile gynoecium) undergo some degree of differentiation and their development ceases shortly after the ovule has been initiated. Staminode and pistillode cells exhibit nuclear integrity although they did not show any accumulation of histone H4 gene transcripts.
These results strongly suggest that the developmental arrest of sterile sex organs and the subsequent unisexuality of date palm flowers result from a cessation of cell division and precocious cell differentiation rather than from cell death.
Date palm; reproductive development; cell division patterns; sex determination
Background and Aims
Monoecious plants have the capacity to allocate resources separately to male and female functions more easily than hermaphrodites. This can be advantageous against environmental stresses such as leaf herbivory. However, studies showing effects of herbivory on male and female functions and on the interaction with the plant's pollinators are limited, particularly in tropical plants. Here, the effects of experimental defoliation were examined in the monoecious shrub Croton suberosus (Euphorbiaceae), a wasp-pollinated species from a Mexican tropical dry forest.
Three defoliation treatments were applied: 0 % (control), 25 % (low) or 75 % (high) of plant leaf area removed. Vegetative (production of new leaves) and reproductive (pistillate and staminate flower production, pollen viability, nectar production, fruit set, and seed set) performance variables, and the abundance and activity of floral visitors were examined.
Defoliated plants overcompensated for tissue loss by producing more new leaves than control plants. Production of staminate flowers gradually decreased with increasing defoliation and the floral sex ratio (staminate : pistillate flowers) was drastically reduced in high-defoliation plants. In contrast, female reproductive performance (pistillate flower production, fruit set and seed set) and pollinator visitation and abundance were not impacted by defoliation.
The asymmetrical effects of defoliation on male and female traits of C. suberosus may be due to the temporal and spatial flexibility in the allocation of resources deployed by monoecious plants. We posit that this helps to maintain the plant's pollination success in the face of leaf herbivory stress.
Euphorbiaceae; floral sex ratio; foliar herbivory; leaf production; nectar production; Neotropical dry forest; plant–insect interactions; pollen production; pollination success; pollinator activity
Background and Aims
The distribution and differentiation times of flowers in monoecious wind-pollinated plants are fundamental for the understanding of their mating patterns and evolution. Two closely related South American Nothofagus species were compared with regard to the differentiation times and positions of staminate and pistillate flowers along their parent growth units (GUs) by quantitative means.
Two samples of GUs that had extended in the 2004–2005 growing season were taken in 2005 and 2006 from trees in the Lanín National Park, Patagonia, Argentina. For the first sample, axillary buds of the parent GUs were dissected and the leaf, bud and flower primordia of these buds were identified. The second sample included all branches derived from the parent GUs in the 2005–2006 growing season.
Both species developed flowering GUs with staminate and/or pistillate flowers; GUs with both flower types were the most common. The position of staminate flowers along GUs was similar between species and close to the proximal end of the GUs. Pistillate flowers were developed more distally along the GUs in N. alpina than in N. obliqua. In N. alpina, the nodes bearing staminate and pistillate flowers were separated by one to several nodes with axillary buds, something not observed in N. obliqua. Markovian models supported this between-species difference. Flowering GUs, including all of their leaves and flowers were entirely preformed in the winter buds.
Staminate and pistillate flowers of N. alpina and N. obliqua are differentiated at precise locations on GUs in the growing season preceding that of their antheses. The differences between N. alpina and N. obliqua (and other South American Nothofagus species) regarding flower distribution might relate to the time of anthesis of each flower type and, in turn, to the probabilities of self-pollination at the GU level.
Branch; bud; growth unit; Markovian models; Nothofagus alpina; N. obliqua; Patagonian forests; pistillate flower; preformation; staminate flower
Background and Aims
Macaranga (Euphorbiaceae) is a large genus of dioecious trees with approx. 260 species. To date, only one pollination study of the genus has reported brood-site pollination by thrips in M. hullettii. In this study, the pollination system of Macaranga tanarius is reported.
The study was conducted on Okinawa and Amami Islands, Japan. Flower visitors on M. tanarius were collected and their pollen load and behaviour on the flowers examined, as well as inflorescence structure and reward for the pollinators.
The most abundant flower visitors found on the male and female inflorescences were Orius atratus (Anthocoridae, Hemiptera), followed by Decomioides schneirlai (Miridae, Hemiptera). Pollen load on O. atratus from flowering pistillate inflorescences was detected as well as from staminate flowers. Orius atratus and D. schneirlai are likely to use the enclosed chambers formed by floral bracts as breeding sites before and during flower anthesis, and feed on nectar on the adaxial surface of flower bracts. The extrafloral nectary has a ball-shaped structure and the contained nectar is not exposed; the hemipterans pierce the ball to suck out the nectar.
The results indicate that the plant is pollinated by flower bugs breeding on the inflorescences. This study may be the first report of pollination systems in which flower bugs are the main pollinators. Similarity of pollination systems between M. hullettii and M. tanarius indicates that the two brood-site pollination systems have the same origin. The pollinator species belongs to a predacious group, whose major prey includes thrips. The pollination system might represent a unique example of evolution from predatory flower visitors feeding on the pollinators (thrips) to the main pollinators.
Amami; brood-site pollination; dioecy; Euphorbiaceae; extrafloral nectary; Hemiptera; Macaranga tanarius; Okinawa; Orius atratus
Background and Aims
Eriocaulaceae (Poales) is currently divided in two subfamilies: Eriocauloideae, which comprises two genera and Paepalanthoideae, with nine genera. The floral anatomy of Actinocephalus polyanthus, Leiothrix fluitans, Paepalanthus chlorocephalus, P. flaccidus and Rondonanthus roraimae was studied here. The flowers of these species of Paepalanthoideae are unisexual, and form capitulum-type inflorescences. Staminate and pistillate flowers are randomly distributed in the capitulum and develop centripetally. This work aims to establish a floral nomenclature for the Eriocaulaceae to provide more information about the taxonomy and phylogeny of the family.
Light microscopy, scanning electron microscopy and chemical tests were used to investigate the floral structures.
Staminate and pistillate flowers are trimerous (except in P. flaccidus, which presents dimerous flowers), and the perianth of all species is differentiated into sepals and petals. Staminate flowers present an androecium with scale-like staminodes (not in R. roraimae) and fertile stamens, and nectariferous pistillodes. Pistillate flowers present scale-like staminodes (except for R. roraimae, which presents elongated and vascularized staminodes), and a gynoecium with a hollow style, ramified in stigmatic and nectariferous portions.
The scale-like staminodes present in the species of Paepalanthoideae indicate a probable reduction of the outer whorl of stamens present in species of Eriocauloideae. Among the Paepalanthoideae genera, Rondonanthus, which is probably basal, shows vascularized staminodes in their pistillate flowers. The occurrence of nectariferous pistillodes in staminate flowers and that of nectariferous portions of the style in pistillate flowers of Paepalanthoideae are emphasized as nectariferous structures in Eriocaulaceae.
Eriocaulaceae; Paepalanthoideae; nectariferous structures; staminodes; staminate flowers; pistillate flowers; floral anatomy; monocotyledons; Poales
The pistil is a place where multiple interactions between cells of different types, origin, and function occur. Ca2+ is one of the key signal molecules in plants and animals. Despite the numerous studies on Ca2+ signalling during pollen-pistil interactions, which constitute one of the main topics of plant physiology, studies on Ca2+ dynamics in the pistil during flower formation are scarce. The purpose of this study was to analyze the contents and in situ localization of Ca2+ at the whole-organ level in the pistil of olive during the whole course of flower development.
The obtained results showed significant changes in Ca2+ levels and distribution during olive pistil development. In the flower buds, the lowest levels of detectable Ca2+ were observed. As flower development proceeded, the Ca2+ amount in the pistil successively increased and reached the highest levels just after anther dehiscence. When the anthers and petals fell down a dramatic but not complete drop in calcium contents occurred in all pistil parts. In situ Ca2+ localization showed a gradual accumulation on the stigma, and further expansion toward the style and the ovary after anther dehiscence. At the post-anthesis phase, the Ca2+ signal on the stigmatic surface decreased, but in the ovary a specific accumulation of calcium was observed only in one of the four ovules. Ultrastructural localization confirmed the presence of Ca2+ in the intracellular matrix and in the exudate secreted by stigmatic papillae.
This is the first report to analyze calcium in the olive pistil during its development. According to our results in situ calcium localization by Fluo-3 AM injection is an effective tool to follow the pistil maturity degree and the spatial organization of calcium-dependent events of sexual reproduction occurring in developing pistil of angiosperms. The progressive increase of the Ca2+ pool during olive pistil development shown by us reflects the degree of pistil maturity. Ca2+ distribution at flower anthesis reflects the spatio-functional relationship of calcium with pollen-stigma interaction, progamic phase, fertilization and stigma senescence.
Maize is unique among cereal grasses because of its monoecious flowering habit. Male flowers are normally restricted to the tassel that terminates the primary shoot, whereas female flowers occur as ears at the terminal nodes of lateral branches. We observed Ki14, a tropical maize inbred that produces an ear tipped by a staminate (male) spike under certain environmental conditions, such as long daylengths. Recombinant inbred lines derived from the cross between temperate line B97, which was never observed to produce a staminate ear tip, and Ki14 segregated for the trait under long daylengths. Some progeny lines that had even longer staminate tips than Ki14 were male fertile. We mapped three QTL controlling staminate ear tip using a two-part (binomial plus normal) model. A major QTL on chromosome 3 had a large effect on penetrance of the trait (whether a line would produce staminate ear tips or not) as well as its severity (the length of the staminate tip). This QTL seems to be linked to, but at a distinct position from, a previously mapped QTL controlling the proportion of staminate florets in ears in progeny from crosses between maize and teosinte. Two additional QTL affecting staminate ear tip severity overlapped with QTL controlling photoperiod response previously mapped in this population. Alleles conferring photoperiod sensitivity for delayed flowering at these QTL seem to enhance the production of staminate ear tips under long daylengths.
maize recombinant inbred lines; floral sex determination; quantitative trait loci
The production of flowers, fruits and seeds demands considerable energy and nutrients, which can limit the allocation of these resources to other plant functions and, thereby, influence survival and future reproduction. The magnitude of the physiological costs of reproduction depends on both the factors limiting seed production (pollen, ovules or resources) and the capacity of plants to compensate for high resource demand.
To assess the magnitude and consequences of reproductive costs, we used shading and defoliation to reduce photosynthate production by fully pollinated plants of a perennial legume, Oxytropis sericea (Fabaceae), and examined the resulting impact on photosynthate allocation, and nectar, fruit and seed production.
Although these leaf manipulations reduced photosynthesis and nectar production, they did not alter photosynthate allocation, as revealed by 13C tracing, or fruit or seed production. That photosynthate allocation to reproductive organs increased >190 % and taproot mass declined by 29 % between flowering and fruiting indicates that reproduction was physiologically costly.
The insensitivity of fruit and seed production to leaf manipulation is consistent with either compensatory mobilization of stored resources or ovule limitation. Seed production differed considerably between the two years of the study in association with contrasting precipitation prior to flowering, perhaps reflecting contrasting limits on reproductive performance.
Defoliation; shading; carbon partitioning; 13C-labelling; compensation; Oxytropis sericea; ovule limitation; reproductive cost
Unlike most genera in the early-divergent angiosperm family Annonaceae, Pseuduvaria exhibits a diversity of floral sex expression. Most species are structurally andromonoecious (or possibly androdioecious), although the hermaphroditic flowers have been inferred to be functionally pistillate, with sterile staminodes. Pseuduvaria presents an ideal model for investigating the evolution of floral sex in early-divergent angiosperms, although detailed empirical studies are currently lacking. The phenology and pollination ecology of the Australian endemic species Pseuduvaria mulgraveana are studied in detail, including evaluations of floral scent chemistry, pollen viability, and floral visitors. Results showed that the flowers are pollinated by small diurnal nitidulid beetles and are protogynous. Pollen from both hermaphroditic and staminate flowers are shown to be equally viable. The structurally hermaphroditic flowers are nevertheless functionally pistillate as anther dehiscence is delayed until after petal abscission and hence after the departure of pollinators. This mechanism to achieve functional unisexuality of flowers has not previously been reported in angiosperms. It is known that protogyny is widespread amongst early-divergent angiosperms, including the Annonaceae, and is effective in preventing autogamy. Delayed anther dehiscence represents a further elaboration of this, and is effective in preventing geitonogamy since very few sexually mature flowers occur simultaneously in an individual. We highlight the necessity for field-based empirical interpretations of functional floral sex expression prior to evaluations of evolutionary processes.
Sexual reproduction in flowering plants is controlled by recognition mechanisms involving the male gametophyte (the pollen) and the female sporophyte (the pistil). Self-incompatibility (SI) involves the recognition and rejection of self- or incompatible pollen by the pistil. In Papaver rhoeas, SI uses a Ca(2+)-based signalling cascade triggered by the S-protein, which is encoded by the stigmatic component of the S-locus. This results in the rapid inhibition of incompatible pollen tube growth. We have identified several targets of the SI signalling cascade, including protein kinases, the actin cytoskeleton and nuclear DNA. Here, we summarize progress made on currently funded projects in our laboratory investigating some of the components targeted by SI, comprising (i) the characterization of a pollen phosphoprotein (p26) that is rapidly phosphorylated upon an incompatible SI response; (ii) the identification and characterization of a pollen mitogen-activated protein kinase (p56), which exhibits enhanced activation during SI; (iii) characterizing components involved in the reorganization and depolymerization of the actin cytoskeleton during the SI response; and (iv) investigating whether the SI response involves a programmed cell death signalling cascade.
Polarized cell elongation is triggered by small molecule cues during development of diverse organisms. During plant reproduction, pollen interactions with the stigma result in the polar outgrowth of a pollen tube, which delivers sperm cells to the female gametophyte to effect double fertilization. In many plants, pistils stimulate pollen germination. However, in Arabidopsis, the effect of pistils on pollen germination and the pistil factors that stimulate pollen germination remain poorly characterized. Here, we demonstrate that stigma, style, and ovules in Arabidopsis pistils stimulate pollen germination. We isolated an Arabidopsis pistil extract fraction that stimulates Arabidopsis pollen germination, and employed ultrahigh resolution ESI FT-ICR and MS/MS techniques to accurately determine the mass (202.126 daltons) of a compound that is specifically present in this pistil extract fraction. Using the molecular formula (C10H19NOS) and tandem mass spectral fragmentation patterns of the m/z (mass to charge ratio) 202.126 ion, we postulated chemical structures, devised protocols, synthesized N-Methanesulfinyl 1- and 2-azadecalins that are close structural mimics of the m/z 202.126 ion, and showed that they are sufficient to stimulate Arabidopsis pollen germination in vitro (30 µM stimulated ~50% germination) and elicit accession-specific response. Although N-Methanesulfinyl 2-azadecalin stimulated pollen germination in three species of Lineage I of Brassicaceae, it did not induce a germination response in Sisymbrium irio (Lineage II of Brassicaceae) and tobacco, indicating that activity of the compound is not random. Our results show that Arabidopsis pistils promote germination by producing azadecalin-like molecules to ensure rapid fertilization by the appropriate pollen.
Pollen; pistil; germination; stimulant; chemical biology; functional mimic
The flower has a finite lifespan that is controlled largely by its role in sexual reproduction. The programmed senescence of flowers allows the plant to systematically degrade the petal cells and remobilize nutrients to developing tissues, including the seeds. This senescence program is tightly controlled by the plant hormone ethylene in some flowers, while in some species the senescence signals are unknown. This review article will examine the role of nutrient remobilization during petal senescence and how this differs among flowers with different flower termination phenotypes.
The flower has a finite lifespan that is controlled largely by its role in sexual reproduction. Once the flower has been pollinated or is no longer receptive to pollination, the petals are programmed to senesce. A majority of the genes that are up-regulated during petal senescence, in both ethylene-sensitive and -insensitive flowers, encode proteins involved in the degradation of nucleic acids, proteins, lipids, fatty acids, and cell wall and membrane components. A smaller subset of these genes has a putative role in remobilizing nutrients, and only a few of these have been studied in detail. During senescence, carbohydrates (primarily sucrose) are transported from petals, and the degradation of macromolecules and organelles also allows the plant to salvage mineral nutrients from the petals before cell death. The remobilization of mineral nutrients from a few species has been investigated and will be reviewed in this article. Ethylene's role in nutrient remobilization is discussed by comparing nutrient changes during the senescence of ethylene-sensitive and -insensitive flowers, and by studies in transgenic petunias (Petunia × hybrida) that are insensitive to ethylene. Gene expression studies indicate that remobilization is a key feature of senescence, but some senescence-associated genes have different expression in leaves and petals. These gene expression patterns, along with differences in the nutrient content of leaves and petals, suggest that there are differences in the mechanisms of cellular degradation and nutrient transport in vegetative and floral organs. Autophagy may be the mechanism for large-scale degradation that allows for recycling during senescence, but it is unclear if this causes cell death. Future research should focus on autophagy and the regulation of ATG genes by ethylene during both leaf and petal senescence. We must identify the mechanisms by which individual mineral nutrients are transported out of senescing corollas in both ethylene-sensitive and -insensitive species.
Abscission; autophagy; cell death; flowers; nitrogen; petals; petunias; phosphorus
Many co-sexual plants segregate female and male function among flowers on an inflorescence through dichogamy or the production of unisexual flowers. Sexual segregation may reduce self-pollination among flowers within inflorescences (geitonogamy), thereby increasing the pollen available for export to other plants. To assess these complementary roles we manipulated the simultaneously hermaphroditic (adichogamous) flowers of Eichhornia paniculata to produce ten-flowered inflorescences with either female above male flowers (female/male inflorescences) or male/female inflorescences, which competed for mating opportunities with five-flowered adichogamous inflorescences. Because of the upward movement of bumble-bees, selfing increased upward in adichogamous inflorescences (overall female selfing rate s+/-s.e.=0.320+/-0.026). Female flowers of male/female inflorescences selfed less than flowers in corresponding positions in adichogamous inflorescences so s fell to 0.135+/-0.027. In contrast, all-female flowers of female/male inflorescences selfed similarly to upper flowers on adichogamous inflorescences, elevating s (0.437+/-0.043). During 1997, male/female inflorescences sired more outcrossed seeds than female/male or adichogamous inflorescences, whereas during 1994 flowers on male/female inflorescences received fewer visits than those of adichogamous inflorescences, reducing their outcross siring success. Hence, sexual segregation limits geitonogamy and enhances outcross siring success when it does not affect pollinator behaviour, illustrating the importance of both female and male function in inflorescence design.
Background and Aims
Flower morphology and inflorescence architecture affect pollinator foraging behaviour and thereby influence the process of pollination and the reproductive success of plants. This study explored possible ecological functions of the lever-like stamens and the floral design in Salvia cyclostegia.
Flower construction was experimentally manipulated by removing either the lower lever arms or the upper fertile thecae of the two stamens from a flower. The two types of manipulated individuals were intermixed with the control ones and randomly distributed in the population.
Removing the sterile lower lever arms significantly reduced handling time per flower of the main pollinator, Bombus personatus. Interestingly, this manipulation did not increase the number of flowers probed per plant visit, but instead reduced it, i.e. shortened the visit sequence of the bumble-bees. Both loss of staminal lever function by removing lower lever arms and exclusion of self pollen by removing upper fertile thecae significantly reduced seed set per flower and seed set per plant. Both the manipulations interacted significantly with inflorescence size for the effect on female reproductive output.
Though the intact flowers demand a long handling time for pollinators, the reversible staminal lever is of advantage by promoting dispersal of pollen and thus the male function. The particular floral design in S. cyclostegia contributes to the floral constancy of B. personatus bumble-bees, with the lower lever arms acting as an optical cue for foraging cognition.
Adaptation; Bombus personatus; experimental flower manipulation; floral constancy; floral design; foraging behaviour; geitonogamy; Salvia
Background and Aims
The functional morphology of Salvia pratensis flowers was re-investigated, after new insights revealed that pollen dispensing is one of the main functions of the staminal lever. In particular, no detailed information was available regarding the process of pollen transfer and the forces arising between the pollen-bearing thecae and the pollinating bee's body. The assumption was made that these forces play a significant role in pollen dispensing.
The functional morphology of S. pratensis flowers and the interaction between flowers and bees (Apis mellifera) were studied by reconstructing stress and strains by using qualitative and semi-quantitative theoretical analysis. Flowers were manipulated to study the spatial arrangement of the filament and lever, and of the head and proboscis of the visiting bee inside the tube. Photographs and films of bee visits on flowers were used to analyse the interaction of pollinator and staminal lever.
The spoon-shaped lower lever of S. pratensis has a small hole through which a bee introduces its proboscis into the corolla tube. Although mentioned for the first time by Kerner von Marilaun in 1891, presented here is the first drawing and the first photograph showing this interaction in detail. The analysis of the interaction of flower visitor and the lever mechanism revealed that the position of bees on different flowers is spatially very similar. Flower morphology constrains postures of legitimately nectar-probing bees within narrow bounds. A theoretical discussion on structural elements and force progression in the flower allows the principles of lightweight architecture in flower morphology to be recognized.
The staminal lever of S. pratensis is a pollen-dispensing device. It seems to influence the amount of pollen deposited on pollinators by determining the forces arising between the pollinator and the pollen. The relevant forces occur either during the first, dynamic phase or during the second, almost static phase of a flower visit.
Flower–pollinator interaction; bee; Apis mellifera; pollination; pollen uptake; see-saw mechanism; biomechanics; pollen dispensing
Females tend to be smaller than males in woody dioecious plant species, but they tend to be larger in herbs. The smaller size of females in woody species has been attributed to higher reproductive costs, yet no satisfactory explanation has been provided for their larger size in herbs. Because herbs have higher nitrogen concentrations in their tissues than woody plants, and because pollen is particularly rich in nitrogen, we predicted that male growth would be more compromised by reproduction than female growth. To test this hypothesis, we conducted three experiments on the annual dioecious herb Mercurialis annua. First, we compared the timing of reproduction between males and females and found that males started flowering earlier than females; early flowering is expected to compromise growth more than later flowering. Second, we compared plants allowed to flower with those prevented from flowering by experimental debudding and found that males incurred a higher reproductive cost than females in terms of both biomass and, particularly, nitrogen. Third, we grew plants under varying levels of nitrogen availability and found that although sexual size dimorphism was unaffected by nitrogen, females, but not males, decreased their relative allocation to both roots and reproduction under high nitrogen availability. We propose that males deal with the high cost of pollen production in terms of nitrogen by allocating biomass to nitrogen-harvesting roots, whereas females pay for carbon-rich seeds and fruits by investing in photosynthetic organs. Sexual dimorphism would thus seem to be the outcome of allocation to above- versus below-ground sinks that supply resources (carbon versus nitrogen) limiting the female and male reproduction differentially.
life history; Mercurialis annua; trade-off; nitrogen allocation; carbon allocation
AtLURE1 defensin-like peptides, which show species-specific evolution, are essential in Arabidopsis for attracting pollen tubes and can function in the breakdown of reproductive isolation barriers.
Genes directly involved in male/female and host/parasite interactions are believed to be under positive selection. The flowering plant Arabidopsis thaliana has more than 300 defensin-like (DEFL) genes, which are likely to be involved in both natural immunity and cell-to-cell communication including pollen–pistil interactions. However, little is known of the relationship between the molecular evolution of DEFL genes and their functions. Here, we identified a recently evolved cluster of DEFL genes in A. thaliana and demonstrated that these DEFL (cysteine-rich peptide [CRP810_1]) peptides, named AtLURE1 peptides, are pollen tube attractants guiding pollen tubes to the ovular micropyle. The AtLURE1 genes formed the sole species-specific cluster among DEFL genes compared to its close relative, A. lyrata. No evidence for positive selection was detected in AtLURE1 genes and their orthologs, implying neutral evolution of AtLURE1 genes. AtLURE1 peptides were specifically expressed in egg-accompanying synergid cells and secreted toward the funicular surface through the micropyle. Genetic analyses showed that gametophytic mutants defective in micropylar guidance (myb98, magatama3, and central cell guidance) do not express AtLURE1 peptides. Downregulation of the expression of these peptides impaired precise pollen tube attraction to the micropylar opening of some populations of ovules. Recombinant AtLURE1 peptides attracted A. thaliana pollen tubes at a higher frequency compared to A. lyrata pollen tubes, suggesting that these peptides are species-preferential attractants in micropylar guidance. In support of this idea, the heterologous expression of a single AtLURE1 peptide in the synergid cell of Torenia fournieri was sufficient to guide A. thaliana pollen tubes to the T. fournieri embryo sac and to permit entry into it. Our results suggest the unique evolution of AtLURE1 genes, which are directly involved in male–female interaction among the DEFL multigene family, and furthermore suggest that these peptides are sufficient to overcome interspecific barriers in gametophytic attraction and penetration.
Defensin-like (DEFL) peptides commonly function as effector peptides and are involved in male-female and host-parasite interactions in eukaryotes. In higher plants, DEFL genes belong to a large multigene family and are highly variable between species. However, little is known about the relationship between the molecular evolution of DEFL genes and their functions. By comparing multiply duplicated DEFL genes between A. thaliana and its close relative A. lyrata, we have now identified pollen tube attractant peptides called AtLURE1 peptides, in A. thaliana. We find that AtLURE1 genes form a species-specific gene cluster and that the AtLURE1 peptides these genes encode are specifically expressed in the synergid (egg-accompanying) cells and are secreted along the path down which the pollen tube elongates to reach the female gametophyte. AtLURE1 peptides attract pollen tubes in a species-preferential manner and their downregulation impairs pollen tube guidance. Interestingly, the genetic introduction of a single AtLURE1 gene from A. thaliana into another plant, T. fournieri, is sufficient to breakdown reproductive isolation barriers in pollen tube guidance and penetration. These results suggest that AtLURE1 peptides, which show species-specific evolution, are key molecules that attract pollen tubes from a plant's own species to the embryo sac to enable successful reproduction.
Although self-incompatibility (SI) in plants has been studied extensively, far less is known about interspecific reproductive barriers. One interspecific barrier, known as unilateral incongruity or incompatibility (UI), occurs when species display unidirectional compatibility in interspecific crosses. In the wild tomato species Solanum pennellii, both SI and self-compatible (SC) populations express UI when crossed with domesticated tomato, offering a useful model system to dissect the molecular mechanisms involved in reproductive barriers. In this study, the timing of reproductive barrier establishment during pistil development was determined in SI and SC accessions of S. pennellii using a semi-in vivo system to track pollen-tube growth in developing styles. Both SI and UI barriers were absent in styles 5 days prior to flower opening, but were established by 2 days before flower opening, with partial barriers detected during a transition period 3–4 days before flower opening. The developmental expression dynamics of known SI factors, S-RNases and HT proteins, was also examined. The accumulation of HT-A protein coincided temporally and spatially with UI barriers in developing pistils. Proteomic analysis of stigma/styles from key developmental stages showed a switch in protein profiles from cell-division-associated proteins in immature stigma/styles to a set of proteins in mature stigma/styles that included S-RNases, HT-A protein and proteins associated with cell-wall loosening and defense responses, which could be involved in pollen–pistil interactions. Other prominent proteins in mature stigma/styles were those involved in lipid metabolism, consistent with the accumulation of lipid-rich material during pistil maturation.
pistil development; reproductive barriers; self-incompatibility; Solanum pennellii; stigma/stylar proteins; unilateral incongruity/incompatibility
MicroRNAs (miRNAs) are a class of endogenous, small, non-coding RNAs that regulate gene expression by mediating gene silencing at transcriptional and post-transcriptional levels in high plants. However, the diversity of miRNAs and their roles in floral development in Japanese apricot (Prunus mume Sieb. et Zucc) remains largely unexplored. Imperfect flowers with pistil abortion seriously decrease production yields. To understand the role of miRNAs in pistil development, pistil development-related miRNAs were identified by Solexa sequencing in Japanese apricot.
Solexa sequencing was used to identify and quantitatively profile small RNAs from perfect and imperfect flower buds of Japanese apricot. A total of 22,561,972 and 24,952,690 reads were sequenced from two small RNA libraries constructed from perfect and imperfect flower buds, respectively. Sixty-one known miRNAs, belonging to 24 families, were identified. Comparative profiling revealed that seven known miRNAs exhibited significant differential expression between perfect and imperfect flower buds. A total of 61 potentially novel miRNAs/new members of known miRNA families were also identified by the presence of mature miRNAs and corresponding miRNA*s in the sRNA libraries. Comparative analysis showed that six potentially novel miRNAs were differentially expressed between perfect and imperfect flower buds. Target predictions of the 13 differentially expressed miRNAs resulted in 212 target genes. Gene ontology (GO) annotation revealed that high-ranking miRNA target genes are those implicated in the developmental process, the regulation of transcription and response to stress.
This study represents the first comparative identification of miRNAomes between perfect and imperfect Japanese apricot flowers. Seven known miRNAs and six potentially novel miRNAs associated with pistil development were identified, using high-throughput sequencing of small RNAs. The findings, both computationally and experimentally, provide valuable information for further functional characterisation of miRNAs associated with pistil development in plants.
Japanese apricot; microRNA; Pistil abortion; qRT-PCR; Solexa sequencing
Background and Aims
Why are sterile anthers and carpels retained in some flowering plants, given their likely costs? To address this question, a cryptically dioecious species, Petasites tricholobus, in which male and female plants each have two floret types that appear pistillate and hermaphroditic, was studied. The aim was to understand the function of sterile hermaphroditic florets in females. In addition, the first examination of functions of sterile female structures in male plants was conducted in the hermaphroditic florets on males of this species. These female structures are exceptionally large in this species despite being sterile.
Differences in floret morphology between the sex morphs were documented and the possible functions of sterile sex organs investigated using manipulative experiments. Tests were carried out to find out if sterile female structures in male florets attract pollinators and if they aid in pollen dispersal, also to find out if the presence and quantity of sterile hermaphroditic florets in females increase pollinator attraction and reproductive success. To investigate what floret types provide nectar, all types of florets were examined under a scanning electron microscope to search for nectaries.
The sterile female structures in male florets did not increase pollinator visits but were essential to secondary pollen presentation, which significantly enhanced pollen dispersal. Sterile pistillate florets on male plants did not contribute to floral display and disappeared in nearly half of the male plants. The sterile hermaphroditic florets on female plants attracted pollinators by producing nectar and enhanced seed production.
The presence of female structures in male florets and hermaphroditic florets on female plants is adaptive despite being sterile, and may be evolutionarily stable. However, the pistillate florets on male plants appear non-adaptive and are presumably in decline. Differential fates of the sterile sex organs in the species are determined by both the historical constraints and the ecological functions.
Cryptic dioecy; sterile sex organ; secondary pollen presentation; pollinator attraction; breeding system evolution; ecological function; Petasites tricholobus
Background and Aims
The number of flowers blooming simultaneously on a plant may have profound consequences for reproductive success. Large floral displays often attract more pollinator visits, increasing outcross pollen receipt. However, pollinators frequently probe more flowers in sequence on large displays, potentially increasing self-pollination and reducing pollen export per flower. To better understand how floral display size influences male and female fitness, we manipulated display phenotypes and then used paternity analysis to quantify siring success and selfing rates.
To facilitate unambiguous assignment of paternity, we established four replicate (cloned) arrays of Mimulus ringens, each consisting of genets with unique combinations of homozygous marker genotypes. In each array, we trimmed displays to two, four, eight or 16 flowers. When fruits ripened, we counted the number of seeds per fruit and assigned paternity to 1935 progeny.
Siring success per flower declined sharply with increasing display size, while female success per flower did not vary with display. The rate of self-fertilization increased for large floral displays, but siring losses due to geitonogamous pollen discounting were much greater than siring gains through increased self-fertilization. As display size increased, each additional seed sired through geitonogamous self-pollination was associated with a loss of 9·7 seeds sired through outcrossing.
Although total fitness increased with floral display size, the marginal return on each additional flower declined steadily as display size increased. Therefore, a plant could maximize fitness by producing small displays over a long flowering period, rather than large displays over a brief flowering period.
Bumble-bee; floral display size; functional gender; geitonogamy; male selfing rate; mating system; Mimulus ringens; paternity analysis; pollen discounting; pollination; self-fertilization; siring success
Background and Aims
The effect of pollination on flower life span has been widely studied, but so far little attention has been paid to the reproductive consequences of delayed pollination in plants with long floral life spans. In the present study, Polygala vayredae was used to answer the following questions. (1) How does male and female success affect the floral longevity of individual flowers? (2) How does delaying fertilization affect the female fitness of this species?
Floral longevity was studied after experimental pollinations involving male and/or female accomplishment, bagging and open pollination. The reproductive costs of a delay in the moment of fertilization were evaluated through fruit set, seed–ovule ratio and seed weight, after pollination of flowers that had been bagged for 2–18 d.
Senescence of the flowers of P. vayredae was activated by pollen reception on the stigmatic papillae, while pollen removal had no effect on floral longevity. Nonetheless, a minimum longevity of 8 d was detected, even after successful pollination and pollen dissemination. This period may be involved with the enhancement of male accrual rates, as the female accomplishment is generally achieved after the first visit. Floral life span of open-pollinated flowers was variable and negatively correlated with pollinator visitation rates. Delayed pollination had a major impact on the reproductive success of the plant, with fruit set, seed–ovule ratio and seed weight being significantly diminished with the increase of flower age at the moment of fertilization.
A strong relationship between pollination and floral longevity was observed. Flowers revealed the ability to extend or reduce their longevity, within some limits, in response to the abundance of efficient pollinators (i.e. reproductive fulfilment rates). Furthermore, with scarce or unpredictable pollinators, a long floral life span could maintain the opportunity for fertilization but would also have reproductive costs on production of offspring. Reduced female fitness late in the flower's life could shift the cost–benefit balance towards a shorter life span, partially counteracting the selection for longer floral life span potentially mediated by scarce pollination services.
Delayed pollination; endemic species; flower longevity; life span; pollen limitation; pollination; pollinator scarcity; Polygala vayredae; Polygalaceae; reproductive consequences; secondary pollen presentation
Precise coordination between stamen and pistil development is essential to make a fertile flower. Mutations impairing stamen filament elongation, pollen maturation, or anther dehiscence will cause male sterility. Deficiency in plant hormone gibberellin (GA) causes male sterility due to accumulation of DELLA proteins, and GA triggers DELLA degradation to promote stamen development. Deficiency in plant hormone jasmonate (JA) also causes male sterility. However, little is known about the relationship between GA and JA in controlling stamen development. Here, we show that MYB21, MYB24, and MYB57 are GA-dependent stamen-enriched genes. Loss-of-function of two DELLAs RGA and RGL2 restores the expression of these three MYB genes together with restoration of stamen filament growth in GA-deficient plants. Genetic analysis showed that the myb21-t1 myb24-t1 myb57-t1 triple mutant confers a short stamen phenotype leading to male sterility. Further genetic and molecular studies demonstrate that GA suppresses DELLAs to mobilize the expression of the key JA biosynthesis gene DAD1, and this is consistent with the observation that the JA content in the young flower buds of the GA-deficient quadruple mutant ga1-3 gai-t6 rga-t2 rgl1-1 is much lower than that in the WT. We conclude that GA promotes JA biosynthesis to control the expression of MYB21, MYB24, and MYB57. Therefore, we have established a hierarchical relationship between GA and JA in that modulation of JA pathway by GA is one of the prerequisites for GA to regulate the normal stamen development in Arabidopsis.
Gibberellin and jasmonate are plant hormones that mediate diverse plant developmental processes and responses to the environment. Deficiency in either gibberellin or jasmonate causes male sterility, in part due to the short stamen filament conferred. In this report, we sought to study the interaction between gibberellin and jasmonate during stamen filament development. We focused on three MYB genes, namely MYB21, MYB24, and MYB57, which have been proven to be essential for stamen filament development in Arabidopsis. These three MYB genes are regulated by both gibberellin and jasmonate. We performed various molecular analyses and found that GA activates the expression of DAD1 and LOX1, two genes essential for jasmonate biosynthesis. The hypothesis of GA regulating JA biosynthesis is proved by the fact that the JA content in the young flower buds of the ga1-3 gai-t6 rga-t2 rgl1-1 quadruple mutant is much lower than that in the WT. This evidence demonstrates that GA promotes the production of jasmonate and high level of jasmonate will induce the expression of MYB21, MYB24, and MYB57 to promote stamen filament development. This is most likely the first molecular and genetic evidence to show how gibberellin and jasmonate interact to control stamen filament development.
Pollination is a crucial step in angiosperm (flowering plant) reproduction. Highly orchestrated pollen–pistil interactions and signalling events enable plant species to avoid inbreeding and outcrossing as a species-specific barrier. In compatible pollination, pollen tubes carrying two sperm cells grow through the pistil transmitting tract and are precisely guided to the ovules, discharging the sperm cells to the embryo sac for fertilization.
In Lilium longiflorum pollination, growing pollen tubes utilize two critical mechanisms, adhesion and chemotropism, for directional growth to the ovules. Among several molecular factors discovered in the past decade, two small, secreted cysteine-rich proteins have been shown to play major roles in pollen tube adhesion and reorientation bioassays: stigma/style cysteine-rich adhesin (SCA, approx. 9·3 kDa) and chemocyanin (approx. 9·8 kDa). SCA, a lipid transfer protein (LTP) secreted from the stylar transmitting tract epidermis, functions in lily pollen tube tip growth as well as in forming the adhesive pectin matrix at the growing pollen tube wall back from the tip. Lily chemocyanin is a plantacyanin family member and acts as a directional cue for reorienting pollen tubes. Recent consecutive studies revealed that Arabidopsis thaliana homologues for SCA and chemocyanin play pivotal roles in tip polarity and directionality of pollen tube growth, respectively. This review outlines the biological roles of various secreted proteins in angiosperm pollination, focusing on plant LTPs and chemocyanin.
Angiosperm fertilization; Arabidopsis thaliana; chemocyanin; cysteine-rich peptides (CRPs); Lilium longiflorum; lipid transfer proteins (LTPs); plantacyanins; pollen tube tip growth; stigma/style cysteine-rich adhesin (SCA)