The unusual gynoecium structure and extragynoecial pollen-tube pathway in <Emphasis Type="Italic">Phytolacca</Emphasis> (Phytolaccaceae)

The fusion of carpels into a unified compound gynoecium is considered a dominant feature of angiosperm evolution and it also occurs by postgenital fusion during the gynoecium development in some apocarpous species. However, we found the reverse process, the separation of carpels from combined carpel primordia, during the development of the gynoecium in Phytolacca. Semithin sectioning and scanning electron microscopy were utilised to observe the structure and development of the gynoecia in Phytolacca acinosa and Phytolacca americana, fluorescence microscopy was utilised to observe the pollen tube growth in the gynoecia of the two species, and the topology method was applied to analyze the relationship between the gynoecium structure and pollen tube pathway. Although the gynoecia of P. acinosa and P. americana are both syncarpous, the degree of carpel fusion in the mature gynoecia of the two syncarpous species is different as a result of variant developmental processes. However, change in the degree of carpel fusion during the development of gynoecia in Phytolacca does not affect pollen tube growth because of the existence of the extragynoecial pollen-tube pathway. Thus, the change in the degree of carpel fusion in Phytolacca is primarily the result of diversification of developmental processes related to selection pressure.     

Intercarpellary growth of pollen tubes in the extragynoecial compitum and its contribution to fruit set in an apocarpous species, Schisandra sphenanthera (Schisandraceae)

? Premise of the study: Apocarpous plants possess carpels that are separated in the gynoecium. Extragynoecial compita, commonly occurring in basal angiosperms, have been proposed to have the potential to increase offspring quantity in apocarpous species through the intercarpellary growth of pollen tubes. To date, the impact of an extragynoecial compitum on fruit or seed set has not been studied in any species. This study investigated the pollen tube pathway between adjacent carpels and its contribution to fruit set in Schisandra sphenanthera. ? Methods: We investigated the fruit set ratio in the field and collected hundreds of gynoecia at their full flowering stage. Pollinated carpel ratio and pollen tube pathway observations were performed using fluorescence optics. ? Key results: Pollen grains germinated and tubes extended along the pseudostyle surface. Some of them turned and entered the ovules at the end of the stigmatic crest, whereas others subsequently grew into neighboring carpels through promontory connections located at the base of the unfused carpels. No tubes were found growing on the surface of the receptacle. More than 24 carpels could be fertilized by pollen tubes from one carpel through hand pollination. The pollinated carpel ratio was significantly lower than the fruit set ratio under natural conditions. ? Conclusions: Pollen tubes from one carpel can easily cross in the extragynoecial compitum between the adjacent carpels of S. sphenanthera, and this intercarpellary growth of pollen tubes can significantly increase the fruit set of apocarpous species, at least in S. sphenanthera.     

Floral development in <Emphasis Type="Italic">Thermopsis turcica</Emphasis>, an unusual multicarpellate papilionoid legume

The vast majority of the species of family Leguminosae have an apocarpous monomerous gynoecium. However, only a few taxa regularly produce multicarpellate gynoecia. The only known species of papilionoid legumes which has both a typical “flag blossom” and more than one carpel is Thermopsis turcica (tribe Thermopsideae). We studied the floral ontogeny of T. turcica with special reference to its gynoecium initiation and development. Flowers arise in simple terminal racemes in a helical order and are subtended by bracts. Bracteoles are initiated but then suppressed. Sepals appear more or less simultaneously. Then, petals emerge and remain retarded in development until later stages. The gynoecium usually includes three carpels with an abaxial one initiating first and two adaxial carpels arising later and developing somewhat asynchronously. The abaxial carpel appears concomitant with the outer stamens and is always oriented with its cleft toward the adaxial side, while the adaxial carpels face each other with their clefts and have them slightly turned to the adaxial side. Rarely uni-, bi- or tetracarpellate flowers arise. Seed productivity of T. turcica is on approximately the same level as in unicarpellate species of Thermopsis hence supporting the fact that the multicarpellate habit is adaptive or at least not harmful in this species.     

Unique growth paths of heterospecific pollen tubes result in late entry into ovules in the gynoecium of Sagittaria (Alismataceae)

• Pollen‐pistil interactions are a fundamental process in the reproductive biology of angiosperms and play a particularly important role in maintaining incipient species that exist in sympatry. However, the majority of previous studies have focused on species with syncarpous gynoecia (fused carpels) and not those with apocarpous gynoecia (unfused carpels).
• In the present study, we investigated the growth of conspecific pollen tubes compared to heterospecific pollen tubes in Sagittaria species, which have apocarpous gynoecia. We conducted controlled pollinations between S. pygmaea and S. trifolia and observed the growth of conspecific and heterospecific pollen tubes under a fluorescence microscope.
• Heterospecific and conspecific pollen tubes arrived at locules within the ovaries near simultaneously. However, conspecific pollen tubes entered into the ovules directly, whereas heterospecific tubes passed through the carpel base and adjacent receptacle tissue, to ultimately fertilize other unfertilized ovules. This longer route taken by heterospecific pollen tubes therefore caused a delay in the time required to enter into the ovules. Furthermore, heterospecific pollen tubes displayed similar growth patterns at early and peak pollination. The growth pattern of heterospecific pollen tubes at late pollination was similar to that of conspecific pollen tubes at peak pollination.
• Heterospecific and conspecific pollen tubes took different routes to fertilize ovules. A delayed entry of heterospecific pollen into ovules may be a novel mechanism of conspecific pollen advantage (CPA) for apocarpous species.

     

Pollen tubes enter neighbouring ovules by way of receptacle tissue,resulting in increased fruit-set in Sagittaria potamogetifolia Merr

Using fluorescence microscopy, deposition of pollen on stigmas and pollen tube growth in the gynoecium of Sagittaria potamogetifolia Merr., a monoecious species with an apocarpous gynoecium, were observed. The maximum rate of pollination averaged 83.9 +/- 4.7 %, and the number of pollen grains per stigma ranged from zero to 30. Pollen tubes grew through one stigma to the base of the ovary at almost the same speed, but generally only one of the pollen tubes then turned towards the ovule and finally entered the nucellus through the micropyle. The other pollen tubes grew through the ovary base and the receptacle tissue into ovules of adjacent carpels whose stigmas were not pollinated or which had been pollinated later. This phenomenon is termed pollen tube 'reallocation' by the authors. To verify the direct effect of the phenomenon on fruit set, artificial pollination experiments were conducted in which two or more pollen grains were placed onto only one stigma in each gynoecium; frequently more than one fruitlet was obtained from each flower treated. The reallocation of pollen tubes among pistils in the gynoecium could effect fertilization of ovules of unpollinated pistils and lead to an increase in sexual reproduction efficiency. It would, to some extent, also increase pollen tube competition among pistils of the whole gynoecium.     

Pollen tube reallocation in two preanthesis cleistogamous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae)

Pollen deposition on stigmas and pollen tube growth in two apocarpous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae), were examined with fluorescence microscopy. The reallocation of pollen tubes among pistils was observed in both species. The percentage of pollinated stigmas per flower was only 22.0% in R. rostratum and 51.0% in S. guyanensis, though the seed/ovule ratios are higher than 65% in both species. The number of pollen grains on each single stigma ranged from 0 to 96 in R. rostratum, and from 0 to 125 in S. guyanensis. When more than one pollen grain deposited on a stigma, all pollen tubes grew to the ovary, but only one of them turned towards the ovule and finally entered the nucleus. The other tubes grew through the receptacle tissue into ovules of adjacent carpels whose stigmas were unpollinated or pollinated later. The intercarpellary growth of pollen tubes could be a mechanism to increase the efficiency of sexual reproduction in an apocarpous gynoecium with low pollination on the pistils.     

Timing of invasive pollen deposition influences pollen tube growth and seed set in a native plant

Invasive plants may threaten the reproductive success of native sympatric plants by modifying the pollination process. One potential mechanism takes place through the deposition of invasive pollen onto native stigmas when pollinators are shared among species. We explore how pollen from the invasive plant Brassica nigra influences pre- and post-fertilization stages in the native plant Phacelia parryi, through a series of hand pollination experiments. These two species share pollinators to a high degree. P. parryi flowers were hand-pollinated with either pure conspecific pollen (the control) or with B. nigra pollen applied prior to, simultaneously with, or following conspecific pollen. Application of B. nigra pollen lowered seed set, with the simultaneous application resulting in the highest reduction. Pollen tube growth was also influenced by the presence of invasive pollen, with fewer conspecific pollen tubes reaching the base of P. parryi styles in treatments where B. nigra pollen was applied prior to or simultaneously with conspecific pollen. The deleterious effects of invasive pollen on native seed set in this study are likely not due to loss of stigmatic receptivity since seed set was less affected when heterospecific pollen was applied prior to conspecific pollen, but may instead involve interactions between interspecific pollen grains on the stigma or within the style. Our study highlights the importance of timing of foreign pollen deposition on native stigmas and suggests that interspecific pollen transfer between native and exotic plants may be an important mechanism of competition for pollination in invaded plant communities.     

Extra-gynoecial pollen-tube growth in apocarpous angiosperms is phylogenetically widespread and probably adaptive

? Fusion of floral carpels (syncarpy) in angiosperms is thought to have allowed for significant improvements in offspring quantity and quality in syncarpous species over gymnosperms and apocarpous (free-carpelled) angiosperms. Given the disadvantages of apocarpy, it remains an evolutionary puzzle why many angiosperm lineages with free carpels (apocarpy) have been so successful and why some lineages show reversals to apocarpy. ? To investigate whether some advantages of syncarpy may accrue in other ways to apocarpous species, we reviewed previous studies of pollen-tube growth in apocarpous species and also documented pollen-tube growth in nine additional apocarpous species in six families. ? Anatomical studies of a scattering of apocarpous paleodicots, monocots, and eudicots show that, after transiting the style, 'extra' pollen tubes exit fully fertilized carpels and grow to other carpels with unfertilized ovules. In many species this occurs via openings in the simple carpels, as we report here for Sagittaria potamogetifolia, Sagittaria pygmaea, Sedum lineare, and Schisandra sphenanthera. ? The finding that extra-gynoecial pollen-tube growth is widespread in apocarpous species eliminates the possibility of a major fitness cost of apocarpy relative to syncarpy and may help to explain the persistence of, and multiple reversals to, apocarpy in the evolutionary history of angiosperms.     

Within-carpel and among-carpel competition during seed development, and selection on carpel number, in the apocarpous perennial herb Helleborus foetidus L. (Ranunculaceae)

Apocarpous flowers share opportunities for post-fertilization ovule selection among more functional levels than syncarpous flowers, because the occurrence of a variable number of unfused carpels adds a new source of variation to the likelihood of successful female reproduction. The extent to which post-fertilization events might differ among these unfused carpels may promote variations in the reproductive strategies of plants. We report a study of the variation, within and among carpels and flowers, in seed production and mass in the apocarpous Helleborus foetidus (Ranunculaceae), in relation to the number of carpels per flower. Differences within and among carpels in female reproductive success were affected by carpel number and pollination environment. When analysing whole flowers as functional units we also found that the magnitude of the differences related to carpel number and pollination treatment actually depended on the “distribution” of pollen types within flowers. Thus, variable within-flower pollination environments, more likely to occur in apocarpous than in syncarpous flowers, may affect the strategies of resource allocation for fruit development at different stages of the reproductive process. Regarding seed production, we found that producing more flowers with four carpels was under directional; however, when mean diaspore mass was considered as a measure of fitness, directional selection was found on producing flowers with two and three carpels (the modal carpel number found in wild populations). We discuss ecological and developmental reasons which could explain the observed pattern, and conclude that selection on an optimum carpel number may be very variable across the species range, as the discussed reasons may impose constraints on eventual evolutionary response, thus contributing to the maintenance of the intra-individual variability in carpel number.     

Pollen tube growth in association with a dry-type stigmatic transmitting tissue and extragynoecial compitum in the basal angiosperm Kadsura longipedunculata (Schisandraceae)

Spatial features of pollen tube growth and the composition of the extracellular matrix (ECM) of transmitting tissue in carpels of Kadsura longipedunculata, a member of the basal angiosperm taxon Schisandraceae, were characterized to identify features of transmitting tissue that might have been important for pollen-carpel interactions during the early history of angiosperms. In addition to growing extracellularly along epidermal cells that make up stigmatic crests of individual carpels, pollen tubes grow on abaxial carpel epidermal cells between unfused carpels along an extragynoecial compitum to subsequently enter an adjacent carpel, a feature important for enhancing seed set in apocarpous species. Histo- and immunochemical data indicated that transmitting tissue ECM is not freely flowing as previously hypothesized. Rather, the ECM is similar to that of a dry-type stigma whereby a cuticular boundary with associated esterase activity confines a matrix containing methyl-esterified homogalacturonans. The Schisandraceae joins an increasing number of basal angiosperm taxa that have a transmitting tissue ECM similar to a dry-type stigma, thereby challenging traditional views that the ancestral pollen tube pathway was similar to a wet-type stigma covered with a freely flowing exudate. Dry-type stigmas are posited to provide tighter control over pollen capture, retention, and germination than wet-type stigmas.     

Low interspecific pollen transfer between invasive aquatic <Emphasis Type="Italic">Ludwigia grandiflora</Emphasis> and native co-flowering plants

Showy invasive alien plants are often integrated in the diet of generalist pollinators and because of the lack of co-evolvement with the native plant community, a high amount of interspecific pollen transfer (IPT) can be expected. We investigated pollinator switching and magnitude plus distance of IPT between the alien aquatic Ludwigia grandiflora and the native Lythrum salicaria in both directions in uninvaded and invaded sites with a different relative abundance of L. grandiflora (% cover of the alien plant: no cover; low cover: <5%; high cover: 50–75%). A field experiment was conducted to include both pollinator interspecific movements and tracking of IPT, using fluorescent dye as a pollen analogue. Despite a substantial overlap in pollinators between L. grandiflora and the native L. salicaria, less than 10% of the observed flights were interspecific. Similar results were found in dye transfer patterns. The proportions of stigmas with conspecific dye were always higher than the proportions of stigmas with heterospecific dye for L grandiflora and L. salicaria. There were no differences in conspecific dye loads for L. salicaria between uninvaded and invaded sites. Conspecific pollen loss (native CPL) and heterospecific pollen deposition (alien HPD) were in general low and species-specific. The distance of HPD ranged respectively from 1.7 to 39 m and from 0.3 to 54.8 m in the low cover and high cover sites while CPL ranged respectively from 6.40 to 68.02 m and from 0.60 to 40.18 m in the low cover and high cover sites. We can conclude that, in this system, CPL and HPD will play a minor role in pollinator-mediated interaction. Furthermore, interspecific competition for pollinators will cover a larger distance than just neighboring individuals. Our results suggest the necessity to consider the combined effect of insect visitation, pollen deposition, relative alien abundance, distance and seed set when investigating pollinator-mediated interactions of invasive plants.     

Isolation and characterization of the C-class MADS-box gene from the distylous pseudo-cereal <Emphasis Type="Italic">Fagopyrum esculentum</Emphasis>

In the model species Arabidopsis thaliana, the floral homeotic C-class gene AGAMOUS (AG) specifies reproductive organ (stamen and carpels) identity and floral meristem determinacy. Gene function analyses in other core eudicots species reveal functional conservation, subfunctionalization and function switch of the C-lineage in this clade. To identify the possible roles of AG-like genes in regulating floral development in distylous species with dimorphic flowers (pin and thrum) and the C function evolution, we isolated and identified an AG ortholog from Fagopyrum esculentum (buckwheat, Family Polygonaceae), an early diverging species of core eudicots preceding the rosids-asterids split. Protein sequence alignment and phylogenetic analysis grouped FaesAG into the euAG lineage. Expression analysis suggested that FaesAG expressed exclusively in developing stamens and gynoecium of pin and thrum flowers. Moreover, FaesAG expression reached a high level in both pin and thrum flowers at the time when the stamens were undergoing rapidly increased in size and microspore mother cells were in meiosis. FaesAG was able to substitute for the endogenous AG gene in specifying stamen and carpel identity and in an Arabidopsis ag-1 mutant. Ectopic expression of FaesAG led to very early flowering, and produced a misshapen inflorescence and abnormal flowers in which sepals had converted into carpels and petals were converted to stamens. Our results confirmed establishment of the complete C-function of the AG orthologous gene preceding the rosids-asterids split, despite the distinct floral traits present in early- and late-diverging lineages of core eudicot angiosperms.     

Structure of the stigma and style of <Emphasis Type="Italic">Callaeum psilophyllum</Emphasis> (Malpighiaceae) and its relation with potential pollinators

The family Malpighiaceae, particularly in the Neotropic, shows a similar floral morphology. Although floral attraction and rewards to pollinators are alike, stigmas and styles show more diversity. The stigmas were described covered with a thin and impermeable cuticle that needs to be ruptured by the mechanical action of the pollinators. However, this characteristic was only mentioned for a few species and the anatomy and ultrastructure of the stigmas were not explored. In this work, we analyze the morphology, anatomy, and ultrastructure of the stigma and style of Callaeum psilophyllum. Moreover, we identify the potential pollinators in order to evaluate how the disposition of the stigmas is related with their size and its role in the exposure of the receptive stigmatic surface. Our observations indicate that Centris flavifrons, C. fuscata, C. tarsata, and C. trigonoides are probably efficient pollinators of C. psilophyllum. The three stigmas are covered by a cuticle that remained intact in bagged flowers. The flowers exposed to visitors show the cuticle broken, more secretion in the intercellular spaces between sub-stigmatic cells and abundant electron-dense components inside vacuoles in stigmatic papillae. This indicates that the stigmas prepares in similar ways to receive pollen grains, but the pollinator action is required to break the cuticle, and once pollen tubes start growing, stigmatic and sub-stigmatic cells release more secretion by a granulocrine process.     

GYNOECIAL ONTOGENY AND MORPHOLOGY,AND POLLEN TUBE PATHWAY IN BLACK MAPLE,ACER SACCHARUM SSP. NIGRUM (ACERACEAE)

The black maple (Acer saccharum Marsh, ssp. nigrum [Michx. f.] Desm.) gynoecium displays classical involute carpel development; carpels form, in mid- to late-summer, as two separate, opposite, hood-shaped primordia bearing naked megasporangia on inrolled carpel margins. Megasporogenesis, integument initiation, and carpel closure occur in spring; carpels fuse, forming a biloculate ovary with a short, hollow style and two divergent, dry, unicellular papillose stigmas. Transmitting tissues consist of developmentally and morphologically similar trichomes that form along the apparent carpel margins. The path from stigma to micropyle is open, but pollen tubes do not grow entirely ectotrophically. Germinating at the tip of a stigma papilla, a tube grows, apparently under the cuticle, to the papilla base. It then grows between stigma cells to the style, emerging to grow ectotrophically through the style to the compitum, where it passes into one of the locules. Within a locule, the tube grows over placenta and obturator to the micropyle, then between megasporangium cells to the female gametophyte, spreading over the surface near the egg. This study adds to our sparse understanding of gynoecium development and transmitting tissue in relation to pollen tube growth in naturally pollinated woody plants.     

Embryological peculiarities of interspecific hybridization in <Emphasis Type="Italic">Pinus sibirica</Emphasis>

Cytoembryological research of the ovules in experiments with interspecific hybridization of Pinus sibirica (pollination be the pollen of P. koraiensis, P. armandii, P. parviflora, P. strobus, P. hokkaidensis, P. wallichiana, P. monticola, and P. сembra) revealed that the development of megagametophytes occurred in them by the usual scenario and resulted in the formation of mature archegonia. Pollen successfully germinated on the nucellus of ovules. However, disturbances were observed in the process of male gametophyte development, and pollen tubes on the nucellus were not visible by the period of archegonia maturation. Fertilization was usually absent. The development of embryonic channel is determined by egg cell maturity. The only exception was the variant of the controlled pollination of Pinus sibirica × P. сembra, in which the embryo has been formed.     

Pollinator-mediated impacts of alien invasive plants on the pollination of native plants: the role of spatial scale and distinct behaviour among pollinator guilds

Alien invasive plant species can affect pollination, reproductive success and population dynamics of co-flowering native species via shared pollinators. Consequences may range from reproductive competition to facilitation, but the ecological drivers determining the type and magnitude of such indirect interactions remain poorly understood. Here, we examine the role of the spatial scale of invader presence and spatially contingent behavioural responses of different pollinator groups as potential key drivers, using the invasive Oxalis pes-caprae and the self-incompatible native annual Diplotaxis erucoides as a model system. Three treatments were assigned to native focal plants: (1) invader present at the landscape scale (hectares) but experimentally removed at the floral neighbourhood scale (pa); (2) invader present at both scales (pp); (3) invader absent at both scales (aa). Interestingly, we found pronounced spatially contingent differences in the responses of pollinators: honeybees and bumblebees were strongly attracted into invaded sites at the landscape scale, translating into native plant visitation facilitation through honeybees, while bumblebees almost exclusively visited Oxalis. Non-corbiculate wild bees, in contrast, showed less pronounced responses in foraging behavior, primarily at the floral neighborhood scale. Average heterospecific (Oxalis) pollen deposition onto stigmas of Diplotaxis was low (<1 %), but higher in the pp than in the pa treatment. Hand-pollination of Diplotaxis with Oxalis and conspecific pollen, however, reduced seed set by more than half when compared to hand-pollination with only conspecific pollen. Seed set of Diplotaxis, finally, was increased by 14 % (reproductive facilitation) in the pp treatment, while it was reduced by 27 % (reproductive competition) in the pa treatment compared to uninvaded populations. Our study highlights the crucial role of spatial scale and pollinator guild driving indirect effects of invasive on co-flowering native plant species.     

Adaptation of the seed reproduction system to conditions of Maritime Antarctic in <Emphasis Type="Italic">Deschampsia antarctica</Emphasis> E. Desv.

Deschampsia antarctica3 E. Desv. is one of the two flowering plants that, along with Colobanthus quitensis (Kunth) Bartl., was able to settle the ice-free areas of Antarctica. In order to identify the possible adaptations of the D. antarctica reproductive system to adverse environmental conditions, comparative cytoembryological analysis of plants of this species growing on the Antarctic Peninsula with plants of the closely related species D. beringensis Hult. from the Kamchatka Peninsula was conducted. It was found that both species are characterized by sexual mode of reproduction, equal size of pollen grains (25.5 ± 2.2 and 26.2 ± 1.9 μm, respectively), same features of the embryo sac structure, and emryo- and endospermogenesis. Interspecies differences have been found in mature embryo sac size (326.8 ± 12.8 and 161.7 ± 10.4 μm), pollen sterility percentage (86.1 ± 8.9 and 35.3 ± 9.2%), and quantity of pollen in the anthers (140 ± 15.3 and 1578 ± 88.6). Possible causes and significance of these differences are discussed. No unique adaptations of seed reproduction system that are inherent exclusively to D. antarctica were found. The D. antarctica reproduction strategy is based on the combination of autogamy (and its extreme form cleistogamy) with production of excess pollen quantity for its mode of pollination.     

FUNCTIONAL SYNCARPY BY INTERCARPELLARY GROWTH OF POLLEN TUBES IN A PRIMITIVE APOCARPOUS ANGIOSPERM,ILLICIUM FLORIDANUM (ILLICIACEAE)

Fluorescence microscopy and histological studies have been used to show that in Illicium floridanum Ellis (Illiciaceae), a primitive apocarpous angiosperm, functional syncarpy is achieved by intercarpellary growth of pollen tubes. After pollen germinates on the separate stigmatic crests of the carpellary whorl, tubes grow within the carpels obliquely down and inward toward the central floral axis which is modified as a stigmalike “apical residuum.” In a restricted shallow region around the base of the apical residuum, some pollen tubes grow out between the unfused margins of the carpels and circumferentially around the surface of the apical residuum from where they may enter neighboring carpels. Some pollen germination and tube growth also occur on the apical residuum itself. The apical residuum with its associated unfused carpel margins acts as an extragynoecial compitum for pollen tube transfer between carpels, and, as such, is believed to represent a mechanism for increasing the efficiency of seed set. The pollen tube pathway of Illicium appears to be a primitive expression of a line of evolutionary development leading to syncarpous gynoecia through stages possibly exemplified by certain members of the Trochodendraceae (lower Hamamelididae).     

Arabidopsis PRC1 core component AtRING1 regulates stem cell-determining carpel development mainly through repression of class I <Emphasis Type="Italic">KNOX</Emphasis> genes

Background

Polycomb repressive complex 2 (PRC2)-catalyzed H3K27me3 marks are tightly associated with the WUS-AG negative feedback loop to terminate floral stem cell fate to promote carpel development, but the roles of Polycomb repressive complex 1 (PRC1) in this event remain largely uncharacterized.

Results

Here we show conspicuous variability in the morphology and number of carpels among individual flowers in the absence of the PRC1 core components AtRING1a and AtRING1b, which contrasts with the wild-type floral meristem consumed by uniform carpel production in Arabidopsis thaliana. Promoter-driven GUS reporter analysis showed that AtRING1a and AtRING1b display a largely similar expression pattern, except in the case of the exclusively maternal-preferred expression of AtRING1b, but not AtRING1a, in the endosperm. Indeterminate carpel development in the atring1a;atring1b double mutant is due to replum/ovule-to-carpel conversion in association with ectopic expression of class I KNOX (KNOX-I) genes. Moreover, AtRING1a and AtRING1b also play a critical role in ovule development, mainly through promoting the degeneration of non-functional megaspores and proper integument formation. Genetic interaction analysis indicates that the AtRING1a/b-regulated KNOX-I pathway acts largely in a complementary manner with the WUS-AG pathway in controlling floral stem cell maintenance and proper carpel development.

Conclusions

Our study uncovers a novel mechanistic pathway through which AtRING1a and AtRING1b repress KNOX-I expression to terminate floral stem cell activities and establish carpel cell fate identities.

     

Ants as floral visitors of <Emphasis Type="Italic">Blutaparon portulacoides</Emphasis> (A. St-Hil.) Mears (Amaranthaceae): an ant pollination system in the Atlantic Rainforest

Ant pollination is a debated topic that requires more attention in order to clarify the role of ants as potential pollinators. Although many authors consider ants as mere nectar robbers, there are studies proving that ants may act as pollinators and that some plants even have flower traits acting as ant attractors. In this study, we evaluated the role of the ants in pollination of Blutaparon portulacoides inflorescences. This plant species has most of the traits favorable for ant pollination, such as short and aggregated inflorescences, and synchronized blooming as well as growing in an environment where ant pollination is likely to occur. Our results show that ants are the most abundant visitors throughout the day and that there is no effect of ant integument on pollen germination. Furthermore, the flower visitor exclusion experiment showed that ants have a role in the pollination of B. portulacoides by promoting seed formation. Ants can have an important part in the pollination of B. portulacoides in a scenario where winged insects are absent or scarce.