Stigmatic Papillae Research Articles

Floral development and the formation of functionally unisexual flowers in Xanthoceras sorbifolium (Sapindaceae), a morphologically andromonoecious tree endemic to northern China

KEY MESSAGE: Xanthoceras sorbifolium is apparently andromonoecious but exhibits a cryptically monoecious breeding system. Sexual differentiation in male and functional female flowers occurs 2 weeks before flowering. Individual trees of Xanthoceras sorbifolium bear male and morphologically bisexual flowers but functionally female flowers, and show labile sex expression. Investigations of the floral development are of significant value for understanding the breeding system and elucidating the systematic relationships. We studied floral development of male and bisexual flowers using scanning electron and light microscopy. The early stages of floral development were essentially the same, and all floral organ primordia were initiated in the two types of flowers of X. sorbifolium. Later, the stamens in bisexual flowers and the gynoecium in male flowers were aborted. We divided floral development into nine stages. Morphological differences between male and bisexual flowers appeared first at stage 8, when the style elongated obviously and the stigma papillae began to expand in the bisexual flowers but not in the male flowers. Ovule development was arrested shortly after formation of megaspore mother cells or during meiosis in the male flowers, whereas anther development was aberrant in the bisexual flowers. Morphologically bisexual flowers of X. sorbifolium do not have male function and are functionally female. Comparison of floral developmental characters did not support the separation of the Chinese monotypic genus Xanthoceras from the family Sapindaceae.

Pollenkitt of some monocotyledons: lipid composition and implications for pollen germination.

The composition of pollenkitt and its role in the progamic phase of reproduction are poorly understood. With the aim of extending knowledge on these topics, we chose to study two monocotyledons rich in pollenkitt, with bi-celled and long-lived pollen and dry-type stigma: Crocus vernus Hill subsp. vernus and Narcissus poeticus L. Fatty acids of pollenkitt were assayed with gas chromatography. Germination tests were performed invivo by pollinating the stigmas with a beard hair under a stereomicroscope, and invitro in liquid culture medium using pollen, either treated or not, with carbon disulphide to remove pollenkitt. The pollen tube percentages were evaluated using fluorescence microscopy techniques. Scanning electron microscopy was used to examine pollen and to follow the early post-pollination stages. Pollenkitt forms bridges between pollen grains but not between grains and stigma papillae. It consists of a mixture of 25 fatty acids, most with long and unsaturated chains, among which are some omega acids. The same acids with different percentages persist on the peritapetal membrane. After its removal, the pollen loses adhesiveness and dries quickly, but retains full capacity for germination on the papillae and can even trigger germination in contiguous pollen grains that do not touch the papillae. The results, while confirming the key role of pollenkitt in protecting pollen and favouring pollination, suggest secondary roles in the progamic phase, and highlight the interactive ability of the pollen regardless of lipid shell. The predominance of fatty acids with 18:3 and 16:0, as already noted in Brassica napus pollenkitt, suggests their hierarchy independent of plant species.

Identification of a role for an E6-like 1 gene in early pollen-stigma interactions in Arabidopsis thaliana.

We describe a function for a novel Arabidopsis gene, E6-like 1 (E6L1), that was identified as a highly expressed gene in the stigma and plays a role in early post-pollination stages. In Arabidopsis, successful pollen-stigma interactions are dependent on rapid recognition of compatible pollen by the stigmatic papillae located on the surface of the pistil and the subsequent regulation of pollen hydration and germination, and followed by the growth of pollen tubes through the stigma surface. Here we have described the function of a novel gene, E6-like 1 (E6L1), that was identified through the analysis of transcriptome datasets, as one of highest expressed genes in the stigma, and furthermore, its expression was largely restricted to the stigma and trichomes. The first E6 gene was initially identified as a highly expressed gene during cotton fiber development, and related E6-like predicted proteins are found throughout the Angiosperms. To date, no orthologous genes have been assigned a biological function. Both the Arabidopsis E6L1 and cotton E6 proteins are predicted to be secreted, and this was confirmed using an E6L1:RFP fusion construct. To further investigate E6L1's function, one T-DNA and two independent CRISPR-generated mutants were analyzed for compatible pollen-stigma interactions, and pollen hydration, pollen adhesion, and seed set were mildly impaired for the e6l1 mutants. This work identifies E6L1 as a novel stigmatic factor that plays a role during the early post-pollination stages in Arabidopsis.

Anther and gynoecium structure and development of male and female gametophytes of Koelreuteria elegans subsp. formosana (Sapindaceae): Phylogenetic implications

Anther and gynoecium structure and embryological information in Koelreuteria and Sapindaceae as a whole remain understudied, as well as the evolution of imperfect flowers in the latter. The aims of this study were to analys in K. elegans subsp. formosana the anther and gynoecium structure and the development of male and female gametophytes in the two floral morphs of putatively imperfect flowers. Standard techniques were applied for LM and SEM. Compared to the normal anther development in staminate flowers, a delayed programmed cell death of tapetum, septum and middle layers on the onset of microspore stage result in indehiscent anthers in the functionally pistillate flowers. Orbicules are reported for the first time in Sapindaceae. Gynoecium development in functionally pistillate flowers is normal, whereas in functionally staminate ones a pistillode with degenerated ovules at the tetrad stage is formed. The pollen tube transmitting tract consists of one layer of epithelial cells with a small lumen in the style and ovary. The anatomy of the latter revealed an axile placentation and complete septum. Reproductive characters of Koelreuteria shared with other Sapindaceae taxa are a secretory binucleate tapetum, simultaneous cytokinesis, unicellular stigmatic papillae, Polygonum type megagametophyte with ephemeral antipodals, and ovules campylotropous, with funicular obturator and amphistomic micropyle. Imperfect flowers in the family are triggered by defective sporophytic tissues impacting on the normal development of microspores and megaspores, disrupting embryological events in staminate and pistillate organs at different developmental stages. Reconstructing ancestral character state under parsimony revealed that two ovules per locule, absence of obturator, presence of hypostase, and binucleate tapetum may be regarded as plesiomorphic for Sapindaceae. These analyses have provided information on the evolution of embryological characters in the family and its phylogenetic significance.

Stigma Functionality and Fertility Are Reduced by Heat and Drought Co-stress in Wheat.

As a consequence of climate change, unpredictable extremely hot and dry periods are becoming more frequent during the early stages of reproductive development in wheat (Triticum aestivum L.). Pollen sterility has long been known as a major determinant of fertility loss under high temperature and water scarcity, but it will be demonstrated here that this is not the exclusive cause and that damage to female reproductive organs also contributes to losses of fertility and production. Changes in the phenology, morphology, and anatomy of female reproductive cells and organs, in the ROS and RNS generation of stigmatic papilla cells, and in fertility and yield components in response to simultaneous high temperature and drought at gametogenesis were studied in two wheat genotypes with contrasting stress responses. The combination of high temperature (32/24°C) and total water withdrawal for 5 days at gametogenesis altered the phenology of the plants, reduced pollen viability, modified the morphology and the anatomy of the pistils, enhanced the generation of ROS and RNS, intensified lipid peroxidation and decreased the NO production of stigmatic papilla cells, all leading to reduced fertility and to production loss in the sensitive genotype, depending on the position of the floret on the spike. Reduced functionality of female and male reproductive parts accounted for 34% and 66%, respectively, of the total generative cell- and organ-triggered fertility loss.

Structure of floral nectaries and comparison of reproductive and vestigial organs in the staminate and pistillate flowers of dioeciousSilene latifolia(Caryophyllaceae)

Silene latifolia Poiret of Eurasia has established in North America, prompting this structural study of its mature unisexual buds and flowers. Floral nectaries, anther and stigma changes, and vestigial reproductive structures were studied using light and scanning electron microscopy. In staminate flowers, anthers dehisced before anthesis and >90% of their pollen was liberated within 36 h. Accumulated in the tubular calyx, nectar descended an anthophore from the stomatal-bearing nectary at the stamen bases. Nectary tissue surrounded the pistillode, a central filamentous organ lacking ovules but tipped by hairs resembling stigmatic papillae. In pistillate flowers, nectar flowed into an inflated calyx. The annular nectary had 10 regularly spaced, stomatal-lined craters and was continuous with the adaxial surfaces of the infertile antisepalous and epipetalous staminodes. Key elements of entomophilous pollination leading to successful sexual reproduction of this invasive species include secondary nectar presentation from disparate floral nectaries that, for pistillate flowers, also incorporate the staminodes; rapid pollen release from anthers; and elongation of papillae by tip growth that enhances each stigma’s receptive surface. Context is also provided for future studies of floral nectary development in this model dioecious species.

Abscisic acid-mediated developmental flexibility of stigmatic papillae in response to ambient humidity in Arabidopsis thaliana.

Stigmatic papillae develop at the apex of the gynoecium and play an important role as a site of pollination. The papillae in Brassicaceae are of the dry and unicellular type, and more than 15,000 genes are expressed in the papillae; however, the molecular and physiological mechanisms of their development remain unknown. We found that the papillae in Arabidopsis thaliana change their length in response to altered ambient humidity: papillae of flowers incubated under high humidity elongated more than those under normal humidity conditions. Genetic analysis and transcriptome data suggest that an abscisic acid-mediated abiotic stress response mechanism regulates papilla length. Our data suggest a flexible regulation of papilla elongation at the post-anthesis stage, in response to abiotic stress, as an adaptation to environmental conditions.

Progress on deciphering the molecular aspects of cell-to-cell communication in Brassica self-incompatibility response.

The sporophytic system of self-incompatibility is a widespread genetic phenomenon in plant species, promoting out-breeding and maintaining genetic diversity. This phenomenon is of commercial importance in hybrid breeding of Brassicaceae crops and is controlled by single S locus with multiple S haplotypes. The molecular genetic studies of Brassica 'S' locus has revealed the presence of three tightly linked loci viz. S-receptor kinase (SRK), S-locus cysteine-rich protein/S-locus protein 11 (SCR/SP11), and S-locus glycoprotein (SLG). On self-pollination, the allele-specific ligand-receptor interaction activates signal transduction in stigma papilla cells and leads to rejection of pollen tube on stigmatic surface. In addition, arm-repeat-containing protein 1 (ARC1), M-locus protein kinase (MLPK), kinase-associated protein phosphatase (KAPP), exocyst complex subunit (Exo70A1) etc. has been identified in Brassica crops and plays a key role in self-incompatibility signaling pathway. Furthermore, the cytoplasmic calcium (Ca2+) influx in papilla cells also mediates self-incompatibility response in Brassicaceae, but how this cytoplasmic Ca2+ influx triggers signal transduction to inhibit pollen hydration is still obscure. There are many other signaling components which are not well characterized yet. Much progress has been made in elucidating the downstream multiple pathways of Brassica self-incompatibility response. Hence, in this review, we have made an effort to describe the recent advances made on understanding the molecular aspects of genetic mechanism of self-incompatibility in Brassicaceae.

Temperature effects on pistil viability and fruit set in sweet cherry

Abstract Field observations suggest high air temperatures during bloom decrease fruit set of many sweet cherry (Prunus avium L.) cultivars. We investigated the role of temperature on stigma receptivity and ovule viability in four sweet cherry cultivars that exhibit variability in productivity and fertility: ‘Sweetheart’ (self-fertile, high productivity), ‘Benton’ (self-fertile, low productivity), ‘Rainier’ (self-sterile, high productivity), and ‘Tieton’ (self-sterile, low productivity). The development of the stigmatic surface, and pollen hydration, germination and tube growth in vivo, were evaluated by hand pollination and used as proxies for stigma receptivity. In addition, the senescence of primary and secondary ovules was analyzed with fluorescence microscopy. The stigmatic papillae began deteriorating by the second day after anthesis and had collapsed by the sixth day post-anthesis across cultivars. Pollen hydration exhibited 5 stages related to the length-width ratios of 2.5:1, 2:1, 1.75:1, 1.5:1 to 1:1. Maximum pollen hydration and pollen germination occurred on the stigmas within 2 days of bloom, depending on cultivar and air temperature. Pollen hydration and germination, and ovule senescence accelerated under warmer temperatures. At 7 days post-anthesis and under 24 °C approximately 80% of ‘Rainier’ ovules were viable compared to 30% in the three other cultivars. Under moderate temperature (18 °C) which mimicked the field average flowering temperature, ovule of the two high productivity cherry cultivars, ‘Rainier’ and ‘Sweetheart’, kept full viabilities for 3–4 days post-anthesis while only 1 day for the two low productivity cultivars ‘Benton’ and ‘Tieton’. These results reveal that low commercial productivity of sweet cherry cultivars in the Pacific Northwest (PNW) of U.S. is likely due to rapid ovule senescence, a condition exacerbated in warm conditions.

The Molecular and Cellular Regulation of Brassicaceae Self-Incompatibility and Self-Pollen Rejection.

In flowering plants, sexual reproduction is actively regulated by cell-cell communication between the male pollen and female pistil, and many species possess self-incompatibilitysystems for the selective rejection of self-pollen to maintain genetic diversity. The Brassicaceae self-incompatibility pathway acts early on when pollen grains have landed on the stigmatic papillae at the top of the pistil. Extensive studies have revealed that self-pollen rejection in the Brassicaceae is initiated by an S-haplotype-specific interaction between two polymorphic proteins: the pollen S-locus protein 11/S cysteine-rich (SP11/SCR) ligand and the stigma S receptor kinase (SRK). While the different S-haplotypes are typically codominant, there are several examples of dominant-recessive interactions, and a small RNA-based regulation of SP11/SCR expression has been uncovered as a mechanism behind these genetic interactions. Recent research has also added to our understanding of various cellular components in the pathway leading from the SP11/SCR-SRK interaction, including two signaling proteins, the M-locus protein kinase (MLPK) and the ARM-repeat containing 1 (ARC1) E3 ligase, as well as calcium fluxes and induction of autophagy in the stigmatic papillae. Finally, a better understanding of the compatible pollen responses that are targeted by the self-incompatibility pathway is starting to emerge, and this will allow us to more fully understand how the Brassicaceae self-incompatibility pathway causes self-pollen rejection. Here, we provide an overview of the field, highlighting recent contributions to our understanding of Brassicaceae self-incompatibility, and draw comparisons to a recently discovered unilateral incompatibility system.

Structure of the stigma and style of Callaeum psilophyllum (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.

Morphological diversity and function of the stigma in Ficus species (Moraceae)

The stigma plays several roles such as pollen hydration and selection, and pollen tube nutrition. In the Ficus-fig wasp mutualism, stigmata have an additional, almost unknown, function by representing a physical interface for both plant and wasp reproduction. We used light and electron microscopy to compare the detailed morphology of the stigmata of nine Ficus species of different sections and with different pollination modes and sexual expressions. Figs were collected at the stage when the stigmata were receptive for pollination. Stigmata in actively pollinated monoecious species have well developed papillae concentrated on the adaxial surface exposed towards the fig cavity. Conversely, the passively pollinated monoecious species have the whole surface of the stigmata covered by somewhat smaller papillae. In both actively and passively pollinated monoecious species these features are consistent, irrespective of style length. In all actively pollinated gynodioecious species, the stigmata of pistillate flowers were tubular or infundibuliform whereas in almost all actively pollinated monoecious species (except F. racemosa) the stigmata were filiform, with one branch or two asymmetric branches. In gynodioecious species the short-styled flowers in “male” figs show a limited receptive surface with small papillae, while the stigmata of long-styled flowers in “female” figs are covered by papillae that extend down the sides of the style, increasing the stigmatic surface. In actively pollinated species, stigmata are cohesive, forming a common surface for pollen tube germination (= synstigma). The synstigma arrangement was quite variable: lax, cohesive or very cohesive, with entanglement by stigmatic papillae and stylar trichomes. Entanglement by stylar trichomes is common in gynodioecious species. The synstigma arrangement did not correlate with phylogeny or breeding system. This study is the first to report a very loose synstigma in actively pollinated monoecious Ficus species. Our analyses revealed that, in Ficus, the synstigma is functionally analogous to an extra-gynoecial compitum. Comparative studies will be required to test further hypotheses about the evolutionary determinants of such variation.

Floral Biology and Embryological Studies are Important for Conservation of Threatened Plants Having Reproductive Bottlenecks:A Case Study of <i>Illicium griffithii</i> Hook. f. & Thomson

Information on reproductive biology of threatened plant species could be useful for conservation, particularly when the species fails to perpetuate in nature due to regeneration failure. In flowering plants, the domain of reproductive biology includes structural details of reproductive units such as flower or inflorescence, formation of viable gametes, pollination dynamics, role of pollinators, pollen–pistil interactions as determined through compatibility, breeding system and mating strategies, fertilization and embryogeny, seed development, dispersal and germination. The importance of reproductive biology in species conservation has been demonstrated through a case study of Illicium griffithii Hook f. & Thomson, a threatened plant species from Arunachal Pradesh. I. griffithii (Illiciaceae) is a member of the ANITA clade and a representative taxon of the three most basal angiosperms. The flowers of I. griffithii are obligate xenogamous, and hence compatible pollen grains only germinate on the stigmatic papillae. Esterase and phosphatase enzymatic activities are absent in most stigmatic surfaces. Therefore, stigmatic receptivity is absent in many carpels. The ovules are anatropous and bitegmic with a four-celled/four-nucleate structure of embryo sac at maturity. Antipodals and filiform apparatus are absent. Presence of mucilage cells in the embryo sac facilitates the entry of pollen tubes into it. Embryolike organization of the endosperm is present, with one pole globular having smaller cells and the other pole roughly filamentous with larger cells resembling that of a suspensor. Extragynoecial compitum is not observed in I. griffithii . The flowers are brooding sites for the midges and the young ovules are eaten by larvae of the midges. Only 10% of the flowers mature into 13-seeded fruits and the carpels of the remaining 90% of the flowers have seeds ranging from 1 to 5. The seedling survival rate is only 7%. In addition to the above-mentioned reproductive bottlenecks, the species is also being over-harvested. It is now categorized as endangered. The findings of the present study on reproductive biology of I. griffithii should help in improving its conservation status.

Self-incompatibility: a pollination control mechanism in plants

Mode of pollination is very important in plant breeding because it determines the genetic constitution, nature of gene action, ease in pollination control and stability of varieties after release. There are several mechanisms that promote cross pollination, among these self-incompatibility (SI) is of special significance as it is used in hybrid seed production. SI is defined as the prevention of fusion of fertile (functional) male and female gametes of the same plant (Gowers, 1989). SI is a system where self-recognition and rejection is the rule that prevents inbreeding depression. Bateman (1952) classified self-incompatibility based on the interaction between pollen grains and pistil as complementary and oppositional system. Lewis (1954) has classified SI into homomorphic and heteromorphic systems. Homomorphic SI is again subdivided into gametophytic (determined by the genotype of gametes) and sporophytic (determined by the genotype of the plant) systems. Molecular studies after 1980’s revealed that at least two genes within S-locus control the SI, among these one unit function as male and the other as female determinant. In Brassicaceae family, the determinant gene encodes a pollen ligand and its stigmatic receptor kinase and their interaction induces incompatible signaling within the stigma papilla cells. In the Solanaceae, Rosaceae, and Scrophulariaceae, the female determinant is ribonuclease and F-box protein, suggesting the involvement of RNA degradation and protein degradation within the system. In the Papaveraceae, the female determinant induces Ca 2+ dependent signaling network that ultimately results in the death of incompatible pollen (Takayama and Isogai, 2005). Genes controlling the SI is multiallelic in nature and number of alleles varies depending upon the crop. Number of alleles reported are five in Theobroma cacao (Knight and Rogers, 1953), 30 in Brassica campestris (Singh, 2012), 32 alleles in Raphanussativus (Karron et al ., 1989). SI is commercially exploited for the production hybrid seeds. Pusa Hybrid-2, Snow Queen and Snow King hybrids of cauliflower, BRH-5, H-44 of cabbage and CCRP8 to CCRP15 (Minimol et al ., 2015a) of cocoa are some of the examples. Kucera et al . (2006) has compared the quality between SI and male sterility hybrids in cauliflower and it was found that SI hybrids are superior in their performance. Minimol et al . (2015b) emphasized the importance of polyclonal garden in cocoa for production of F 1 hybrid seeds by utilizing the self-incompatibility. Rego and Rego (2013) evaluated the efficiency of three methods of overcoming self-incompatibility in passion fruit and found fruit set of 16.67 and 10 per cent in bud and double pollination, respectively. The main limitations in exploiting SI is the maintenance of inbreds, however, it can be overcome by some temporary methods such as bud pollination, salt sprays and irradiation methods.

Estimating pollinator performance of visitors to the self-incompatible crop-plant <i>Brassica rapa</i> by single visit deposition and pollen germination: a comparison of methods.

Estimating the pollen-deposition effectiveness of flower visitors is fundamental to understanding their performance as pollinators. While estimates of visitation rates, pollen loads, and single visit deposition (SVD) are all useful proxies for performance, and so help to reveal the relative effectiveness of different visitors, none take into account the breeding system of the plants, or the quality of pollen deposited. Here we compare the performance of visitors to the self-incompatible plant Brassica rapa (turnip) using SVD and pollen germination. We also report the first use of the staining of Brassica rapa stigma papilla cells (known to reveal a specific reaction to self-pollen) to compare self-pollen deposition between insect visitors. We found that most of the pollen grains deposited by insect visitors (and therefore counted by SVD methods) were non-germinating self-pollen. A smaller proportion of grains were outcrossed and so germinated. There was also a significant positive relationship between environmental conditions (wind speed) and pollen deposition, but not pollen germination.Both methods identified Bombus spp. as the best-performing visitors on turnip flowers, followed by Eristalis spp., whereas performance estimates for Episyrphus balteatus and ‘other hoverflies’ were no higher than controls for both methods. This study provides further insight into the methodology for estimating pollinator performance, especially in plants when only cross-pollen can germinate.

Exocyst, exosomes, and autophagy in the regulation of Brassicaceae pollen-stigma interactions.

Brassicaceae pollen-stigma interactions have been extensively studied in Brassica and Arabidopsis species to identify cellular events triggered in the stigmatic papillae by pollen contact. Compatible pollinations are linked to the activation of basal cellular responses in the stigmatic papillae, which include calcium gradients, actin networks, and polarized secretion. The occurrence of these cellular events in stigmatic papillae coincides with the stages of pollen hydration and pollen tube entry into the stigmatic papillar cell wall. However, the form of the vesicle trafficking appears to differ between species, with vesicle-like structures detected in Arabidopsis species while exosomes were found to be secreted in Brassica species. Around the same timeframe, self-incompatible pollen recognition leads altered cellular responses in the stigmatic papillae to interfere with basal compatible pollen responses and disrupt regulated secretion, causing self-pollen rejection. Here, the literature on the changing cellular dynamics in the stigmatic papillae following pollination is reviewed and discussed in the context of other well-characterized examples of polarized secretion in plants.

SEX EXPRESSION, BREEDING SYSTEM AND POLLINATORS OF Piper caldense (PIPERACEAE) IN THE BRAZILIAN ATLANTIC FOREST

Las especies neotropicales de Piper presentan flores bisexuales, condición considerada basal al interior de este género pantropical. Sin embargo, fueron observadas especies neotropicales con flores unisexuales (estaminadas), además de las bisexuales. La dicogamia es común en el género y se encuentra asociada a la autocompatibilidad o incompatibilidad, al igual que la entomofilia. Fue analizada una población natural de Piper caldense en un área de bosque Atlántico (municipio de Viçosa, Minas Gerais, sureste de Brasil). Las observaciones preliminares indicaron que esta especie produce dos tipos florales, para lo cual fue analizada la sexualidad de las flores en 50 espigas a través de estudios morfológicos, anatómicos y análisis con microscopia electrónica de barrido. La longevidad y dinámica de exposición de las papilas estigmáticas y la secuencia de liberación de los granos de polen en los cuatro estambres fueron observados para verificar el grado de dicogamia. El sistema reproductivo se evaluó mediante el test de autopolinización espontánea y polinización abierta. Los visitantes florales fueron identificados y se registró su frecuencia de visita. Piper caldense presenta flores bisexuales y estaminadas, cada tipo floral se encuentra en espigas separadas. Estas espigas se encuentran en la misma planta, lo que indica que es andromonoica; adicionalmente, fueron observadas plantas que solo presentaban espigas con flores estaminadas, lo que se constituye en una combinación sexual inédita para el género. La exposición gradual y secuencial de las papilas estigmáticas asociada con la liberación asincrónica de los granos de polen (un estambre por día) indica una protoginia incompleta. Los test de polinización mostraron que la especie es autoincompatible. Las abejas sociales, principalmente Apis mellifera y Melipona spp., fueron los polinizadores principales. Este estudio refuerza la necesidad de asociar los analisis morfológicos con la funcionalidad de las estructuras florales, además indica los cambios futuros en estudios que abordan características reproductivas asociadas a la filogenia de las especies de Piper.

The boron transporter BnaC4.BOR1;1c is critical for inflorescence development and fertility under boron limitation in Brassica napus.

Boron (B) is an essential micronutrient for plants, but the molecular mechanisms underlying the uptake and distribution of B in allotetraploid rapeseed (Brassica napus) are unclear. Here, we identified a B transporter of rapeseed, BnaC4.BOR1;1c, which is expressed in shoot nodes and involved in distributing B to the reproductive organs. Transgenic Arabidopsis plants containing a BnaC4.BOR1;1c promoter-driven GUS reporter gene showed strong GUS activity in roots, nodal regions of the shoots and immature floral buds. Overexpressing BnaC4.BOR1;1c in Arabidopsis wild type or in bor1-1 mutants promoted wild-type growth and rescued the bor1-1 mutant phenotype. Conversely, knockdown of BnaC4.BOR1;1c in a B-efficient rapeseed line reduced B accumulation in flower organs, eventually resulting in severe sterility and seed yield loss. BnaC4.BOR1;1c RNAi plants exhibited large amounts of disintegrated stigma papilla cells with thickened cell walls accompanied by abnormal proliferation of lignification under low-B conditions, indicating that the sterility may be a result of altered cell wall properties in flower organs. Taken together, our results demonstrate that BnaC4.BOR1;1c is a AtBOR1-homologous B transporter gene expressing in both roots and shoot nodes that is essential for the developing inflorescence tissues, which highlights its diverse functions in allotetraploid rapeseed compared with diploid model plant Arabidopsis.

Inflorescence and floral development in Trochodendron aralioides (Trochodendraceae)

In the early development of Trochodendron aralioides (Trochodendraceae) inflorescences lateral flowers are initiated after the appearance of the floral pherophylls (subtending bracts). The terminal flower is preceded by metaxyphylls and is initiated earlier than the uppermost lateral flowers of the botryoid inflorescence. Small scales (interpreted as rudimentary perianth organs) precede the stamens. These scales are more distinct in the terminal flower than in the lateral flowers. In the radially symmetrical terminal flower, small scales (or metaxyphylls) and stamens are initiated in a spiral during early development. At anthesis, stamen phyllotaxis appears irregular or approximately whorled as a result of the rapid elongation and irregular slight curvature of the stamen filaments which distorts the originally regular pattern. Finally, the numerous carpels arise simultaneously in a single whorl. It takes about 9 months for flowers to develop and the 2-year reproductive cycle of T. aralioides is typical of many trees. The floral development of T. aralioides is compared with that of other basal eudicots. The bottle-shaped, unicellular stigmatic papillae and long, decurrent stigma of basally united carpels are similar to those of the Buxales¸ suggesting a close relationship.

Pollen Acceptance or Rejection: A Tale of Two Pathways.

While the molecular and cellular basis of self-incompatibility leading to self-pollen rejection in the Brassicaceae has been extensively studied, relatively little attention has been paid to compatible pollen recognition and the corresponding cellular responses in the stigmatic papillae. This is now changing because research has started to uncover steps in the Brassicaceae 'basal compatible pollen response pathway' in the stigma leading to pollen hydration and germination. Furthermore, recent studies suggest that self-incompatible pollen activates both the basal compatible pathway and the self-incompatibility pathway in the stigma, with the self-incompatibility response ultimately prevailing to reject self-pollen. We review here recent discoveries in both pathways and discuss how compatible pollen is accepted by the stigma versus the rejection of self-incompatible pollen.