Floral ontogeny and gene protein localization rules out euanthial interpretation of reproductive units in Lepironia (Cyperaceae,Mapanioideae, Chrysitricheae)

Background and Aims

In the sedge subfamily Mapanioideae there are considerable discrepancies between the standard trimerous monocot floral architecture expected and the complex floral and inflorescence morphologies seen. Decades of debate about whether the basic reproductive units are single flowers or pseudanthia have not resolved the question. This paper evaluates current knowledge about Mapaniid reproductive structures and presents an ontogenetic study of the Mapaniid genus Lepironia with the first floral protein expression maps for the family, localizing the products of the APETALA1/FRUITFULL-like (AP1/FUL) MADS-box genes with the aim of shedding light on this conundrum.

Methods

A range of reproductive developmental stages, from spikelet primordia through to infructescence material, were processed for anatomical and immunohistochemical analyses.

Key Results

The basic reproductive unit is subtended by a bract and possesses two prophyll-like structures, the first organs to be initiated on the primordium, which grow rapidly, enclosing two whorls of initiating leaf-like structures with intervening stamens and a central gynoecium, formed from an annular primordium. The subtending bract and prophyll-like structures possess very different morphologies from that of the internal leaf-like structures and do not show AP1/FUL-like protein localization, which is otherwise strongly localized in the internal leaf-like structures, stamens and gynoecia.

Conclusions

Results support the synanthial hypothesis as the evolutionary origin of the reproductive unit. Thus, the basic reproductive unit in Lepironia is an extremely condensed pseudanthium, of staminate flowers surrounding a central terminal pistillate female flower. Early in development the reproductive unit becomes enclosed by a split-prophyll, with the whole structure subtended by a bract.     

Synorganisation without organ fusion in the flowers of Geranium robertianum (Geraniaceae) and its not so trivial obdiplostemony

Background and Aims

Synorganisation of floral organs, an important means in angiosperm flower evolution, is mostly realized by congenital or post-genital organ fusion. Intimate synorganisation of many floral organs without fusion, as present in Geranium robertianum, is poorly known and needs to be studied. Obdiplostemony, the seemingly reversed position of two stamen whorls, widely distributed in core eudicots, has been the subject of much attention, but there is confusion in the literature. Obdiplostemony occurs in Geranium and whether and how it is involved in this synorganisation is explored here.

Methods

Floral development and architecture were studied with light microscopy based on microtome section series and with scanning electron microscopy.

Key Results

Intimate synorganisation of floral organs is effected by the formation of five separate nectar canals for the proboscis of pollinators. Each nectar canal is formed by six adjacent organs from four organ whorls. In addition, the sepals are hooked together by the formation of longitudinal ribs and grooves, and provide a firm scaffold for the canals. Obdiplostemony provides a guide rail within each canal formed by the flanks of the antepetalous stamen filaments.

Conclusions

Intimate synorganisation in flowers can be realized without any fusion, and obdiplostemony may play a role in this synorganisation.     

Functional implications of the staminal lever mechanism in Salvia cyclostegia (Lamiaceae)

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.

Methods

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.

Key Results

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.

Conclusions

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.     

Functional aspects of floral nectar secretion of Ananas ananassoides, an ornithophilous bromeliad from the Brazilian savanna

Background and Aims

Several members of Bromeliaceae show adaptations for hummingbird pollination in the Neotropics; however, the relationships between floral structure, nectar production, pollination and pollinators are poorly understood. The main goal of this study was to analyse the functional aspects of nectar secretion related to interaction with pollinators by evaluating floral biology, cellular and sub-cellular anatomy of the septal nectary and nectar composition of Ananas ananassoides, including an experimental approach to nectar dynamics.

Methods

Observations on floral anthesis and visitors were conducted in a population of A. ananassoides in the Brazilian savanna. Nectary samples were processed using standard methods for light and transmission electron microscopy. The main metabolites in nectary tissue were detected via histochemistry. Sugar composition was analysed by high-performance liquid chromatography (HPLC). The accumulated nectar was determined from bagged flowers (‘unvisited’), and floral response to repeated nectar removal was evaluated in an experimental design simulating multiple visits by pollinators to the same flowers (‘visited’) over the course of anthesis.

Key Results

The hummingbirds Hylocharis chrysura and Thalurania glaucopis were the most frequent pollinators. The interlocular septal nectary, composed of three lenticular canals, extends from the ovary base to the style base. It consists of a secretory epithelium and nectary parenchyma rich in starch grains, which are hydrolysed during nectar secretion. The median volume of nectar in recently opened ‘unvisited’ flowers was 27·0 µL, with a mean (sucrose-dominated) sugar concentration of 30·5 %. Anthesis lasts approx. 11 h, and nectar secretion begins before sunrise. In ‘visited’ flowers (experimentally emptied every hour) the nectar total production per flower was significantly higher than in the ‘unvisited’ flowers (control) in terms of volume (t = 4·94, P = 0·0001) and mass of sugar (t = 2·95, P = 0·007), and the concentration was significantly lower (t = 8·04, P = 0·0001).

Conclusions

The data suggest that the total production of floral nectar in A. ananassoides is linked to the pollinators'' activity and that the rapid renewal of nectar is related to the nectary morphological features.     

Floral morphogenesis in Sinofranchetia (Lardizabalaceae) and its systematic significance

The floral organs of Sinofranchetia chinensis Hemsl. (Lardizabalaceae) are all spiral in initiation. Stamen and petal (nectar‐leaf) primordia initiate independently and are different in shape. The petals and three stamens in the first whorl are retarded in the early developmental stages. The carpel primordia are conduplicate; the stigma is formed around the upper part of the ventral slit and the style is not differentiated. The functionally unisexual flowers are bisexual in organization in the early developmental stages. The development of the flowers on the inflorescence is spiral and centripetal. Some floral characteristics of Sinofranchetia appear to be plesiomorphic in Lardizabalaceae. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 160 , 82–92.     

Correlation between number and position of floral organs in Arabidopsis

Background and Aims

The study of variation in number, position and type of floral organs may serve as a key to understanding the mechanisms underlying their variation, and will make it possible to improve the analysis of gene function in model plant species by means of a more accurate characterization of mutant phenotypes. The present analysis was carried out in order to understand the correlation between number and position of floral organs in Arabidopsis thaliana.

Methods

An analysis of number and position of organs in flowers of wild type as well as in a series of mutations with floral organ position alterations was carried out, using light and electron microscopy. Variation common to different genotypes was analysed by means of individual diagrams, upon which generalized diagrams depicting variation in number and position of organs, were built by superimposition.

Key Results and Conclusions

It is shown that in the Arabidopsis flower a correlation exists between positions of petals and sepals, as well as between positions of stamens and carpels, whereas the position of carpels does not seem to depend on number and position of petals and stamens. This suggests that the position of organs in the basal (sepals) and apical (carpels) parts of the flower are determined before that in the intermediate zone. This assumption is consistent with the results of mathematical modelling and is supposed to be the consequence of stem-cell activity in the flower.     

First record of bat-pollination in the species-rich genus Tillandsia (Bromeliaceae)

Background and Aims

Bromeliaceae is a species-rich neotropical plant family that uses a variety of pollinators, principally vertebrates. Tillandsia is the most diverse genus, and includes more than one-third of all bromeliad species. Within this genus, the majority of species rely on diurnal pollination by hummingbirds; however, the flowers of some Tillandsia species show some characteristics typical for pollination by nocturnal animals, particularly bats and moths. In this study an examination is made of the floral and reproductive biology of the epiphytic bromeliad Tillandsia macropetala in a fragment of humid montane forest in central Veracruz, Mexico.

Methods

The reproductive system of the species, duration of anthesis, production of nectar and floral scent, as well as diurnal and nocturnal floral visitors and their effectiveness in pollination were determined.

Key Results

Tillandsia macropetala is a self-compatible species that achieves a higher fruit production through outcrossing. Nectar production is restricted to the night, and only nocturnal visits result in the development of fruits. The most frequent visitor (75 % of visits) and the only pollinator of this bromeliad (in 96 % of visits) was the nectarivorous bat Anoura geoffroyi (Phyllostomidae: Glossophaginae).

Conclusions

This is the first report of chiropterophily within the genus Tillandsia. The results on the pollination biology of this bromeliad suggest an ongoing evolutionary switch from pollination by birds or moths to bats.     

Floral traits mediate the vulnerability of aloes to pollen theft and inefficient pollination by bees

Background and Aims

Pollen-collecting bees are among the most important pollinators globally, but are also the most common pollen thieves and can significantly reduce plant reproduction. The pollination efficiency of pollen collectors depends on the frequency of their visits to female(-phase) flowers, contact with stigmas and deposition of pollen of sufficient quantity and quality to fertilize ovules. Here we investigate the relative importance of these components, and the hypothesis that floral and inflorescence characteristics mediate the pollination role of pollen collection by bees.

Methods

For ten Aloe species that differ extensively in floral and inflorescence traits, we experimentally excluded potential bird pollinators to quantify the contributions of insect visitors to pollen removal, pollen deposition and seed production. We measured corolla width and depth to determine nectar accessibility, and the phenology of anther dehiscence and stigma receptivity to quantify herkogamy and dichogamy. Further, we compiled all published bird-exclusion studies of aloes, and compared insect pollination success with floral morphology.

Key Results

Species varied from exclusively insect pollinated, to exclusively bird pollinated but subject to extensive pollen theft by insects. Nectar inaccessibility and strong dichogamy inhibited pollination by pollen-collecting bees by discouraging visits to female-phase (i.e. pollenless) flowers. For species with large inflorescences of pollen-rich flowers, pollen collectors successfully deposited pollen, but of such low quality (probably self-pollen) that they made almost no contribution to seed set. Indeed, considering all published bird-exclusion studies (17 species in total), insect pollination efficiency varied significantly with floral shape.

Conclusions

Species-specific floral and inflorescence characteristics, especially nectar accessibility and dichogamy, control the efficiency of pollen-collecting bees as pollinators of aloes.     

The evolution of floral nectaries in Disa (Orchidaceae: Disinae): recapitulation or diversifying innovation?

Background and Aims

The Orchidaceae have a history of recurring convergent evolution in floral function as nectar production has evolved repeatedly from an ancestral nectarless state. However, orchids exhibit considerable diversity in nectary type, position and morphology, indicating that this convergence arose from alternative adaptive solutions. Using the genus Disa, this study asks whether repeated evolution of floral nectaries involved recapitulation of the same nectary type or diversifying innovation. Epidermis morphology of closely related nectar-producing and nectarless species is also compared in order to identify histological changes that accompanied the gain or loss of nectar production.

Methods

The micromorphology of nectaries and positionally equivalent tissues in nectarless species was examined with light and scanning electron microscopy. This information was subjected to phylogenetic analyses to reconstruct nectary evolution and compare characteristics of nectar-producing and nectarless species.

Key Results

Two nectary types evolved in Disa. Nectar exudation by modified stomata in floral spurs evolved twice, whereas exudation by a secretory epidermis evolved six times in different perianth segments. The spur epidermis of nectarless species exhibited considerable micromorphological variation, including strongly textured surfaces and non-secreting stomata in some species. Epidermis morphology of nectar-producing species did not differ consistently from that of rewardless species at the magnifications used in this study, suggesting that transitions from rewardlessness to nectar production are not necessarily accompanied by visible morphological changes but only require sub-cellular modification.

Conclusions

Independent nectary evolution in Disa involved both repeated recapitulation of secretory epidermis, which is present in the sister genus Brownleea, and innovation of stomatal nectaries. These contrasting nectary types and positional diversity within types imply weak genetic, developmental or physiological constraints in ancestral, nectarless Disa. Such functional convergence generated by morphologically diverse solutions probably also underlies the extensive diversity of nectary types and positions in the Orchidaceae.     

Xylan-degrading enzymes in male and female flower nectar of Cucurbita pepo

Background and Aims

Nectar is a very complex mixture of substances. Some components (sugars and amino acids) are considered primary alimentary rewards for animals and have been investigated and characterized in numerous species for many years. In contrast, nectar proteins have been the subject of few studies and little is known of their function. Only very recently have detailed studies and characterization of nectar proteins been undertaken, and then for only a very few species. This current work represents a first step in the identification of a protein profile for the floral nectar of Cucurbita pepo. In this regard, the species studied is of particular interest in that it is monoecious with unisexual flowers and, consequently, it is possible that nectar proteins derived from male and female flowers may differ.

Methods

Manually excised spots from two-dimensional (2-D) electrophoresis were subjected to in-gel protein digestion. The resulting peptides were sequenced using nanoscale LC–ESI/MS-MS (liquid chromatography–electrospray ionization/tandem mass spectrometry). An MS/MS ions search was carried out in Swiss-Prot and NCBInr databases using MASCOT software.

Key Results

Two-dimensional electrophoresis revealed a total of 24 spots and a different protein profile for male and female flower nectar. Four main proteins recognized by 2-D electrophoresis most closely resemble β-d-xylosidases from Arabidopsis thaliana and have some homology to a β-d-xylosidase from Medicago varia. They were present in similar quantities in male and female flowers and had the same molecular weight, but with slightly different isoelectric points.

Conclusions

A putative function for xylosidases in floral nectar of C. pepo is proposed, namely that they may be involved in degrading the oligosaccharides released by the nectary cell walls in response to hydrolytic enzymes produced by invading micro-organisms. Several types of oligosaccharides have been reported to increase the pathogenic potential of micro-organisms. Thus, it is possible that such a mechanism may reduce the virulence of pathogens present in nectar.