Changes in RNA levels in the shoot apex of Silene during the transition to flowering

Changes in RNA concentration in the shoot apical meristem during induction and the transition to flowering were measured histochemically in Silene coeli-rosa (L.) Godron, a long-day plant. In the apices of plants induced by 7 long days the RNA concentration increased to about 25 per cent higher than in non-induced plants. Three long days did not induce flowering but resulted in a transient rise in RNA concentration. When plants were given long days interrupted by varying numbers of short days successful induction was accompanied by a sustained increase in RNA concentration but those treatments which were not inductive gave only transient increases in RNA. Gibberellic acid had no effect on induction or apical growth rates but increased the RNA concentration by 50 per cent or more in both induced and non-induced plants. Plants induced to flower at 13° C had the same RNA concentration and growth rate at the apex as in non-induced plants at 20° C. Since changes in RNA concentration in the apex could occur without changes in growth rate and without flowering, and induction could occur without a change in RNA concentration or growth rate, it is suggested that the increase in RNA and growth rate which normally occur at the transition to flowering might not be essential for the formation of a flower but may be more closely related to the rapid growth associated with the formation of the inflorescence.Abbreviations LD long day - SD short-day     

Vascular floral anatomy of the east asianHeloniopsis orientalis (Thunb.) C. Tanaka (Liliaceae-Helonieae)

Observations on the vascular floral anatomy, carpel morphology and floral biology ofHeloniopsis orientalis are presented. The lower flowering pedicel has six large bundles which lack an enclosing sclerenchymatous sheath. At mid-pedicel, branch bundles originate via radial divisions from each of these bundles. Subsequently, there is a vascular ring of 12 bundles below the receptacle. The six smaller bundles which are derived from alternate pedicel bundles eventually establish all of the ventral gynoecium supply. The six larger bundles supply the tepals, stamens and dorsal gynoecial vasculature. The simple dorsals do not branch or fuse in their vertical ascent. The ventral and placental supplies are far more complex. Fusion occurs between paired sets of the six smaller pedicel bundles along the septal radii and results in a submarginal laminal ventral network. An independent ventral plexus is formed in each septum and from each plexus two septal axials, of which the innermost has a reversed xylem-phloem disposition, and four placental bundles are derived. Two placental bundles are associated with each septal axial. Basally the septa are fused centrally, but are freed at mid-gymoecial height. The broadly tri-lobed, tri-carpellate gynoecium is depressed terminally where the erect, hollow style with its capitate stigma is attached. Dorsal grooves are present: the fruit is loculicidally dehiscent. There are no septal glands due to complete lateral fusion of the septal wings. Basally each of the six equal tepals has a saccate nectary. The similarity in vascular anatomy and carpel morphology of the AsianHeloniopsis and eastern North American endemic,Helonias bullata, justifies their position in the same tribe. Research and publication supported in part by the M. Graham Netting Research Fund through a grant from the Cordelia Scaife May Charitable Trust, the U. S.—Japan Cooperative Science Program Grant GF-41367, the Japan Society for the Promotion of Science, and Grant-in-Aid No. 934053 from the Ministry of Education, Japan.     

In vitro pollen embryogenesis in Nicotiana tabacum L. and its relation to pollen sterility,sex balance,and floral induction of the pollen donor plants

Pollen sterility, sex balance, and floral induction of the pollen donor plants were tested for a possible relation to embryogenesis from in vitro cultured tobacco pollen (Nicotiana tabacum L. var. Badischer Burley). The pollen grains destined to become embryos in culture (P-grains) were sterile for the donor plants as judged by their staining reaction with acetocarmine and fluorescin-diacetate, and by an in vitro germination test. They were produced in high frequency in flowers which exhibited a shift in sex balance towards femaleness. Sex balance could be measured by the relative length of pistil to stamens. High P-grain frequency, high pollen sterility, and a shift in sex balance towards femaleness could be induced by raising the donor plants under short days and/or low temperature (18–15° C) as compared to long days at 24° C. Short days and/or low temperature also reinforced floral induction, revealing that the tobacco variety Badischer Burley is a quantitative short day and low temperature plant and that the variety follows the rule that conditions of strong floral induction shift sex balance towards femaleness. At 12° C and short days, contabescent flowers were formed with completely sterile anthers containing a few and mostly collapsed P-grains. Based on these results, it is now possible to predict conditions by which haploids via pollen embryogenesis might be produced in high frequency from low-yielding and recalcitrant species.Abbreviations DPF dead pollen grain frequency - LD24 long days at 24° C - PD pollen dimorphism - P:S ratio of pistil to stamen length - SD15 short days at 15° C     

Involvement of indole-3-acetic acid in the circadian growth of the first internode of Arabidopsis

The extension rate of the first inflorescence node of Arabidopsis was measured during light/dark or continuous light exposure and was found to exhibit oscillations which showed a circadian rhythmicity. Decapitation induced a strong inhibition of stem extension. Subsequent application of IAA restored growth and the associated extension–rate oscillations. In addition, IAA treatments, after decapitation, re-established the circadian rhythmicity visible in the intact plants during free run. This indicates that the upper zone of the inflorescence has a major influence on the extension rate of floral stems and implies a role for auxin. Application of N-(1-naphthyl)phthalamic acid, an IAA transport inhibitor, to an intact floral stem inhibited growth and the rhythmicity in the extension rate oscillations, indicating that IAA polar transport may play a role in the dynamics of stem elongation. Furthermore, IAA-aspartate application, after decapitation, did not restore growth and rhythmicity. Nevertheless, biochemical analysis of IAA and IAA-aspartate demonstrated circadian fluctuations of the endogenous levels of both compounds. These observations suggest that IAA metabolism is an essential factor in the regulation of the circadian growth rhythm of Arabidopsis floral stems. Received: 21 September 1998 / Accepted: 23 January 1999     

The frequency of plasmodesmata increases early in the whole shoot apical meristem of Sinapis alba L. during floral transition

 The frequency of plasmodesmata increases in the shoot apical meristem of plants of Sinapis alba L. induced to flower by exposure to a single long day. This increase is observed within all cell layers (L1, L2, L3) as well as at the interfaces between these layers, and it occurs in both the central and peripheral zones of the shoot apical meristem. The extra plasmodesmata are formed only transiently, from 28 to 48 h after the start of the long day, and acropetally since they are detectable in L3 4 h before they are seen in L1 and L2. These observations indicate that (i) in the Sinapis shoot apical meristem at floral transition, there is an unfolding of a single field with increased plasmodesmatal connectivity, and (ii) this event is an early effect of the arrival at this meristem of the floral stimulus of leaf origin. Since (i) the wave of increased frequency of plasmodesmata is 12 h later than the wave of increased mitotic frequency (A. Jacqmard et al. 1998, Plant cell proliferation and its regulation in growth and development, pp. 67–78; Wiley), and (ii) the increase in frequency of plasmodesmata is observed in all cell walls, including in walls not deriving from recent divisions (periclinal walls separating the cell layers), it is concluded that the extra plasmodesmata seen at floral transition do not arise in the forming cell plate during mitosis and are thus of secondary origin. Received: 4 October 1999 / Accepted: 23 December 1999     

Preferences of pollinators and herbivores in gynodioecious Geranium sylvaticum

BACKGROUND AND AIMS: For the maintenance of gynodioecy (i.e. the coexistence of female and hermaphroditic plants), females need to compensate for the lack of pollen production through higher seed production or better progeny quality compared to hermaphrodites. In Geranium sylvaticum, females produce more seeds per flower than hermaphrodites. This difference in seed production might be modified by biological interactions with pollinators and herbivores that may favour one sex and thus affect the maintenance of gynodioecy. METHODS: Sexual dimorphism in flower size and flowering phenology, and in attractiveness to pollinators, pre-dispersal seed predators and floral herbivores were examined in natural populations of G. sylvaticum. KEY RESULTS: Pollinators preferred hermaphrodites 25 % more often than females in two of the three study populations, and floral herbivores attacked hermaphrodites 15 % more often than females in two of the six study populations. These preferences might be explained by the larger flower size of hermaphrodites. In contrast, seed predators did not prefer either sex. CONCLUSIONS: The data suggest that pollinator preference does not benefit females, whereas the higher floral herbivory of hermaphrodites might enhance the maintenance of females in G. sylvaticum. Thus, although the data support the view that ecological factors may contribute to the maintenance of gynodioecy, they also suggest that these contributions may vary across populations and that they may function in opposite directions.     

Effects of number of flowers per raceme and number of racemes per plant on bumblebee visits and female reproductive success in <Emphasis Type="Italic">Salvia nipponica</Emphasis> (Labiatae)

We studied the adaptive arrangement of raceme flowers in dense and sparse habitats of Salvia nipponica. We recorded bumblebee visits and collected mature seeds to estimate outcrossing rates from allozyme genotypes. To examine the resource investment in flowering stalks, we measured their length and dry mass. We found that a greater number of open flowers in a raceme enhanced bumblebee visits and successive probings in both density plots. However, greater height of a raceme did not enhance these in plots of either density. In the high-density plot, both a greater mean number of open flowers in a raceme and a greater number of flowering racemes on a plant enhanced bumblebee visits, successive probings, and also enhanced outcrossing rate in spite of an increase in successive probings. Although the number of flowers per raceme had a greater positive effect on seed-to-ovule ratios than did the number of racemes, the number of flowers per raceme may be constrained because a longer flowering stalk needs greater buckling strength. Hence, it may be advantageous for plants in high-density plots to increase both number of flowers per raceme and number of racemes. In the low-density plot, a greater number of flowering racemes on a plant enhanced pollinator visits, but a greater mean number of open flowers in a raceme did not. Hence, it may be advantageous for plants to increase the number of racemes while keeping the numbers of flowers per raceme small. Thus, the adaptive number of flowers per raceme and number of racemes per plant may differ between high- and low-density plots.     

Characterization of <Emphasis Type="Italic">SpAPETALA3</Emphasis> and <Emphasis Type="Italic">SpPISTILLATA</Emphasis>, B class floral identity genes in <Emphasis Type="Italic">Spinacia oleracea</Emphasis>, and their relationship to sexual dimorphism

Floral organ identity B class genes are generally recognized as being required for development of petals and stamens in angiosperm flowers. Spinach flowers are distinguished in their complete absence of petals in both sexes, and the absence of a developed stamen whorl in female flowers. As such, we hypothesized that differential expression of B class floral identity genes is integral to the sexual dimorphism in spinach flowers. We isolated two spinach orthologs of Arabidopsis B class genes by 3 and 5 RACE. Homology assignments were tested by comparisons of percent amino acid identities, searches for diagnostic consensus amino acid residues, conserved motifs, and phylogenetic groupings. In situ hybridization studies demonstrate that both spinach B class genes are expressed throughout the male floral meristem in early stages, and continue to be expressed in sepal primordia in reduced amounts at later stages of development. They are also highly expressed in the third whorl primordia when they arise and continue to be expressed in these tissues through the development of mature anthers. In contrast, neither gene can be detected in any stage in female flowers by in situ analyses, although northern blot experiments indicate low levels of SpAP3 within the inflorescence. The early, strong expressions of both B class floral identity genes in male floral primordia and their absence in female flowers demonstrate that B class gene expression precedes the origination of third whorl primordia (stamen) in males and is associated with the establishment of sexual floral dimorphism as it initiates in the first (sepal) whorl. These observations suggest that regulation of B class floral identity genes has a role in the development of sexual dimorphism and dioecy in spinach rather than being a secondary result of organ abortion.Electronic Supplementary Material Supplementary material is available for this article at Edited by G. Jürgens     

Relationships between species’ floral traits and pollinator visitation in a temperate grassland

Knowledge about plant–plant interactions for pollinator service at the plant community level is still scarce, although such interactions may be important to seed production and hence the population dynamics of individual plant species and the species compositions of communities. An important step towards a better understanding of pollination interactions at the community level is to assess if the variation in floral traits among plant species explain the variation in flower visitation frequency among those species. We investigated the relative importance of various floral traits for the visitation frequency of all insects, and bumblebees and flies separately, to plant species by measuring the visitation frequency to all insect-pollinated species in a community during an entire flowering season. Visitation frequency was identified to be strongly positive related to the visual display area and the date of peak flowering of plant species. Categorical variables, such as flower form and symmetry, were important to the visitation frequency of flies only. We constructed floral similarity measures based on the species’ floral traits and found that the floral similarity for all species’ traits combined and the continuous traits separately were positively related to individual visitation frequency. On the other hand, plant species with similar categorical floral traits did not have similar visitation frequencies. In conclusion, our results show that continuous traits, such as flower size and/or density, are more important for the variation in visitation frequency among plant species than thought earlier. Furthermore, differences in visitation frequency among pollinator groups give a poor support to the expectations derived from the classical pollination syndromes.     

Sequence evolution and sex-specific expression patterns of the C class floral identity gene, <Emphasis Type="Italic">SpAGAMOUS</Emphasis>, in dioecious <Emphasis Type="Italic">Spinacia oleracea</Emphasis> L

Development in dioecious cultivated spinach, Spinacia oleracea, is distinguished by the absence of alternative reproductive organ primordia in male and female flowers. Given the highly derived floral developmental program in spinach, we wished to characterize a spinach C class floral identity gene and to determine the patterns of sequence evolution as well as compare the spatial and temporal expression patterns with those of AGAMOUS. The isolated cDNA sequence clusters phylogenetically within the AGAMOUS/FARINELLI C class clade. In comparison with the SLM1 sequence from the related Silene latifolia, amino acid replacements are highly conservative and non-randomly distributed, being predominantly found in hinge regions or on exposed surfaces of helices. The spinach gene (SpAGAMOUS) appears to be exclusively expressed in reproductive tissues and not in vegetative organs. Initial expression of SpAGAMOUS is similar in male and female floral primordia. However, upon initiation of the first whorl organs, SpAGAMOUS becomes restricted to meristemic regions from which the reproductive primordia will develop. This results in an early gender-specific pattern. Thus, the spinach C class gene is differentially expressed prior to reproductive organ development and is, at least, correlated with, if not directly involved in, the sexual dimorphism in spinach.Electronic Supplementary Material Supplementary material is available for this article at