Feasibility of a Unitary Quantum Dynamics in the Gowdy T3T^3 Cosmological Model

It has been pointed out that it is impossible to obtain a unitary implementation of the dynamics for the polarized Gowdy $T^{3}$ cosmologies in an otherwise satisfactory, nonperturbative canonical quantization proposed for these spacetimes. By introducing suitable techniques to deal with deparametrized models in cosmology that possess an explicit time dependence (as it is the case for the toroidal Gowdy model), we present in this paper a detailed analysis about the roots of this failure of unitarity. We investigate the impediments to a unitary implementation of the evolution by considering modifications to the dynamics. These modifications may be regarded as perturbations. We show in a precise manner why and where unitary implementability fails in our system, and prove that the obstructions are extremely sensitive to modifications in the Hamiltonian that dictates the time evolution of the symmetry-reduced model. We are able to characterize to a certain extent how far the model is from unitarity. Moreover, we demonstrate that the dynamics can actually be approximated as much as one wants by means of unitary transformations.Comment: 12 pages, version accepted for publication in Physical Review

Flavonols are not essential for fertilization in Arabidopsis thaliana

Flavonols are plant metabolites suggested to serve a vital role in fertilization of higher plants. Petunia and maize plants mutated in their flavonol biosynthesis are not able to set seed after self-pollination. We have investigated the role of these compounds in Arabidopsis thaliana. Like in all other plant species, high levels of flavonols could be detected in pollen of wild-type A. thaliana. No flavonols were detected in reproductive organs of the A. thaliana tt4 mutant in which the ohs gene is mutated. Surprisingly, this mutant did set seed after self-fertilization and no pollen tube growth aberrations were observed in vivo. The role of flavonols during fertilization of Arabidopsis is discussed

Hexose Transport in Growing Petunia Pollen Tubes and Characterization of a Pollen-Specific, Putative Monosaccharide Transporter

We investigated the molecular and physiological processes of sugar uptake and metabolism during pollen tube growth and plant fertilization. In vitro germination assays showed that petunia (Petunia hybrida) pollen can germinate and grow not only in medium containing sucrose (Suc) as a carbon source, but also in medium containing the monosaccharides glucose (Glc) or fructose (Fru). Furthermore, high-performance liquid chromatography analysis demonstrated a rapid and complete conversion of Suc into equimolar amounts of Glc and Fru when pollen was cultured in a medium containing 2% Suc. This indicates the presence of wall-bound invertase activity and uptake of sugars in the form of monosaccharides by the growing pollen tube. A cDNA designated pmt1 (petunia monosaccharide transporter 1), which is highly homologous to plant monosaccharide transporters, was isolated from petunia. Pmt1 belongs to a small gene family and is expressed specifically in the male gametophyte, but not in any other vegetative or floral tissues. Pmt1 is activated after the first pollen mitosis, and high levels of mRNA accumulate in mature and germinating pollen. A model describing the transport of sugars to the style, the conversion of Suc into Glc and Fru, and the active uptake by a monosaccharide transporter into the pollen tube is presented

Pollen tubes of flavonol-deficient Petunia show striking alterations in wall structure leading to tube disruption

Despite the vital role that flavonols play in fertilization and pollen tube growth of a number of species such as petunia and maize, their function is still unclear. Pollen tubes of the flavonol-deficient transformant T17.02 of Petunia hybrida L. are able to germinate and start growing in vitro, but eventually disrupt at the tip approximately 2 h after germination. In order to establish the possible role of flavonols in this process, wild-type and flavonol-deficient pollen tubes were subjected to cytological and ultrastructural analyses and screened for differences. The results showed that before disruption of the flavonol-deficient pollen tubes, the structure of the primary wall at the tip dramatically changed from layered to granular. Secretory vesicles at the tip still fused with the wall but lost their capacity to melt into the wall and to form layers. Instead they remained as dark, electron-dense granular structures surrounded by an electron-translucent matrix. Apparently the matrix is not able to sustain the wall's coherence and as a consequence the tube disrupts. No other remarkable cytological or ultrastructural differences between the transformant and the wild-type pollen tubes could be found before tip disruption. Even a morphometric analysis of abundance and distribution of endoplasmic reticulum, dictyosomes and mitochondria did not reveal any significant difference. However, for the first time, obvious morphological differences were observed in the wall of the flavonol-deficient pollen tubes. We conclude that flavonols act on precursors of the pollen tube wall of petunia and interfere with a cross-linking system in the wall, possibly via extensins

ODORANT1 Regulates Fragrance Biosynthesis in Petunia Flowers

Floral scent is important to plant reproduction because it attracts pollinators to the sexual organs. Therefore, volatile emission is usually tuned to the foraging activity of the pollinators. In Petunia hybrida, volatile benzenoids determine the floral aroma. Although the pathways for benzenoid biosynthesis have been characterized, the enzymes involved are less well understood. How production and emission are regulated is unknown. By targeted transcriptome analyses, we identified ODORANT1 (ODO1), a member of the R2R3-type MYB family, as a candidate for the regulation of volatile benzenoids in Petunia hybrida cv W115 (Mitchell) flowers. These flowers are only fragrant in the evening and at night. Transcript levels of ODO1 increased before the onset of volatile emission and decreased when volatile emission declined. Downregulation of ODO1 in transgenic P. hybrida Mitchell plants strongly reduced volatile benzenoid levels through decreased synthesis of precursors from the shikimate pathway. The transcript levels of several genes in this pathway were reduced by suppression of ODO1 expression. Moreover, ODO1 could activate the promoter of the 5-enol-pyruvylshikimate-3-phosphate synthase gene. Flower pigmentation, which is furnished from the same shikimate precursors, was not influenced because color and scent biosynthesis occur at different developmental stages. Our studies identify ODO1 as a key regulator of floral scent biosynthesis