Chalcone Synthase and Flavonol Accumulation in Stigmas and Anthers of Petunia hybrida

Molecular cloning and expression analysis of dihydroflavonol 4-reductase gene in flower organs of Forsythia x intermedia

The expression, during flower development, of the gene encoding the anthocyanin pathway key enzyme dihydroflavonol 4-reductase (DFR) was investigated in floral organs of Forsythia x intermedia cv. 'Spring Glory'. Full-length DFR and partial chalcone synthase (CHS) cDNAs, the gene of interest and a flavonoid pathway control gene respectively, were obtained from petal RNA by reverse transcription PCR. Whereas for CHS northern blot analysis enabled the study of its expression pattern, competitive PCR assays were necessary to quantify DFR mRNA levels in wild-type plants and in petals of 2 transgenic clones containing a CaMV 35S promoter-driven DFR gene of Antirrhinum majus. Results indicated a peak of CHS and DFR transcript levels in petals at the very early stages of anthesis, and different expression patterns in anthers and sepals. In comparison to wild-type plants, transformants showed a more intense anthocyanin pigmentation of some vegetative organs, and a dramatic increase in DFR transcript concentration and enzymatic activity in petals. However, petals of transformed plants did not accumulate any anthocyanins. These results indicate that other genes and/or regulatory factors should be considered responsible for the lack of anthocyanin production in Forsythia petals.

Expression of chalcone synthase and chalcone isomerase proteins in Arabidopsis seedlings

Antibodies have been developed against the first two enzymes of flavonoid biosynthesis in Arabidopsis thaliana. Chalcone synthase (CHS) and chalcone isomerase (CHI) were overexpressed and purified from Escherichia coli as fusion proteins with glutathione S-transferase from Schistosoma japonicum. The recombinant proteins were then used to immunize chickens and the resulting IgY fraction was purified from egg yolks. Immunoblots of crude protein extracts from Arabidopsis seedlings carrying wild-type and null alleles for CHS and CHI showed that the resulting antibody preparations provide useful tools for characterizing expression of the flavonoid pathway at the protein level. An initial analysis of expression patterns in seedlings shows that CHS and CHI proteins are present at high levels during a brief period of early seedling germination that just precedes the transient accumulation of flavonoid end-products.

POLLEN GERMINATION AND TUBE GROWTH

Many aspects of Angiosperm pollen germination and tube growth are discussed including mechanisms of dehydration and rehydration, in vitro germination, pollen coat compounds, the dynamic involvement of cytoskeletal elements (actin, microtubules), calcium ion fluxes, extracellular matrix elements (stylar arabinogalactan proteins), and control mechanisms of gene expression in dehydrating and germinating pollen. We focus on the recent developments in pollen biology that help us understand how the male gamete survives and accomplishes its successful delivery to the ovule of the sperm to effect sexual reproduction.

Characterization of flavonol synthase and leucoanthocyanidin dioxygenase genes in Arabidopsis. Further evidence for differential regulation of "early" and "late" genes

As part of an ongoing investigation into the organization and regulation of the flavonoid biosynthetic pathway, two Arabidopsis thaliana expressed sequence tag (EST) clones (153O10T7 and YAY780) with high homology to leucoanthocyanidin dioxygenase (LDOX) or flavonol synthase (FLS) were identified. EST YAY780 was sequenced and found to encode a protein 49 to 78% identical to all LDOX sequences in the database. EST 153O10T7 was used to isolate a genomic clone encoding a protein with 59 to 61% sequence identity to petunia (Petunia hybrida) and potato (Solanum tuberosum) FLS. DNA blot analysis was used to screen the Arabidopsis genome for sequences related to FLS and LDOX and to determine the positions of the two clones on the RI map. The expression patterns of FLS and LDOX in etiolated seedlings moved to white light and in two putative regulatory mutants (ttg and tt8) were determined by RNA blot analysis. These studies indicate that FLS is an "early" flavonoid gene in Arabidopsis seedlings, whereas LDOX is a "late" gene. Furthermore, FLS is the first flavonoid enzyme identified in Arabidopsis that may be encoded by a gene family.

Structure and regulation of the maize Bronze2 promoter

The maize Bronze2 (Bz2) gene encodes a glutathione S-transferase that is required for anthocyanin pigment accumulation. Two classes of regulatory proteins, R and C1, are required for transcriptional activation of Bz2 and several additional structural genes. Functional domains of the Bz2 promoter were identified using Bz2 promoter-driven luciferase reporter genes electroporated into maize protoplasts together with R and C1 expression plasmids. Complete regulation was conferred by 224 nt of the Bz2 promoter. Within this region at least two separable regions are independently capable of conferring regulation by R and C1. Predicted regulatory elements corresponding to two classes of sequence motifs, the Myb-box homologous 'C1-motif', TAACTG/CAGTTA, and the G-box and E-box homologous 'R-motif', CACGTG, were shown to be important for full R and C1 activation of the Bz2 promoter. Expression of reconstructed Bz2 genes with mutated promoters was quantified using RNase protection, and this analysis confirmed results obtained using reporter genes.

Flavonol 3-O-glycosyltransferases associated with petunia pollen produce gametophyte-specific flavonol diglycosides

Wild-type petunia pollen accumulates high levels of flavonol 3-O-glycosides. Pollen from conditionally male-fertile petunia has no flavonols and is unable to germinate. Pollen function is restored both in vivo and in vitro by providing flavonol aglycones, but not flavonol glycosides, to the pollen. In the present study, incubation of an in vitro suspension of conditionally male-fertile pollen with kaempferol or quercetin resulted in the accumulation of kaempferol and quercetin 3-O-glycosides in the pollen. We identified two glycosyltransferase activities associated with the intact pollen grain that catalyze the formation of a gametophyte-specific class of flavonol glycosides. Feeding studies showed that product formation was highly specific for flavonols with an unsubstituted 3-hydroxyl group and was not dependent on an external source of UDP-hexose. Ultraviolet spectral analysis, fast atom bombardment mass spectrometry, 1H-nuclear magnetic resonance, and 13C-nuclear magnetic resonance identified the products as kaempferol and quercetin 3-O-(2"- O-beta-D-glucopyranosyl)-beta-D-galactopyranoside, identical with the flavonol 3-O-glycosides present in wild-type pollen. The sugars are linked in a 1-->2 configuration that results in a pollen-specific class of compounds. To retain both glycosyltransferase activities in a cell-free extract, it was necessary to add triton X-100, suggesting that one or both of the proteins may be associated with a pollen membrane. A model for flavonol glycoside biosynthesis and uptake into the pollen is discussed in terms of the germination requirement for flavonols.

Pollination- or Wound-Induced Kaempferol Accumulation in Petunia Stigmas Enhances Seed Production

Flavonols are essential for pollen germination and tube growth in petunia and can be supplied by either the pollen or stigma at pollination. HPLC analysis and a sensitive bioassay demonstrated that both pollination and wounding induce flavonol accumulation, especially kaempferol, in the outer cell layers and exudate of the stigma. Pollination and wounding induced nearly identical flavonol kinetics and patterns of accumulation in the same target tissue, suggesting that they share elements of a common signal transduction pathway. The wound response was systemic, because kaempferol accumulated in the stigma when distal tissues, such as the corolla, stamens, or sepals, were wounded. We have exploited the germination requirement for flavonols and the high level of kaempferol that accumulates after wounding to enhance plant fecundity. Seed set was significantly increased by mechanically wounding the corolla and stamens prior to the application of pollen to the stigma. A reproductive role for a plant secondary metabolite and the specific function of stigmatic kaempferol are discussed from an evolutionary perspective.

Pollination- or Wound-Induced Kaempferol Accumulation in Petunia Stigmas Enhances Seed Production

Flavonols are essential for pollen germination and tube growth in petunia and can be supplied by either the pollen or stigma at pollination. HPLC analysis and a sensitive bioassay demonstrated that both pollination and wounding induce flavonol accumulation, especially kaempferol, in the outer cell layers and exudate of the stigma. Pollination and wounding induced nearly identical flavonol kinetics and patterns of accumulation in the same target tissue, suggesting that they share elements of a common signal transduction pathway. The wound response was systemic, because kaempferol accumulated in the stigma when distal tissues, such as the corolla, stamens, or sepals, were wounded. We have exploited the germination requirement for flavonols and the high level of kaempferol that accumulates after wounding to enhance plant fecundity. Seed set was significantly increased by mechanically wounding the corolla and stamens prior to the application of pollen to the stigma. A reproductive role for a plant secondary metabolite and the specific function of stigmatic kaempferol are discussed from an evolutionary perspective.