Nectaries are secretory organs that are widely present in flowering plants that function to attract floral pollinators. Owing to diversity in nectary positions and structures, they are thought to have originated multiple times during angiosperm evolution, with their potential contribution to the diversification of flowering plants and pollinating animals being considerable. We investigated the genetic basis of diverse nectary forms in eudicot angiosperm species using CRABS CLAW (CRC), a gene required for nectaries in Arabidopsis. CRC expression is conserved in morphologically different nectaries from several core eudicot species and is required for nectary development in both rosids and asterids,two major phylogenetic lineages of eudicots. However, in a basal eudicot species, no evidence of CRC expression in nectaries was found. Considering the phylogenetic distribution of nectary positions and CRC expression analyses in eudicots, we propose that diverse nectaries in core eudicots share conserved CRC gene regulation, and that derived nectary positions in eudicots have altered regulation of CRC. As the ancestral function of CRC lies in the regulation of carpel development, it may have been co-opted as a regulator of nectary development within the eudicots, concomitant with the association of nectaries with reproductive organs in derived lineages.

Agrobacterium-mediated infection of petunia (Petunia hybrida) plants with tobacco rattle virus (TRV) bearing fragments of Petunia genes resulted in systemic infection and virus-induced gene silencing (VIGS) of the homologous host genes. Infection with TRV containing a phytoene desaturase (PDS) fragment resulted in reduced abundance of PDS transcripts and typical photobleaching of photosynthetic tissues. Infection with TRV containing a chalcone synthase (CHS) fragment resulted in silencing of anthocyanin production in infected flowers. The silencing phenotype ranged from scattered white spots on the normal purple background to entirely white flowers. Symptoms in the V26 cultivar were a diffuse mosaic, but infection of some purple-flowered commercial cultivars resulted in large white sectors and even entirely white flowers. Abundance of CHS transcripts in the white flowers was less than 4% of that in purple flowers on the same plant. Infection with TRV containing a tandem construct of PDS and CHS resulted in leaf photobleaching and white patterns on the flowers. Transcripts of CHS and PDS were reduced both in leaves and in flowers confirming simultaneous silencing of both genes by the tandem construct. We tested the effects of infection with TRV containing CHS and a fragment of a petunia gene encoding for 1-aminocyclopropane-1-carboxylate oxidase (ACO4) Abundance of transcripts encoding ACO4 and ACO1 were reduced (by 5% and 20%, respectively) in infected flowers. Whether the flowers were treated with ACC or pollinated, the white (silenced) flowers or flower sectors produced less ethylene and senesced later than purple (non-silenced) tissues. These results indicate the value of VIGS with tandem constructs containing CHS as reporter and a target gene as a tool for examining the function of floral-associated genes.

SummaryProhibitins, highly conserved mitochondrial proteins, have been shown to play important roles in cell cycling and senescence in animals and yeast. Sequences with high similarity to prohibitins have been identified in a number of plant species, but their function has not yet been demonstrated. The deduced amino acid sequences of PhPHB1 and PhPHB2, sequences that we identified in a petunia floral expressed sequence tag (EST) database, show high similarity to those of prohibitin-1 and prohibitin-2 proteins, respectively, reported from yeast, animals and plants. Southern analysis suggested that these genes were members of small gene families with at least two prohibitin-1 homologs and four prohibitin-2 homologs. When we downregulated expression of prohibitin-1 using a Tobacco rattle virus-based (TRV), virus-induced gene silencing system (VIGS), we observed plants with smaller and distorted leaves and flowers. Cells in silenced flowers were larger than in control flowers, indicating a substantial reduction in the number of cell divisions that took place during corolla development. The life of silenced flowers was shorter than that of controls, whether on the plant or detached. The respiration of silenced flowers was higher than that of controls, and we observed a marked increase in the abundance of transcripts of a catalase and a small heat-shock protein in the silenced flowers. Our data indicate that prohibitins play a key role in plant development and senescence.

95616 (J.-C.C., M.S.R.)To understand how auxin regulates sensitivity of abscission zone (AZ) tissues to ethylene, we used a polymerase chain reactionbased subtractive approach to identify gene transcripts in Mirabilis jalapa AZs that changed in abundance during the time the zones became competent to abscise in response to exogenous ethylene. Transcript expression was then examined in leaf and stem AZs over the period they became ethylene competent following indole-3-acetic acid (IAA) depletion either by leaf deblading, treatment with the IAA transport inhibitor naphthylphthalamic acid, or cutting the stem above a node (decapitation). Transcripts down-regulated by deblading/decapitation included Mj-Aux/IAA1 and Mj-Aux/IAA2, encoding Aux/IAA proteins, and three other transcripts showing highest identity to a polygalacturonase inhibitor protein, a b-expansin, and a b-tubulin. Application of IAA to the cut end of petioles or stumps inhibited abscission, and prevented the decline in the levels of transcripts in both AZs. Transcripts up-regulated in the AZ following deblading/decapitation or treatment with naphthylphthalamic acid were isolated from plants pretreated with 1-methylcyclopropene before deblading to help select against ethylene-induced genes. Some of the up-regulated transcripts showed identity to proteins associated with ethylene or stress responses, while others did not show homology to known sequences. Sucrose infiltration of stem stumps enhanced abscission following ethylene treatment and also enhanced the induction of some of the up-regulated genes. Our results demonstrate a correlation between acquisition of competence to respond to ethylene in both leaf and stem AZs, and decline in abundance of auxin regulatory gene transcripts.