| Pollination of flowers triggers a variety of developmental events known as the postpollination syndrome, including senescence of unnecessary floral organs, ethylene production, and ovary development. Several inter-related aspects of postpollination development in Phalaenopsis orchid flowers were addressed. First, the nature of the signal perceived by the orchid flower following pollination was examined. A biological assay for pollen mimicking activity was developed, and a variety of exogenous as well as endogenous substances were tested for their activity in the bioassay to characterize the primary pollen signal. These results support a role for auxin in the primary pollination response. Second, the regulation of ethylene biosynthesis with a particular focus on the last enzyme in the biosynthetic pathway, ACC oxidase, was examined. Two ACC oxidase cDNAs were identified, and the temporal and spatial expression of D-ACO1 was examined. The regulation of this gene was dissected and found to be auxin-induced, an effect that is mediated through ethylene in most organs of the flower. A model for the interorgan regulation of ethylene biosynthesis is presented to explain the coordination of ethylene-regulated events. Lastly, the Phalaenopsis orchid system was employed to study ovule development at the level of gene expression. A differential screening approach was used to identify seven cDNAs that were candidates for involvement in developmental regulation or differentiation of cells within the ovule. Of seven clones examined, five were expressed in a stage- and tissue-specific manner in ovules and were further characterized. Homology to sequences of known function, as well as temporal and spatial patterns of expression of the genes encoding these cDNAs, suggest hypothetical roles in ovule development. Clone O39 encodes a putative homeobox transcription factor that is expressed early in the differentiation of the ovule primordium, while O40 encodes a cytochrome P450 monooxygenase (CYP78A2) that is pollen tube specific. O108 encodes a protein of unknown function that is expressed exclusively in the outer layer of the outer integument and in the female gametophyte of mature ovules. O126 encodes a glycine-rich protein that is expressed in mature ovules, and O141 encodes a cysteine proteinase that is expressed in the outer integument of ovules during seed formation. These genes provide new information about ovule development by delineating previously unrecognized biochemical compartments in the ovule and will provide the basic tools for future molecular studies of ovule development. |
