Research in the O'Neill laboratory focuses on plant reproductive development. The specific research themes are: 1) Postpollination flower development including interorgan regulation of ethylene biosynthesis and pollen signaling, 2) Ovule and female gametophyte development , and 3) Photoperiodic control of flowering. Click on any one of these topics to learn more and to find links to visual data on each research theme.
Postpollination flower development
Pollination of flowers is a key regulatory event in the life cycle of the flowering plant. The pollination event initiates an entire syndrome of developmental changes that contribute to successful reproduction. Components of the postpollination developmental syndrome include perianth senescence, pigmentation changes, ovary growth, and in certain cases, ovule differentiation. Our studies of postpollination development in orchids have focused on the role of pollination-associated factors in initiating and signaling the postpollination response. These studies indicated that ethylene with auxin regulate the various aspects of postpollination development, especially the coordinate development of the female and male gametophytes (Zhang and O'Neill, 1993). Parallel studies demonstrated that the postpollination syndrome in orchids, ranging from perianth senescence to ovule differentiation in the ovary, involve coordinated interorgan regulation of expression of the two key ethylene biosynthetic genes, ACC synthase and ACC oxidase (O'Neill et al., 1993; Nadeau et al., 1993). Currently, we are focusing on the primary signal-response coupling event that occurs between the pollen and the stigma and on identifying the primary signal molecule(s) in the pollen that initiate the postpollination response. For this work we are focusing on auxin as a component of the primary pollen-borne signal and on an auxin-induced ACC synthase gene expressed uniquely in the stigma following pollination.
Ovule and Female Gametophyte Development
A major current focus of our research on postpollination development is targeted towards understanding the molecular events regulating ovule and female gametophyte development (Link). Our studies of pollination-induced ovule differentiation in the Phalaenopsis orchid flower have established the basic information of timing and hormonal stimuli to allow further characterization of gene expression associated with the timing of major developmental transitions during ovule development (Zhang and O'Neill, 1993) . We have isolated ovule developmental stage-specific mRNA, constructed stage-specific cDNA libraries capturing key developmental events occurring during the archesporial cell, megaspore mother cell and mature embryo sac stages, and have identified a large number of ovule-specific, differentially expressed cDNA clones corresponding to the major ovule developmental transitions of megasporogenesis and embryo sac development. Eight ovule- and stage-specific genes have been characterized in detail (Nadeau et al., 1996). One of these genes (039) is particularly important because it encodes a homeodomain protein that may function as one of several master regulatory protein involved in specifying the ovule developmental pathway in higher plants.
RNA In Situ Hybridization of Clones O39, O40 , O108 , O141. See domain of expression
Genetic Control of Ovule Development can be Generalized from Orchids to Arabidopsis and Other Flowering Plant Species
One of the future goals of our research is to elucidate the function of several ovule stage-specific genes using Arabidopsis thaliana as a model genetic system suitable for transgenic plant studies. Although orchid flowers are unusual in that ovule development is delayed and triggered by pollination, once initiated, the processes of megasporogenesis and megagametogenesis are similar to many other plant species with the development of the mature embryo sac being of the Polygonum type (See Orchid Gametophyte Development). Recently, we have found that transcripts homologous to orchid ovule stage-specific genes are present in Arabidopsis flower buds and are in the process of studying these genes in greater detail. It appears likely that the molecular tools developed in orchids to study ovule and ovary development can be used to isolate functionally homologous genes from Arabidopsis in order to test their function in a system better suited for genetic analysis. The information derived from both species will be transferable to a wide range of crop species that share similar aspects of ovule and embryo sac development.
Photoperiodic Control of Flowering
Gene Expression
Flowering involves the developmental transition of the shoot meristem from vegetative to reproductive growth. In many plant species, this developmental transition is regulated by photoperiod as shown in this model of photoperiodic floral induction in a typical short-day plant. We are investigating the molecular and biochemical basis of flowering in the qualitative short-day species, Pharbitis nil . (O'Neill, 1992). We have identified genes whose expression is correlated with photoperiodic induction of flowering that have the following characteristics: 1) their expression at the level of mRNA abundance is regulated by photoperiodic change, specifically the transition to darkness, 2) they exhibit a rhythmic pattern of expression with a circadian periodicity, 3) their accumulation is altered by night break treatment that inhibits flowering, and 4) their expression in certain cases is localized to photoperiodically-sensitive tissues, the site of floral induction (O'Neill et al., 1994; Zheng et al., 1993). One of these genes encodes a putative basic leucine zipper transcription factor, a focus of our current research in both Pharbitis and Arabidopsis .
Phytochrome
The recent discovery of multiple genes encoding phytochrome has renewed an interest in the role of phytochrome in photoperiodic floral induction and has raised the intriguing possibility that one or more of these phytochromes may be specifically involved in the photoperiodic timekeeping mechanism that underlies floral induction. To explore this possibility, we have initiated research on the role of individual phytochrome genes and their gene products (mRNA and protein) in our well-characterized photoperiodic model system, Pharbitis nil. Thus far we have cloned four different phytochrome genes and have examined their pattern of expression during photoperiodic treatments that induce or inhibit flowering. For further details, see the research summary in Zheng and O'Neill (1996).
Dark Signaling
Studies to identify potential chemical messengers of the florally induced state have lead to our interest in a novel class of potential signal molecules in plants. We have identified the methoxylated indoleamine hormone, N -acetyl-5-methoxytryptamine, or melatonin, in tissue from higher plants and are now in the process of studying its synthetic pathway and biological activity. In mammals, melatonin has been determined to function as a "night signal" or "hormone of darkness" in that this molecule appears to be the primary chemical messenger regulating photoperiodic phenomena. Click here to learn more about our work on melatonin as a dark signal in plants.
