Andrew G. Stephenson, Ph.D. - US grants

Many plant species, including numerous economically important ones, regularly shed (abort) a large portion of their fruit crop. Recent study of fruit abortion on the common and important forage crop, birdsfoot trefoil, shows that this plant not only selectively aborts those fruits with the fewest seeds but, by doing so, it improves the average quality of its seed (progeny). In another study of common zucchini squash, the number of pollen grains placed onto the stigmas of the flowers were varied. Again those fruits with the fewest seeds were the most likely to abort. Moreover, the fruits with the most seeds produced progeny that germinated more rapidly, were more vigorous as seedlings, and had greater flower and fruit production as adults than did the fruits with fewer seeds. Dr. Stephenson proposes to examine the hypothesis that these observed differences in seed quality are due to genetic differences in the pollen grains that fathered the seed. That is, when there are more pollen tubes than eggs, only the best pollen fathers mature seed. In contrast, when there are fewer pollen tubes than eggs, both the good and the mediocre pollen fathers mature seed. Therefore, by aborting those fruits with the few seeds (produced when there were fewer pollen tubes than eggs) the plants eliminate those progeny fathered by the mediocre pollen while retaining the fruits fathered only by the best pollen. These findings should not only far reaching implications for basic evolutionary biology and ecology but will also have significance for applied areas of plant science. For example, the ability to rapidly screen large numbers of pollen grains for agriculturally desirable traits, using only limited space, represents enormous potential gains for plant breeders. Such screening could prove to be less expensive, more rapid, and easier to implement than many of the other state-of-the-art techniques, such as recombinant DNA and tissue culture.

9318224 Stephenson A large portion of the genome of the microgametophytes (pollen) is both transcribed and translated during development, germination and pollen tube growth. Consequently, the performance of pollen (speed and germination, pollen tube growth rate and the ability to penetrate ovules and achieve fertilization) can potentially be affected by its genetic composition, by its interactions with the pistil, by environmental conditions and by the resources provided to pollen by the pollen producing parent. This proposal seeks support to continue our detailed investigations of the factors that affect pollen performance, the relationship between pollen performance and non-random fertilization, and the effects of non-random fertilization on the progeny performance. The investigators will continue ongoing studies of pollen competition in Cucurbita texana, to determine if there is heritable variation for pollen performance and to determine if the effects of pollen competition on progeny vigor extend to subsequent generations. They will also determine the potential for pollen competition to occur and the effects of pollen competition on progeny vigor in natural populations of C. foetidissima growing in the Jornada Long-Term Ecological Research Site near Las Cruces, New Mexico. They will also determine if the growing conditions of the pollen producing parent affect the performance of the pollen. They will use C. texana and C. foetidissima to examine the effects of leaf herbivory by cucumber beetles (Diabrotica spp.) on pollen production, pollen grain size and the ability of the pollen to sire seeds when deposited into stigmas in mixture. This research will conclude studies of pollen competition and its effects on progeny performance and will result in the most carefully controlled and comprehensive set of experiments in this field of research. The herbivory studies will greatly extend the now very limited understanding of environmental factors that affect pollen performance.

This proposal requests support for a portion of the travel costs of speakers invited to participate in the 9th Annual Penn State Symposium in Plant Physiology to be held May 19- 21, 1994, in State College, Pennsylvania. The topic for 1994 is "Pollen-pistil interactions". This symposium will bring together outstanding scientists who will provide critical analyses and syntheses of recent research concerning the development of pollen and pistils, pollen- stigma interactions (e.g., the nature of the pollen coat and sporophytic self-incompatibility), the mechanisms of growth in pollen tubes and analagous systems such as root hairs and fungal hyphae, pollen-style interactions (e.g., post- pollination signaling and gametophytic self- incompatibility), pollen selection (non-random fertilization) and the processes of fertilization. This topic is of tremendous interest to both basic and applied plant scientists. One of the goals for this symposium is to foster interaction and communication among scientists of all ranks working in academia, government and industry spanning the spectrum from basic to applied research.

9419722 Stephenson Recent studies indicate that phosphorus fertility can control plant reproduction by controlling the characteristics of the pollen. Symbiotic fungi, called mycorrhizal fungi, can increase the uptake of phosphorus into plants. This research will determine whether the symbiotic fungi influence plant reproduction via an effect on pollen quality. The project will greatly advance our understanding of the response of plants to environmental stress (phosphorus infertility) and the role of mycorrhizal fungi in that response. The study will provide insight into the evolution of natural plant populations, especially in regards to local adaptation and the genetic substructure of populations. The research should have important ramifications for the understanding of pollination of crops, the flow of genes among crops and their wild relatives, and the development of techniques for pollen selection for agriculturally desirable traits.

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Stephenson

Many plant species possess genetic self-incompatibility (SI) systems whereby fertilization by self-pollen is prevented. This allows plants to avoid the deleterious effects of inbreeding but may reduce seed production when cross pollen is unavailable. Previous studies have shown that bellflowers (Campanula rapunculoides) are SI when they first open but become more self-compatible as the flowers' age. In short, bellflowers have low levels of inbreeding when cross-pollen is available and high seed production when it is not. This research examines the molecular, genetic, and ecological causes of this age dependent breakdown in SI. The first project will isolate, clone, and sequence the DNA that specifies SI (S-locus). The second will quantify the amount of inbreeding depression that is due to genes that are closely linked to the S-locus. The third examines the fitness of plants with strong and weak SI in two environments.

These projects will provide insights into (a) the mechanism by which self-pollen is recognized and rejected by flowers, (b the origin, maintenance and evolution of the S-locus, and (c) the relationship between fitness and the breeding system. These basic findings should also assist plant breeders, conservation ecologists, and those interested in the escape of genes from genetically modified organisms.

Stephenson 9806691 Inbreeding depression is the reduction in fitness of inbred progeny (progeny resulting from self-pollinations or pollinations by close relatives) relative to outcrossed progeny. An impressive body of theory predicts that inbreeding depression is a major force in the evolution of floral traits (size, shape, color, odor, etc. of flowers) and mating patterns in plants. Over the past century many studies have shown that inbreeding reduces relative survivorship and the relative number and quality of the seeds produced by plants. However, most studies have ignored the effects of inbreeding on the male (pollen) function of plants despite the fact that most plants are hermaphrodites (produce both pollen and seeds). Moreover, we know little about how the magnitude of inbreeding depression changes with growing (environmental) conditions. Finally, there remains much debate concerning the genetic basis of inbreeding depression. Consequently, we will perform a series of projects designed 1) to quantify and compare the magnitude of inbreeding depression on survivorship and reproduction through both the male and female functions, 2) to examine the relationship between inbreeding depression and environmental stress, 3) to determine the relationship between inbreeding and fitness across four levels of inbreeding, and 4) to determine the proximate cause of inbreeding depression on the male function. The data obtained from the projects in this proposal will result in the first comprehensive study of the effects of inbreeding on the male function of plants; they will generate testable predictions (and the seeds necessary to test these predictions) concerning the genetic basis of inbreeding depression; they will allow us to better predict the direction in which floral traits will evolve under different environmental conditions; and they will allow us to critically evaluate the theory surrounding the evolution of plant mating patterns. From a practical perspective, our findings will have important ramifications for agriculture (e.g., the hybrid seed industry uses inbred lines of plants as the major component in their breeding programs), conservation biology (e.g., the preservation of endangered plants that now exist only in small, highly inbred populations), and forestry (where pollen selection is of utmost importance because traditional plant breeding techniques fail due to the long generation time of trees).

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Ma
We are seeking support for the travel costs of speakers invited to participate in the 17th Penn State Symposium in Plant Physiology to be held May 16-18, 2002 in State College, Pennsylvania. This symposium is entitled "Plant reproduction: from evolutionary and physiological analyses to molecular and cellular studies." Plant reproductive biology has been a subject of great interest and intrigue for many generations. In recent years, there have been very rapid advances in our understanding of plant development not only from molecular genetic studies, but also from evolutionary and physiological analyses. The goal of this meeting is to provide an interdisciplinary forum for leading researchers working in diverse areas of plant reproductive biology to present their recent findings, to share their ideas, and to stimulate each other in discussions. The symposium will bring together about 20 outstanding scientists from around the world, many of whom do not regularly attend the same meetings because of their wide-ranging specialties. In addition to the invited talks, we anticipate having approximately 50 contributed posters summarizing current research by scientists, postdoctoral researchers, and graduate students. The results presented at the symposium will be summarized in a meeting report to be published by one of the leading plant journals.
This symposium will provide a valuable service to the research community. We are not aware of any other symposia that have considered this topic in such an integrated and comprehensive manner. By bringing together scientists using different approaches to study the problems in plant reproduction, the symposium will facilitate novel collaborations and experimental approaches, and will further stimulate progress in this fast advancing and growing field.

Many plants suffer from diseases spread by insect herbivores. The damage experienced by plants depends both on their attractiveness to insects and on their resistance to disease. These, in turn, are influenced by the plants'genes. These studies investigate interactions among herbivory by beetles and aphids, infection by bacterial and viral diseases, and the level of genetic inbreeding in wild gourds, which are free-living forms of cultivated squashes. Field and greenhouse experiments will be conducted in which inbreeding, herbivory, and exposure to disease will be controlled. Serological analyses and transmission bioassays will determine infection levels in the plants.

The proposed studies both will shed light on basic ecological and evolutionary issues such the consequences of inbreeding in natural populations, the establishment and spread of diseases in natural populations, and the dynamics of food chains, and will be relevant for the management and conservation of species with small populations. Moreover, because the same herbivores and pathogens that attack squashes, melons, and cucumbers will be studied, the findings will be useful in the development of integrated pest management programs for cucurbit crop species. Furthermore, knowledge of the interactions among inbreeding, herbivores and pathogens is especially relevant now that transgenic squash cultivars with resistance to insect transmitted viruses are being cultivated. If these genes escape into wild gourd populations, information on the effects of viral pathogens will assist in predicting the impact of these transgenes in nature.

RNA viruses have high rates of mutation and so typically have high genetic variation and high capacity to evolve rapidly in response to host defenses. The evolutionary genetics of RNA viruses are important because the group includes significant pathogens of many plants and animals, including humans, domestic animals, and crop plants. This project will study populations of the Zucchini Yellow Mosaic Virus, an RNA virus that is an important pathogen of squashes, melons, and cucumbers, and will determine the influence of mode of transmission from host to host and growth within a host on the evolutionary genetics of the virus using modern gene sequencing technologies.

This project will document genetic diversity of an important crop pathogen and will illuminate the general question of how the dynamics of population growth within hosts and during transmission from one host to another influence genetic diversity of populations of an RNA virus. Results will inform management strategies for a group of pathogens that cause extensive agricultural losses. The project also provides research-based training opportunities for undergraduates.

Many plant species self-pollinate (a severe form of inbreeding) and the leaves of all plant species are damaged by insect herbivores. Both inbreeding and herbivory reduce the growth and reproduction of plants but the effects of inbreeding on plant resistance to herbivores is largely unexplored. The proposed studies will use horsenettle plants to examine differences between inbred and outbred plants in the expression of anti-herbivore genes (using microarrays), the induction of key plant defensive chemicals (using gas chromatography and mass spectrometry) that help to protect the plants from herbivores, and the impact of these defensive chemicals on the feeding behavior of herbivores and their natural enemies (predatory insects).

Horsenettle is a close relative of crop species such as tomatoes, potatoes, eggplant, and peppers and it shares many of the same insect herbivores as these crops. Horsenettle, however, is a widespread invasive weed that is a problem in 43 states. It is an important weed in agricultural fields and it harbors pests and diseases that reduce the yield of these crop species. In addition to producing a complete study of the effects of inbreeding on plant resistance, the proposed research will produce critical information on the colonization and establishment stages of the life cycle of this important weed; provide insight into potential biocontrol agents of horsenettle; and elucidate the potential roles of chemical defenses in the development of sustainable management strategies informed by a sophisticated understanding of underlying genetic and chemical mechanisms.