DISSERTATION RESEARCH: Evolution of Mangrove Seed Physiology

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Proposal: 9623313 PI: Kellogg and Farnsworth A comprehensive understanding of mechanisms of seed drying and dormancy is critical to developing appropriate storage, planting, and germination protocols for farms, arboreta, botanical gardens, and seed conservation banks. The evolution of desiccation-intolerance, and its developmental manifestations, vivipary and recalcitrance, will be studied in seeds of four phylogenetically independent mangrove lineages. Embryos of dormant (orthodox) seeds normally lose water prior to entering metabolic quiescence; by contrast, "recalcitrant" seeds that are intolerant of desiccation show no dormancy, and some sprout viviparously on the parent plant. While rare among upland angiosperms, this phenomenon appears frequently among coastal mangroves, rice, and wet- forest trees, with important implications for agriculture and seed conservation. We hypothesize that desiccation- intolerance results from one of three alternative evolutionary modifications that are not mutually exclusive: (1) decreased levels of the plant hormone abscisic acid, ABA, during embryogenesis (which controls both osmotic adjustment of the whole plant to flooding, and dormancy in orthodox seeds); (2) differential levels of dehydrin proteins, carbohydrate reserves, or other hormones in recalcitrant seeds; (3) altered anatomy of the seed coat, maternal transfer tissues, or embryo/endosperm relationship that may free the embryo from influences of physical or biochemical germination inhibitors. To test among these hypotheses, we will examine the hormonal, physiological, and anatomical bases of desiccation-intolerance that are shared among four viviparous mangrove families, in a study of seeds through development and germination. Seeds of viviparous species will be compared with seeds of closely related, non- viviparous taxa to determine whether similar, functionall y significant changes in seed structure and physiology have arisen consistently in this pattern of parallel evolution. This integrated approach will elucidate the nature of this evolutionary convergence at several biological levels; clarify relationships between anatomy, hormonal physiology, proteins and sugar metabolism during development of viviparous and orthodox seeds; and provide some of the first documentation of the role of hormones in the evolution of plant life histories. These comparative data on multiple, interacting factors controlling seed dormancy and recalcitrance can usefully inform strategies for plant conservation, propagation, and agriculture.