Reductions in abscisic acid are linked with viviparous reproduction in mangroves

We investigate physiological mechanisms behind the convergent evolutionary loss of seed dormancy in plant lineages, focusing on mangroves as a model system. More than 60 angiosperm families, including several mangrove taxa, contain species with seeds that are intolerant of drying and do not undergo dormancy. These desiccation-intolerant species occur with disproportionate frequency in wet or coastal tropical habitats. In plants, the hormone abscisic acid (ABA) coordinates both the development of desiccation tolerance during the onset of seed dormancy and whole-organism responses to flooding. Thus, changes in ABA levels and/or modes of action in different plant compartments are implicated in the repeated evolutionary loss of seed dormancy among species of wet habitats. We compare ontogenetic dynamics of ABA levels in embryonic, maternal, and mature vegetative tissue of four phylogenetically independent pairs of related viviparous mangroves and nonviviparous nonmangroves. We demonstrate that ABA levels are consistently lower in embryos of viviparous mangrove taxa than embryos of nonmangrove, nonviviparous sister taxa. In contrast, elevated tissue concentrations of ABA characterize leaves of all mangrove species tested, while ABA levels in maternal tissues vary among mangrove species. These commonalities suggest a functionally important trade-off between the maintenance of embryonic development and the adjustment of the parent tree to salinity stress. This study yields comparative data on seed physiology in naturally occurring desiccation-intolerant species, for which these data are currently scarce, and demonstrates a potentially significant role of phytohormones in the evolution of plant life histories.