Evolution of Vernalization and Photoperiod-Regulated Genetic Networks in the Grass Subfamily Pooideae

Flowering time is a carefully regulated trait that integrates cues from temperature and photoperiod to coordinate flowering at favorable times of the year. This dissertation aims to understand the evolution of genetic architecture that facilitated the transition of Pooideae, a subfamily of grass, from the tropics to the temperate northern hemisphere approximately 50 million years ago. Two traits hypothesized to have facilitated this evolutionary shift are the use of long-term low-temperature (vernalization) to ready plants for flowering, and long-day photoperiods to induce flowering. In chapter one I review literature on the regulation of grass flowering by vernalization and photoperiod, and in chapters two and three I determine the role of VERNALIZATION 1 (VRN1) and VRN2, known to confer vernalization responsiveness in core Pooideae crop species, in flowering time across Pooideae. In chapter four, I then test predictions of the hypothesis that the Brachypodium distachyon miR5200 ortholog in the ancestor of Pooideae was important for suppressing short day flowering through its negative regulation of flowering time integrator FLOWERING LOCUS T (FT)/VERNALIZATION3 (VRN3). In combination with other studies, my data demonstrate that VRN1-mediated vernalization responsiveness evolved early in the Pooideae, while VRN2-mediated vernalization responsiveness appears to have evolved much later in the diversification of Pooideae. Although miR5200 likely evolved early in the Pooideae, its transcriptional regulation by short day photoperiod appears derived within Brachypodium distachyon. This work answers important questions about the evolutionary origin of temperature- and photoperiod-mediated flowering in an economically important clade that contains crop species such as wheat (Triticum aestivum) and barley (Hordeum vulgare). Directions for future work on this topic are discussed in chapter 5.