EMPERORS OF THE SUN: Linking the evolution of flowering time in long days with a major niche transition in the grass subfamily Pooideae

A fundamental question in evolutionary biology is how plants adapt to the environment? We aimed at providing some answer to this by studying Pooideae, a species rich subfamily of the grasses that dominates the grass flora in Northern temperate regions. The Northern temperate climate is characterized by shifting seasons with cold winters, mild summers and strong variation in day length throughout the year. The ability to synchronize flowering time to favorable environmental conditions is essential for the survival and reproduction of plants. We proposed that flowering in response to seasonal changes in photoperiod is major part of adaptation to temperate regions. Model species of the temperate grasses have a two-step induction of flowering: through winter (low temperature and /or short days (SD)) they gain competency to react to inductive cues that in turn will lead to flowering in long days (LD) in the spring. However, whether this two-step induction of flowering is common for all the grasses of the Pooideae has so far been unknown. We studied the the effect of vernalization and photoperiod (SD/LD) for induction of flowering in a large scale experiment and found that responses are not conserved in the phylogeny. The absolute requirement for vernalization and long days for induction of flowering is characteristic only for a restricted group of grasses. This group includes known cereals like wheat and barley and forage grasses like ryegrass and fescues. The long-day flowering evolved early in Pooideae and may have played an important role in the initial transitioning to temperate regions. We also identify transitions between long-day and short-day flowering. These events are rare, but are important for shifts between regions. The molecular mechanisms underlying these transitions follow each transition and is thus specific to the different lineage. We also identified that the species can use short days as a sign of winter instead of low temperatures and that this can prepare the grasses for flowering in the spring when daylength increase.

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In this project we set out to answer a fundamental question in evolutionary biology: how do species adapt to new environments? We do so by studying the Pooideae subfamily of grasses, which dominate the grass flora in northern temperate regions. The abilit y to synchronize flowering time to favorable environmental conditions is essential for the survival and increased fitness of plants. We propose that flowering time in response to seasonal changes in photoperiod is a major part of adaptation to temperate r egions. Most temperate grasses have a two-step induction of flowering: through winter they gain competency to react to inductive cues that in turn will lead to flowering in long days (LD) in the spring. Competency to flower is induced either by a period o f cold temperatures or short days (SD), while flowering is to a large extent induced by photoperiod. We intend to study both the importance of SD in rendering the temperate grasses competent to react to inductive photoperiodic cues, and the effect of phot operiod for induction of flowering. We will reconstruct the phylogeny of Pooideae and map SD competency to flower and LD induction of flowering on the phylogeny using a set of 40 phylogenetically diverse species to see if the characters are ancestral to P ooideae and thus can be interpreted as a critical adaptation for the control of flowering time in temperate grasses. Furthermore we will compare transcriptomes of a subset of the LD responsive species to determine if molecular responses to LD are conserve d throughout the evolution of Pooideae, and similarly in a subset of species that show SD competency to flower. We will also study the expression profiles of candidate genes for SD competency in a larger set of Pooideae species. The expected outcome of th is project is an expanded knowledge of the role of flowering time, and molecular mechanisms behind, grass adaptation to temperate regions that extends beyond traditional insights gained from a few model species.