Evolution Of Drought And Low Temperature Responses In Temperate Pooideae Grasses: Timings, Determinants, And Intersections

Anthropogenically-mediated rises in atmospheric CO2 and global average temperatures is leading to increasingly severe drought and extreme weather events, the latter including unseasonal bouts of low and high temperatures. In order for plant breeders and conservation biologists to predict future responses to global warming, they must understand the ecological and evolutionary processes that shaped plant tolerance to stressful environments in the past. This is particularly true for grasses (Poaceae) that dominate approximately one-third of the Earth\u27s vegetative cover, live in some of the world\u27s harshest terrestrial environments, and are tremendously important, both ecologically and economically. One of the largest subfamily of grasses, Pooideae, comprises around 4,000 species and contains some of the world\u27s most important temperate crops, including wheat (Triticum sp.), barley (Hordeum vulgare), oats (Avena sativa), and ryegrass (Lolium sp.). Pooideae are collectively known for their high drought and cold tolerance, but the factors that have led to one or more origins of this stress tolerance are largely unknown.The primary goal of my thesis is to evaluate the evolutionary history of, and relationship between, drought and low temperature adaptation in Pooideae. Following a literature review on how plants have evolved to tolerate stressful environments (chapter one), I test whether variation in stress tolerance is driven by differences in the native climate of Pooideae species and/or differences in stomatal traits that affect water use efficiency (chapter two). My findings suggest that Pooideae tolerance to aridity and above-freezing cold can at least partly be explained by climate of origin, but not by variation in stomatal traits. For chapters three and four, I test predictions of the hypothesis that the last common ancestor of Pooideae was adapted to a cold highland microclimate, and that an increase in drought and freezing tolerance evolved later, after the diversification of Pooideae tribes. The cold-before-drought hypothesis was supported by both physiological trait and differential gene expression data, and I also found evidence that cold tolerance genes acted as precursors for the novel acquisition of increased drought tolerance multiple times independently (chapter three). In chapter four, concurrent evolution of drought and freezing tolerance was also supported, but I found diminutive support for increased drought tolerance evolving as a consequence of cross-tolerance provided by the freezing tolerance pathway. My final chapter elucidates how this work advances our understanding of the role of both ancient and more recent climates in shaping Pooideae stress tolerance, identifies novel candidate genes providing cross-tolerance to multiple stressors, and highlights the importance of pre-adaptation driving plant responses to novel climate conditions