Responses of an alpine plant to warming temperatures: from plasticity to molecular pathways

Rapid changes to climatic parameters, including temperature, pose substantial threat to species persistence and biodiversity. Climate warming has been more rapid in alpine systems than lowland environments, however studies of responses to high temperature stress in alpine plants are lacking; particularly transcriptomic and genomic studies are nearly absent. Being sessile, plants must respond and adapt to changing environmental conditions in situ. In this regard, phenotypic plasticity is described as a rapid and adaptive mechanism to respond to climate change, however many fundamental questions about plasticity are yet unsolved, including its genetic and molecular underpinnings. Answering said fundamental questions, such as whether plasticity is adaptive, will help us better understand which species can be resilient to climate change. The overall objectives of this thesis were to (1) characterize the plastic phenotypic response of the alpine herb Wahlenbergia ceracea to warming and heatwaves using a whole-of-life cycle approach to address fundamental questions about plasticity; and (2) to examine changes in gene expression in response to warmer temperatures as an intermediate component of the molecular basis of plasticity. To determine whether the alpine plant W. ceracea will be able to tolerate projected, future warmer temperatures and heatwaves I examine plasticity in a range of relevant traits in lineages (the group of half-siblings obtained with targeted crosses) from low and high elevation (Chapter 2). When grown under warmer temperatures, W. ceracea plants showed signs of acclimation by means of higher thermal tolerance (Tcrit) but this did not confer resilience to heatwaves. High- and low-elevation lines exhibited limited genetic variation in plasticity, suggesting canalisation of the plastic response. To assess the effect of warming on the seed size vs number trade-off and on germination responses, I conduct germination assays under relatively cool and warm temperatures using seeds produced by parental individuals grown under cool and warm temperatures (Chapter 3). The reductions in seed size and number in parents grown under warm temperatures resulted in the breakdown of the seed size vs number trade-off, but this did not affect germination, despite correlations of these seed traits with germination traits. Warming increased germination, particularly of larger seeds, but overall resulted in more than fourfold reduction in parental fitness. To investigate the molecular pathways regulated in response to temperature and involved in physiological acclimation, I examine the differential gene expression of tolerant and sensitive lines grown under cool and warm temperatures using RNA-seq (Chapter 4). Under warm growth temperatures, plants upregulated post-translational processes and downregulated processes related to photosynthesis. Interestingly, tolerant lines showed stronger downregulation of genes involved in photosynthesis light harvesting, electron transport chain and photosystem II repair/assembly. In Chapter 5 I bring together the previous chapters by linking gene expression to mean trait values and plastic changes therein. I found that co-expressed genes involved in photosynthetic processes were correlated with the physiological parameter Fv/Fm. Genes involved in cell wall and cytoskeleton organisation were correlated with the number of reproductive stems. My thesis represents one of the few works in plant ecology addressing intraspecific variation in plasticity and its relationship with fitness and transcriptomic regulation in a native plant. Overall, the results showed that significant plasticity in many traits did not confer resilience to warmer conditions, as fitness was substantially reduced. The transcriptomic analyses highlighted candidate genes potentially involved in photosynthetic acclimation as measured by chlorophyll fluorescence, opening avenues for future research