Physiological and transcriptomic responses of soybean reproductive development to global climate change

The increase in anthropogenic greenhouse gas emissions has led to warming of the Earth's surface and changes in annual precipitation over land. Additionally, tropospheric O3 concentrations ([O 3]) have more than doubled since the Industrial Revolution. Tropospheric O3 is a secondary air pollutant formed from the photochemical oxidation of methane, carbon monoxide and volatile organic compounds (VOCs) in the presence of nitrogen oxides (NOx). High temperatures, drought stress and seasonally high [O3] already challenge agricultural production in many of the world's growing regions, and global climate and atmospheric change will exacerbate these challenges in a way that may limit the ability of the world to sustain adequate food production. Current research on climate change effects on crop yield have focused on leaf-level processes, while less effort has been placed on understanding the response of reproductive organs to abiotic stress. Therefore, the aim of this thesis research was to examine the physiological and genomic responses of reproductive tissues to elevated temperature, elevated [O3] and increased drought stress.;In Chapter 2, the effects of elevated O3 on reproductive processes in plants were quantitatively assessed from peer-reviewed literature using meta-analytic techniques. Elevated [O3] negatively impacted multiple stages of reproductive development, including seed and fruit properties, pollen germination and tube growth. Daytime elevated [O3] between 70 to 100 ppb decreased plant yield by 27%. In this analysis, elevated [O3 ] did not impact the number of flowers or inflorescences produced by the plants, suggesting elevated [O3] may elicit distinct responses in different reproductive tissues. To further examine the differential effects of abiotic stress on reproductive tissues, transcriptomic studies were performed on the seed coat, flower and pod tissues of soybean (Glycine max L. Merr.).;In Chapter 3, RNA-Seq was used to assess stress-induced changes in gene expression in the soybean seed coat, the structure that regulates photosynthate transport from phloem sieve tube into developing seeds. Seed coats responded to drought, elevated temperature and elevated [O3] by increasing the transcript abundance of WRKY and MYB transcription factors, receptor-like kinases and genes related to protein degradation. While there was strong overlap among stresses at the gene family level, each individual abiotic stress elicited a distinct transcriptomic response. Although all three stresses decreased photosynthetic carbon assimilation, no change in the level of seed coat sucrose compared to cotyledon hexose was observed. In addition, seed coat genes related to carbon and nitrogen transport increased in response to all three abiotic stress treatments. These results suggest that the seed coat responded to abiotic stress in a manner consistent with maintaining sink strength in order to ensure proper seed development. Furthermore, the increase in transcript abundance of genes related to photoassimilate transport in the seed coat indicates a direct mechanism for maintenance of sink strength in response to limiting leaf carbon.;To further explore the transcriptomic responses of soybean to abiotic stress, the flower and pod transcriptome were sequenced in soybeans grown at elevated [O3]. Previous work demonstrated that pod number, not flower number, is responsive to elevated [O3], indicating a differential effect of O3 on these two reproductive tissues. Furthermore, the genomic responses to elevated [O3] in flowers and pods has not been explored. This is important because these tissues ultimately determine the quantity and quality of future food supply. RNA-Seq analysis revealed that flower and pod tissues had distinct responses to elevated [O3]. Flower tissues responded to elevated [O3] through increased expression of genes involved in remodeling of the plant extracellular matrix, while pod tissues increased expression of several receptor-like kinases previously shown to be involved in dehiscence of reproductive organs. Some conserved transcriptional responses to elevated [O3] in vegetative and reproductive tissues included genes involved in post-translational modification and protein phosphorylation. Chapter 4 characterized soybean flower and pod transcriptomic responses to abiotic stress for the first time, and identified potential genetic targets for improving soybean response to elevated [O3].