| dc.description.abstract | Leaf hairs (trichomes; Fig. 1) are small and rigid epidermal structures that serve in herbivore defense, temperature regulation, boundary layer fortification, and UV-B protection, and can even act as mechanosensory switches indicating insect herbivore presence (Fig. 2). As such, leaf trichomes have impacts on overall plant physiology, photosynthetic efficiency, fitness, and plant-environment interactions. It has been found that leaf trichomes shift in density on the leaf surface when grown at elevated [CO2] in a number of species (Fig. 3). Elevated [CO2] decreases trichome densities by as much as 60% in some species (wheat, Arabidopsis) and increases densities by as much as 57% in other species (Brassica rapa, Medicago truncatula). However, the responses of trichomes to elevated [CO2] remain critically understudied, and little is known about the molecular/developmental mechanisms driving these responses. As trichomes are physiologically important structures for numerous food crop species (e.g. wheat, soybean) and critical ecological species, it is imperative that further research be dedicated to understanding the implications of shifting trichome densities in an elevated [CO2] environment of the future. The goal of this dissertation is to further understand the phenotypic responses driving trichome density shifts at elevated [CO2], and molecular mechanisms that potentially underlie trichome density shifts at elevated [CO2]. Our first aim sought to quantify trichome density responses to elevated [CO2] across the entire Arabidopsis leaf, and not a smaller subsection of the leaf as previous publications have. Furthermore, we analyzed multiple leaves spanning whole-plant development, to observe how trichome densities respond as the plant develops in an elevated [CO2] environment. Finally, we were able to relate full-leaf trichome numbers with leaf area and trichome density under elevated [CO2], to tease apart the relationships between the trichome initiation and trichome densities at elevated [CO2]. Here we show that there trichome number shifts nor leaf area shifts exclusively underlie trichome density shifts under elevated [CO2], dependent upon the genotype. We found significant genotypic variation in trichome density, trichome number, and leaf area responses to elevated [CO2]. Furthermore, we found significant variation in elevated [CO2] response between developmental stages, in regards to trichome density, number, and leaf area. Our second aim was to determine the effect of elevated [CO2] on trichome patterning across the leaf surface, which remains wholly unstudied. Previous to our work, no publications have investigated interactions between trichome patterning and environmental perturbation, therefore our phenotyping work here is novel. We show that elevated [CO2] can significantly shift the patterning of trichomes across the leaf surface. Furthermore, we show evidence that elevated [CO2] may alter trichome-to-trichome spatial relationships. Finally, we find that the impacts of elevated [CO2] on trichome patterning was highly variable across Arabidopsis lines, as well as across developmental stages within lines. There exists little published on potential molecular pathways underpinning the response of trichome densities to elevated [CO2]. As such, our final aim was to present potential molecular/developmental mechanisms behind elevated [CO2]-trichome responses, grounded heavily in trichome genetics, and elevated [CO2] literature. Furthermore, we sought to investigate differential gene expression in growing primordial leaves across current and elevated [CO2], to provide candidate genes that may be involved in the response of trichome densities to elevated [CO2]. Most importantly, we show that unlike other environmental perturbations, elevated [CO2] may not directly affect core trichome gene transcription to alter trichome numbers, and we provide candidate genes for future research avenues. Our research advances our understanding of the phenotypic impacts of elevated [CO2], and the magnitude and variation in the density and patterning of important leaf micro-structures. Through extensive phenotyping we are able to provide a novel and detailed picture of trichome density and trichome patterning responses under elevated [CO2]. As trichomes mitigate herbivory damage, improve water use efficiency, affect fungal infection rates, and increase photosynthetic efficiency, we believe that a deep understanding of how trichome densities may respond to environmental change is an important addition to the field. Finally, we construct a molecular framework and provide novel results concerning potential molecular pathways underlying elevated [CO2] driven trichome density shifts. | |
