College of Agriculture & Natural Resources
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item SPATIO-TEMPORAL ANALYSIS OF PHOTOTROPISM IN ARABIDOPSIS SEEDLINGS(2019) Pritchard, Candace; Murphy, Angus S; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optimization of light capture during seedling development is a major determinant of plant fitness. As seedlings emerge from the soil, the processes of photomorphogenesis and phototropism optimize deployment of structures that capture light for photosynthesis. Photomorphogenesis produces hypocotyl thickening, cotyledon expansion, and chloroplast maturation. Concurrent phototropic responses initiated by blue light position the expanding cotyledons to maximize photosynthesis. The mechanisms underlying both processes have been explored for more than 140 years, but are still not fully understood. This dissertation seeks to provide a better understanding of phototropism by exploring the timing and localization of the constituent mechanisms downstream of the well-characterized perception of blue light by the PHOTOTROPIN photoreceptors. The experiments described herein characterize temporally and spatially distinct processes involved in asymmetric auxin accumulations that lead to differential hypocotyl elongation. To better identify the link between early perception and later auxin transport and elongation events, an open-air system was used to remove seedling hindrance and provide better spatio-temporal resolution. These experiments confirmed the more rapid bending conferred by loss of the ATP Binding Cassette class B (ABCB) 19 auxin efflux transporter and loss of differential elongation in the mid hypocotyl elongation zone in higher order pinformed mutants. However, apart from the enhancement of phototropic bending observed in abcb19 and pin4 mutants, no auxin transport mutants tested showed alterations in early phototropic responses, and no mutant exhibited a delay in the onset of phototropic bending. Recently identified CBC1 and CBC2 (CONVERGENCE OF BLUE LIGHT (BL) AND CO2 1/2) have been shown to act in downstream signaling during phot1-mediated regulation of stomatal conductance. Similarly, during phototropism cbc1cbc2 double mutants show early defects in phot1-mediated phototropism. Further, CBC1 and CBC2 have been shown to regulate S-type anion channels. Analysis of S-type anion channel mutants also reveals defects in early bending responses. These results point to blue light-dependent regulation of anion channel activity having an important role during the earliest stages of phototropism.Item SEASONAL NITROGEN CYCLING AND HOMEOSTASIS IN POPULUS: SOURCE-SINK COMMUNICATION(2018) Li, Gen; Coleman, Gary D; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrogen (N) is an essential nutrient for plant growth, development and reproduction. Seasonal N cycling is an adaption to nutrient limitation and a feature of the perennial lifestyle of trees. Poplar (Populus) is a model system used to study forest tree genetics and molecular biology, including seasonal N cycling. The accumulation of Bark storage proteins (BSP) is a central feature of seasonal N cycling in poplar, yet our understanding of the contribution of the BSP storage pool to N remobilization during growth and mechanisms that regulate BSP accumulation, catabolism and N remobilization is limited. The research presented in this dissertation is directed towards advancing knowledge of the regulation of seasonal N cycling in poplar using a combination of experimental approaches. The role of the N storage pool to N remobilization was examined through N sink manipulations and the specific role of BSP storage was investigated by N source reduction via RNAi mediated knockdown of BSP accumulation. From this it was found that N remobilization from BSP significantly contributes to shoot growth following dormancy and initial shoot growth is source limited. Poplar bark transcriptome analyses during regrowth following dormancy revealed an enrichment for up-regulated genes associated with auxin transport and signaling. Based on the transcriptome analysis experiments that manipulate auxin production or polar auxin transport were performed and the results indicate that BSP catabolism and N remobilization likely involves polar auxin transport from expanding buds and shoots and auxin-mediated regulation of protease gene expression. Analysis of DNA microarrays of bark RNA during short-day (SD) induction of BSP gene expression was used to identify putative regulatory factors that may play a role of BSP accumulation. The transcriptome analyses indicated that SD represses the expression of genes involved in ethylene production as well as a reduction in bark ethylene biosynthesis. Additionally, treatment of excised stems with ACC or ethephon repressed BSP gene expression while AVG induced BSP gene expression. This repression was reduced in ethylene-insensitive poplars expressing Arabidopsis dominant gain-of-function allele etr1-1. Furthermore, transient expression of ERF12 and ERF41, two transcription factors with the greatest induction in SD treatment, in transgenic tobacco stably transformed with BSPA promoter fused with GUS resulted in enhanced GUS activity suggesting ERF12 and ERF41 may act as positive regulators of BSP gene expression. Since glutamine plays a pivotal role in N partitioning during N storage and remobilization, the possible role of the PII glutamine sensor was studied and it was found that the transcript levels of PII increased in bark during SD-induced leaf senescence and BSP accumulation. PII knockdown poplars using RNAi showed reduced glutamine-induced BSP gene expression. Moreover, glutamine-induced BSP gene expression was also inhibited by 2-OG, a PII-NAGK interaction antagonist, suggesting a possible role of PII-glutamine sensing in BSP accumulation. Taken together, this study provides important insights into the mechanism of seasonal N accumulation and remobilization in poplar.Item Influence of Nitrogen and Sink Competition on Shoot Growth of Poplar(2016) Egekwu, Chioma; Coleman, Gary D; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Terrestrial and oceanic biomass carbon sinks help reduce anthropogenic CO2 emissions and mitigate the long-term effect of increasing atmospheric CO2. Woody plants have large carbon pools because of their long residence time, however N availability can negatively impact tree responses to elevated CO2. Seasonal cycling of internal N in trees is a component that contributes to fitness especially in N limited environments. It involves resorption from senescing leaves of deciduous trees and storage as vegetative storage proteins (VSP) in perennial organs. Populus is a model organism for tree biology that efficiently recycles N. Bark storage proteins (BSP) are the most abundant VSP that serves as seasonal N reserves. Here I show how poplar growth is influenced by N availability and how growth is influenced by shoot competition for stored N reserves. I also provide data that indicates that auxin mediates BSP catabolism during renewed shoot growth. Understanding the components of N accumulation, remobilization and utilization can provide insights leading to increasing N use efficiency (NUE) of perennial plants.