Plant Science & Landscape Architecture Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2797
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Item FUSARIUM SPECIES OF CUCUMIS MELO IN THE MID-ATLANTIC REGION OF THE US AND THEIR IMPACT ON SALMONELLA ENTERICA NEWPORT SURVIVAL AND INTERNALIZATION ON VARIOUS MELON CULTIVARS.(2019) Korir, Robert Cheruiyot; Everts, Dr. Kathryne L; Micallef, Dr. Shirley A; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fruit rots caused by Fusarium spp. can lead to economic yield losses on melon (Cucumis melo). However, which Fusarium spp. are the most prevalent in Maryland and Delaware has not been documented. Several Salmonella enterica subsp. serovar Newport (S. Newport) outbreaks on melon have occurred over the past 25 years. Fusarium spp. infestation on melon have potential impact on survival and colonization of Salmonella. Our objectives were to identify Fusarium spp. infestations on melons within the Delmarva region, and evaluate their impact on survival and internalization of S. Newport on various melon cultivars. Fifty-six isolates were molecularly identified, according to Fusarium-ID online database, as Fusarium spp. (Fusarium fujikuroi-20, Fusarium proliferatum-18, Fusarium oxysporum-15, Fusarium graminearium-2, Fusarium verticilloides-1). Our findings revealed that most of the Fusarium isolates we collected were not pathogenic to melon fruit. We evaluated the impact of four Fusarium spp. (F. armeniacum, F. oxysporum, F. fujikuroi, and F. proliferatum) on S. Newport survival in five melon cultivars; ‘Arava’ (C. melo var. reticulatus, Galia), ‘Athena’ (var. reticulatus, muskmelon), ‘Dulce Nectar’ (var. inodorus, honeydew), ‘Jaune de Canaries’ (var. inodorus, Canary), and ‘Sivan’ (var. cantalupensis, Charentais). Impact of F. proliferatum on survival and interlization of S. Newport was evaluated on honeydew (smooth) and cantaloupe (netted) melons. Generally, Fusarium did not impact the survival of S. Newport, however greater survival of S. Newport was observed on the netted cultivars compared to the smooth surface melons. Fusarium fujikuroi significantly enhanced survival of Salmonella when inoculated on riper ‘Jaune de Canaries’ melons (above ¾ slip). However, when the experiments were replicated with less ripe (about ¾ slip) melon, F. fujikuroi did not significantly influence the growth of S. Newport. Salmonella Newport internalized in all treatments and the cantaloupe and honeydew melons, but variation in population levels were observed across the treatments. Overall, Fusarium proliferatum did not impact internalization of S. Newport on either melon type. This may be attributed to that Fusarium species used during this study were non-pathogenic. Salmonella Newport recovered gradually decreased with time. Fusarium species on melon, influence S. Newport colonization differently. Also, melon rind type affects the ability of S. Newport to survive and colonize differently.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 SALMONELLA ENTERICA STRATEGIES FOR PERSISTENCE ON TOMATO (SOLANUM LYCOPERSICUM) AND SEROVAR DYNAMICS IN SURFACE AND RECLAIMED WATER(2019) Ferelli, Angela Marie Cecelia; Micallef, Shirley A.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)While select aspects of Salmonella enterica subspecies enterica persistence in agricultural matrices have been illustrated, serovar specific survival strategies in surface water, transmission, and persistence on plants are multifaceted and remain only partially examined. In the present work, we utilized an interdisciplinary approach to illustrate novel mechanisms by which S. enterica may adapt to plants as an alternative host. Furthermore, we leveraged the wealth of diversity in S. enterica serovars to investigate specific dynamics and drivers of persistence in water and transfer onto produce crops. Through biochemical, gene expression, and plant challenge assays of both tomato (Solanum lycopersicum) vegetative and fruit organs, we found that plant-derived NO was generated in response to S. Newport recognition. Furthermore, bacterial gene expression on both leaves and fruit was indicative of adaptation to a novel environment including upregulation in NO detoxification machinery, indicating plant-derived NO as a novel bacterial stress. NO tolerance of various S. enterica was then evaluated to investigate drivers of “produce associated’ S. enterica adaptation to the plant niche. We identified that plant derived NO can negatively affect titers of all S. enterica serovars tested and that serovar specific tolerance to NO in vitro was apparent in a concentration and exposure time dependent manner. Finally, the survival of various S. enterica in surface and reclaimed water was investigated while evaluating the potential for transition to viable but non-culturable (VBNC) organisms. Furthermore, surface water used for irrigation, a common water environment for S. enterica, was investigated as a priming reservoir for various S. enterica serovars for enhanced transmission onto tomato crops. Persistence in water included VBNC subpopulations and was driven by water type. Transfer success onto tomato was driven by serovar, and prolonged incubation in water increased the transfer ability of serovars that initially transferred poorly onto tomato. Finally, attachment to polystyrene and water oxidation-reduction potential were identified as possible indicators of foodborne pathogen transfer success onto tomato. Moving forward, a greater understanding of the environmental queues used by S. enterica subspecies enterica responding to the agricultural environment will aid researchers in developing S. enterica targeted on-farm management techniques to ensure safe yet sustainable fresh produce cultivation practices.Item IDENTIFYING HIGHLY CONSERVED PATHOGENICITY GENES IN CHESTNUT BLIGHT AND POWDERY MILDEW FUNGI AS TARGETS FOR NOVEL FORMS OF HOST RESISTANCE(2019) Levine, Bruce Jonathan; Xiao, Shunyuan; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A bioinformatic search of the genomes of chestnut blight fungus, Cryphonectria parasitica (Cp), and the Arabidopsis powdery mildew fungus, Golovinomyces cichoracearum (Gc), yielded six suspected pathogenicity genes with homologues in both species. Deletion of these genes by homologous gene replacement was attempted in Cp, with one success, TG4. The TG4-knockout strain showed changes in phenotype and reduced fungal virulence against chestnut. TG4 appears to be a promising target for host-induced gene silencing (HIGS) in transgenic American chestnut. The use of homologues from genetically tractable species like Cp can help overcome the obstacles to performing reverse genetics on intractable, biotrophic fungi such as Gc. Experiments underway involving the silencing and ectopic overexpression of the Gc homologues of the target genes provide a rapid method to study Cp genes, including to screen additional candidate genes as future targets for HIGS.Item ECOPHYSIOLOGICAL AND SOCIAL FUNCTIONS OF URBAN FOREST PATCHES(2019) Sonti, Nancy Falxa; Sullivan, Joseph H; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Urban trees provide many ecosystem services to cities: alleviating the urban heat island effect, absorbing stormwater runoff, and contributing to residents’ social and psychological well-being. The production of these benefits is influenced by tree growth and physiological function within the urban ecosystem, and also by the social-ecological context in which urban forest patches exist. This dissertation investigates the ecophysiological and social functions of urban forest patches of the eastern United States using a multidisciplinary approach that combines diverse empirical methods across varied timescales and geographies. Using data collected from urban and reference forest patch sites, this dissertation begins by addressing the following questions: How does native tree growth and physiology vary between urban and reference forest patches? Are there differences in ecophysiological responses by tree species and by city within the eastern United States? Air temperature and soil data from each field site are analyzed alongside tree ring and leaf-level physiological data. Next, results from a controlled growth chamber experiment are presented to examine how Baltimore’s urban vs. reference forest soils and air temperature interact to affect seed germination and seedling growth of white oak (Quercus alba), a dominant species in the region. Finally, the social functions of forest patches are investigated using qualitative data from semi-structured interviews conducted with Baltimore residents. Overall, urban forest patches were found to support robust growth and physiological function of white oak (Quercus alba L.) and red maple (Acer rubrum L.) trees, with differences by species and site type (urban vs. reference), and over time. In particular, urban soils appear to support greater biomass and photosynthesis rates than reference soils. Regardless of the favorable ecophysiological conditions of urban forest patches, community awareness and engagement with these sites will be critical to their continued protection and management. Qualitative interview data revealed local residents’ strongly ambivalent attitudes towards urban wilderness, with only limited differences by homeownership and property management regime. In sum, dissertation documents important ecophysiological and social functions of urban forest patches, with implications for the continued provision of benefits to urban and rural communities.Item Evaluation of organic inputs for reducing dollar spot disease on cool-season turfgrasses(2018) Beckley, Cody James; Roberts, Joseph A; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lolium perenne, Poa annua, and Agrostis stolonifera are turfgrass species commonly grown on golf course fairways; however, they are susceptible to dollar spot (Clarireedia spp.). Field studies were conduction to assess: 1) the effects of organic fertilizer treatments and fungicide programs on dollar spot severity; and 2) the impact of organic amendments on dollar spot severity and residual fungicide efficacy. Alternating applications of organic and conventional fungicides reduced seasonal dollar spot severity to the same degree as conventional fungicides. Dollar spot was more severe in Lolium perenne and Poa annua treated with organic fungicides. On A. stolonifera, organic biosolids compost, biochar, and vermicompost amendments suppressed dollar spot to the same degree as conventional fertilizer in year one of the trial, while dollar spot was more severe on A. stolonifera fertilized with organic biosolids compost in year two. Fertilizer treatments had no effect on residual fungicide efficacy on A. stolonifera.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 A COMPARATIVE ANALYSIS OF RANDOM FOREST AND LOGISTIC REGRESSION FOR WEED RISK ASSESSMENT(2018) Harris, Chinchu; Peer, Wendy; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Invasive species have largely negative impacts on the environment and the economy. The management and regulation of invasive plants are facilitated using screening tools, such as weed risk assessments (WRAs) to predict the invasive potential of non-native plants. The identification of these species and their subsequent regulation on importation helps to reduce the risk of future ecosystem and economic costs. Globally, there are many different types of highly useful WRAs already available. However, in this day of big data and powerful predictive analytics, there is an increasing demand for the development of new and more robust screening tools. In this thesis, I use the machine learning algorithm, Random forests, to develop a new WRA. I show that random forest model has greater predictive accuracies than an existing logistic regression model and that random forest is a better learner. In addition, variable importance analysis was performed to identify factors associated with invasive status classification of non-native plants. The study suggests that random forests make powerful weed risk screening tools and should be utilized for assessing invasive risk potential along with other WRAs. An integrative approach for evaluating weed risk can greatly serve to facilitate the WRA process.Item Understanding the Influence of Microbial Sources and Time on the Developing Creeping Bentgrass Microbiome(2018) Doherty, Joseph Ryan; Roberts, Joseph A; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microbial communities are intimately intertwined with many processes affecting the health of plants. There is increasing interest in utilizing microbial communities to increase plant health while reducing management inputs. To that end, bacterial and fungal communities associated with creeping bentgrass were evaluated using next-generation sequencing technologies. Evaluating the impact of resident seed and soil microbial communities revealed introductions of microbes from the seed despite a strong influence from the soil. Observing long-term population dynamics revealed no shifts in fungal diversity over six months, while bacterial diversity increased from emergence to two months post-emergence. Across both studies taxonomic profiling revealed that bacterial and fungal communities were consistently dominated by just a few groups. In both studies, ordination analyses revealed clustering of samples by sampling time. These results show that changes in the microbiome are driven by rare species, and that the turfgrass microbiome is resilient to change over time.Item ANALYSIS OF ARABIDOPSIS ABCB AUXIN TRANSPORTER MUTANTS REVEALS A PRIMARY ROLE IN MEMBRANE EXCLUSION(2018) Jenness, Mark Kubo; Murphy, Angus; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polar transport of the phytohormone auxin regulates multiple of aspects of plant growth and development. A subset of plant ATP-binding cassette subfamily B (ABCB) transporters mediate cellular auxin export. Loss of these transporters results in reduced polar auxin movement and altered plant architecture but no significant defects in embryogenesis or organ formation. Several of lines of evidence suggest that isotropically-localized ABCB transporters mediate auxin exclusion from the plasma membrane and prevention of reuptake after directional PIN-mediated efflux. Examination of the Arabidopsis auxin transporters ABCB1 and ABCB19 indicates a primary role in exclusion from small auxin producing cells in apical regions and prevention of leakage from polar auxin transport streams. Analysis of abcb mutants identifies a contribution from ABCB21 in restricting auxin to within the root vasculature in seedlings. In mature tissues, ABCB6, ABCB21, and ABCB11 make additional contributions to polar auxin transport in inflorescence stems, leaves, and flowers, respectively. The results presented herein reflect an evolutionarily conserved function for ABCB transporters in maintaining polar transport streams and prevention of cellular reuptake via exclusion.