Hohenheim, Univ., Diss., 2008.

No description available.

Tübingen, Universiẗat, Diss., 2002.

<p></p><p>Sorghum (<i>Sorghum bicolor) </i>is a well-known agronomic crop of global importance. The demand for sorghum as a food crop makes it the fifth most important cereal in the world. The grain of sorghum is utilized for food and feed, whereas the sorghum biomass may have many other uses such as for fodder, bioenergy or even for construction. Globally, sorghum is consumed as a food crop and used for home construction primarily in the developing world. The grain and biomass yield of sorghum is drastically reduced by the parasitic plant <i>Striga hermonthica </i>which is endemic to Sub-Saharan Africa. To date, only one sorghum gene, <i>LGS1</i>, has been characterized as a genetic mechanism that reduces <i>S. hermonthica</i> parasitism by altering the strigolactone composition of the host root exudates which results in a reduction of the parasites ability to germinate. To establish more durable resistance additional genetic variation needs to be identified that reduces the <i>S. hermonthica </i>parasitism in sorghum, but also reduces the parasitic weed seed bank by promoting suicidal germination. To that end, the PP37 multi-parent advanced generation inter-cross (MAGIC) population was developed, originally as a recurrent selection population that was developed to recombine sorghum accessions with different putative resistance mechanisms to <i>S. hermonthica. </i>Whole genome sequences were developed for approximately 1,006 individuals of the PP37 MAGIC population. The population was phenotyped for <i>S. hermonthica </i>resistance during the 2016 and 2017 growing season in Northwestern Ethiopia. There was significant spatial variation in the <i>S. hermonthica </i>natural infestations that were partially attenuated for with artificial inoculation. The data was used to conduct a genome-wide association study that detected several subthreshold peaks, including the previously mapped <i>LGS1. </i>The highly quantitative nature of <i>S. hermonthica </i>resistance confounded with the complex spatial variation in the parasite infestations across a given location make it difficult to detect highly heritable variation across years and environments. </p> <p> In addition to <i>S. hermonthica </i>resistance, the plant architecture of the PP37 MAGIC was also assessed at a location in Northwestern Ethiopia that is free of the parasite, as it significantly reduces plant height. To asses plant architecture the total plant height, the height of the panicle base, flag leaf height, and pre-flag leaf height were collected using a relatively high-throughput barcoded measurement system. Sorghum head exertion and panicle length were derived from this data. The actual measures of plant architecture and the derived traits were used to conduct a genome-wide association study. The high heritability of this trait demonstrated the statistical power of the PP37 mapping population. Highly significant peaks were detected that resolved the <i>dwarf3</i> locus and an uncharacterized qHT7.1 that had only been previously resolved using a recombinant inbred line population. Furthermore, a novel significant locus was associated with exertion on chromosome 1. The random mating that was utilized to develop the PP37 MAGIC has broken the population structure that when present can hinder our ability associate regions of the genome to a given phenotype. As a result, novel candidate gene lists have been developed as an outcome of this research that refined the potential genes that need to be explored to validate qHT7.1 and the novel association on chromosome 1. </p> <p>This research demonstrated the power of MAGIC populations in determining the genomic regions that influence complex phenotypes, that facilitates future work in sorghum genetic improvement through plant breeding. This research however also demonstrates a large international research effort. The nuisances and lessons learned while conducting this international research project are also discussed to help facilitate and guide similar research projects in the future. The broader impacts of this research on the society at large are also discussed, to highlight the unique potential broader impacts of international research in the plant sciences. The broader impacts of this research include germplasm development and extensive human capacity building in plant breeding genetics for developing country students and aspiring scientists. Overall this research attempts to serve as a model for highlighting the interdisciplinary nature and complexity of conducting international plant science research, while also making significant strides in improving our understanding the genetic architecture of quantitative traits of agronomic importance in sorghum.</p><br><p></p>

Philosophiae Doctor - PhD / The invasion by Striga in most cereal crop fields in Africa has posed an acute threat to food security and socioeconomic integrity. Consequently, numerous technological and research developments have been made to minimize and even control the Striga impacts on crop production. So far, efforts to control Striga have primarily focused on the manipulation of the genetics of the host crops, as well as understanding the phenological and physiological traits, along with the chemical composition of the weed.

Plants have evolved different communication mechanisms that convey information encoded in chemical signals, both internally and to surrounding organisms. Two such signals are strigolactones (SLs) and nitric oxide (NO). SLs are plant hormones that shape plant architecture according to nutrition availability and mediate interactions with beneficial arbuscular mycorrhizal fungi. For this second purpose, plant roots release SLs into the soil where they also trigger seed germination in root parasitic weeds, such as Striga hermonthica. Attachment of Striga causes severe damage to crops yield, particularly in sub-Saharan Africa. One way to control this threat to food security in infested African regions is to develop SL antagonists, which can inhibit Striga germination. Recently, we have shown that Triton X-100 can bind to the Striga SL-receptor, HYPOSENSITIVE to LIGHT 7 (ShHTL7). In addition, triazole urea compounds have been shown to specifically bind to Oryza sativa SL-receptor DWARF-14 (OsD14), blocking SL signalling. Here we used the structures of Triton X-100 and triazole urea to design two isomers and putative ShHTL7 inhibitors: KK023-N1 and KK023-N2. We demonstrate that these compounds antagonize SL signalling in S. hermonthica via specific binding to ShHTL7, and that application of KK023-N1 results in a 38% reduction in Striga germination under greenhouse conditions. Furthermore, we discovered a histidine residue (H51) in ShHTL7, which may be involved in SL perception in addition to known residues. Substitution of H51 to asparagine (N) led to a significant reduction in ShHTL7 hydrolysis activity, indicating the importance of this H residue. Our work provides a starting point for designing new series of SLs inhibitors to combat Striga, and improve food security worldwide. NO is a gaseous signaling molecule involved in regulating plant development and adaptive responses to biotic and abiotic stresses. In this work, we characterized AtLRB3, a Light Response Bric-a-Brac/Tramtrack/Broad Complex (BTB) family protein, and showed that it contains a Heme Nitric Oxide/Oxygen (H-NOX) domain that can sense NO, providing an evidence of the existence of NO binding proteins in planta.

Strigolactones (SLs) are a class of plant hormones regulating several aspects of plant growth and development according to nutrient availability, particularly the modulation of root and shoot architectures. Under nutrient deficiency, SLs are abundantly released into the soil to recruit a plant-beneficial partner, arbuscular mycorrhizal fungi (AMF), and establish plant-AMF symbiosis that provides the plant with minerals and water. However, released SLs are also seed germination signals for the root parasitic plants Orobanchacea family and pave their way to the host plants’ roots. “New comers” in the field of plant hormones, their large structural variety intrigues and led to ask why plants produce many different types of SLs. In this work, we generated tools that can help to link the SL structural diversity with their biological function(s). The most common way to evaluate SL activity is based on their ability to be parasitic seeds’ germination stimulants. Despite being the most sensitive assay for SL quantification, parasitic seed-based bioassays are laborious and time-consuming as performed usually manually. Therefore, we developed a detection algorithm, SeedQuant, which identifies and counts germinated and non-germinated seeds 600 times faster than a trained human; thus, reducing the data processing from days down to minutes. To gain quantitative insights in SL perception, depending on the structural diversity, we developed a precise and detailed protocol for the use of a genetically encoded biosensor in Arabidopsis protoplast, StrigoQuant. StrigoQuant takes advantage of the SL-dependent degradation of the repressor protein AtSMXL6 coupled with luciferase reporter proteins. We also tried to adapt this molecular sensor to the rice repressor protein D53, but the use of rice protoplasts made it very challenging. Moreover, to better understand the later steps in SL biosynthesis in vivo, we generated CRISPR/Cas9-based rice mutants and shed light on the biological function of different SLs at the organismal level. MAX1-900 mutants defined the minor role of the canonical SL 4-deoxyorobanchol (4DO) - a major plant SL - in determining rice architecture, while being a crucial contributor to rhizospheric interactions. Finally, we reviewed other strategies to decipher plant signaling pathways in general.

The parasitic weeds belonging to the genus Striga are among the world's most tenacious, prolific and destructive agricultural pests. Crop loss estimates due to Striga infestations can reach 100 percent. Furthermore, the weeds' affinity for low-fertility soils and low rainfall means that those farming the most marginal lands are most severely affected. Nonetheless, subsistence farmer have yet to adopt seemingly beneficial control practices to any appreciable degree. This paper develops a bio-economic model capable of identifying: (1) affordable, effective Striga control practices consistent with the resource constraints of subsistence farmers; and (2) barriers to the adoption of those practices. The model is comprised of two components: a biological component modeling Striga population dynamics, and an economic component representing the production opportunity set, resource constraints, and price parameters farmers face. The model is applied to two zones in Northwestern Mali, Sirakorola and Mourdiah, and solved using non-linear, dynamic programming. Data collected by the USAID IPM-CRSP/Mali project are used to specify the economic parameters of the model. A new technique for estimating the lower bound of a farmer's production planning horizon is also developed and employed in the application of the model to Sirakorola and Mourdiah. The results of several model scenarios indicate that the availability of information regarding the efficacy of Striga control practices is a primary barrier to their adoption by subsistence farmers. The movement of Striga seed between fields, however, is of limited importance. The "optimal control practices" identified by the model depend on the size and demographic composition of the production unit (UP), the zone in which the UP is located, and the cash budget available to the UP. At low budget levels, the model suggests planting millet without fertilizer at a high density in Sirakorola and a low density in Mourdiah. At high budget levels, the model suggests planting millet at a high density in both zones while applying urea. The benefits of adopting the optimal set of practices are presented in both nutritional and financial terms, and can reach as much as a ten-fold increase in the nutritional content of and financial returns to a harvest. / Ph. D.

La présente recherche avait pour but de révéler la diversité génétique qui existe au sein et entre différentes populations d’Afrique de l’Ouest du parasite épirhize Striga gesnerioides. Cette plante parasite plusieurs espèces appartenant à différentes familles de dicotylédones, dont le niébé (Vigna unguiculata), une légumineuse alimentaire constituant la majeure partie des protéines retrouvées dans la diète de plusieurs populations rurales des zones semi-arides d’Afrique de l’Ouest. Certains cultivars résistants ont déjà été identifiés et sont utilisés dans les programmes d’amélioration variétale. Il existe cependant plusieurs biotypes, ou races physiologiques, de S. gesnerioides, lesquels diffèrent quant à leur virulence. En utilisant trois types de marqueurs moléculaires différents, soit des marqueurs AFLP, ISSR et cpSSR, nous avons mis en évidence la quasi-absence de variabilité au sein des populations de Striga étudiées, ainsi que la très faible diversité qui existe entre les différentes populations du parasite. Nous n’avons pas non plus trouvé de marqueurs permettant de discriminer entre les races. Il semble exister une certaine structure dans la distribution géographique des populations, mais aucun groupe monophylétique n’a été obtenu sur une base « raciale », indiquant que la virulence ne joue pas encore un rôle dans leur différentiation. Quelques hypothèses ont été émises pour expliquer la faible diversité et l’absence de marqueur de races, dont le mode de reproduction autogame du parasite, ainsi qu’une origine probablement récente de la forme de Striga gesnerioides parasitant le niébé. / The goal of the present study was to reveal the genetic diversity within and among different West African populations of the root parasite Striga gesnerioides. This plant parasitizes many species from different dicotyledonous families, including cowpea (Vigna unguiculata), an important legume crop and the major dietary protein source for many people of the semi-arid regions of West Africa. Some resistant cowpea varieties have been identified and are used in breeding programs. However, based on host-parasite interactions in the field, various races of S. gesnerioides attacking cowpea have been identified. Using three different types of molecular markers, AFLP, ISSR and cpSSR, we showed that there is almost no genetic variability within populations. The variability between the populations was also extremely low and did not allow discrimination of the five races. A few populations were more closely related, and there was a certain geographical structure but no “racial” clustering could be seen, enhancing the fact that virulence is not yet involved in the genetic differentiation process. Possible causes of the extremely low level of genetic variability seen in S. gesnerioides are proposed including the autogamous mode of reproduction of the parasite and the hypothesis that the cowpea strain has only quite recently arisen.

Striga, a genus of obligate parasitic weeds in the family Orobanchaceae, has been identified as the most important biological factor limiting agricultural productivity in sub-Saharan Africa. Germination of Striga seeds is triggered by strigolactone root exudates from host plants. Strigolactones also induce hyphal branching in arbuscular mycorrhizal (AM) fungi, which are important for plant uptake of phosphorus in low phosphorus soils. Mechanisms of Striga resistance based on reduced strigolactone production may also convey resistance to AM fungi which would require higher inputs of phosphorus fertilizer to attain optimal crop growth. There is evidence for genetic differences in mycorrhizal responsiveness in other grain crops; therefore it is beneficial for breeders to be aware of these differences when developing Striga-resistant sorghum cultivars. This research aims to determine phosphorus and mycorrhizal responsiveness of sorghum genotypes important for or developed by breeders working on Striga resistance. Phosphorus response curves were determined for twelve sorghum genotypes using pasteurized low phosphorus soil amended to achieve four different phosphorus levels. Simple linear regression was performed on root and shoot dry weight data. Results indicate variability in phosphorus responsiveness within Striga resistant and susceptible genotypes. Seven of these genotypes were selected for continued research, which analyzed responsiveness to phosphorous and differences in mycorrhizal responsiveness in relation to reported mechanisms of Striga resistance. Treatments included three levels of phosphorus amendments and the addition of Funneliformis mosseae inoculum. All genotypes were strongly responsive to P amendment when grown without AM fungi and showed a decrease in responsiveness to P when inoculated with F. mosseae. Trends for all genotypes indicate a greater uptake of P, Zn, and Mg by mycorrhizal plants as compared to nonmycorrhizal plants. All seven genotypes were responsive to mycorrhizae, with a significant increase in biomass for all genotypes, especially at the lowest phosphorus level. The responsiveness to the mycorrhizal fungus does not appear to be directly related to the susceptibility of genotypes to the parasitic weed Striga. / Graduation date: 2012