No description available.

Controlled environment agriculture offers a solution to challenges including less available land, water deficits, and consumer demand for pesticide free produce. However, control of soil-borne diseases is a major limiting factor. The goal of this dissertation was to examine predictions of the hypothesis that border cells function to protect plant health by controlling microorganisms associated with plants grown in hydroponic culture. Border cells separate from root tips upon immersion in water, and appear to have important roles in the defense mechanisms of plant roots. The general objectives were (1) to study the delivery of border cells in hydroponics; (2) to evaluate interactions between border cells and microorganisms in hydroponics; and (3) to explore approaches to alter border cell production for improved root disease control. In this study it was confirmed that border cells can be released continuously into the solution of hydroponic culture suggesting that plants grown in this system may use extra energy in the production of new border cells. Free border cells interacted with microorganisms present in the hydroponic solution by secreting an extracellular capsule. Previous studies showed that proteins are a key component of this capsule, including lectins. The interaction of pea lectin and Nectria haematococca spores therefore was explored. Results demonstrated that pea lectin agglutinates fungal spores in a hapten-specific manner, and inhibits their germination. Lectin had no negative effect on root development suggesting that it could be used as a potential control for soil-borne diseases in hydroponics. To control the production of border cells, subsequent studies measured the impact of a transient exposure of root tips to different metabolites secreted by root caps and border cells. Exposure to specific metabolites altered the production of border cells without measurable effects on root growth and development. This is in contrast to results obtained with altered gene expression. For example, gene silencing of a border cell specific gene resulted in altered root growth.

Plant roots are central to C- and N-cycling in soil. However, (i) plants differ strongly in tissue recalcitrance (e.g. lignin content) affecting their mineralization in soil, and (ii) rhizodeposits also vary strongly in terms of the metabolites that they contain. Therefore, (i) we used 13C labelled ryegrass root and shoot residues as substrates to investigate the impact of tissue recalcitrance on soil processes through controlled incubation of soil, (ii) we assessed variations in root C-deposition between barley genotypes and their respective impacts on soil processes using 13CO2 labelled plants, (iii) using 13C/15N enriched ryegrass root residues as tracer material, we investigated the impacts of barley genotypes on mineralization of recently incorporated plant residues in soil and plant uptake of the residue-derived N, and (iv) we applied a quantitative trait loci analysis approach to identify barley chromosome regions affecting soil microbial biomass and other soil and root related traits. In the first study, addition of root residues resulted in reduced C-mineralization rates, soil microbial activity and soil organic matter (SOM) priming relative to shoot residues. Planted experiments revealed (i) genotype effects on plant-, SOM- and residuederived surface soil CO2-C efflux and showed that incorporation of plant derived-C to the silt-and-clay soil fraction varied between genotypes, indicating relative stabilization of root derived-C as a result of barley genotype, (ii) that plant uptake of residue released N between genotypes was linked to genotype impacts on residue mineralization, and (iii) barley chromosome regions that influence plant-derived microbial biomass C. These results (i) suggest that greater plant tissue recalcitrance can lower soil C-emissions and increase C-storage in soil, and (ii) demonstrate the barley genetic influence on soil microbial communities and C- and N-cycling, which could be useful in crop breeding to improve soil microbial interactions, and thus promote sustainable crop production systems.

L'auxine régule la croissance et le développement des plantes grâce aux facteurs de transcription de la famille des "AUXIN RESPONSE FACTOR" (ARF). Chez Arabidopsis thaliana en particulier, ARF5, 6, 7, 8 et 19 activent l'expression de gènes cibles en réponse à l'auxine. Ces cinq ARF activateurs contrôlent de façon plus ou moins redondante des processus divers au cours du développement de la plante, notamment la régulation des croissances au niveau des méristèmes racinaires et aériens ainsi que la formation des racines latérales ou des méristèmes axillaires.Chacun de ces cinq ARF activateurs présente des patrons d'expression uniques dans chacun des tissus racinaires et aériens, en association avec leurs fonctions particulières. Il est probable que cette expression tissu-spécifique trouve son origine dans un contrôle différencié de leur transcription. Dans cette étude, des régulateurs amonts de la transcription de ARF5, 6, 7, 8 et 19 ont été identifiés par une méthode haut-débit de crible simple hybride en levure (Y1H). Une procédure d'expression transitoire en protoplastes a permis de confirmer que l'expression de chaque ARF activateur est contrôlée par des régulateurs spécifiques, dont la majorité se comportent comme des répresseurs de la transcription des ARF in planta. Parmi les régulateurs identifiés, les facteurs de transcription ont été étudiés grâce à des mutants pour préciser les interactions in planta. Ces mutants montrent des phénotypes développementaux typiques de perturbations de l'auxine dans les racines et les tiges : altérations des cinétiques de croissance, de l'émergence des organes latéraux ou de réponses à l'auxine et modification de l'expression des ARF activateurs.Par ailleurs, ce travail aborde également les dialogues entre les voies de signalisation de l'auxine et des cytokinines, et en particulier le rôle de ces interactions dans le développement des racines et des tiges. Une des interactions identifiées dans le crible Y1H est la répression de ARF7 par CRF10, un gène membre de la famille des "Cytokinin Response Factors". Nous avons mis en évidence l'importance de cette interaction pour le maintien de l'architecture du méristème apical racinaire, pour la sénescence des feuille et pour la réponse phototropique à la lumière bleue dans les hypocotyles. / Auxin regulates plant growth and development through the transcription factors of the AUXIN RESPONSE FACTOR (ARF) gene family. Most notably in Arabidopsis thaliana ARF5, 6, 7, 8 and 19 activate expression of target genes in response to auxin. These five ARF activators control both variable and overlapping processes during plant development including regulation of growth at the root and the shoot apical meristems, lateral root and axillary shoot formation. Each of the five ARF activators shows unique tissue-specific expression patterns in the root and the shoot associated with their distinct functions. This tissue-specific expression is likely derived from the differences in the control of ARF activator transcription. In this study the upstream regulators of ARF5, 6, 7, 8 and 19 transcription were identified. This was achieved by utilizing a high-throughput yeast one-hybrid (Y1H) method. The transient protoplast assay revealed that each ARF activator is controlled by specific transcriptional regulators and that the majority of these regulators are repressors of ARF transcription in planta. Mutants of the regulatory transcription factors were utilized to additionally investigate the interactions in planta. These mutants display auxin-related developmental phenotypes in the root and the shoot including alternations in growth kinetics, emergence of lateral organs, responses to auxin and altered expression of ARF activators. Furthermore, this study additionally focuses on cross-talk between the auxin and cytokinin signaling pathways and its role in root and shoot development. One of the interactions identified in the Y1H screen is a repression of ARF7 by CRF10, a member of the Cytokinin Response Factors gene family. The importance of this interaction in maintaining architecture of the root apical meristem, in leaf senescence and in the phototropic response to blue light in hypocotyls is studied.

No description available.

This study is the first to consider how the combination of root physical effects, microbial production of EPS, and root effects on the hydrodynamic boundary layer could influence streambank soil erodibility. Specifically, the goal of this research was to quantify the physical and biological effects of roots on streambank fluvial erosion. A series of laboratory-scale erosion tests were conducted using a mini jet erosion testing device and a recirculating flume channel to address this goal. Several soil and vegetation factors that influence fluvial entrainment, like extracellular polymeric substances (EPS), soil aggregate stability and root length density, were measured following erosion testing. For flume experiments, three streambank boundary conditions were constructed to simulate unvegetated streambanks, as well as streambanks with herbaceous and woody roots. Soil treatments were also created to represent unamended and organic matter (OM) amended soil either without roots (bare soil), with synthetic roots, or with living roots (Panicum virgatum). Median soil erosion rates along the simulated rooted boundaries were two to ten times higher compared to the unvegetated boundary due to protruding root impacts on the boundary layer. In flume experiments, median erosion rates were 30% to 72% lower for unamended soils containing compacted synthetic root fibers as compared to bare soil samples. Adding both OM and fibers to the soil had a greater effect; the median erosion rate reductions of live rooted treatments (95% to 100%) and synthetic rooted + OM treatments (86% to 100%) were similar and statistically lower than bare soil controls. Stimulated microbial production of EPS proteins were significantly correlated with increased erosion resistance in OM-amended treatments while OM treatments had significantly lower EPS carbohydrates compared to unamended treatments. In summary, while sparsely spaced roots exposed on streambanks may increase soil erosion rates due to impacts on the hydrodynamic boundary layer, overall results highlight how the synergistic relationship between root fibers and soil microbes can significantly reduce streambank soil erodibility due to fiber reinforcement and EPS production. / Doctor of Philosophy / Plant roots are known to protect streambank soils from erosion by water; however, exactly how roots provide this protection has remained unclear. Among other things, roots can influence streambank soil erosion by holding soil together through a thick root network, interacting with soil microorganisms to stimulate the release of "sticky" organic compounds called extracellular polymeric substances (EPS), and altering the force of the water against the streambank. This research aimed to quantify and compare the relative importance of these three mechanisms on streambank soil erosion using a mini jet erosion testing device and an indoor recirculating flume channel. To do this in the flume, three walls were constructed to simulate unvegetated streambanks, as well as streambanks with herbaceous and woody roots. In greenhouse settings, soil treatments were also created to represent unamended and organic matter (OM) amended soil either without roots (bare soil), with artificial roots, or with living roots (Panicum virgatum). While roots protruding out of streambanks appeared to increase median soil erosion rates due to the impact of roots on near-bank flow, artificial roots in the soil and OM amended soils reduced soil erosion rates. Specifically, OM amendments stimulated the production of EPS proteins, leading to improved soil stability and increased soil resistance to erosion by water. Overall results highlight how the synergistic relationship between root fibers (living roots and artificial roots) and soil microbes can significantly reduce streambank soil erodibility due to root binding and microbial production of EPS. While plant roots naturally provide both fibers and EPS to soils, these materials could be incorporated into fill soils during construction to rapidly increase soil erosion resistance following levee construction and stream restoration projects.