No description available.

<p><strong>ABSTRACT</strong></p> <p>As plants are sessile organisms, detecting the presence of neighboring plants and exhibiting competitive behavior to acquire limiting resources is crucial. One of the ways plants respond to belowground competition is by allocation to fine roots in order to acquire the limited resources. However, this phenotypic plasticity can be costly as it assigns resources away from reproduction. Being able to recognize the relatedness of one′s neighbours and preferentially compete with strangers is a beneficial trait that can minimize the costs of competition with relatives and increases inclusive fitness. Many studies have looked at the association between resource availability and competition in plants while others have observed kin recognition in several plants species. However, no one has yet studied the effect of resource availability on kin recognition in plants. Here, I looked at root architecture to test if there is an association between kin recognition and nutrient availability in <em>Cakile edentula</em>.</p> <p>I found that the root system architecture is highly plastic and complex, showing variable responses to neighbour identity signals and resource availability. The results from the four experiments demonstrate that the responses of <em>C. edentula </em>to neighbour relatedness are dependent on nutrient availability. Additionally, this study also indicates that kin recognition in <em>C. edentula</em> does not require root contact; instead it occurs through a signal found in soluble compounds excreted from plants, possibly root exudates, as observed in <em>Arabidopsis thaliana</em> <em>(</em>Biedrzycki et al. 2010).</p> <p>In conclusion, this study provides novel findings regarding the dynamics of root behavior in response to nutrient availability and the relatedness of neighbours.</p> / Master of Science (MSc)

The aims of this study were to determine the efficacy of the KontrolFlexTM AccufileTM compared to the READY•STEEL™ Flexofile® using an electronic apex locator and to evaluate the instrument’s design via scanning electron microscopy. Actual canal lengths of thirty extracted teeth were determined under magnification. An alginate model was used to determine experimental canal lengths with the Root ZX II apex locator and size #10 Accufiles and Flexofiles. Differences between actual lengths and experimental lengths were compared with Student’s t test. The average experimental lengths were short of the actual length by -0.10 mm (±0.34) and -0.12 mm (±0.16) for the Accufile and Flexofile, respectively with no statistical difference (p > 0.05). SEM images revealed similar non-cutting “batt” tips and a square cross-section for the Accufile and triangular for the Flexofile. Both files provided similarly high levels of reliability when used with the Root ZX II electronic apex locator.

Root-feeding insects can have top-down influence on vegetative composition and ecosystem processes; however, they may respond to bottom-up factors such as soil resources, site productivity, and disturbance.  My research addressed the following questions: (1) Do disturbance (fire), vegetative composition, soil resources, and fine root standing mass influence the structure of root-feeding insect assemblages? (2) What types of roots do root-feeding insects eat, and do they forage selectively?  (3) Do root-feeding insects influence fine root productivity?  To address these questions, I studied root-feeding insect assemblages in longleaf pine wiregrass (Pinus palustris-Aristida stricta) ecosystems of southwestern Georgia, U.S.A.  On a random basis, study sites were burned at least every other year (B), or left unburned (UB) for about 9 years.  Fine root productivity and root-feeding insect abundances were sampled repeatedly across 54 random plots in UB and B sites.  In Chapter 2, I characterized spatial and temporal patterns of root-feeding insect abundance, understory plant composition, soil resource availability, and fine root standing mass within each plot.  Insect population densities were low overall, but abundance, patchiness, and diversity were greater in UB sites.  Abundance patterns were significantly related to vegetative composition.  In Chapter 3, I quantified the diet of root-feeding insects by measuring the natural abundance of carbon (C) and nitrogen (N) stable isotopes in insects and fine roots.  Using 13C abundance, I examined the contribution of warm season grass roots to insect diet, relative to the proportion of warm season grass roots within adjacent root standing crop samples; 15N abundance was used to detect omnivory.  Overall, insects appeared to be non-selective herbivores and omnivores that may alter foraging behavior to maintain a mixed diet (i.e. reducing or increasing warm season grass consumption when its abundance was high or low, respectively).  The extent of omnivory varied within and among taxa.  In Chapter 4, I estimated the top-down influence of root-feeding insects on fine root productivity by comparison of ingrowth cores with or without an insecticide treatment.  I detected a weak positive effect of herbivores on the productivity of non-grass fine roots (< 10% of fine root productivity). / Ph. D.

In Medicago truncatula, the MtNPF1.7 transporter has been shown to be essential for root morphology and nodulation development. The allelic MtNPF1.7 mutants, Mtnip-1 (A497V), Mtnip-3 (E171K), and Mtlatd (W341STOP), show altered lateral root growth and compromised legume-rhizobium symbiosis. To assess the role of a series of distinct amino acids in the transporter's function, in silico structural predictions were combined with in planta complementation of the severely defective Mtnip-1 mutant plants. The findings support hypotheses about the functional importance of the ExxE(R/K) motif including an essential role for the first glutamic acid of the motif in proton(s) and possibly substrate transport. The results also question the existence of a putative TMH4-TMH10 salt bridge, which may not form in MtNPF1.7. Results reveal that a motif conserved among MFS proteins, Motif A, is essential for function. Hypothetically, the Motif A participates in intradomain packing of transmembrane helices and stabilizing one conformation during transport. The mutated valine (A497V) in Mtnip-1 may interfere with the lateral helix. Mutating a residue (L253) on the lateral helix with reduced side chain restored Mtnip-1 function. The predicted residue (Q351) for substrate binding is not essential for protein function. To probe the possibility that MtNPF1.7 transports auxin, two heterologous assay systems were attempted. The first was a Xenopus laevis oocyte assay. However, MtNPF1.7 expressed in oocytes failed to show substrate transport, which may due to low expression levels of proteins on the membrane or may be caused by other factors. Second, yeast (Saccharomyces cerevisiae) strains expressing MtNPF1.7 were constructed. They showed an increased flux of radiolabeled IAA and differential susceptibility to 5-fluoroindole-3-acetic acid (F-IAA), a toxic IAA-like compound. These results suggested that MtNPF1.7 may function as an auxin transporter in yeast. Unexpectedly, the Mtnip-1 (A497V) and Mtnip-3 (E171K) proteins when expressed in yeast also showed influx of F-IAA transport in yeast, suggesting the auxin transport remains functional in the mutants, potentially contradicting other results suggesting Mtnip-1 is non-functional. To address the possible role of MtNPF1.7 in auxin responses, the auxin reporter DR5-GUS was used and mutant plants were subjected to gravitropism tests. The auxin reporter DR5::GUS displayed different expression in Mtnip-1 mutant in comparison with DR5::GUS expression in wild-type. In addition, the spatial expression of pMtNPF1.7-GUS partly co-localized with the expression of DR5:GUS in roots and nodules. The Mtnip-1 plants showed altered gravitropic responses to gravistimulus compared to wild-type and Mtnip-3 plants. The results suggest that MtNPF1.7, recognized as a nitrate transporter, may be able to transport auxin and may participate in auxin distribution during M. truncatula root and nodule development. These research findings helps elucidate the complex function of plant NPF transporters.

L'obiettivo della ricerca è stato quello di identificare la coltura bioenergetica con il maggior potenziale di sequestro del carbonio (C); sono state considerate tre colture perenni arboree (pioppo, robinia e salice) e tre colture erbacee perenni (canna comune , miscanto e panico ) al sesto anno dal loro impianto e coltivate nello stesso ambiente. In primo luogo sono state misurate le variazioni dei tassi del C organico del suolo (COS) per il primo 1 m, mentre per i primi 30 cm di suolo è stato stimato il grado di stabilita del COS valutando sette frazioni di COS che presentano differenti gradi di stabilizzazione; in secondo luogo, sono stati caratterizzati gli apparati radicali delle sei specie per la stessa profondità di suolo, per valutare dove le specie accumulano la biomassa radicale lungo il profilo di suolo. I risultati confermano che l’impianto di colture bioenergetiche perenni su superfici precedentemente dedite a colture annuali gestite convenzionalmente rappresenta una opzione valida per sequestrare C nel soulo. Tuttavia, è stata osservata una diversa capacità di sequestro di C tra specie arboree ed erbacee: le specie arboree hanno dimostrato aumentre il contenuto di COS nel primo strato di suolo ( 0-10 cm di suolo), ma la loro capacità di allocare biomassa radicale negli strati profondi del suolo è limitata; mentre, la specie erbacee allocano un’alta quantità di biomassa radicale negli strati profondi del suolo, ma solo il panico ed il miscanto hanno aumentato il contenuto di C nel primo strato di suolo. / The objective of the present research was to identify the bioenergy crop with the greatest carbon sequestration potential among three perennial woody crops (poplar, black locust and willow) and three perennial herbaceous crops (giant reed, miscanthus and switchgrass) at the sixth year from plantation and in the same location. First of all the SOC stock variations for the first 1 m soil depth and the quantification of seven soil C fractions related to SOC stabilization level of the first 30 cm of soil were assessed; secondly, a characterization of the root system and the traits which affect the carbon allocation in soil were considered. The results confirm that the establishment of perennial bioenergy crops in previous arable fields can be a suitable option to sequester carbon (C) belowground. However, a different C sequestration capacity was observed between woody and herbaceous crops: woody species showed the greatest SOC sequestration potential in the first soil layer (0-10 cm of soil) but their ability to allocate root biomass in the deeper soil layers was limited; while, the herbaceous species allocated a high amount of root biomass in the deeper soil layers, but only switchgrass and miscanthus sequester C in the first soil layer.

As the title suggests, the purpose of this investigation is to establish a clear picture of what constitutes a verbal root in Xhosa, and to classify the various means whereby a simple verbal root may be extended to modify its meaning. / African Languages / D. Litt. et Phil. (Bantu Languages)

No description available.

Cette thèse s’applique à améliorer la compréhension de l’absorption du cadmium (Cd) par le maïs et le tabouret calaminaire, dans l’objectif de mieux modéliser son transfert vers les plantes comestibles ou hyperaccumulatrices. Le modèle utilisé étant sensible aux caractéristiques d’absorption racinaire, le premier objectif était de développer une méthode rigoureuse de mesure de ces paramètres. Deux protocoles ont été mis au point pour décrire précisément, en fonction de la concentration de Cd en solution, l’influx net de Cd dans les parois et dans le milieu intracellulaire des racines. Les résultats ont mis en évidence, pour la première fois, l’existence d’un système de transport à faible affinité (LATS) qui agit en même temps que le système de transport à forte affinité pour le Cd (HATS). Les nouveaux paramètres cinétiques mesurés n’ont cependant pas amélioré significativement le modèle : le prélèvement de Cd par le maïs est surestimé de 100%, et son prélèvement par le tabouret calaminaire est sous-estimé de 66%. Plusieurs facteurs ont alors été étudiés pour comprendre les raisons de ce décalage. Nous avons montré que les conditions dans lesquelles ont été mesurés les paramètres cinétiques présentaient des caractéristiques capables de modifier radicalement leurs valeurs : la composition ionique de la solution d’exposition au Cd, la concentration de Cd durant la croissance et la structure racinaire engendrée par l’hydroponie. Cette thèse suggère de cultiver les plantes en aéroponie et de mesurer les paramètres cinétiques dans une composition ionique représentative du voisinage des parois et des membranes racinaires en sol, composition qu’il reste encore à déterminer. / This thesis aimed to improve the comprehension of cadmium (Cd) absorption by maize and alpine pennycress plants in order to model better its transfer into edible and hyperaccumulating plants. Since the model used is sensitive to the characteristics of root absorption, the first objective was to develop a rigorous method for measuring those parameters. Two protocols were finalized to describe precisely Cd net influx in both root compartments (apoplast and symplast) according to Cd concentration in solution. The results highlighted, for the first time, the existence of a low-affinity transport system (LATS) that works at the same time with the high-affinity transport system (HATS). However, the kinetics parameters measured through these experimentations did not succeed in improving significantly the model: Cd uptake by maize is overestimated by 100%, and Cd uptake by alpine pennycress is under-estimated by 66%. Several factors were investigated in order to understand the reasons of this difference. We showed that the experimental conditions used to measure the kinetics parameters present characteristics that are able to modify their values significantly: the ionic composition of the solution of exposition to Cd, Cd concentration during growth, and the root structure that forms in hydroponics. This thesis suggests to choose aeroponics as controlled culture condition, and to measure the kinetics parameters in an ionic composition that is representative of the close vicinity of the root cell walls and membranes in soil. This composition remains to be investigated.

Le comportement mécanique d’un sol constitue un des facteurs prépondérants affectant la croissance et le développement racinaire. L’augmentation de la résistance mécanique d’un sol affecte l’architecture du système racinaire, la croissance axiale et radiale des racines, et l’orientation de la croissance. Bien que ces réponses soient décrites sur diverses espèces végétales dans la littérature, leur mise en place et les mécanismes de signalisation sous-jacents restent à ce jour méconnus. L’objectif principal de cette thèse est d’étudier la mise en place des réponses des racines primaires d’Arabidopsis thaliana à des variations de contraintes mécaniques de son substrat. La littérature fait état jusqu’ici de deux voies de signalisation mécanique possibles, l’une impliquant une mécanoperception au niveau de la coiffe racinaire, et l’autre faisant intervenir entres autres l’activation de canaux mécano-sensibles au niveau de l’apex racinaire. Pour étudier leur implication respective, nous avons caractérisé les réponses à une augmentation des contraintes mécaniques de racines primaires issues de la lignée Arabidopsis thaliana sauvage et de lignées mutantes affectées au niveau de l’organisation de la coiffe (fez-2, smb-3, brn1-1 brn2-1) ou au niveau de canaux ioniques mécanosensibles (mca1-null, mslΔ5). Nous avons développé un système expérimental original basé sur le suivi de croissance et d’orientation des racines dans des milieux de culture à base de Phytagel. Ces milieux de culture permettent de mimer d’une part une augmentation à long terme et simultanée des contraintes mécaniques axiales, radiales et des frottements et d’autre part une augmentation à court terme des contraintes mécaniques axiales. Tout d’abord, nous avons observé que la mise en place des réponses de croissance des racines primaires d’Arabidopsis thaliana à ces augmentations de contraintes mécaniques reposait sur une interaction entre des processus mécaniques (flambement) et biologiques (régulations de la zone de croissance). Ensuite, nous avons mis en évidence que la coiffe racinaire participait à la mise en place des réponses des racines d’Arabidopsis thaliana face à une augmentation des contraintes mécaniques. Nos résultats suggèrent que la coiffe pourrait constituer un siège de la mécanoperception. Par ailleurs, nos résultats sur les lignées mca1-null et mslΔ5 suggèrent que les canaux ioniques MCA1 et MSLs participeraient également à la mise en place des réponses racinaires à l’augmentation des contraintes mécaniques. / Root growth and development are highly modulated by soil mechanical properties such as texture, structure and bulk density. Increases in soil penetration resistance affect root system architecture, root cell production and elongation, root diameter, and root tip orientation. Although root responses to changes in mechanical stresses are well described in several plant species, their establishment and the signaling pathways underlying these responses remains misunderstood. The main objective of this thesis is to study the establishment of Arabidopsis thaliana primary root responses to changes in mechanical stresses. So far, studies reported two distinct putative mechanical signaling pathways involving either (i) the root cap as a main mechanosensing site or (ii) the activity of stretch-activated ion channels localized on the cell plasma membranes along the root apex. According to previous studies, we studied the implication of the root cap and of the stretch-activated channels of the MCA and MSL families in the root responses to increases in mechanical stresses. We developed an original experimental set-up based on the monitoring of growth and orientation of Arabidopsis thaliana primary roots in one-layer and two-layer Phytagel based growth media. The one-layer growth media mimicked a long-term increase in frictions and in radial and axial mechanical stresses whereas the two-layer growth media mimicked a short-term increase in axial mechanical stresses. We characterized penetration abilities, growth rate and tip orientation of Arabidopsis thaliana wild type roots (Col-0) and mutant roots showing defects in root cap organization (fez-2, smb-3, brn1-1 brn2-1) or in stretch-activated ion channels activity (mca1-null, mslΔ5) in one-layer and two-layer media. Firstly, we observed that the establishment of root growth responses to increases in mechanical stresses involved both mechanical (buckling) and biological (growth zone modulations) mechanisms. Then, our experiments demonstrated that alterations in root cap organization led to altered root growth responses to increases in mechanical stresses. Our results suggest that the root cap acts as a mechanosensing site. Moreover, the results obtained with the mca1-null and mslΔ5 Arabidopsis lines suggest that the MCA1 and MSLs ion channels also participate in the establishment of root responses to increases in mechanical stresses.