Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Alien invasive plants are of concern in the world because of their potential to spread into the natural environment. Invasion patterns observed in plant species can be attributed among other things to favourable environmental conditions. Understanding invasion dynamics of alien invasive plants can help in timely intervention initiatives. In Mpumalanga, Lilium formosanum (Liliaceae) appears to be an emerging plant invader and is spreading in the natural environment. The study investigated the invasion extent and predictor variables which could explain abundance patterns of L. formosanum in the invasive range in Mpumalanga province in South Africa and further predict regions of the world that could be climatically suitable. Lilium formosanum was surveyed along 11 major routes leading into the towns of Sabie and Graskop in Mpumalanga from a slow moving vehicle. A total of 241 kilometres was surveyed. Lilium formosanum was found to be invasive from the towns of Sabie and Graskop. Climatically suitable areas were predicted using DIVA-GIS and climate data was obtained from WORLDCLIM database while occurrence records were obtained from the Southern African Plant Invaders Atlas, Australia’s virtual herbarium and the Global Biodiversity Information Facility. In conclusion, distance, altitude and route were found to have an influence on the abundance of L. formosanum in Mpumalanga Province. Climatically suitable areas included the eastern coastal belt, northern provinces and the interior parts of South Africa including major parts of Limpopo and Mpumalanga. / Dissertation (MSc)--University of Pretoria, 2014. / Plant Production and Soil Science / MSc / Unrestricted

La fertilisation chez les plantes dépend de la livraison des cellules spermatiques contenues dans le pollen à l’ovule. Au contact du stigmate, le grain de pollen s’hydrate et forme une protubérance, le tube pollinique, chargé de livrer les noyaux spermatiques à l’ovule. Le tube pollinique est une cellule à croissance rapide, anisotrope et non autotrophe; ainsi tout au long de sa croissance à travers l’apoplaste du tissu pistillaire, le tube pollinique puise ses sources de carbohydrates et de minéraux du pistil. Ces éléments servent à la synthèse des constituants de la paroi qui seront acheminés par des vésicules de sécrétion jusqu’à l’apex du tube. Ce dernier doit aussi résister à des pressions mécaniques pour maintenir sa forme cylindrique et doit répondre à différents signaux directionnels pour pouvoir atteindre l’ovule. Mon projet de doctorat était de comprendre le rôle du cytosquelette dans la croissance anisotrope du tube pollinique et d’identifier les éléments responsables de sa croissance et de son guidage. Le cytosquelette du tube pollinique est composé des microfilaments d’actine et des microtubules. Pour assurer une bonne croissance des tubes polliniques in vitro, les carbohydrates et les éléments de croissance doivent être ajoutés au milieu à des concentrations bien spécifiques. J’ai donc optimisé les conditions de croissance du pollen d’Arabidopsis thaliana et de Camellia japonica qui ont été utilisés avec le pollen de Lilium longiflorum comme modèles pour mes expériences. J’ai développé une méthode rapide et efficace de fixation et de marquage du tube pollinique basée sur la technologie des microondes. J’ai aussi utilisé des outils pharmacologiques, mécaniques et moléculaires couplés à différentes techniques de microscopie pour comprendre le rôle du cytosquelette d’actine lors de la croissance et le tropisme du tube pollinique. J’ai trouvé que le cytosquelette d’actine et plus précisément l’anneau d’actine localisé dans la partie sub-apicale du tube est fortement impliqué dans la croissance et le maintien de l’architecture du tube à travers le contrôle de la livraison des vésicules de sécrétion. J’ai construit une chambre galvanotropique qui peut être montée sur un microscope inversé et qui sert à envoyer des signaux tropistiques bien précis à des tubes polliniques en croissance. J’ai trouvé que les filaments d’actine sont impliqués dans la capacité du tube pollinique à changer de direction. Ce comportement tropistique dépend de la concentration du calcium dans le milieu de croissance et du flux de calcium à travers des canaux calciques. Le gradient de calcium établi dans le tube pollinique affecte l’activité de certaines protéines qui se lient à l’actine et dont le rôle est la réorganisation des filaments d’actine. Parmi ces protéines, il y a celles de dépolymérisation de l’actine (ADF) dont deux spécifiquement exprimées dans le gamétophyte mâle d’Arabidopsis (ADF7 et ADF10). Par marquage avec des proteins fluorescents, j’ai trouvé que l’ADF7 et l’ADF10 ont des expressions différentielles pendant la microsporogenèse et la germination et croissance du tube pollinique et qu’elles partagent entre elles des rôles importants durant ces différents stades. / Fertilization in plants depends on the delivery of the sperm cells in the pollen grain through the pollen tube to the ovule. The pollen tube is a highly anisotropic, fast growing cellular protuberance. Because the pollen tube is non autotrophic, it requires a steady supply of carbohydrates and minerals supplied by the pistil to sustain its growth. These elements serve for the synthesis of cell wall material, delivered to the site of cell wall assembly in secretory vesicles that are transported along the actin cytoskeleton and deposited at the growing apex of the tube. The tube has to resist external deformation forces in order to maintain its cylindrical shape and to respond to various directional signals in order to reach its target. My objectives were to identify the role of the cytoskeleton in the anisotropic growth of the pollen tube and to determine how the tube responds to directional cues. The cytoskeleton in the pollen tube consists of microfilaments and microtubules, both forming long filamentous elements. For in vitro growing pollen tubes, carbohydrates and growth minerals have to be added to the growth medium in specific amounts order to sustain pollen tube growth. I optimized the growth conditions of Arabidopsis thaliana and Camellia japonica pollen tubes which, in addition to pollen from Lilium longiflorum, were used as model species for my experiments. I developed a microwave based, fast and efficient fixation and labelling protocol for pollen tubes. I used pharmacological, mechanical, molecular and microscopical tools to study the role of the cytoskeleton in pollen tube growth and tropism. I found that the actin cytoskeleton, and more specifically the subapical actin fringe, plays an important role in the regulation of pollen tube growth and architecture through the controlled delivery of secretory vesicles to the growing apex. I constructed a galvanotropic chamber that can be mounted on an inverted microscope to induce controlled tropic triggers. I found that the actin cytoskeleton is also involved in the ability of the pollen tube to change its direction. This tropic behaviour was shown to be dependent on the concentration of calcium ions in the growth medium and calcium influx through calcium channels. The cytosolic calcium gradient in the pollen tube regulates the activity of various actin binding proteins that are responsible for remodelling the actin cytoskeleton. Among these proteins are two Arabidopsis gametophyte-specific actin depolymerizing factors (ADFs) that I tagged with two intrinsically fluorescent proteins. I found that ADF7 and ADF10 are differentially expressed during microsporogenesis and pollen tube germination and growth and that they likely divide important functions between them.

No description available.

Lily leaf spot is an emerging disease of the globally rare Lilium grayi S. Wats., a species endemic to the southern Appalachian Blue Ridge, USA. The species is considered Threatened or Endangered in the three states where it naturally occurs (North Carolina, Tennessee, Virginia). Disease leads to premature senescence of aboveground tissues and curtailment of sexual reproduction. Spore morphology, completion of Koch’s postulates, and DNA sequence data showed the causative agent of lily leaf spot to be Pseudocercosporella inconspicua (G. Winter) U. Braun, a Lilium-specific basidiomycete. Diagnostic disease symptomatology includes amphigenous necrotic lesions with tan to green margins encircling a white to grey powdery mass of conidia. Studies conducted in the field at Roan Mountain, NC/TN, in the largest known population, showed that a visual disease diagnosis based on morphology can be highly accurate in predicting P. inconspicua infection, and that high concentrations of conidia of P. inconspicua are strongly associated with infected L. grayi, but are largely absent or few on uninfected L. grayi and other species. Field inoculation trials using infected L. grayi leaf tissue as inocula resulted in transmission of disease and induction of premature senescence under natural conditions.

A population of the rare Southern Appalachian endemic species Lilium grayi, (Gray’s lily) Roan Mountain, TN/NC was monitored for 2 years to determine the cause and impact of an early season collapse. High concentrations of the Lilium spp. host-specific fungal phytopathogen, Pseudocercosporella inconspicua (G. Winter) U. Braun were associated with 19/20 symptomatic and 0/30 asymptomatic plants. Strength of the association between pathogen and disease and the replication of disease symptoms in 4/4 healthy hosts showed that P. inconspicua was the causal agent of the disease referred to as lily leaf spot. Disease had a severe impact on the population with 59% of mature and 98% of adolescent plants undergoing early senescence. Only 32% of mature plants produced capsules and they were frequently diseased. A recurring spatiotemporal pattern typical of an infectious disease suggested that the lily leaf spot disease is capable of causing sequential annual epidemics of unknown long-term consequences to the stability of the host population.

Lilium grayi (Gray’s Lily), a southern Appalachian endemic species, is threatened by a Lilium-specific fungal pathogen, Pseudocercosporella inconspicua. The disease is characterized by tan lesions that can cause early senescence, while also lowering seed production and viability. This project tested for P. inconspicua conidia and accessed health at nine locations. The disease was present and ubiquitous across the range of L. grayi. Through identification of P. inconspicua conidia in the field, L. superbum (Turk’s Cap Lily) was identified as an additional host, while L. michauxii (Michaux’s Lily) was disease-free. However, infection was inducible in both species. With the disease widespread in L. superbum and this species represented by many large populations, L. superbum may act as disease reservoir, further complicating the outlook for L. grayi. The disease should be considered an epidemic because of its impact on individual plants, its commonness within populations, and its ubiquity across the geographical range.