RNA-seq analysis of the Quercus suber root response to drought

Dissertação de mestrado em Plant Molecular Biology, Biotechnology and Bioentrepreneurship

Two's company, three's a crowd: experimental evaluation of the evolutionary maintenance of trioecy in Mercurialis annua (Euphorbiaceae).

Trioecy is an uncommon sexual system in which males, females, and hermaphrodites co-occur as three clearly different gender classes. The evolutionary stability of trioecy is unclear, but would depend on factors such as hermaphroditic sex allocation and rates of outcrossing vs. selfing. Here, trioecious populations of Mercurialis annua are described for the first time. We examined the frequencies of females, males and hermaphrodites across ten natural populations and evaluated the association between the frequency of females and plant densities. Previous studies have shown that selfing rates in this species are density-dependent and are reduced in the presence of males, which produce substantially more pollen than hermaphrodites. Accordingly, we examined the evolutionary stability of trioecy using an experiment in which we (a) indirectly manipulated selfing rates by altering plant densities and the frequency of males in a fully factorial manner across 20 experimental plots and (b) examined the effect of these manipulations on the frequency of the three sex phenotypes in the next generation of plants. In the parental generation, we measured the seed and pollen allocations of hermaphrodites and compared them with allocations by unisexual plants. In natural populations, females occurred at higher frequencies in denser patches, a finding consistent with our expectations. Under our experimental conditions, however, no combination of plant densities and male frequencies was associated with increased frequencies of females. Our results suggest that the factors that regulate female frequencies in trioecious populations of M. annua are independent of those regulating male frequencies (density), and that the stable co-existence of all three sex phenotypes within populations is unlikely.

Cardenolides, induced responses, and interactions between above- and belowground herbivores of milkweed (Asclepias spp.).

Theory has long predicted allocation patterns for plant defense against herbivory, but only recently have both above- and belowground plant defenses been considered simultaneously. Milkweeds in the genus Asclepias are a classic chemically defended clade of plants with toxic cardenolides (cardiac glycosides) and pressurized latex employed as anti-herbivore weapons. Here we combine a comparative approach to investigate broadscale patterns in allocation to root vs. shoot defenses across species with a species-specific experimental approach to identify the consequences of defense allocational shifts on a specialist herbivore. Our results show phylogenetic conservatism for inducibility of shoot cardenolides by an aboveground herbivore, with only four closely related tropical species showing significant induction; the eight temperate species examined were not inducible. Allocation to root and shoot cardenolides was positively correlated across species, and this relationship was maintained after accounting for phylogenetic nonindependence. In contrast to long-standing theoretical predictions, we found no evidence for a trade-off between constitutive and induced cardenolides; indeed the two were positively correlated across species in both roots and shoots. Finally, specialist root and shoot herbivores of common milkweed (A. syriaca) had opposing effects on latex production, and these effects had consequences for caterpillar growth consistent with latex providing resistance. Although cardenolides were not affected by our treatments, A. syriaca allocated 40% more cardenolides to shoots over roots. We conclude that constitutive and inducible defenses are not trading off across plant species, and shoots of Asclepias are more inducible than roots. Phylogenetic conservatism cannot explain the observed patterns of cardenolide levels across species, but inducibility per se was conserved in a tropical clade. Finally, given that above- and belowground herbivores can systemically alter the defensive phenotype of plants, we concur with recent calls for a whole-plant perspective in testing models of plant defense allocation.

Differential allocation and deployment of direct and indirect defences by Vicia sepium along elevation gradients

Dissecting drivers of plant defence investment remains central for understanding the assemblage of communities across different habitats. There is increasing evidence that direct defence strategies against herbivores, including secondary metabolites production, differ along ecological gradients in response to variation in biotic and abiotic conditions. In contrast, intraspecific variation in indirect defences remains unexplored. Here, we investigated variation in herbivory rate, resistance to herbivores, and indirect defences in ant-attracting Vicia species along the elevation gradient of the Alps. Specifically, we compared volatile organic compounds (VOCs) and ant attraction in high and low elevation ecotypes. Consistent with adaptation to the lower herbivory conditions that we detected at higher elevations in the field, high elevation plants were visited by fewer ants and were more susceptible to herbivore attack. In parallel, constitutive volatile organic compound production and subsequent ant attraction were lower in the high elevation ecotypes. We observed an elevation-driven trade-off between constitutive and inducible production of VOCs and ant attraction along the environmental cline. At higher elevations, inducible defences increased, while constitutive defence decreased, suggesting that the high elevation ecotypes compensate for lower indirect constitutive defences only after herbivore attack. Synthesis. Overall, direct and indirect defences of plants vary along elevation gradients. Our findings show that plant allocation to defences are subject to trade-offs depending on local conditions, and point to a feedback mechanism linking local herbivore pressure, predator abundance and the defence investment of plants.

Location Determinants of New Firms: Does Skill Level of Human Capital Really Matter?

This paper is about the role played by stock of human capital on location decisions of new manufacturing plants. We analyse the effect of several skill levels (from basic school to PhD) on decisions about the location of plants in various industries and, therefore, of different technological levels. We also test whether spatial aggregation level biases the results and determine the most appropriate areas to be considered in analyses of these phenomena. Our main statistical source is the Register of Manufacturing Establishments of Catalonia (REIC), which has plant-level microdata on the locations of new manufacturing plants. Keywords: agglomeration economies, industrial location, human capital, count-data models, spatial econometrics.

Antitumor and Antiviral Activity of Colombian Medicinal Plant Extracts

Extracts of nine species of plants traditionally used in Colombia for the treatment of a variety of diseases were tested in vitro for their potential antitumor (cytotoxicity) and antiherpetic activity. MTT (Tetrazolium blue) and Neutral Red colorimetric assays were used to evaluate the reduction of viability of cell cultures in presence and absence of the extracts. MTT was also used to evaluate the effects of the extracts on the lytic activity of herpes simplex virus type 2 (HSV-2). The 50% cytotoxic concentration (CC50) and the 50% inhibitory concentration of the viral effect (EC50) for each extract were calculated by linear regression analysis. Extracts from Annona muricata, A. cherimolia and Rollinia membranacea, known for their cytotoxicity were used as positive controls. Likewise, acyclovir and heparin were used as positive controls of antiherpetic activity. Methanolic extract from Annona sp. on HEp-2 cells presented a CC50 value at 72 hr of 49.6x103mg/ml. Neither of the other extracts examined showed a significant cytotoxicity. The aqueous extract from Beta vulgaris, the ethanol extract from Callisia grasilis and the methanol extract Annona sp. showed some antiherpetic activity with acceptable therapeutic indexes (the ratio of CC50 to EC50). These species are good candidates for further activity-monitored fractionation to identify active principles.

Reconstructing the origins of high-alpine niches and cushion life form in the genus Androsace S.L. (Primulaceae).

Relatively, few species have been able to colonize extremely cold alpine environments. We investigate the role played by the cushion life form in the evolution of climatic niches in the plant genus Androsace s.l., which spreads across the mountain ranges of the Northern Hemisphere. Using robust methods that account for phylogenetic uncertainty, intraspecific variability of climatic requirements and different life-history evolution scenarios, we show that climatic niches of Androsace s.l. exhibit low phylogenetic signal and that they evolved relatively recently and punctually. Models of niche evolution fitted onto phylogenies show that the cushion life form has been a key innovation providing the opportunity to occupy extremely cold environments, thus contributing to rapid climatic niche diversification in the genus Androsace s.l. We then propose a plausible scenario for the adaptation of plants to alpine habitats.

Trypanocidal activity of Meliaceae and Rutaceae plant extracts

The in vitro trypanocidal activity of 22 extracts and 43 fractions of plants belonging to the families Meliaceae and Rutaceae was evaluated. The extracts from leaves of Conchocarphus heterophyllus and branches of Trichilia ramalhoi were the most active. The trypanocidal activity seems to be increased by fractionation of the extracts. Fractions from C. heterophyllus and Galipea carinata were the most active and a 100% lysis of the parasites was observed for five fractions. From one of them were isolated two flavonoids: flavone and 7-methoxyflavone, which showed weak trypanocidal activity. The results obtained from the extracts and fractions revealed that the order Rutales is a promising source for the search of new drugs for Chagas disease. Phytochemical studies with the other active fractions are underway in order to isolate compounds, which could be associated with observed activities.

The glycoinositol-phospholipids of Phytomonas.

The Phytomonas spp. are trypanosomatid parasites of plants. A polar glycolipid fraction of a Phytomonas sp., isolated from the plant Euphorbia characias and grown in culture, was fractionated into four major glycolipid species (Phy 1-4). The glycolipids were analysed by chemical and enzymic modifications, composition and methylation analyses, electrospray mass spectrometry and microsequencing after HNO2 deamination and NaB3H4 reduction. The water-soluble headgroup of the Phy2 glycolipid was also analysed by 1H NMR. All four glycolipids were shown to be glycoinositol-phospholipids (GIPLs) with phosphatidylinositol (PI) moieties containing the fully saturated alkylacylglycerol lipids 1-O-hexadecyl-2-O-palmitoylglycerol and 1-O-hexadecyl-2-O-stearoylglycerol. The structures of the Phy 1-4 GIPLs are: Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN alpha 1-6PI, Glc alpha 1-2(NH2-CH2CH2-HPO4-)Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN alpha 1-6PI, [formula: see text] Glc alpha 1-2(NH2CH2CH2-HPO4-)Man alpha 1-2Man alpha 1-6Man alpha 1-4(NH2-CH2CH2-HPO4-)GlcN alpha 1-6PI [formula: see text] and Glc alpha 1-2Glc alpha 1-2(NH2CH2-CH2-HPO4-)Man alpha 1-2Man alpha 1-6Man alpha 1-4(NH2CH2CH2-HPO4-)-GlcN alpha 1-6PI. [formula: see text] The Phytomonas GIPLs represent a novel series of structures. This is the first description of the chemical structure of cell-surface molecules of this plant pathogen. The Phytomonas GIPLs are compared with those of other trypanosomatid parasites and are discussed with respect to trypanosomatid phylogenetic relationships.

CHARACTERISATION OF ARABIDOPSIS THALIANA PERMEABLE CUTICLE MUTANTS SHOWING A STRONG RESISTANCE TO BOTRYTIS CINEREA

La cuticule des plantes, composée de cutine, un polyester lipidique complexe et de cires cuticulaires, couvre l'épiderme de la plupart des parties aériennes des plantes. Elle est constituée d'une barrière hydrophobique primaire qui minimise les pertes en eau et en soluté et protège l'organisme de différents stress environnementaux tels que les rayons UV, la dessiccation et l'infection par des pathogènes. Elle est aussi impliquée dans la délimitation des organes durant le développement. La cutine est un polyester qui, dans la plupart des espèces végétales, est principalement composé d'acides gras ω-hydroxylés composé de 16 à 18 carbones. Cependant, la cutine des feuilles d'Arabidopsis a une composition différente et est principalement constituée d'acides dicarboxyliques à 16-18 carbones. Les cires sont présentes dans le polyester de la cutine ou le recouvrent. Chez Arabidopsis, un nombre de mutants, tel que 1er, bdg, hth, att1, wbc11, et des plantes transgéniques avec différents changement dans la structure de la cuticule dans les feuilles et la tige, ont récemment été décrits et servent d'outils pour étudier la relation entre la structure et la fonction de la cuticule.7 mutants d'Arabidopsis ont été isolés par une méthode de coloration qui permet de détecter une augmentation dans la perméabilité cuticulaire. Ces mutants ont été appelés pec pour permeable cuticle.Pour la première partie de mon projet, j'ai principalement travaillé avec pec9/bre1 (permeable cuticle 9/botrytis resistance 1). PEC9/BRE1 a été identifié comme étant LACS2 (LONG CHAIN ACYL-CoA SYNTHETASE 2). Dans ce mutant, la cuticule n'est pas visible sous microscopie électronique et la quantité en acides gras omega- hydroxylés et en leurs dérivés est fortement réduite. Ces altérations conduisent à une plus grande perméabilité de la cuticule qui est mise en évidence par une plus grande sensibilité à la sécheresse et aux xénobiotiques et une coloration plus rapide par bleu de toluidine. Le mutant Iacs2 démontre aussi une grande capacité de résistance à l'infection du champignon nécrotrophique B. cinerea. Cette résistance est due à l'extrusion sur les feuilles d'un composé antifongique durant l'infection. Ce travail a été publié dans EMBO journal (Bessire et al., 2007, EMBO Journal).Mon second projet était principalement concentré sur pec1, un autre mutant isolé par le premier crible. La caractérisation de pec1 a révélé des phénotypes similaires à ceux de Iacs2, mais à chaque fois dans des proportions moindres : sensibilité accrue à la sécheresse et aux herbicides, plus grande perméabilité au bleu de toluidine et au calcofluor white, altération de la structure cuticulaire et résistance à B. cinerea à travers la même activité antifongique. PEC1 a été identifié comme étant AtPDR4. Ce gène code pour un transporteur ABC de la famille PDR ("Pleiotropic Drugs Resistance") qui sont des transporteurs ayants un large spectre de substrats. Le mutant se différencie de Iacs2, en cela que la composition en acides gras de la cuticule n'est pas autant altérée. C'est principalement le dihydroxypalmitate des fleurs dont la quantité est réduite. L'expression du gène marqué avec une GFP sous le contrôle du promoteur endogène a permis de localiser le transporteur au niveau de la membrane plasmique des cellules de l'épiderme, de manière polaire. En effet, la protéine est principalement dirigée vers l'extérieure de la plante, là où se trouve la cuticule, suggérant une implication d'AtPDR4 dans le transport de composants de la cuticule. Ce travail est actuellement soumis à Plant Cell.Une étude phylogénétique a aussi montré qu'AtPDR4 était très proche d'OsPDR6 du riz. Le mutant du riz a d'ailleurs montré des phénotypes de nanisme et de perméabilité similaire au mutant chez Arabidopsis.AbstractThe cuticle, consisting principally of cutin and cuticular waxes, is a hydrophobic layer of lipidic nature, which covers all aerial parts of plants and protects them from different abiotic and biotic stresses. Recently, the research in this area has given us a better understanding of the structure and the formation of the cuticle. The Arabidopsis mutants permeable cuticle 1 (peel) and botrytis resistance 1 (brel) were identified in two screens to identify permeable cuticles. The screens used the fluorescent dye calcofluor to measure permeability and also resistance to the fungal pathogen Botrytis. These mutants have highly permeable cuticle characteristics such as higher water loss, intake of chemicals through the cuticle, higher resistance to Botrytis cinerea infection, and organ fusion.BRE1 was cloned and found to be LACS2, a gene previously identified which is important in the formation and biosynthetic pathway of the cuticle. In brel, the amount of the major component of cutin in Arabidopsis leaves and stems, dicarboxylic acids, is five times lower than in the wild type. Moreover, the permeability of the cuticle allows the release of antifungal compounds at the leaf surface that inhibits the growth of two necrotrophic fungi: Botrytis cinerea and Sclerotinia sclerotiorum.PEC1 was identified as AtPDR4, a gene that codes for a plasma membrane transporter of the Pleiotropic Drug Resistance family, a sub-family of the ABC- transporters. AtPDR4 is strongly expressed in the epidermis of expanding tissues. In the epidermis it is located in a polar manner on the external plasma membrane, facing the cuticle. Analysis of the monomer composition of the cutin reveals that in this mutant the amount of hydroxy-acids and dihydroxy-palmitate is 2-3 times lower in flowers, in which organ these cutin monomers are the major components. Thus AtPDR4 is thought to function as a putative cutin monomer transporter.

Winter survival of immature instars of Mansonia indubitans Dyar & Shannon and Mansonia titillans Walker (Diptera: Culicidae), in Buenos Aires, Argentina

We conducted a whole year research on the ecology of Mansonia indubitans and Ma. titillans in Macáes Pond, Costanera Sur Reserve, Buenos Aires, Argentina. The usage of different floating plants by immature instars and their overwintering was analized. The percentage of usage of the available floating macrophytes (Pistia, Limnobium, and Salvinia) by the larvae and pupae was studied. Also, we defined positivity (P+) as the percentage of plants with immature instars for each plant genus on a monthly basis. Ma. immature instars were captured throughout the year and Pistia was the resource most commonly exploited by the mosquitoes. The percentage of fourth-instar larvae and pupae on Pistia roots with respect to total immature instars captured was assessed on a monthly and seasonal basis. The proportion of fourth-instar larvae and pupae from both species of Mansonia on water lettuce roots, showed significant differences between months and seasons. Our results suggest that the populations of Ma. indubitans and Ma. titillans in Macáes Pond, survive during winter mainly as fourth-instar larvae.

Different arbuscular mycorrhizal fungi alter coexistence and resource distribution between co-occurring plants

It is often thought that the coexistence of plants and plant diversity is determined by resource heterogeneity of the abiotic environment. However, the presence and heterogeneity of biotic plant resources, such as arbuscular mycorrhizal fungi (AMF), could also affect plant species coexistence. In this study, Brachypodium pinnatum and Prunella vulgaris were grown together in pots and biotic resource heterogeneity was simulated by inoculating these pots with one of three different AMF taxa, with a mixture of these three taxa, or pots remained uninoculated. The AMF acted as biotic plant resources since the biomass of plants in pots inoculated with AMF was on average 11.8 times higher than uninoculated pots. The way in which the two plant species coexisted, and the distribution of phosphorus and nitrogen between the plant species, varied strongly depending on which AMF were present. The results showed that the composition of AMF communities determines how plant species coexist and to which plant species nutrients are allocated. Biotic plant resources such as AMF should therefore be considered as one of the factors that determine how plant species coexist and how soil resources are distributed among co-occurring plant species.

Genetic variability in Arbuscular Mycorrhizal Fungi : effect on gene transcription of "Oryza Sativa"

AbstractArbuscular Mycorrhizal Fungi (AMF) form obligate symbioses with the majority of land plants. These fungi influence the diversity and productivity of plants. AMF are unusual organisms, harbouring genetically different nuclei in a common cytoplasm (known as heterokaryosis). Genetic variability has been shown between AMF individuals coming from the same population. Recent findings showed that genetic exchange between genetically different AMF individuals was possible. Additionnaly, segregation was shown to occur at spore formation in AMF. These two processes were shown to increase genetic variability between AMF individuals.Because of the difficulty to study these organisms, almost nothing is known about the effect of intra-specific genetic variability in AMF on the plant transcriptome. The aim of this thesis was to bring insights into the effect of intra-specific genetic variability in AMF on plant gene transcription. We demonstrated that genetic exchange could influence expression of some symbiosis specific plant genes and the timing of the colonization of the fungi in plant roots. We also showed that segregation could have a large impact on plant gene transcription. Taken together, these results demonstrated that AMF intra-specific variability could profoundly affect the life of plants by altering various molecular pathways. Moreover, results obtained on rice open a field of research on AMF genetics in impromvment of growth in agricultural plants and should be taken into account for future experiments.RésuméLes champignons endomycorhiziens arbusculaires (CEA) forment une symbiose obligatoire avec la majorité des plantes sur terre. Ces champignons peuvent influencer la diversité et la productivité des plantes avec lesquelles ils forment la symbiose. Les CEA sont des organismes particuliers de part le fait qu'ils possèdent des noyaux génétiquement différents (appelés hétérocaryosis) dans un cytoplasme commun. Il a été montré qu'il existait de la variabilité génétique intra-specific chez les CEA. De plus, des études recentes ont montré que l'échange génétique chez les CEA était possible entre des individus génétiquement différents tout comme la ségrégation qui a aussi été démontrée au moment de la formation des nouvelles spores chez les CEA. Ces deux processus ont été montrés comme pouvant créer aussi de la variabilité génétique intra-specific.Du fait de la difficulté de travailler avec les CEA et à cause de la nouveauté de ces recherches, très peu de choses sont connues sur l'effet de l'échange génétique et de la ségrégation chez les CEA sur les plantes, et particulièrement au niveau moléculaire. Le but de cette thèse a été d'apporter la lumière sur les effets de la viariabilité génétique intra-specific chez les CEA, sur la transcription des gènes chez la plante. Nous avons pu montrer que l'échange génétique pouvait avoir des effets sur l'expression de gènes spécifiques à cette symbiose mais aussi pouvait influencer le timing de colonisation des racines de plantes par les CEA. Nous avons aussi montré que la ségrégation pouvait grandement influencer le transcriptome complet de la plante, et pas seulement les voies métaboliques spécifiques à la symbiose comme cela avait été montré auparavant.L'ensemble de ces résultats démontre l'importance de la variation intra-specific chez les CEA sur les plantes et leur implication sur leur cycle de vie en changeant l'expression de voies métaboliques. De plus, ces résultats obtenus sur le riz ouvrent un champ de recherches sur les plantes destinées à l'agriculture et devraient être pris en compte pour des expériences futures.

Genetic diversity and host plant preferences revealed by simple sequence repeat and mitochondrial markers in a population of the arbuscular mycorrhizal fungus Glomus intraradices.

Arbuscular mycorrhizal fungi (AMF) are important symbionts of plants that improve plant nutrient acquisition and promote plant diversity. Although within-species genetic differences among AMF have been shown to differentially affect plant growth, very little is actually known about the degree of genetic diversity in AMF populations. This is largely because of difficulties in isolation and cultivation of the fungi in a clean system allowing reliable genotyping to be performed. A population of the arbuscular mycorrhizal fungus Glomus intraradices growing in an in vitro cultivation system was studied using newly developed simple sequence repeat (SSR), nuclear gene intron and mitochondrial ribosomal gene intron markers. The markers revealed a strong differentiation at the nuclear and mitochondrial level among isolates. Genotypes were nonrandomly distributed among four plots showing genetic subdivisions in the field. Meanwhile, identical genotypes were found in geographically distant locations. AMF genotypes showed significant preferences to different host plant species (Glycine max, Helianthus annuus and Allium porrum) used before the fungal in vitro culture establishment. Host plants in a field could provide a heterogeneous environment favouring certain genotypes. Such preferences may partly explain within-population patterns of genetic diversity.

Genetics of cadmium hyperaccumulation variance in Thlaspi caerulescens natural populations from Switzerland

SUMMARY Heavy metal presence in the environment is a serious concern since some of them can be toxic to plants, animals and humans once accumulated along the food chain. Cadmium (Cd) is one of the most toxic heavy metal. It is naturally present in soils at various levels and its concentration can be increased by human activities. Several plants however have naturally developed strategies allowing them to grow on heavy metal enriched soils. One of them consists in the accumulation and sequestration of heavy metals in the above-ground biomass. Some plants present in addition an extreme strategy by which they accumulate a limited number of heavy metals in their shoots in amounts 100 times superior to those expected for a non-accumulating plant in the same conditions. Understanding the genetic basis of the hyperaccumulation trait - particularly for Cd - remains an important challenge which may lead to biotechnological applications in the soil phytoremediation. In this thesis, Thlaspi caerulescens J. & C. Presl (Brassicaceae) was used as a model plant to study the Cd hyperaccumulation trait, owing to its physiological and genetic characteristics. Twenty-four wild populations were sampled in different regions of Switzerland. They were characterized for environmental and soil parameters as well as intrinsic characteristics of plants (i.e. metal concentrations in shoots). They were as well genetically characterized by AFLPs, plastid DNA polymorphism and genes markers (CAPS and microsatellites) mainly developed in this thesis. Some of the investigated genes were putatively linked to the Cd hyperaccumulation trait. Since the study of the Cd hyperaccumulation in the field is important as it allows the identification of patterns of selection, the present work offered a methodology to define the Cd hyperaccumulation capacity of populations from different habitats permitting thus their comparison in the field. We showed that Cd, Zn, Fe and Cu accumulations were linked and that populations with higher Cd hyperaccumulation capacity had higher shoot and reproductive fitness. Using our genetic data, statistical methods (Beaumont & Nichols's procedure, partial Mantel tests) were applied to identify genomic signatures of natural selection related to the Cd hyperaccumulation capacity. A significant genetic difference between populations related to their Cd hyperaccumulation capacity was revealed based on somè specific markers (AFLP and candidate genes). Polymorphism at the gene encoding IRTl (Iron-transporter also participating to the transport of Zn) was suggested as explaining part of the variation in Cd hyperaccumulation capacity of populations supporting previous physiological investigations. RÉSUMÉ La présence de métaux lourds dans l'environnement est un phénomène préoccupant. En effet, certains métaux lourds - comme le cadmium (Cd) -sont toxiques pour les plantes, les animaux et enfin, accumulés le long de la chaîne alimentaire, pour les hommes. Le Cd est naturellement présent dans le sol et sa concentration peut être accrue par différentes activités humaines. Certaines plantes ont cependant développé des stratégies leur permettant de pousser sur des sols contaminés en métaux lourds. Parmi elles, certaines accumulent et séquestrent les métaux lourds dans leurs parties aériennes. D`autres présentent une stratégie encore plus extrême. Elles accumulent un nombre limité de métaux lourds en quantités 100 fois supérieures à celles attendues pour des espèces non-accumulatrices sous de mêmes conditions. La compréhension des bases génétiques de l'hyperaccumulation -particulièrement celle du Cd - représente un défi important avec des applications concrètes en biotechnologies, tout particulièrement dans le but appliqué de la phytoremediation des sols contaminés. Dans cette thèse, Thlaspi caerulescens J. & C. Presl (Brassicaceae) a été utilisé comme modèle pour l'étude de l'hyperaccumulation du Cd de par ses caractéristiques physiologiques et génétiques. Vingt-quatre populations naturelles ont été échantillonnées en Suisse et pour chacune d'elles les paramètres environnementaux, pédologique et les caractéristiques intrinsèques aux plantes (concentrations en métaux lourds) ont été déterminés. Les populations ont été caractérisées génétiquement par des AFLP, des marqueurs chloroplastiques et des marqueurs de gènes spécifiques, particulièrement ceux potentiellement liés à l'hyperaccumulation du Cd (CAPS et microsatellites). La plupart ont été développés au cours de cette thèse. L'étude de l'hyperaccumulation du Cd en conditions naturelles est importante car elle permet d'identifier la marque, éventuelle de sélection naturelle. Ce travail offre ainsi une méthodologie pour définir et comparer la capacité des populations à hyperaccumuler le Cd dans différents habitats. Nous avons montré que les accumulations du Cd, Zn, Fe et Cu sont liées et que les populations ayant une grande capacité d'hyperaccumuler le Cd ont également une meilleure fitness végétative et reproductive. Des méthodes statistiques (l'approche de Beaumont & Nichols, tests de Martel partiels) ont été utilisées sur les données génétiques pour identifier la signature génomique de la sélection naturelle liée à la capacité d'hyperaccumuler le Cd. Une différenciation génétique des populations liée à leur capacité d'hyperaccumuler le Cd a été mise en évidence sur certains marqueurs spécifiques. En accord avec les études physiologiques connues, le polymorphisme au gène codant IRT1 (un transporteur de Fe impliqué dans le transport du Zn) pourrait expliquer une partie de la variance de la capacité des populations à hyperaccumuler le Cd.