Neobeguea mahafalensis Leroy. and Cedrelopsis grevei Baill. are the two Madagascan species which were investigated in this work. Neobeguea mahafalensis Leroy. belongs to the Neobeguea genus of the Meliaceae family and is a plant unique to Madagascar. It is commonly referred to as "Handy" by the native people of Madagascar, who use it as a medicinal plant. The stem-bark of N mahafalensis which has been analysed in this work, was collected from the dry, thorny forests of the deep south of Madagascar. Most species found in this region are unique in the world and are highly specialised in adapting to the very dry climate. The hexane extract was found to yield a limonoid (compound I) which has not been previously reported as a natural product. The known pentacyclic triterpenoid, j3-amyrin (compound II) and stigmasterol (compound III), were also isolated. Cedrelopsis grevei Bail!. is one of seven species of the genus Cedrelopsis which are confined to Madagascar. This species, commonly referred to as "Katrafay" by the Madagascan people, has undergone intensive chemical investigation as prior inclusion ofthis species in the Meliaceae family has always been questionable. The South African species Ptaeroxylon obliquum (Thung.) Radlk., is a member of the Ptaeroxylaceaefamily which is found to be so chemically similar to Cedrelopsis that the latter has been placed in the Ptaeroxylaceae family. The stem bark of Cedrelopsis grevei was obtained from the dry southern part of Madagascar and yielded a variety of chromones and coumarins, as well as stigmasterol (compound III) and the pentacyclic triterpenoid, j3-amyrin (compound II). Two chromones were isolated from Cedrelopsis grevei, namely ptaeroxylinol (compound IV) and ptaeroglycol (compound V). Ptaeroglycol has been previously isolated _from this species as well as from Ptaeroxylon obliquum whereas ptaeroxylinol has only byen isolated from Ptaeroxylon obliquum. Six coumarins were isolated from Cedrelopsis grevei in tillS study, all of which were 6,7-dioxygenated coumarins. These included the known compounds, scoparone (compound VI), O-methylcedrelopsin (compound VIII), norbraylin (compound X) and cedrelopsin (compound IX), as well as compound VII and compound XI which . have not been reported previously. No limonoids were isolated from this species in this investigation. / Thesis (M.Sc.)-University of Natal, 1997.

Despite their ecological and economic importance, the majority of plant species and their conservation status are poorly known. Only 4% of plants have been assessed globally and listed on the IUCN Red List of Threatened Species; and without plant conservation assessments, many plant species will not feature in conservation planning. Herbarium collection information can significantly increase the number of plant conservation assessments. Thus, the aims of this thesis were: (1) to investigate how the quality of herbarium-based conservation assessments can be optimised; (2) to assess the extent to which herbarium-based conservation assessments reflect the reality on the ground; and (3) to scientifically validate genetic and spatial underpinning of IUCN criteria. Preliminary range-based assessments of the Leguminosae of Madagascar achieved a result consistent with the final conservation rating for over 95% of species when using up to fifteen herbarium specimens. Bioclimatic modelling of range shifts based on future climate change predicted that, in the worst case scenario, up to one third of endemic Leguminosae in Madagascar will be threatened with extinction over the next 100 years. An analysis of the population structure of species of Delonix s.l. (Leguminosae) showed that combining spatial analysis with population genetic data provides a more complete picture of landscape-level population dynamics and the impacts on conservation status. Moreover, range-wide genetic analysis of AFLP markers for four species of Delonix demonstrated a genetic basis for IUCN categories distinguishing between threatened and non-threatened species. Although genetic data are currently not often incorporated in conservation assessments, they are crucial in making accurate management decisions and creating effective action plans for conservation. Only by using all available scientific resources can informed conservation decisions be made and the survival of plants and their associated ecosystems be ensured.

Plants are continuously confronted with a multitude attack either abiotic but also biotic in nature. Interestingly, despite the abundance of bacteria that plant has to face, only few are able to induce death or disease in the host plant. It is therefore likely that, in addition to secondary metabolites with antimicrobial properties, plants also synthesize secondary metabolites which are able to inhibit the expression of virulence genes in bacteria without affecting either growth or viability, which allows plants to host willingly or not bacterial populations. This work focuses on the identification of such metabolites in Malagasy plants (genera Dalbergia and Combretum) and the demonstration of their inhibitory effect on the expression of virulence genes in two different pathosystems: Rhodococcus fascians (a phytopathogen) and Pseudomonas aeruginosa (an opportunistic pathogen). Thus, two metabolites were isolated using a combination of chromatographic techniques coupled with tests that evaluate the expression of certain genes involved in the virulence mechanisms of these bacteria. The first is a new prenylated isoflavanone, named perbergin, isolated from the bark extract of D. pervillei. It was shown that the perbergin target attR gene expression, encoding a LysR-type transcriptional regulator that plays a key role in regulating the expression of virulence genes of R. fascians and the transition from an epiphytic to a pathogenic lifestyle. Therefore, we have also shown that the expression of all virulence genes known to date in R. fascians is also affected while the expression of genes involved in epiphytic fitness of the bacteria is not altered. In addition, the application of perbergin at the time of infection of plants susceptible to R. fascians shows that this molecule reduces in vivo the virulence of R. fascians, highlighting the potential of perbergin as an anti-infective agent. The second is a flavonoid known as catechin, isolated from the bark extract of C. albiflorum. Catechin significantly inhibits the expression of genes that regulate the mechanism of quorum sensing in P. aeruginosa such as lasI, LasR, rhlI and rhlR but also lasB and rhlA which expression depends on quorum sensing. Therefore, the production of virulence factors such as pyocyanin and elastase is significantly affected. Because of the limited number of our arsenal of antibiotics and their increasing ineffectiveness, the identification of these compounds create a path to an alternative in the fight against pathogenic bacteria and multidrug resistance of pathogenic bacteria to antibiotics. Our results also demonstrate the richness of Malagasy plants as (re)sources of new therapeutic molecules and the importance of widening the range of bacterial targets to be investigated to develop new strategies to fight within the endless war that we are waging against bacteria pathogens.<p><p>Les plantes sont continuellement confrontées à une multitude d’attaques qu’elles soient de nature abiotique ou surtout biotique. Il est intéressant de noter que malgré la multitude de bactéries auxquelles les plantes doivent faire face, seules quelques unes sont capables d’induire la mort ou une maladie chez la plante hôte. Il est dès lors fort probable que, outre les métabolites secondaires ayant des propriétés antimicrobiennes, les plantes synthétisent également des métabolites secondaires capables d’inhiber l’expression des gènes de virulence chez les bactéries sans toutefois affecter ni leur croissance ni leur viabilité, ce qui permet aux plantes de contenir les populations bactériennes qu’elles hébergent de gré ou de force. Ce travail porte sur l’identification de ce type de métabolites dans des plantes malgaches (genres Dalbergia et Combretum) et la démonstration de leurs effets inhibiteurs sur l’expression de gènes de virulence chez deux pathosystèmes différents: Rhodococcus fascians (un phytopathogène) et Pseudomonas aeruginosa (un pathogène opportuniste). Ainsi, deux métabolites ont été isolés en utilisant une combinaison de techniques chromatographiques couplées avec des tests qui évaluent l’expression de certains gènes impliqués dans les mécanismes de virulence de ces bactéries. Le premier est un nouvel isoflavanone prénylé, nommé perbergine, isolé à partir de l’extrait d’écorces de D. pervillei. Il a été montré que la perbergine cible l’expression du gène attR, codant un régulateur transcriptionnel de type LysR qui joue un rôle clé dans la régulation de l’expression des gènes de virulence de R. fascians et qui assure la transition entre un mode de vie épiphyte et le mode pathogène. En conséquence, nous avons également montré que l’expression de l’ensemble des gènes de virulence connu à ce jour chez R. fascians est également affectée alors que l’expression de gènes impliqués dans l’aptitude épiphyte de la bactérie n’est pas altérée. Par ailleurs, l’application de perbergine au moment de l’infection de plantes sensibles à R. fascians montre que cette molécule atténue la virulence de R. fascians in vivo, mettant en exergue le potentiel de la perbergine comme agent anti-infectieux. Le deuxième est un flavonoïde, connu sous le nom de catéchine, isolé de l’extrait d’écorces de C. albiflorum. La catéchine inhibe significativement l’expression des gènes régulateurs du mécanisme du quorum sensing chez P. aeruginosa tels que lasI, lasR, rhlI et rhlR et également lasB et rhlA dont l’expression dépend du quorum sensing. En conséquence, la production des facteurs de virulence tels que la pyocyanine et l’élastase est significativement affectée. Compte tenu de l’appauvrissement de notre arsenal d’antibiotiques et de leur inefficacité croissante, l’identification de ces composés ouvre une voie alternative de lutte contre les bactéries pathogènes et la multirésistance des bactéries pathogènes aux antibiotiques. Nos résultats démontrent également la richesse des plantes malgaches comme (res)sources de nouvelles molécules thérapeutiques et l’importance d’élargir le champ des cibles bactériennes à investiguer pour développer de nouvelles stratégies de lutte dans la guerre sans fin que nous menons contre les bactéries pathogènes. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished