Recherche de déterminants génétiques de la date de floraison chez la Légumineuse modèle, Medicago truncatula

Jean-Baptiste Pierre 1
P3F - Unité de Recherche Pluridisciplinaire Prairies et Plantes Fourragères
Abstract : The aerial morphogenesis includes traits such as growth, development or phenology, and has an important effect on yield and quality of legume crops. Among these traits, flowering is a major event of the life cycle and therefore crucial for the reproductive success. It corresponds to the irreversible transition of the meristem that generates plant leaves and stems into a floral meristem. The morphogenetic regulation of this phenomenon is a complex network of signals. Legume crops often have complex genomes such as the perennial forage species of alfalfa (Medicago sativa), that is tetraploid and allogamous and pea (P. sativum), with a large genome. Specific studies can be conducted on the model legume Medicago truncatula which is an annual autogamous and diploid species, and has a short reproductive cycle. Many genetic and genomic resources are available in this species that has a high degree of synteny with alfalfa and pea. In addition, genes involved in the determinism on the flowering date were described in A. thaliana and in pea. The aim of the thesis was to identify genomic regions and genes in M. truncatula using the knowledge and tools developed in M. truncatula, A. thaliana and P. sativum for genetic determinism of the flowering date. After analysing the photoperiod effect on flowering date of a set of lines, a "positional candidate genes" strategy has been implemented. After the analysis of the genetic variability of the date of flowering in response to photoperiod, the methodology consisted into searching QTL (Quantitative Trait Locus) of flowering date in three connected populations of recombinant lines, conducting a meta-analysis QTL to detect common regions involved in the control of this trait between populations, realising a fine mapping of a major QTL and to identify major candidate genes in its confidence interval. The expression of these genes was compared between two parental lines to relate traits with genes that are differentially expressed. The flowering date of eight lines was measured in growth chambers under two photoperiods: 12 hours and 18 hours of light. The data showed that there was genetic variability for the flowering date among the eight lines, that flowering was earlier under long days that under short days and that there was an interaction between line and photoperiod. On chromosome 7, a major QTL for flowering date was detected in three populations of recombinant lines explaining between 10 and 60% of the variation observed. A meta-analysis on the three populations revealed a consensus QTL with a confidence interval of only 0.9 cM. Fine mapping of this genome region was conducted in a pseudo-F2 population (1663 plants) derived from a heterozygous F6 plant at the QTL from the population LR4. Six genes homologous to flowering genes described in A. thaliana were identified in the confidence interval (2.4 cM) of the QTL detected in the region of fine mapping. Their full-length sequences revealed polymorphism between the two parents : for MtCO homologous to CONSTANS, and for MtFTLc homologous to FT. In contrast, there was no polymorphism observed for two other FT homologues (MtFTLa and MtFTLb) nor PKS. An analysis of the differential expression by semi-quantitative RT-PCR of the six candidate genes was performed in two parental lines contrasting for flowering date. Only MtCO gene was differentially expressed between these two lines. This gene is currently the best candidate to explain the variation of the trait detected at the QTL on chromosome 7 in these populations.
Document type :
Sciences agricoles. Agrocampus - Ecole nationale supérieure d'agronomie de rennes, 2008. Français
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Jean-Baptiste Pierre. Recherche de déterminants génétiques de la date de floraison chez la Légumineuse modèle, Medicago truncatula. Sciences agricoles. Agrocampus - Ecole nationale supérieure d'agronomie de rennes, 2008. Français. 〈tel-00653807〉



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