Etude de la résistance du Coffea arabica au nématode Meloidogyne exigua conférée par le gène Mex-1 et mise au point des outils pour son analyse fonctionnelle

En Amérique Latine, les nématodes à galles (Meloidogyne sp.) sont des parasites fréquents sur les racines des caféiers (Coffea arabica) et provoquent des baisses de rendement importantes. Concernant Meloidogyne exigua, la lutte génétique consiste à introgresser dans les variétés cultivées des gènes de résistance (gènes R) provenant de l'espèce C. canephora. Le gène nommé Mex-1, identifié récemment est en cours de clonage. Cette thèse avait pour objectif de développer des outils (transgénèse et "bio-essais") pour réaliser la validation fonctionnelle des gènes candidats de résistance à M. exigua. Différentes expérimentations en pépinière et en champs ont permis de prouver que l'expression du gène Mex-1 pouvait être dominant incomplète. Une méthode de transformation basée sur l'utilisation d'Agrobacterium rhizogenes, a été mise au point permettant la régénération rapide et efficace de racines transformées sur des hypocotyles d'embryons zygotiques et la production consécutive de plantes composites co-transformées (50-60%) sur lesquelles peuvent être appliqués les tests de résistance au nématode en conditions contrôlées. Les conditions d'entretien à long terme des racines transformées 'Hairy roots' ont été définies. La caractérisation morphologique de 62 dones transformés a permis l'identification de deux phénotypes altérés. L'utilisation des gènes rapporteurs gus et gfp a permis une sélection visuelle des racines transformées et a également permis de préciser le mode et la stabilité de l'expression du promoteur CaMV35S au niveau racinaire. Les résultats sont discutés pour apprécier l'efficacité des outils développés et la stratégie pour la validation fonctionnelle du gène Mex-1. L'exploitation de cet outil est également envisagée pour l'analyse fonctionnelle d'autres gènes de résistance aux nématodes à galles, ou bien de gènes de résistance aux stress abiotiques (sécheresse) ou impliqués dans le développement de la racine. (Résumé d'auteur

G x E interactions on yield and quality in Coffea arabica: New F1 hybrids outperform American cultivars

Conventional American cultivars of coffee are no longer adapted to global warming. Finding highly productive and stable cultivars in different environments without neglecting quality characteristics has become a priority for breeders. In this study, new Arabica F1 hybrids clones were compared to conventional American varieties in seven contrasting environments, for yield, rust incidence and volume of the canopy. The quality was assessed through size, weight of 100 beans, biochemical analysis (24 aroma precursors and 31 volatiles compounds) and sensory analysis. Conventional varieties were the least productive, producing 50% less than the best hybrid. The AMMI model analysis pointed out five hybrids as the most stable and productive. Two F1 hybrids clones, H1-Centroamericano and H16-Mundo Maya, were superior to the most planted American cultivar in Latin and Central America showing a high yield performance and stability performance. H1-Centroamerica and Starmaya contain more d-limonene than Caturra, while Starmaya contain more 3-methylbutanoic acid than the control. Those two latter volatiles compounds are linked with good cup quality in previous studies. In terms of sensory analysis, Starmaya and H1-Centroamericano scored better than control

Coffee Somatic Embryogenesis: How Did Research, Experience Gained and Innovations Promote the Commercial Propagation of Elite Clones From the Two Cultivated Species?

Since the 1990s, somatic embryogenesis (SE) has enabled the propagation of selected varieties, Arabica F1 hybrid and Robusta clones, originating from the two cultivated coffee species, Coffea arabica and Coffea canephora, respectively. This paper shows how mostly empirical research has led to successful industrial transfers launched in the 2000s in Latin America, Africa, and Asia. Coffee SE can be considered as a model for other woody perennial crops for the following reasons: (i) a high biological efficiency has been demonstrated for propagated varieties at all developmental stages, and (ii) somaclonal variation is understood and mastered thanks to intensive research combining molecular markers and field observations. Coffee SE is also a useful model given the strong economic constraints that are specific to this species. In brief, SE faced four difficulties: (i) the high cost of SE derived plants compared to the cost of seedlings of conventional varieties, (ii) the logistic problems involved in reaching small-scale coffee growers, (iii) the need for certification, and (iv) the lack of solvency among small-scale producers. Nursery activities were professionalized by introducing varietal certification, quality control with regard to horticultural problems and somaclonal variation, and sanitary control for Xylella fastidiosa. In addition, different technology transfers were made to ensure worldwide dissemination of improved F1 Arabica hybrids and Robusta clones. Innovations have been decisive for successful scaling-up and reduction of production costs, such as the development of temporary immersion bioreactors for the mass production of pre-germinated embryos, their direct sowing on horticultural soil, and the propagation of rejuvenated SE plants by rooted mini-cuttings. Today, SE is a powerful tool that is widely used in coffee for biotechnological applications including propagation and genetic transformation. Basic research has recently started taking advantage of optimized SE protocols. Based on -omics methodologies, research aims to decipher the molecular events involved in the key developmental switches of coffee SE. In parallel, a high-throughput screening of active molecules on SE appears to be a promising tool to speed-up the optimization of SE protocols