NORD : IMPACTS DES BIOTECHNOLOGIES RECHERCHES Approches moléculaires de la qualité et du développement des graines

Developed countries: The impact of biotechnologies Research Molecular analysis of seed quality and development

UMR 204, Inra/INAPG, Biologie des semences Inra, route de Saint-Cyr, 78026 Versailles Cedex, France

Résumé

Les semences constituent le maillon essentiel de la production végétale car elles sont à la fois le vecteur de la multiplication des plantes et la source de denrées alimentaires. L’agriculture moderne exige le respect des critères de qualité des semences qui ne sont évidemment pas les mêmes suivant que l’utilisation des semences sera alimentaire, industrielle ou de simple semis. Du point de vue alimentaire ou industriel, la qualité se définit en fonction de la nature et du rendement de l’extraction du produit d’intérêt. Lorsque la graine est utilisée comme semence, la qualité est appréciée suivant des critères qui sont la conformité génétique, l’état sanitaire et la capacité germinative. Les normes qualitatives alors retenues concourent à l’obtention de peuplements homogènes et vigoureux. La semence idéale doit évidemment combiner ces différents critères de qualité. Les bases biologiques qui sous-tendent la plupart des critères de qualité sont encore très mal connues et leur élucidation demeure un défi scientifique majeur.

Abstract

Seeds are essential for plant propagation and crop production and must satisfy the increasingly diverse requirements of both modern agriculture and the agro-industry. The biological bases of seed quality are still poorly understood. Progress in this area will no doubt emerge from the identification of genes that control quality parameters, through the tremendous advances made using genomic based techniques. In this way, the use of 50,000 Arabidopsis thaliana T-DNA mutant lines, developed in France, at the Inra Versailles has enabled us to isolate genes whose functions are essential for seed development and maturation, e.g. storage compound synthesis and deposition, and tegument composition. Mutants have been isolated that are either strongly impaired in protein glycosylation (glycosidase I mutant) or whose lipid composition is modified (diacylglycerol acyltransferase: DGAT mutant). The corresponding cloned genes should prove useful in biotechnological applications for the reduction of the allergenicity of engineered proteins produced in plants and for modifying seed lipid composition respectively. New Transparent Testa genes have also been obtained that play a role in the control of pigment deposition in the seed testa. Engineering the composition of seed coats is a real challenge for the improvement of the nutritional value of seeds for animal feeding. The zeaxanthin epoxidase gene has been cloned, as this is required for the biosynthesis of the hormone abscisic acid (ABA). Using this gene it has been possible to engineer ABA levels and seed dormancy in transgenic plants. To identify other genes playing a role in dormancy control, a systematic screen is being employed to identify genes differentially expressed between dormant (D) and non-dormant (ND) seeds of Nicotiana plumbaginifolia using “cDNA-AFLP”. Six hundred genes have been identified in this way and their expression analysed by macro arrays. Most of them can be classified in four groups based on their functions, from imbibition through to germination: cellular repair genes; signal transduction genes; housekeeping genes; storage compound degradation genes. A reverse genetic screen for knockout mutants in the T-DNA mutant collection is in progress for some of these genes, in order to confirm their role in the control of seed dormancy and germination.