Genome characteristics of facultatively symbiotic Frankia sp. strains reflect host range and host plant biogeography

Soil bacteria that also form mutualistic symbioses in plants encounter two major levels of selection. One occurs during adaptation to and survival in soil, and the other occurs in concert with host plant speciation and adaptation. Actinobacteria from the genus Frankia are facultative symbionts that form N2-fixing root nodules on diverse and globally distributed angiosperms in the “actinorhizal” symbioses. Three closely related clades of Frankia sp. strains are recognized; members of each clade infect a subset of plants from among eight angiosperm families. We sequenced the genomes from three strains; their sizes varied from 5.43 Mbp for a narrow host range strain (Frankia sp. strain HFPCcI3) to 7.50 Mbp for a medium host range strain (Frankia alni strain ACN14a) to 9.04 Mbp for a broad host range strain (Frankia sp. strain EAN1pec.) This size divergence is the largest yet reported for such closely related soil bacteria (97.8%–98.9% identity of 16S rRNA genes). The extent of gene deletion, duplication, and acquisition is in concert with the biogeographic history of the symbioses and host plant speciation. Host plant isolation favored genome contraction, whereas host plant diversification favored genome expansion. The results support the idea that major genome expansions as well as reductions can occur in facultative symbiotic soil bacteria as they respond to new environments in the context of their symbioses

Implication of 5'flanking sequence elements in expression of a plant tRNAleu gene

International audienc

Implication of 5'flanking sequence elements in expression of a plant tRNAleu gene

International audienc

Positional cloning of disease resistance genes in grapevine

Chapitre 8International audienceThe introduction of genes that confer resistance to major grapevine pests and pathogens into the highly susceptible V. vinifera cultivars used for wine production worldwide would be of signifi cant economic and environmental benefi t. The availability of genetic resources for grapevine has led to a dramatic increase in research on the mapping of resistance loci. Often the outcome of this work is the development of markers for marker-assisted selection of new disease-resistant interspecific hybrids. However, positional cloning of these resistance genes offers the possibility to introduce these genes into existing elite wine grape cultivars by grapevine transformation without affecting wine quality which has been a major obstacle in the adoption of hybrid vines generated by classical breeding. Furthermore positional cloning offers the possibility to introduce resistance genes into V. vinifera cultivars from species which cannot be achieved through hybridization. Finally, cloning of resistance genes provides sequence information that can be used to design perfect genetic markers which will maximize the effi ciency of marker assisted selection approaches. This chapter describes the strategy used, and technical challenges encountered, during the mapping and positional cloning of powdery mildew and downy mildew resistance genes from the wild North American grape species Muscadinia rotundifolia