Author:

Abstract:

Bacterial leaf symbiosis, characterized by the formation of leaf nodules in which bacterial symbionts are housed, is a rare and intimate interaction between plants and bacteria. The phenomenon has been reported in two eudicot families, i.e. Primulaceae and Rubiaceae. Rubiaceae have the largest number of species with bacterial leaf nodules, found in three, distantly related, genera, i.e. Pavetta, Psychotria and Sericanthe. In Primulaceae, leaf nodulated species are limited to the genus Ardisia.In both families, the bacteria-plant interaction was shown to be obligate and closed; the endosymbionts are necessary for the survival of the host plant and are transferred vertically from one generation to the other through the seeds. These observations provide opportunities for further and more detailed research on the origin of leaf symbiosis in angiosperms and the co-evolution between bacteria and host.Phylogenetic analyses provide evidence that bacterial leaf nodulation has evolved at least four times independently in angiosperms and that it has a single origin in the genera Ardisia, Pavetta, Psychotria and Sericanthe. The molecular dating analyses indicate thatall leaf nodulated plant groups originated more or less at the same era during the middle Miocene. Our results also show that every leaf nodulating Rubiaceae and Primulaceae species is associated with a single host specific Burkholderia symbiont (beta-Proteobacteria). Phylogenetic analyses of the symbiotic lineages show at least two major evolutionary origins, i.e. one origin in the ancestor of Rubiaceae endosymbionts and one in the nodulating Primulaceae endosymbionts. More interestingly, a co-speciation analysis in Ardisia suggests a long-term vertical symbiont transmission supporting the idea that endosymbionts are transferred from one generation to the other through the seeds. In Rubiaceae, however, endosymbionts and host do not share a commonevolutionary history rejecting the co-speciation hypothesis. The absence and presence of co-speciation in, respectively, Rubiaceae and Primulaceae is somewhat surprising, as leaf symbiosis has been described as a closed and obligate interaction in both families. The discordance between the tree topologies of leaf nodulating Rubiaceae and their endosymbionts might be explained by frequent external infections of soil bacteria. Direct evidence for horizontal symbiont transmission is the observation of soil Burkholderia species that are closely related with leaf nodulated endosymbionts. Because plant development seems largely dependent on metabolites of the endosymbiont, it is hypothesized that endosymbiont replacement may occur occasionally when host plants fail to transfer their original bacteria through the seeds.