The evolution of the nitrogen fixing root nodule symbiosis; how to live from air
Van Leeuwenhoek Lecture on BioScience.
Ton Bisseling is a molecular biologist. He is Professor at Wageningen University and Research Centre.
He obtained a degree in biology from the Radboud University Nijmegen and earned his PhD in molecular biology at Wageningen University.
In 1998 he became professor and head of the molecular biology laboratory at the same institution.
During his career he has done research on the symbiosis between root nodules and rhizobia bacteria.
Long before the current era, early farmers used species from the Legume family (such as lentil, soybean and bean) in crop rotation. Consumers valued the nutritious protein rich seeds, while farmers understood that legumes would improve yield of the following year’s non-legume crop, such as wheat, barley and maize. The importance of this agricultural practice is already documented in ancient Roman literature (Marcus Cato, De Agri Cultura). After the discovery that the beneficial property of legumes is their ability to reduce atmospheric nitrogen into ammonia it became a dream to transfer this property to non-legumes.
Legumes establish a symbiosis with rhizobium bacteria and this is one of the most intimate endosymbiotic interactions on earth. Rhizobium reprogrammes fully differentiated root cells and these can differentiate into different cell types that form a new organ, the root nodule. The rhizobia are hosted in specialized cell as transient organelles and there they are able to reduce atmospheric N2 into ammonia. This is a source of nitrogen that can be used by the plant. This is the most important biological process by which nitrogen becomes available for plants, but in fact for all living organisms. In this seminar I will discuss recent insights in the evolution of nodule symbiosis and its impact for engineering this trait on non-legume crops.
In addition to legumes several other plant species can establish a nodule symbiosis with N fixing Frankia bacteria. These are named actinorhizal plants. Strikingly all these nodule forming plants are relatively closely related, as they all belong to a single (N-fixation) clade. Within this clade most legumes have the ability to establish a nodule symbiosis. However, the actinorhizal plants are scattered between numerous plants that are unable to form this nodule symbiosis. A gain of nodulation in several nodulating genera was considered more likely than numerous loss. Therefore it is generally assumed that within this N fixation clade nodulation evolved several times independently, To explain that all nodulating plants belong to a single clade, it is postulated that the youngest common ancestor of this clade obtained a predisposition to evolve nodulation.
Within this seminar I will discuss the evolutionary relationship between the nodule symbioses and a different endosymbiosis; the interaction of plants with arbuscular mycorrhizal (AM) fungi. This symbiosis evolved once in basal plants (relatives of liverworts ~ 450 M years ago) and has been maintained in most higher plants. In this symbiosis the fungi form a large fungal network outside the plant and in the root they form highly branched hyphae inside cortical cells. There the exchange of nutrients and cabohydrates between plant and fungi takes place. During evolution rhizobia have learned to produce similar signal molecules as the AM fungi and they activate the same signaling pathway. This common signaling pathway is also used in the formation of nodules of actinorhizal plants. The mechanism to host AM fungi intracellular is controlled by a specific (symbiotic) exocytosis pathway and this pathway is also recruited within the nodule symbiosis. The implication of the recruitment of AM fungal symbiosis pathways during nodule evolution is that the fast majority of plants have these pathways. So why did they not evolve a nodule symbiosis?
To obtain further insight in the evolution of nodule symbiosis we studied Parasponia, This is the only genus outside the legume family that also can establish a nodule symbiosis with rhizobium. We assumed that it evolved the nodulation trait rather recent as its very close sister genus Trema cannot establish the symbiosis. Comparative genomics of several Trema and Paraponia species did not reveal any gene that could contribute to the gain of the nodulation trait. In contrast it showed the loss of several genes in Trema that are essential for nodulation. This study and another phylogenomics analysis on several species within the N-fixation clade will be used to discuss the evolution of the nodule symbiosis within the N-fixation clade.
Please reserve the following dates in your diary (all Thursdays at 16 h.):
October 18 2018, Marta Miaczynska (Warsaw, Int. Inst. of Molecular and Cell Biology)
November 29 2018, Steve Brown (Zürich, Institute of Pharmacology and Toxicology)
January 31 2019, Tjeerd Barf (Oss, Acerta Pharma)