Abstract
Like human bodies, plants are surrounded by a highly diverse microbiome. The significance of the plant microbiome should also not be underestimated because of its considerable impact on plant life. The number of microbes associated with the plant often surpasses the number of plant cells and while some of them are harmful, many microbes are beneficial to their host plant. Beneficial microbes hold the potential to increase the plant’s resistance to drought and pathogens as well as to provide essential nutrients. Ultimately, this leads to enhanced plant growth as well as productivity and an overall increase in plant fitness.
While the beneficial interaction between extant plants and their microbiome is well-documented, the microbiome has also been instrumental to the colonization of the terrestrial environment. In particular, it is thought that the transition to land would not have been possible without beneficial interaction with arbuscular-mycorrhiza (AM) fungi and/or other microbes including bacteria. Presently, only two groups of beneficial plant-microbe associations are extensively studied; plant symbioses with AM fungi and with nodule-forming bacteria. A promising, yet not well-understood mutualistic symbiosis is the partnership between plants and cyanobacteria. Endophytic, diazotrophic symbioses between Nostocales cyanobacteria and plants occur in all major land plant lineages but are limited to a few genera and species. While the physiological and molecular biology of the model cyanobiont Nostoc punctiforme has been thoroughly studied over the course of the symbiosis, little is known about how the host plant controls the interaction, nor do we understand the conditions and prerequisites needed to host the nitrogen-fixing cyanobacteria. The state-of-the-art model host to study the plant-cyanobacteria interaction is the hornwort Anthoceros agrestis. Despite decades of study on the hornwort-cyanobacteria system, the molecular pathways employed by the host plant in initiating and regulating the symbiotic interaction are poorly understood. Methodological advancement over the last ten years has made A. agrestis a genetically and experimentally tractable model system.
At the beginning of my thesis, I introduce a definition of symbiosis because its meaning varies between disciplines studying plant-microbe interactions. To understand how the plant host regulates the symbiotic interaction, I investigated the transcriptomic response of A. agrestis during the initiation, establishment, and maintenance of the symbiosis with the cyanobiont N. punctiforme. To enable further study of the key players identified, I developed an efficient protocol to isolate and transiently transform hornwort protoplasts and established a hormogonia reporter strain of N. punctiforme. This reporter strain enables the quantification of hormogonia formation, which are the infection units of the cyanobacteria, in a high throughput setting.
Neubauer, Anna