Profiles

Ulla Rasmussen

Ulla Rasmussen

Docent, Universitetslektor

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Works at Department of Ecology, Environment and Plant Sciences
Telephone 08-16 37 79
Email ulla.rasmussen@su.se
Visiting address Lilla Frescativägen 5
Room N 422
Postal address Institutionen för ekologi miljö och botanik 106 91 Stockholm

Research

Cyanobacteria are photosynthetic prokaryotes forming the ecologically most important group of bacteria on earth. They are of great evolutionary antiquity and have shaped our present day Earth in two important historical events: introducing oxygen into the atmosphere and being the ancestors of chloroplasts in green plants.

Present-day cyanobacteria enter into efficient nitrogen-fixing symbioses with representatives of all four of the major phylogenetic divisions of terrestrial plants: primitive spore-producing bryophytes (hornworts and liverworts) and ferns (Azolla) and the two classes of seed-producing plants: gymnosperms (cycads), and angiosperms (Gunnera). The cyanobacterial genus Nostoc is the most symbiotically competent cyanobacteria and, besides having a broad host range, a single Nostoc isolate has the capacity to enter into an intracellular (Gunnera) or extracellular symbiosis (cycads and bryophytes). This broad host range and capacity places cyanobacteria (in particular Nostoc) in an exclusive position among other symbiotic nitrogen fixers, and thus as a potential candidate for creating artificial symbiosis with food crops and other plants directly relevant for humanity.

For deepening our understanding of cyanobacteria and their interactions with plants, our main research focuses on: (I) the molecular mechanism underlying the communication between the two partners - prior to infection and for the maintenance of a mature symbiotic stage, and (II) the symbiosis between cyanobacteria and feather moss in the boreal forest and their impact on nitrogen input into the ecosystem.

(I) Cyanobacterial-plant communication

A prerequisite for establishment of a cyanobacterial symbiosis is that the cyanobacteria have to: (a) differentiate into a motile stage – hormogonia. This event is host controlled by the secretion of a hormogonia incusing factor (HIF) which still has not been identified – is it a “common universal factor” or is it host specific? Furthermore, (b) the hormogonia have to be attracted/guided towards the host plant. We have from a recent study shown that the feather moss is likely to secrete species-specific chemo-attractants when N-limited (Bay et al. 2013). We have chosen cyanobacterial-moss interactions as a model system for generating knowledge on how the partners communicate. The main reason for choosing this model system is that this cyanobacterial symbiosis is the symbiosis which has the most important impact on the ecosystem, and further the cyanobacteria are epiphytically attached to the leaf of the moss. The latter feature, i.e. not requiring special symbiotic tissues, might be important for eventually generating artificial symbiosis with other host plants.

We are using a novel experimental setup that allows for communication between the moss and cyanobacterial partners, with and without colonization. This setup allows the initial signaling phase (1) to be differentiated from the subsequent establishment phase (2) of the interaction. Cyanobacterial strains that are able to form symbiotic associations (symbiotic competent) are compared to strains that are not symbiotic competent.

 

 

We are combining transcriptomic and proteomic analysis to characterize the feather moss-cyanobacteria association. The result of this project will be a functional genomic model of both the moss and cyanobacteria with detailed information on the genetic control for establishment of a functional nitrogen fixing cyanobacterial symbiosis.

Collaborators:  Christopher L. Dupont and  Philip Weyman, Craig Venter Institute, La Jolla, CA USA; Rhona K. Stuart, Physical and Life Science Directorate, Lawrence Livermore National Laboratory, CA, USA

 

(II) Cyanobacteria - feather moss symbiosis in the boreal forest

In the boreal forest, cyanobacteria living in symbiosis with pleurocarpous feather mosses (e.g. Pleurozium schreberi and Hylocomium splendens) serve as a major input of N into the ecosystem. Our research aims to provide insights into how and to what extent N2-fixation rates performed by cyanobacteria in symbiosis with P. schreberi and H. splendens are related to the abundance and composition of the cyanobacterial community and whether this is consistent over the growing season. This knowledge will help to understand the variability in nitrogen fixation rated measured in the boreal forest.

Collaborators: Marie-Charlotte Nilsson and David A. Wardle, Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden

 

Present group members:

Denis Warshan, PhD student

Eric Pederson, Postdoc

 

Selected publications:

Warshan, D., Espinoza, J.L., Stuart, R.K., Richter, R.A., Kim, S-J., Shapiro, N., Woyke, T., Kyrpides, N.C., Barry, K., Singan, V., Lindquist, E., Ansong, C., Purvine, S.O., Brewer, H.M., Weyman, P.D., Dupont, C.L., Rasmussen, U. (2017) Feathermoss and epiphytic Nostoc cooperate differently: expanding the spectrum of plant–cyanobacteria symbiosis. The ISME Journal http://www.nature.com/ismej/journal/vaop/ncurrent/full/ismej2017134a.html

Warshan, D., Bay, G., Nahar, N., Wardle, D.A., Nilsson, M-C., Rasmussen, U. (2016). Seasonal variation in nifH abundance and expression of cyanobacterial communities associated with boreal feather mosses. The ISME Journal
http://www.nature.com/ismej/journal/vaop/ncurrent/pdf/ismej201617a.pdf

Bay, G., Nahar, N., Oubre, M., Whitehouse, M. J., Wardle, D. A., Zackrisson, O., Nilsson, M-C., Rasmussen, U. (2013). Boreal feather mosses secrete chemical signals to gain nitrogen. New Phytologist 200: 54–60

Zheng, W., Rasmussen, U., Zheng, S., Bao, X., Chen, B., Gao, Y., Guan, X., Larsson, J., Bergman, B.: Multiple Modes of Cell Death Discovered in a Prokaryotic (Cyanobacterial) Endosymbiont. PLoS ONE 8(6) (2013): e66147. doi:10.1371/journal.pone.0066147

Ininbergs, K., Bay, G., Rasmussen, U., Wardle, D.A., Nilsson, M-C. (2011). Composition and diversity of nifH genes of nitrogen-fixing cyanobacteria associated with boreal forest feather mosses. New Phytologist 192: 507-517

Full publication list

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Last updated: February 21, 2018

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