Projects in Per-Olof Lundquists group

Function of nitrogen-fixing symbiotic root nodules

N₂-fixing root nodule symbioses are formed by plants belonging to the legume family and by a group of about 220 tree and shrub species called actinorhizal plants. Actinorhizal root nodules are produced by the host plant after infection of roots by the filamentous soil bacterium Frankia (Actinomycetales). The symbiosis is beneficial for the host plant through the ability of Frankia to reduce nitrogen gas (N₂) into ammonium, which becomes a major source of nitrogen for the host. Nitrogen is a nutrient generally required in high amounts and N₂ fixation is an important part of the overall nitrogen budget of plant ecosystems. My overall research goal is to understand basic processes in root nodules and to understand factors that regulate and determine their efficiency. My model plant is the actinorhizal plant grey alder (Alnus incana). Current research projects deal with primary nitrogen and carbon metabolism and regulation of expression of genes encoding the enzymes involved. Primary metabolism provides energy for N₂ fixation and energy and carbon backbones for assimilation of ammonium. Primary metabolism also contributes as a physiological mechanism to stabilizing the O₂ environment in the root nodules, which is crucialdue to the O₂-lability of the N₂-reducing enzyme nitrogenase as shown in earlier studies. By applying nuclear magnetic resonance (NMR) techniques it was possible to obtain a model of assimilation pathways of nitrogen in root nodules and to demonstrate novel root nodule metabolites. This work has contributed to the development of an interactive database of 1D and 2D NMR data hosted by the Linköping University. One amino acid made in high amounts in root nodules and which functions as transport form of nitrogen is the amino acid citrulline, which is also a precursor to arginine. An irreversible step in the biosynthesis of citrulline is catalyzed by carbamoyl phosphate synthase (CPS), an enzyme also necessary in pyrimidine de novo biosynthesis. Characterization of a cDNA clone isolated from Alnus nodules encoding the small subunit of CPS suggests a similar organization as in bacteria. Expression of this gene is greatly enhanced in root nodules and localized to the plant cells containing N₂-fixing Frankia. Other genes with a nodule-enhanced expression pattern have also been identified. Interactions between metabolites and gene expression could be one way through which information about root nodule efficiency is transduced to control maintenance of nodules. The responses of gene expression to changes in intracellular levels of ammonium produced by N₂ fixation is therefore investigated, as well as further characterization of CPS. Another ongoing project concerns the development of an actinorhizal model species for further molecular studies of root nodule symbiosis. Other activities are as co-adviser of a PhD student in studies of arbuscular mycorrhiza of potato within the IMOP research network, and collaboration in studies of nutrient-regulated aspects of storage root formation on cassava.

Agencies that support the work

Carl Tryggers Stiftelse, Nilsson-Ehle-fonden, Stiftelsen Carl-Fredrik von Horns Fond.

Updated 2007-04-30