In actinorhizal symbioses, nitrogen-fixing soil actinobacteria of the genus Frankia induce the formation of nodules on the roots of their host plants. One of these host plants are Australian trees of the genus Casuarina. Actinorhizal nodules are composed of modified lateral roots with infected cells in the expanded cortex. In these infected cells, Frankia bacteria are hosted intracellularly and fix air dinitrogen while being supplied with carbon sources by the host.

Cross section through C. glauca nodule lobe, stained with Toluidine blue.
Cross section through C. glauca nodule lobe, stained with Toluidine blue.

In nodules of Casuarina sp., the primary walls of infected cells become lignified upon infection. This interferes with oxygen diffusion in that infected cells become microaerobic. We want to understand the process of the induction of lignification, and how the production of stress lignin is different from the production of lignin in, e.g., xylem development.

The genome and root and nodule transcriptomes of Casuarina glauca are currently being sequenced at the Beijing Genomics Institute (BGI) and will soon become available. So the analysis of stress lignin becomes accessible:

  • Literature study to find the enyzmes involved in lignin production
  • Compare the expression levels of the corresponding genes in the transcriptomes of roots and nodules of C. glauca
  • In parallel, extract lignin from roots and nodules of C. glauca and analyse the composition. Both roots and nodules contain xylem lignin, only nodules contain stress lignin. Which metabolites are specific to stress lignin?
  • Integrate the biochemical and molecular information: which gene is only involved in the production of stress lignin?
  • Analyse the promoters of these genes – do they share sequences with genes expressed specifically in infected cells of actinorhizal nodules?
  • Prepare RNAi constructs of three candidate genes using the C. glauca symbiotic haemoglobin promoter (the induction of stress lignin should be switched off in infected cells, and we know that the C. glauca symHb promoter is active in this cell type and the CaMV35S promoter is not)
  • Perform hairy root transformation and examine the differentiation of Frankia, and biological nitrogen fixation, in nodules induced on transgenic hairy roots

If interested, please contact:

Katharina Pawlowski

Department of Ecology, Environment and Plant Sciences

katharina.pawlowski@su.se