© 2008 The Author(s).
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RESEARCH PAPER |
Endogenous PttHb1 and PttTrHb, and heterologous Vitreoscilla vhb haemoglobin gene expression in hybrid aspen roots with ectomycorrhizal interaction
1Department of Biology, PO Box 3000, 90014 University of Oulu, Finland
2Department of Biological and Environmental Sciences, PO Box 56, 00014 University of Helsinki, Finland
3Institute of Microbiology, ETH-Zürich, CH-8093 Zürich, Switzerland
4Department of Applied Biology, PO Box 27, 00014 University of Helsinki, Finland
* To whom correspondence should be addressed. E-mail: Soile.Jokipii{at}oulu.fi
Present knowledge on plant non-symbiotic class-1 (Hb1) and truncated (TrHb) haemoglobin genes is almost entirely based on herbaceous species while the corresponding tree haemoglobin genes are not well known. The function of these genes has recently been linked with endosymbioses between plants and microbes. In this work, the coding sequences of hybrid aspen (Populus tremulaxtremuloides) PttHb1 and PttTrHb were characterized, indicating that the key residues of haem and ligand binding of both genes were conserved in the deduced amino acid sequences. The expression of PttHb1 and PttTrHb was examined in parallel with that of the heterologous Vitreoscilla haemoglobin gene (vhb) during ectomycorrhiza/ectomycorrhizal (ECM) interaction. Both ECM fungi studied, Leccinum populinum and Xerocomus subtomentosus, enhanced root formation and subsequent growth of roots of all hybrid aspen lines, but only L. populinum was able to form mycorrhizas. Real-time PCR results show that the dual culture with the ECM fungus, with or without emergence of symbiotic structures, increased the expression of both PttHb1 and PttTrHb in the roots of non-transgenic hybrid aspens. PttHb1 and PttTrHb had expression peaks 5 h and 2 d after inoculation, respectively, pointing to different functions for these genes during interaction with root growth-improving fungi. In contrast, ECM fungi were not able to enhance the expression of hybrid aspen endogenous haemoglobin genes in the VHb lines, which may be a consequence of the compensating action of heterologous haemoglobin.
Key words: Ectomycorrhizas, non-symbiotic haemoglobin, Populus tremulaxtremuloides, real-time PCR, truncated haemoglobin, Vitreoscilla haemoglobin (VHb)
The nucleotide sequences of hybrid aspen PttHb1 and PttTrHb coding regions that were reported in this paper have been submitted to the NCBI database under accession numbers EF180083 [GenBank] and EF180084 [GenBank] , respectively.
Received 27 December 2007; Revised 18 March 2008 Accepted 18 March 2008