Article

Plant and Soil

, Volume 266, Issue 1, pp 205-230

Root-based N2-fixing symbioses: Legumes, actinorhizal plants, Parasponia sp. and cycads

  • J. Kevin VesseyAffiliated withDepartment of Plant Science, University of Manitoba Email author 
  • , Katharina PawlowskiAffiliated withPlant Biochemistry, Albrecht von Haller Institute for Plant Sciences, Göttingen University
  • , Birgitta BergmanAffiliated withDepartment of Botany, Stockholm University

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Abstract

In the mutualistic symbioses between legumes and rhizobia, actinorhizal plants and Frankia, Parasponia sp. and rhizobia, and cycads and cyanobacteria, the N2-fixing microsymbionts exist in specialized structures (nodules or cyanobacterial zones) within the roots of their host plants. Despite the phylogenetic diversity among both the hosts and the microsymbionts of these symbioses, certain developmental and physiological imperatives must be met for successful mutualisms. In this review, phylogenetic and ecological aspects of the four symbioses are first addressed, and then the symbioses are contrasted and compared in regard to infection and symbio-organ development, supply of carbon to the microsymbionts, regulation of O2 flux to the microsymbionts, and transfer of fixed-N to the hosts. Although similarities exist in the genetics, development, and functioning of the symbioses, it is evident that there is great diversity in many aspects of these root-based N2-fixing symbioses. Each symbiosis can be admired for the elegant means by which the host plant and microsymbiont integrate to form the mutualistic relationships so important to the functioning of the biosphere.

Key words

actinorhizal plants coralloid roots cyanobacteria cycads Frankia legume mutualism N2 fixation nod genes nif genes nodulation Nostoc Parasponia Rhizobium symbioses