Creation of New Nitrogen-Fixing Cyanobacterial Associations

Abstract

Creation of N2-fixing crop plants is the ultimate goal of N2 fixation research, which aims to decrease our dependence on chemical N fertilisers for food production. The ability to use N2 as an N source is restricted to some prokaryotes only. Despite the fact that N2 makes up the bulk of the earth’s atmosphere, plants remain dependent on an exogenous supply of combined N for their growth and development. Nitrogen-fixing cyanobacteria, particularly Nostoc, form symbioses with plants ranging from algae to angiosperms and provide fixed N to the host (Rai et al. 2000). Nitrogenfixing cyanobacteria also occur epiphytically (phyllosphere and/or rhizosphere) on plants growing in aquatic and high-humidity environments (Rai 1990; Freiberg 1999; Whitton and Potts 2000). There are no natural symbioses between cyanobacteria or Frankia and crop plants, and rhizobia associate only with legume crops. Although transfer and integration of nif genes into crop plants is the long-term objective, a promising alternative is to create artificial N2-fixing symbioses or associations. Considering the low level of integration achieved during the natural evolution of cyanobacterial–plant symbioses, the creation of an obligate and self-perpetuating N2-fixing symbiosis is a tall order. In fact, none has been created as yet. The creation of facultative associations involving cyanobacteria located extracellularly on or in the plant seems more promising as an alternative. With their ability to colonise a wide range of plants and plant tissues or organs, their capacity for aerobic N2 fixation, and their flexible modes of C and N nutrition, diazotrophic cyanobacteria are a more likely candidate than rhizobia to form productive associations with cereals. Nostoc is the most common cyanobiont in natural symbioses and, therefore, the ideal choice. Unicellular forms, by contrast, are rarely found as N2-fixing partners in natural symbioses. Attempts to create artificial symbioses using unicellular forms, as well as attempts to create intracellular symbioses, have not been successful (see Rai 1990; Rai et al. 2000). However, co-cultivation of plant seedlings (e.g. from wheat, corn, sugar beet, rice and marine mangroves) and N2-fixing cyanobacteria (particularly Nostoc) in liquid and sand cultures, can lead to colonisation of the roots and support seedling growth (see Toledo et al. 1995; Rai et al. 2000; Whitton and Potts 2000).