Table compiled from Meeks (2009) and Adams (2000) and references therein aPercentages given are the value in the cyanobiont immediately after isolation from the host, compared with that in the free-living strain
Table compiled from Meeks (2009) and Adams (2000) and references therein aPercentages given are the value in the cyanobiont immediately after isolation from the host, compared with that in the free-living strain living cultures (Adams 2000; Rai et al. 2000; Meeks 2009), yet immediately after isolation from the plant the primary cyanobiont has approximately 85% of the photosynthetic rate of free-living cyanobacteria (Table 2); why this should be is not known.
The capacity of cyanobacteria for nitrogen fixation has clear potential applications in agriculture for the supply of combined nitrogen to plants, but there are no cyanobacterial endosymbioses with crop plants. Only Azolla has been used as a green manure in agriculture, being grown in rice fields, releasing its nitrogen to the soil upon death and decay. However, attempts to induce novel interactions between cyanobacteria and plants have been numerous. Cyanobacteria have been introduced into higher plant protoplasts (see: Rai et al. 2000; Adams 2000; Gusev et al. 2002; Bergman et al. 2007a), or co-cultured with plant tissue cultures, plant regenerates and cuttings from a variety of plants (Gantar 2000b; Gorelova 2001, 2006; Lobakova et al. 2001a, b; Gorelova and Korzhenevskaya 2002; Gorelova and Kleimenov 2003; Gorelova and Baulina 2009; see also Rai et al. 2000 and Gusev et al. 2002 for a discussion of the earlier literature).
Some Nostoc hormogonia can be attracted by exudates of non-host plants (Nilsson et al. 2006) and cyanobacteria can also colonise the surface of the roots of rice (Nilsson et al. 2002; 2005) and wheat (Karthikeyan et al. 2007, 2009). Colonisation of wheat seedling roots can enhance plant nitrogen content and root growth (for a discussion of this, see Rai et al. 2000; Adams 2000; Gusev et al. 2002; Bergman et al. 2007a) and cyanobacteria can be induced to grow within root tissues by mechanical damage of the root (Gantar 2000a). Little is known about the extent of such interactions in the field, but the enhancement of such "natural" interactions may offer the best hope of employing cyanobacteria to enhance crop growth and replace at least a proportion of the current artificial fertiliser use. This might involve, for example, producing crop plants with enhanced release of factors that stimulate hormogonia development and chemoattraction, thus increasing root colonisation. Certainly, this would be a great deal less technically challenging than the creation of novel endosymbiotic associations with crop plants, in which the cyanobiont is retained between host generations. Indeed, in the long evolutionary history of cyanobacteria-plant symbioses Azolla is the only such symbiosis to have evolved.
Because most research to date has focussed on the cyanobacteria, our understanding of the plant host involvement in these symbioses is relatively poor. The recent work of Khamar et al. (2010) examining changes in the expression of Gunnera genes encoding enzymes for starch and sucrose hydrolysis is the first real attempt to assess changes in host gene expression in response to the symbiotic state. Nevertheless, we still know relatively little about the chemical signals and molecular mechanisms involved in the establishment of a stable cyanobacteria-plant symbiosis.
Acknowledgements Our thanks go to the authors who gave permission for us to use the images reproduced in this chapter.
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