Specificity in natural and artificial cyanobacterial symbioses

Abstract: Cyanobacteria, primarily of the genus Nostoc, form symbioses with a wide variety of plants. The diversity of cyanobacterial strains in these symbioses, and the specificity of interaction, vary greatly. A high diversity at the strain level has been suggested for bryophytes and cycads, whereas a low diversity is found in lichens. In contrast, a high specificity is present in lichen symbioses and a low in cycads and bryophytes. Using short tandemly repeated repetitive (STRR) sequences in PCR fingerprinting, the cyanobacterial diversity and specificity found in symbioses with Gunnera (angiosperm) and Azolla (fern) were elucidated. Eleven Gunnera species with a world-wide distribution, were found to contain a high diversity of Nostoc strains. Ten groups and five un-grouped isolates were found among the 45 tested cyanobacterial isolates. Since more than one Nostoc strain was found within one symbiotic unit of the Gunnera (the gland), a low specificity was suggested. In contrast, only one cyanobacterial strain occurred within each host species of all seven tested species of Azolla, suggesting a high specificity of interaction. Understanding the specificity of natural symbioses, such as with Gunnera and Azolla, is of importance when creating artificial plant associations. An association between nitrogen-fixing cyanobacteria and the non-host plant rice would be agriculturally beneficial since chemical fertilizers could be avoided. Under laboratory conditions, the possibility of creating such an association was explored through screening of associative capacity of 57 naturally symbiotic Nostoc strains. Thirty-seven percent of the tested Nostoc strains associated tightly and in large amounts to the rice roots. The nitrogen fixation activity increased two-fold in associated cyanobacteria compared with free-living. Four strains found to associate tightly with rice roots were further screened for competitive fitness. The individual strains were identified using hetR-DGGE (denaturing gradient gel electrophoresis) analysis. One strain (8964:3) solely colonized the roots in competition with the other strains. The competitive fitness of this strain was shown to be dependent on its fast growth and spread by hormogonia. However, the number of hormogonia produced, and the chemotactic attraction towards rice, was not crucial for enhancing the competitive fitness, nor did the most competitive strains inhibit the growth of the less competitive. Since not all tested strains formed association in the primary screening, specificity seems to occur even in the formation of artificial associations with the non-host rice.