Characterisation of Plant Exudates Inducing Chemotaxis in Nitrogen-Fixing Cyanobacteria

Abstract

Cyanobacteria form symbiotic associations with a wide range of plants including cy-cads, the angiosperm Gunnera, the water fern Azolla, and bryophytes such as the liverwort Blasia and the hornwort Anthoceros (Bergman et al., 1992, 1996). The cyanobacterial symbionts in these plant symbioses are almost always members of the genus Nostoc that possess two important characteristics: they are capable of nitrogen fixation, in differentiated cells known as heterocysts (Wolk et al., 1994), and they produce specialised filaments known as hormogonia (Tandeau de Marsac, 1994). The latter are motile filaments that serve as the infective agents in most if not all the plant symbioses, and that develop from immotile parent trichomes in response to a variety of environmental stimuli (Tandeau de Marsac, 1994) including signals from potential plant hosts. For example, a hormogonia inducing factor is excreted by the hornwort Anthoceros when grown free of its symbiotic cyanobacteria in combined nitrogen-free medium (Campbell and Meeks, 1989). Similarly, the acidic mucilage secreted by Gunnera stem glands contains a hormogonia inducing activity thought to be a small, heat-labile protein (Rasmussen et al., 1994; Bergman et al., 1996). Even the roots of wheat, which forms only loose associations with cyanobacteria, release hormogonia inducing factors (Gantar et al., 1993).