J. Wouters et al., The effect of exogenous carbohydrates on nitrogen fixation and hetR expression in Nostoc PCC 9229 forming symbiosis with Gunnera, SYMBIOSIS, 28(1), 2000, pp. 63-76
The cyanobacterium Nostoc PCC 9229 forms an intracellular nitrogen-fixing s
ymbiosis with the angiosperm Gunnera. In symbiosis the cyanobacterium is en
closed in darkness and receives carbon from the plant in an unknown form. O
ut of five putative plant carbohydrate sources tested in vitro, fructose an
d glucose were found to support nitrogen fixation in darkness. The other th
ree dextrin, sucrose and Gunnera sp. mucilage could not induce nitrogenase
activity in darkness. The stimulatory effect by fructose was also observed
in illuminated samples. After four weeks incubation in darkness, nitrogenas
e was still active in cultures when fructose was added and multiple thick-w
alled nitrogen-fixing cells (heterocysts) were observed, and chlorophyll le
vels unchanged. The expression as shown by Northern blot analysis revealed
that fructose influenced the gene expression of hetR, a gene necessary for
heterocyst formation, in darkness. Fructose and glucose may therefore be th
e carbohydrates supplied by the host plant to induce heterocyst differentia
tion and nitrogen fixation in the cyanobiont Nostoc PCC 9229.