Annals of Botany

Can they fix it? Yes they can!

Fixation of nitrogen by Rhizobium and Bradyrhizobium in symbiotic associations with legumes and by Frankia endosymbionts of Alnus and Casuarina is a well-known phenomenon. However, these symbiotic bacteria represent only a small proportion of N-fixing prokaryotes. There are many free-living, soil-dwelling bacteria that fix N and there is evidence that plants may benefit from this if the bacteria are located in the rhizosphere. This is especially true of mycorrhizal systems in which uptake of nitrogenous compounds is enhanced. Furthermore it is now clear that N-fixing bacteria may live inside tuberculate ectomycorrhizae. These are mycorrhizae in which tight clusters of mycorrhizal root tips are enclosed by a hyphal sheath to form a tubercle. Paul et al. (British Columbia, Canada and Uppsala, Sweden, pp. 1101–1109) have previously shown that N-fixing bacteria of the species Paenibacillus vinicolor and Methylobacterium mesophilicum live inside the the tuberculate ectomycorrhizae formed by Suillis tomentosus on roots of Pinus contorta. They have now investigated the nitrogenase (N-fixing) activity (as measured by the acetylene reduction assay) in these mycorrhizae. In all three study sites in British Columbia, roots with tuberculate ectomycorrhizae exhibited significant nitrogenase activity, with roots from younger stands showing higher activity than roots from older stands. There was some seasonal variation with activities in summer being much higher than those in spring. This result surprised the authors: they expected a spring-time peak, followed by a decline of activity in the dry summer. Non-mycorrhizal roots and roots with non-tuberculate mycorrhizae did not exhibit nitrogenase activity. Thus, only the structures within which the bacteria were living were active in N-fixation. The authors regard this system as a truly symbiotic relationship dependent on a particular mycorrhizal morphology. Further, although the nitrogenase activity is only approx. 10 % of that observed in the Frankia-Alnus symbiosis, it is clear that it contributes significantly to the N economy of P. contorta, especially in nutrient-poor soils.