3.5.2  A range of N2-fixing associations

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Nitrogen-fixing bacteria can be free-living in water or on solid substrates like soil or rocks. Sandstone buildings discolour black, for example, because of the presence of an N2-fixing cyanobacterium. Nitrogen released by decay of such organisms can then be used by plants.

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Figure 3.18 An effective symbiosis between white clover (Trifolium repens) and nitrogen-fixing bacteria (Rhizobium leguminosarum bv. trifolii) supports vigorous growth in host plants on a nitrogen-free growing medium (right side of illustration). In that case, a positive interaction has occurred between host and bacterial genes for both nodule formation (Nod+) and nitrogen fixation (Fix+). By contrast, a genetic mutant Nod+Fix- (centre specimen) results in formation of a few rudimentary nodules that lack nitrogen-fixing capacity, while host plants show no nodulation response to Nod- bacteria (left side of illustration). Scale bar = 1 cm (Photograph courtesy Barry Rolfe)

However, some plants have evolved a tighter relationship with N2-fixing bacteria, involving an exchange of carbon and nitrogen between plant host and partner. Several different symbioses of this type have evolved (Table 3.2). Roots or leaves of some plants form a loose association with N2-fixing bacteria, with plant exudates used as a carbon source by the bacteria. In Azolla, the cyanobacterium Anabaena is located in cavities on the underside of modified leaves, with a secretory trichome delivering sugars and absorbing fixed nitrogen. In other plants, the N2 fixer is located in intercellular spaces of the host plant, as reported for sugar cane. These less intimate associations supply host plants with substantial amounts of nitrogen.

In more highly developed associations, plants localise the symbiotic association within a modified root or ‘nodule’ (Figure 3.18). In cycads, the microsymbiont Anabaena is located in intercellular spaces of the mid-cortex of short, highly branched, modified roots (Figure 3.19a and b). In another class of symbioses, the actinorhizal plants, the micro-symbiont Frankia (an actinomycete, or filamentous bacterium) is located within the cortical cells of a modified root. This group includes the genus Casuarina. Parasponia is the only non-legume known to form an association with the rod-shaped bacterium Rhizobium. Unlike legumes, the Parasponia nodule has a central vascular bundle and the microsymbiont is always encapsulated within cellulosic material (termed a ‘persistent infection thread’).

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Figure 3.19 Nodule anatomy showing (a) a cycad (Macrozamia miquellii) nodule consisting of a central vascular strand (VB) and an infected cortical region (stars); (b) a cyanobacterium, a microsymbiont, is located in the intercellular spaces of this infected cortex; (c) a nodule of the river-oak (Casuarina cunninghamii) consisting of a central vascular bundle with infected cells in the cortex (arrows) identified by subersation and lignification of their walls (section stained with berberine sulphate and viewed under epi-fluorescence optics); (d) scanning electron micrograph of an actinomycete microsymbiont (a filamentous bacterium) encapsulated within threads (arrow) throughout the plant cytoplasm; (e) a legume (Macroptilium lathyroides) nodule consisting of a central infected region with scattered infected cells (arrows) enclosed in a cortex. Vascular strands (VB) are present in the cortex; (f) transmission electron micrograph of a soybean (Glycine max L.) nodule containing a microsymbiont enveloped by plasma rneinbrane to form ‘symbiosomes’ — packets of bacteria within the cell cytoplasm (arrows). Scale bar = 100 µm in (a), (b), (c) and (e); 5 µm in (d) and (f). (Images courtesy K. Walsh)

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