The Rhizobium -legume symbiosis

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Abstract
Physiological changes involved in the differentiation of bacteroids within the nodule. Certainly the addition of nodules to fixed nitrogen in the form of NH + 4 or NO - 3 does repress N 2 -ase, although the concentrations required are greater than for repression in free-living nitrogen-fixing bacteria (Klamberger 1977). Various hypotheses have been put forward to account for the repression of N 2 -ase activity in the nodule. One hypothesis was that in the presence of exogenous nitrate the plant’s nitrate reductase competed for photosynthate, thereby rendering N 2 -ase inactive because it was starved of an energy supply (Oghoghorie & Pate 1971). Evidence against this was the finding (Chen & Phillips 1976) that addition of NO - 3 to pea leaves did not inhibit N 2 -ase, and that, if the plants were exposed to an atmosphere enriched with CO 2 , the extent of N 2 -ase repression when NO - 3 was applied to the roots was the same as in a normal atmosphere despite the increase in the plants’ photosynthetic capability. Thus, if there is any competition for photosynthate between N 2 -ase and nitrate reductase, it must be localized. This model does not explain why NH + 4 is also efficient as a repressor of N 2 -ase. Recently Bisseling, van den Bos & van Kamman (1978) showed that under conditions where NH 4 NO 3 repressed N 2 -ase activity in pea nodules, it did not affect synthesis of the enzyme. They proposed that the inhibition of N 2 -ase in the nodule was indirect, being mediated by an inhibition of the synthesis of leg-haemoglobin.

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