Denitrification and Anaerobic, Nitrate-dependent Acetylene Reduction in Cowpea Rhizobium

Abstract

The dissimilatory reduction of nitrate to nitrous oxide and dinitrogen has recently been discovered in cowpea rhizobia and Rhizobium japonicum (Zablotowicz et al., 1978). The general sequence of denitrification, as proposed by Payne (1 973), of NO,-+ NO2+ NO -+ N,0+N2 is the likely pathway of denitrification in Rhizobium in both free-living and bacteroid forms. The slower growing, alkali-producing ' cowpea-type ' rhizobia probably evolved from tropical soils which remained saturated with water for extended periods of time (Norris, 1964). Nodulation in cowpeas is sensitive to short periods of anoxia (Sallee & Smith, 1969), and waterlogging depresses nitrogenase activity by decreasing respiratory activity (Sprent, 1971). Ethanol accumulation induced in soybean nodules may be partially responsible for the subsequent disintegration of nodule integrity (Sprent & Gallacher, 1976). Minchin & Pate (1975) found that, under waterlogging conditions, peas (Pisum sativum) exhibited extreme degradation of nodule tissue in 14 d, while functional bacteroids in cowpeas (Vigna) could be observed after 32 d. Though better O2 transport in cowpeas might be a possible explanation for the differences, as suggested by Minchin & Pate (1975), the possession of a dissimilatory nitrate reductase in the cowpea rhizobia cannot be precluded. Such an enzyme has been found in suspensions of R. japonicum bacteroids, though an active dissimilatory nitrite reductase was not detected (Daniel & Appleby, 1972). Rhizobium japonicum can use nitrate as an alternative electron acceptor to oxygen to generate ATP for nitrogenase activity, and nitrogenase and nitrate reductase activities remain constant until nitrite accumulates and inhibits nitrogenase activity (Rigaud et al., 1973).