Nitrate and nitrite reduction by alfalfa root nodules: Accumulation of nitrite in Rhizobium melioti bacteroids and senescence of nodules |
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Authors: | M Becana P M Aparicio-Tejo M Sánchez-Díaz |
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Institution: | Dpto. de Fisiologia Vegetal, Univ. de Navarra, Apartado 273, Pamplona, Spain. |
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Abstract: | Addition of NO−3 rapidly induced senescence of root nodules in alfalfa ( Medicago sativa L. cv. Aragon). Loss of nodule dry matter began at the lowest NO−3 concentration (10 m M ) but degradation of bacteroid proteins was only detected when nodules were supplied with NO−3 concentrations above 20 m M . Bacteroids from Rhizobium meliloti contained high specific activities of nitrate reductase (NR) and nitrite reductase (NiR). Both enzymes were presumably substrate-induced although substantial enzyme activities were present in the absence of NO−3 Typical specific activities for soluble NR and NiR of bacteroids under NO−3 free conditions were 1.2 and 1.4 μmol (mg protein)−1h−1, respectively. In the presence of NO−3, the specific activity of NR was considerably greater than that of NiR, thus causing NO−2 accumulation in bacteroids. Nitrite levels in the bacteroids were linearly correlated with specific activities of NR and NiR, indicating that NO−2 is formed by bacteroid NR and that this NO−2 in turn, induces bacteroid NiR. Accumulation of NO−2 within bacteroids also indicates that NO−2 inhibits nodule activity after feeding plants with NO−3 |
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Keywords: | Bacteroid proteins Medicago sativa nitrate reductase nitrite reductase |
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