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Published on April 27, 2007; 10.1104/pp.107.099648

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Received March 15, 2007
Accepted April 13, 2007

The Response of Carbon Metabolism and Antioxidant Defenses of Alfalfa Nodules to Drought Stress and to the Subsequent Recovery of Plants

Loreto Naya , Ruben Ladrera , Javier Ramos , Esther M. González , Cesar Arrese-Igor , Frank R. Minchin , and Manuel Becana *

Departamento de Nutrición Vegetal, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, Apartado 202, 50080 Zaragoza, Spain; Departamento de Ciencias del Medio Natural, Universidad Pública de Navarra, Campus de Arrosadía, 31006 Pamplona, Spain; Institute of Grassland and Envirnomental Research, Plas Gogerddan, Aberystwyth, Dyfed SY23 3EB, United Kingdom

* Corresponding author; email: becana{at}eead.csic.es.

Alfalfa (Medicago sativa) plants were exposed to drought to examine the involvement of carbon metabolism and oxidative stress in the decline of nitrogenase (N2ase) activity. Exposure of plants to a moderate drought (leaf water potential of -1.3 MPa) had no effect on sucrose synthase (SS) activity, but caused inhibition of N2ase activity (-43%), accumulation of succinate (+36%) and Suc (+58%), and upregulation of genes encoding cytosolic CuZn-superoxide dismutase (SOD), plastid FeSOD, cytosolic glutathione reductase, and bacterial MnSOD and catalases B and C. Intensification of stress (-2.1 MPa) decreased N2ase (-82%) and SS (-30%) activities and increased malate (+40%), succinate (+68%), and Suc (+435). There was also upregulation (mRNA level) of cytosolic ascorbate peroxidase and downregulation of SS, homoglutathione synthetase, and bacterial catalase A. Drought stress did not affect nifH mRNA level or leghemoglobin (Lb) expression, but decreased MoFe- and Fe-proteins. Rewatering of plants led to a partial recovery of the activity (75%) and proteins (>64%) of N2ase, a complete recovery of Suc, and a decrease of malate (-48%) relative to control. The increase in O2 diffusion resistance, the decrease in N2ase-linked respiration and N2ase proteins, the accumulation of respiratory substrates and oxidized lipids and proteins, and the upregulation of antioxidant genes reveal that bacteroids have their respiratory activity impaired and that oxidative stress occurs in nodules under drought conditions prior to any detectable effect on SS or Lb. We conclude that a limitation in metabolic capacity of bacteroids and oxidative damage of cellular components are contributing factors to the inhibition of N2ase activity in alfalfa nodules.




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