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Salinity tolerance is related to cyanide-resistant alternative respiration in Medicago truncatula under sudden severe stress.

Salt respiration is defined as the increase of respiration under early salt stress. However, the response of respiration varies depending on the degree of salt tolerance and salt stress. It has been hypothesized that the activity of the alternative pathway may increase preventing over-reduction of the ubiquinone pool in response to salinity, which in turn can increase respiration. Three genotypes of Medicago truncatula are reputed as differently responsive to salinity: TN1.11, A17 and TN6.18. We used the oxygen-isotope fractionation technique to study the in vivo respiratory activities of the cytochrome oxidase pathway (COP) and the alternative oxidase pathway (AOP) in leaves and roots of these genotypes treated with severe salt stress (300 mM) during 1 and 3 days. In parallel, AOX capacity, gas exchange measurements, relative water content and metabolomics were determined in control and treated plants. Our study shows for first time that salt respiration is induced by the triggered AOP in response to salinity. Moreover, this phenomenon coincides with increased levels of metabolites such as amino and organic acids, and is shown to be related with higher photosynthetic rate and water content in TN6.18.