Photosynthetic down-regulation in N-2-fixing alfalfa under elevated CO2 alters rubisco content and decreases nodule metabolism via nitrogenase and tricarboxylic acid cycle
2014
Erice, Gorka | Sanz-Sáez, Alvaro | Aroca, Ricardo | Manuel Ruiz-Lozano, Juan | Avice, Jean-Christophe | Jose Irigoyen, Juan | Sanchez-Diaz, Manuel | Aranjuelo, Iker
Although responsiveness of N-2-fixing plants to elevated CO2 conditions have been analyzed in previous studies, important uncertainties remain in relation to the effect enhanced CO2 in nodule proteomic profile and its implication in leaf responsiveness. The aim of our study was to deepen our understanding of the relationship between leaf and nodule metabolism of N-2-fixing alfalfa plants after long-term exposure to elevated CO2. After 30-day exposure to elevated CO2, plants showed photosynthetic down-regulation with reductions in the light-saturated rate of CO2 assimilation (A (sat)) and the maximum rate of rubisco carboxylation (Vc(max)). Under elevated CO2 conditions, the rubisco availability limited potential photosynthesis by around 12 %, which represented the majority of the observed fall in Vc(max). Photosynthetic down-regulation has been associated with decreased N availability even if those plants are capable to assimilate N-2. Diminishment in shoot N demand (as reflected by the lower rubisco and leaf N content) suggests that the lower aboveground N requirements affected negatively nodule performance. In this condition, specific nodule activity was reduced due to an effect on nodule metabolism that manifested as a lower amount of nitrogenase reductase. Moreover, the nodule proteomic approach also revealed that nodule functioning was altered simultaneously in various enzyme quantity apart from nitrogenase. At elevated CO2, the tricarboxylic acid cycle was also altered with a reduced amount of isocitrate synthase protein. The nodule proteome analysis also revealed the relaxation of the antioxidant system as shown by a decline in the amount of catalase and isoflavone reductase protein.
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