Functional Plant Biology Functional Plant Biology Society
Plant function and evolutionary biology
RESEARCH ARTICLE

On the relationship between C and N fixation and amino acid synthesis in nodulated alfalfa (Medicago sativa)

Gemma Molero A B F , Guillaume Tcherkez C D , Jose Luis Araus B , Salvador Nogués B and Iker Aranjuelo B E
+ Author Affiliations
- Author Affiliations

A International Maize and Wheat Improvement Center (CIMMYT), El Batán, Texcoco, CP 56130, Mexico.

B Unitat de Fisologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain.

C Plateforme Métabolisme-Métabolome, IFR 87, Bât. 630, Université Paris Sud, 91405 Orsay cedex, France.

D Institut Universitaire de France, 103 Boulevard Saint Michel, 75005 Paris cedex, France.

E Instituto de Agrobiotecnología (IdAB), Universidad Pública de Navarra-CSIC-Gobierno de Navarra, Campus de Arrosadía, E-31192-Mutilva Baja, Spain.

F Corresponding author. Email: g.molero@cgiar.org

Functional Plant Biology 41(4) 331-341 https://doi.org/10.1071/FP13189
Submitted: 21 June 2013  Accepted: 31 October 2013   Published: 13 January 2014

Abstract

Legumes such as alfalfa (Medicago sativa L.) are vital N2-fixing crops accounting for a global N2 fixation of ~35 Mt N year–1. Although enzymatic and molecular mechanisms of nodule N2 fixation are now well documented, some uncertainty remains as to whether N2 fixation is strictly coupled with photosynthetic carbon fixation. That is, the metabolic origin and redistribution of carbon skeletons used to incorporate nitrogen are still relatively undefined. Here, we conducted isotopic labelling with both 15N2 and 13C-depleted CO2 on alfalfa plants grown under controlled conditions and took advantage of isotope ratio mass spectrometry to investigate the relationship between carbon and nitrogen turn-over in respired CO2, total organic matter and amino acids. Our results indicate that CO2 evolved by respiration had an isotopic composition similar to that in organic matter regardless of the organ considered, suggesting that the turn-over of respiratory pools strictly followed photosynthetic input. However, carbon turn-over was nearly three times greater than N turn-over in total organic matter, suggesting that new organic material synthesised was less N-rich than pre-existing organic material (due to progressive nitrogen elemental dilution) or that N remobilisation occurred to sustain growth. This pattern was not consistent with the total commitment into free amino acids where the input of new C and N appeared to be stoichiometric. The labelling pattern in Asn was complex, with contrasted C and N commitments in different organs, suggesting that neosynthesis and redistribution of new Asn molecules required metabolic remobilisation. We conclude that the production of new organic material during alfalfa growth depends on both C and N remobilisation in different organs. At the plant level, this remobilisation is complicated by allocation and metabolism in the different organs.

Additional keywords: carbon exchange, carbon isotopes, nitrogen fixation, nitrogen 15 isotope, photosynthesis, respiration.


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