The model symbiotic association between Medicago truncatula cv. Jemalong and Rhizobium meliloti strain 2011 leads to N-stressed plants when symbiotic N2 fixation is the main N source for plant growth

Abstract

A better knowledge of the nitrogen nutrition of Medicago truncatula at the whole plant level and its modulation by environmental factors is a crucial step to reach a complete understanding of legume nitrogen nutrition. This study was based on the symbiotic system that is the most commonly used by the research community (M. truncatula cv. Jemalong A17 x Rhizobium meliloti strain 2011). Plant nitrogen nutrition was analysed in relation to carbon nutrition, under a range of nitrate concentrations in the nutrient solution and different light conditions. This study shows that this 'model symbiotic association' does not allow the plant to meet its nitrogen requirements, when dinitrogen fixation is the main nitrogen source for plant growth. A strong interaction between nitrogen and carbon nutrition was shown: when plant nitrogen requirements were not sustained, plant leaf area was much affected whereas photosynthesis per unit leaf area remained relatively stable. Both total nitrogen uptake and leaf area increased with increasing nitrate concentration in the nutrient solution; the magnitude of these responses varied according to the light conditions. Interestingly, the plant nitrogen nutrition level remained nearly unaffected by the light conditions. The observed nitrogen-limitation in this 'model symbiotic association' is an important finding for the research community. Based on practical recommendations regarding both the experimental conditions and the phenotypic traits to consider, a methodological framework was proposed to (i) help genomicists to assess plant nitrogen nutrition better, and (ii) assist in the detection of new genetic variants affected for nitrogen uptake in large-scale phenotyping studies.

The Autoregulation Gene SUNN Mediates Changes in Root Organ Formation in Response to Nitrogen through Alteration of Shoot-to-Root Auxin Transport

“…Interestingly, inoculation did not significantly increase tissue [N] in the plants growing in the absence of nitrate. Even though nodules were pink at the time of harvest at 20 d post germination, it is possible that N supply from nodulation was not sufficient to cover the plant's N needs, as reported previously for M. truncatula (Moreau et al, 2008).…”

“…This nitrate concentration was chosen because it was sufficient to inhibit nodulation (data not shown) and N fixation (Moreau et al, 2008) in the wild type to the same extent as higher (5 or 10 mM) N concentrations and at the same time increased internal N concentrations significantly (see below). As increasing nitrate concentrations delay nodulation in M. truncatula (Moreau et al, 2008), we kept nitrate concentrations to a minimum to avoid the asynchronous behavior of plants with different nitrate treatments. Half of the plants were inoculated with rhizobia and the other half were mock inoculated 5 d after germination.…”

Using Functional Traits to Assess the Services Provided by Cover Plants

“…Nitrogenase activity was reported to be regulated by the O 2 permeability of nodules (Hartwig, 1998) and can be qualitatively assessed by the internal color of nodules (a pink color indicates the production of leghaemoglobin that is present only in N 2 -fixing nodules). A qualitative estimation of the plant fixating activity made on a visual scale basis that takes into account the number of nodules, their size and their internal color were proposed to assess the ability of plants to fix atmospheric N (e.g., Moreau et al, 2008). However, such a notation requires previous knowledge on each species.…”

Plant N Fluxes and Modulation by Nitrogen, Heat and Water Stresses: A Review Based on Comparison of Legumes and Non Legume Plants

“…For legumes, soil mineral N availability is the main environmental factor affecting symbiotic N 2 fixation, and consequently the relative contributions of soil mineral N assimilation by the root and symbiotic N 2 fixation to overall plant N uptake (i.e. the N source) (Moreau et al, 2008;Streeter & Wong, 1988;Voisin et al, 2002b).…”

“…For example, in soybean, N nutrition can be limiting is case where no N fertiliser is supplied at sowing (Crozat et al, 1994;Gan et al, 2002). As another example, the symbiotic association between Medicago truncatula cv Jemalong and Rhizobium meliloto strain 2011 (Moreau et al, 2008) or Sm 1021 (Terpolilli et al, 2008), which are most frequently used in genomic studies, lead to N stressed plants when N 2 fixation is the main N source for plant growth. Symbiotic N 2 Fixation is an original adaptive feature of legumes to low mineral N supply.…”