Abstract
The symbiotic association between Medicago truncatula and Sinorhizobium meliloti is a well-established model system in the legume–Rhizobium community. Despite its wide use, the symbiotic efficiency of this model has been recently questioned and an alternative microsymbiont, S. medicae, has been proposed. However, little is known about the physiological mechanisms behind the higher symbiotic efficiency of S. medicae WSM419. In the present study, we inoculated M. truncatula Jemalong A17 with either S. medicae WSM419 or S. meliloti 2011 and compared plant growth, photosynthesis, N2-fixation rates, and plant nodule carbon and nitrogen metabolic activities in the two systems. M. truncatula plants in symbiosis with S. medicae showed increased biomass and photosynthesis rates per plant. Plants grown in symbiosis with S. medicae WSM419 also showed higher N2-fixation rates, which were correlated with a larger nodule biomass, while nodule number was similar in both systems. In terms of plant nodule metabolism, M. truncatula–S. medicae WSM419 nodules showed increased sucrose-catabolic activity, mostly associated with sucrose synthase, accompanied by a reduced starch content, whereas nitrogen-assimilation activities were comparable to those measured in nodules infected with S. meliloti 2011. Taken together, these results suggest that S. medicae WSM419 is able to enhance plant carbon catabolism in M. truncatula nodules, which allows for the maintaining of high symbiotic N2-fixation rates, better growth and improved general plant performance.
Highlights
One of the most studied plant–microbe symbiosis is the one established between members of the Leguminosae family and soil bacteria from diverse genera collectively termed rhizobia
In terms of plant nodule metabolism, M. truncatula–S. medicae WSM419 nodules showed increased sucrose-catabolic activity, mostly associated with sucrose synthase, accompanied by a reduced starch content, whereas nitrogenassimilation activities were comparable to those measured in nodules infected with S. meliloti 2011. These results suggest that S. medicae WSM419 is able to enhance plant carbon catabolism in M. truncatula nodules, which allows for the maintaining of high symbiotic N2-fixation rates, better growth and improved general plant performance
In general terms, M. truncatula plants inoculated with S. medicae WSM419 outperformed those inoculated with S. meliloti 2011
Summary
One of the most studied plant–microbe symbiosis is the one established between members of the Leguminosae family and soil bacteria from diverse genera collectively termed rhizobia. When compatible symbiotic partners interact, the microsymbiont is able to invade the host root hair cells, typically (but not exclusively) through infection threads, reaching the root cortex, where they are released and differentiate into nitrogen-fixing forms; the bacteroids. In such differentiated forms, bacteria express an enzyme complex, the nitrogenase, which catalyzes the reduction of atmospheric dinitrogen (N2) to ammonium during the highly energy-demanding process known as symbiotic N2-fixation. The development of mutant collections (Tadege et al, 2008; Calderini et al, 2011), optimization of transformation techniques (Boisson-Dernier et al., 2001) and availability of its genome sequence (Young et al, 2011) have greatly contributed to progress in the field
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