Abstract
The legume-rhizobium symbiosis is a major supplier of fixed nitrogen in the biosphere and constitutes a key step of the nitrogen biogeochemical cycle. In some legume species belonging to the Inverted Repeat Lacking Clade (IRLC) and the Dalbergioids, the differentiation of rhizobia into intracellular nitrogen-fixing bacteroids is terminal and involves pronounced cell enlargement and genome endoreduplication, in addition to a strong loss of viability. In the Medicago truncatula-Sinorhizobium spp. system, the extent of bacteroid differentiation correlates with the level of symbiotic efficiency. Here, we used different physiological measurements to compare the symbiotic efficiency of photosynthetic bradyrhizobia in different Aeschynomene spp. (Dalbergioids) hosts inducing different bacteroid morphotypes associated with increasing ploidy levels. The strongly differentiated spherical bacteroids were more efficient than the less strongly differentiated elongated ones, providing a higher mass gain to their hosts. However, symbiotic efficiency is not solely correlated with the extent of bacteroid differentiation especially in spherical bacteroid-inducing plants, suggesting the existence of other factors controlling symbiotic efficiency.
Highlights
Nitrogen-fixing symbioses constitute a major process in the nitrogen biogeochemical cycle, accounting for the main input of fixed nitrogen in terrestrial ecosystems (Gruber and Galloway, 2008)
Before analyzing the symbiotic efficiency of E-type and S-type bacteroids, we first determined the functionality of the symbiosis in the 16 associations formed by A. afraspera, A. evenia, and A. indica and 6 Bradyrhizobium strains (BTAi1, ORS278, ORS285, ORS287, ORS335, and ORS357; the two first strains are unable to form nodules on A. afraspera because they lack the required Nod factors for nodulation of these plants (Giraud et al, 2007; Chaintreuil et al, 2013)
Functional nodules were formed which displayed a pink coloration of the infected zone due to leghemoglobin production as observed in the handsectioned nodules, the coloration was paler in the case of A. evenia nodulated by Bradyrhizobium sp
Summary
Nitrogen-fixing symbioses constitute a major process in the nitrogen biogeochemical cycle, accounting for the main input of fixed nitrogen in terrestrial ecosystems (Gruber and Galloway, 2008). After mutual recognition of the partners through a molecular dialogue, a coordinated program of organogenesis and infection results in a mature nodule where plant symbiotic cells are filled with intracellular bacteria that differentiate into nitrogen-fixing bacteroids (Kondorosi et al, 2013). In some legume clades such as the Inverted Repeat Lacking Clade (IRLC) to which Medicago truncatula belongs, bacteroids undergo a strong cell enlargement, associated to an increased membrane permeability. During this process, elongated bacteroids become polyploid and their capacity to resume growth is compromised. Their differentiation is considered to be terminal
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