Abstract
Under nitrogen-limiting conditions, legumes are able to interact symbiotically with bacteria of the Rhizobiaceae family. This interaction gives rise to a new organ, named a root nodule. Root nodules are characterized by an increased glutathione (GSH) and homoglutathione (hGSH) content compared to roots. These low molecular thiols are very important in the biological nitrogen fixation. In order to characterize the modification of nodule activity induced by the microsymbiont glutathione deficiency, physiological, biochemical, and gene expression modifications were analyzed in nodules after the inoculation of Medicago truncatula with the SmgshB mutant of Sinorhizobium meliloti which is deficient in GSH production. The decline in nitrogen fixation efficiency was correlated to the reduction in plant shoot biomass. Flow cytometry analysis showed that SmgshB bacteroids present a higher DNA content than free living bacteria. Live/dead microscopic analysis showed an early bacteroid degradation in SmgshB nodules compared to control nodules which is correlated to a lower bacteroid content at 20 dpi. Finally, the expression of two marker genes involved in nitrogen fixation metabolism, Leghemoglobin and Nodule Cysteine Rich Peptide 001, decreased significantly in mutant nodules at 20 dpi. In contrast, the expression of two marker genes involved in the nodule senescence, Cysteine Protease 6 and Purple Acid Protease, increased significantly in mutant nodules at 10 dpi strengthening the idea that an early senescence process occurs in SmgshB nodules. In conclusion, our results showed that bacterial GSH deficiency does not impair bacterial differentiation but induces an early nodule senescence.
Highlights
Nitrogen (N) is the major macronutrient needed for plant growth
The nitrogen fixation efficiency was significantly reduced by 53% and 21% in S. meliloti gshB (SmgshB) inoculated plants compared to control plants at 10 and 20 dpi, respectively (Figure 1A)
Phenotypic analysis of root nodules showed that SmgshB nodules presented a less pronounced pink color compared to control ones and present an extended green color on the distal part of nodule at 20 dpi (Figure 1B)
Summary
Nitrogen (N) is the major macronutrient needed for plant growth. whereas N is the most abundant element in the atmosphere, it is generally the most limiting plant nutrient in soils. Legume plants are able to interact symbiotically with rhizobia to perform nitrogen fixing symbiosis (NFS). In these mutualistic associations, bacteria deliver nitrogen source to the plants while plants provide bacteria with photosynthates, nutrients required for rhizobium metabolism and a specialized ecological niche that favors their growth. The nitrogenfixing symbiosis involves the de novo formation of a new organ, the nodule, associated to a reprogramming of the plant development and metabolism. This allows the intracellular accommodation of several thousands of differentiated bacteria, within nodule cells, called bacteroids which reduce atmospheric nitrogen. NO has been shown to regulate the glutathione (GSH) synthesis in roots (Innocenti et al, 2007)
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