Abstract
Pea forms symbiotic nodules with Rhizobium leguminosarum sv. viciae (Rlv). In the field, pea roots can be exposed to multiple compatible Rlv strains. Little is known about the mechanisms underlying the competitiveness for nodulation of Rlv strains and the ability of pea to choose between diverse compatible Rlv strains. The variability of pea-Rlv partner choice was investigated by co-inoculation with a mixture of five diverse Rlv strains of a 104-pea collection representative of the variability encountered in the genus Pisum. The nitrogen fixation efficiency conferred by each strain was determined in additional mono-inoculation experiments on a subset of 18 pea lines displaying contrasted Rlv choice. Differences in Rlv choice were observed within the pea collection according to their genetic or geographical diversities. The competitiveness for nodulation of a given pea-Rlv association evaluated in the multi-inoculated experiment was poorly correlated with its nitrogen fixation efficiency determined in mono-inoculation. Both plant and bacterial genetic determinants contribute to pea-Rlv partner choice. No evidence was found for co-selection of competitiveness for nodulation and nitrogen fixation efficiency. Plant and inoculant for an improved symbiotic association in the field must be selected not only on nitrogen fixation efficiency but also for competitiveness for nodulation.
Highlights
Legumes are a sustainable source of protein for both human and animal diets
This study investigates the genetic variability of partner choice in the pea-Rhizobium leguminosarum sv. viciae (Rlv) symbiosis
The slope of the regression line was lower than one indicating that increase in shoot dry matter index was associated with moderate change in competitiveness. This is the first time that the genetic diversity for pea-Rlv partner choice has been investigated within a pea collection representative of the variability within the genus Pisum and co-inoculated with a mixture of diverse Rlv strains
Summary
Legumes are a sustainable source of protein for both human and animal diets Owing to their ability to establish symbiosis with nitrogen-fixing bacteria, their cultivation is vital for reducing the use of nitrogen fertilizers, a major cause of agricultural greenhouse gas emissions and energy consumption (Jensen and Hauggaard-Nielsen, 2003; Galloway et al, 2008). Genetic Diversity of Pea-Rhizobium leguminosarum Partner Choice fixation (BNF) obtained from grain legume crops (pulses and oilseed legumes) represents a quarter of the N applied to arable lands annually as chemical fertilizers (Herridge et al, 2008). Despite these benefits, grain legumes are under-cultivated in European agricultural systems. Improving the regularity of grain legume yield is a major objective from an agroecological standpoint, and one avenue to achieve it could be through improved symbiosis
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