Interaction between Medicago truncatula and Pseudomonas fluorescens: evaluation of costs and benefits across an elevated atmospheric CO(2)

Abstract

Soil microorganisms play a key role in both plants nutrition and health. Their relation with plant varies from mutualism to parasitism, according to the balance of costs and benefits for the two partners of the interaction. These interactions involved the liberation of plant organic compounds via rhizodeposition. Modification of atmospheric CO(2) concentration may affect rhizodeposition and as a consequence trophic interactions that bind plants and microorganisms. Positive effect of elevated CO(2) on plants are rather well known but consequences for micoorganisms and their interactions with plants are still poorly understood. A gnotobiotic system has been developed to study the interaction between Medicago truncatula Jemalong J5 and the mutualistic bacteria Pseudomonas fluorescens strain C7R12 under two atmospheric CO(2) concentrations: ambient (365 ppm) versus enriched (750 ppm). Costs and benefits for each partner have been determined over time by measuring plant development and growth, the C and N contents of the various plant parts and the density of the bacteria in rhizosphere compartments. Following the increase in CO(2), there was a beneficial effect of P. fluorescens C7R12 on development, vegetative growth, and C/N content of M. truncatula. Concerning plant reproduction, an early seed production was noticed in presence of the bacterial strain combined with increased atmospheric CO(2) conditions. Paradoxically, this transient increase in seed production was correlated with a decrease in bacterial density in the rhizosphere soil, revealing a cost of increased CO(2) for the bacterial strain. This shift of costs-benefits ratio disappeared later during the plant growth. In conclusion, the increase in CO(2) concentration modifies transiently the cost-benefit balance in favor of the plant. These results may be explained either by a competition between the two partners or a change in bacterial physiology. The ecosystem functioning depends on the stability of many plant-microbe associations that abiotic factors can disrupt.

Figure 1. Average number of leaves of Medicago truncatula over its growth.

The number of leaves depends on the CO₂ concentration and the inoculation with Pseudomonas fluorescens C7R12. An ANOVA for repeated measures was made because this trait was measured several times on the same plants. Standard errors are represented by vertical bars. V and Fp corresponds to the plant developmental stages analysed in the experiment.

Figure 2. Average C/N ratio for shoot and root of Medicago truncatula.

Average C/N ratio for the shoot (A) and the root (B) depends on the developmental stage (Fp, Pf, see text) and the condition of inoculation (Inoculated or not with Pseudomonas fluorescens C7R12). Data were log-transformed to achieve normality and equal variances respectively tested by Shapiro-Wilk and Bartlett tests. Carbon-Nitrogen ratio, measured during the second experiment replicate only, was analysed by linear model with condition of inoculation, CO₂ concentration and developmental stage as fixed factors and with their interactions. Standard errors are represented by vertical bars.

Figure 3. Average percentage of Nitrogen in Medicago truncatula shoot and root.

Average percentage of Nitrogen in the shoot (A) and the root (B) depends on the developmental stage (Fp, Pf, see text) and the condition of inoculation (Inoculated or not with Pseudomonas fluorescens C7R12). The percentage of N, measured during the second experiment replicate only, was analysed by linear model with condition of inoculation, CO₂ concentration and developmental stage as fixed factors and with their interactions. Standard errors are represented by vertical bars.

Figure 4. Average number of seeds per plant.

Average number of seeds per plant depends on the CO₂ concentration (ambient, 365 ppm or enriched, 750 ppm), the condition of inoculation (I: inoculated or NI: not with Pseudomonas fluorescens C7R12) and the developmental stage of Medicago truncatula (Fp, Pf, see text). Data were log-transformed to achieve normality and equal variances respectively tested by Shapiro-Wilk and Bartlett tests. Number of seeds, measured during the second experiment replicate only, was analysed by linear model with condition of inoculation, CO₂ concentration and developmental stage as fixed factors and with their interactions. Standard errors are represented by vertical bars.

Figure 5. Carbon-Nitrogen ratio of Medicago truncatula seeds.

Carbon-Nitrogen ratio of seeds depends on (A) the CO₂ concentration (ambient, 365 ppm or enriched, 750 ppm) and (B) the condition of inoculation (I: inoculated or NI: not with Pseudomonas fluorescens C7R12). Data were log-transformed to achieve normality and equal variances respectively tested by Shapiro-Wilk and Bartlett tests. Carbon-Nitrogen ratio, measured during the second experiment replicate only, was analysed by linear model with condition of inoculation, CO₂ concentration and developmental stage as fixed factors and with their interactions. Standard errors are represented by vertical bars.

Figure 6. Average density of Pseudomonas fluorescens C7R12 over time.

The density of the bacterial strain was measured for the developmental stages chosen (V: vegetative; Fp: Flowers, some pods; Pf: Pods, some flowers) in compartments (A) rhizosphere soil and (B) root. Data were log-transformed to achieve normality and equal variances respectively tested by Shapiro-Wilk and Bartlett tests. The densities of microorganisms were analysed with ANOVA using as explanatory fixed factors the developmental stage and CO₂ concentration and their interactions, nested within the replicate experiment factor. Standard errors are represented by vertical bars.