Developmental plasticity of Brachypodium distachyon (L.) P. Beauv. in response to P-solubilizing bacteria inoculation

Introduction. Mineral phosphorus (P) fertilizers should be sparingly used to slow
down the depletion of rock phosphate, a finite and non-renewable resource. Plants
cope with stresses in their environment and heterogeneous soil conditions by adjusting
their phenotype, what defines the phenotypic plasticity. The use of fertilizers in
agriculture could be reduced by integrating plastic root system traits into crop
breeding strategies. Bacterial bioinoculants are also considered as an interesting
strategy to increase the nutrient use efficiency of plants and reach desired phenotypes.
This project studied the impact of inoculation of the model plant Brachypodium
distachyon with P solubilizing bacteria on the plant phosphorus use efficiency,
focusing on enhancement of phosphorus bioavailability and modulation of the root
system plasticity.
Materials and methods. The first step of this project consisted in the
characterization of the response of B. distachyon to inorganic P deficiency, in order
to define levels of inorganic P resulting in contrasted plant phenotypes. Then the
ability of selected bacterial strains to solubilize poorly available P forms (tricalcium
phosphate and hydroxyapatite) was quantified. Finally, based on the results of the
previous steps, the response of B. distachyon to contrasted P supplies and inoculation
with P-solubilizing bacteria was studied by focusing on the plant developmental
plasticity and P use efficiency. Allometry analyses were performed to study plasticity
in the biomass allocation pattern and persistent homology analyses were conducted to
detect differences in root system morphology.
Results. A plastic response in B. distachyon biomass allocation pattern was
observed, by prioritizing root over shoot development under poorly soluble P
conditions. All the bacterial strains were able to solubilize tricalcium phosphate and
hydroxyapatite in the solubilization test. However, inoculation of the plants with
bacteria reduced the shoot productivity. On the other hand, the root system
development was maintained. Both P condition and inoculation with bacteria
impacted the root system morphology. P use efficiency in B. distachyon was not
improved by the modulation of its developmental plasticity induced by the bacteria.
Conclusion. The results support the hypothesis that P-solubilizing bacteria can
modulate the plastic response of B. distachyon in response to limited P condition. The
methods used to study the plant plasticity were useful and should be considered as
potential tools to investigate the effects of bioinoculants on plant nutrient use
efficiency. The experimental system can greatly impact plant-bacteria interaction.
Experimental conditions as close as possible to agronomic ones are recommended to
work with bacterial bioinoculants.