Effect of maize diversification on its associative symbiosis with Pseudomonas fluorescens rhizobacteria – SymbioMaize

An experimental system was developed in order to grow, in parallel, 15 different maize lines in axenic conditions and to collect exudates. The composition of exudates will be characterized using various chromatographic methods. Their ability or not to induce the expression of bacterial genes in a model bacterial strain will be studied in vitro using fluorescent reporter systems. Results will then be validated by inoculating maize with a model strain in pot experiments.

We have shown that the adaptation of maize to temperate climatic conditions during its domestication has been accompanied by major changes in its physiology and, in particular, its secondary metabolism. This may have some impact on the ability of maize to interact with soil symbiotic bacteria. The presence of certain secondary metabolites in the roots that we identified as involved in the differentiation of maize genetic groups could serve as tools for the selection of maize varieties most able to benefit from soil symbiotic populations, in conditions of low fertilizer inputs.

Overall, this project will enable a better understanding of the functioning of the interaction between maize and soil bacteria and of the signification of maize diversification for this interaction. It should permit the identification of maize genotypes that are most effective at interacting with soil bacteria in temperate conddition and at benefiting from the latter. This might open new avenues for maize breeding in a context of sustainable agriculture.

Vacheron, J., E. Combes-Meynet, V. Walker, B. Gouesnard, D. Muller, Y. Moënne-Loccoz, C. Prigent-Combaret (2016) Expression on roots and contribution to maize phytostimulation of 1-aminocyclopropane-1-decarboxylate deaminase gene acdS in Pseudomonas fluorescens F113. Plant Soil. 407:187–202.
Vacheron, J., Y. Moënne-Loccoz, A. Dubost, M. Gonçalves-Martins, D. Muller, C. Prigent-Combaret (2016) Fluorescent Pseudomonas strains with only few plant-beneficial properties are favored in the maize rhizosphere. Front Plant Sci.7:1212.
Vacheron, J., G. Desbrosses, M.-L. Bouffaud, B. Touraine, Y. Moënne-Loccoz, D. Muller, L. Legendre, F. Wisniewski-Dyé, and C. Prigent-Combaret. 2013. Plant growth-promoting rhizobacteria and root system functioning. Frontiers in Plant Science 4:356.

The efficacy of the associative symbiosis (syn. cooperation) between cereals and Plant Growth-Promoting Rhizobacteria (PGPR) differs according to the genotypes of the two partners, but the functional traits responsible for these differences are poorly understood. This is especially true on the plant side, even though the likely role of root exudates has been often put forward. Cereal diversification has led to extensive modifications of crop physiological properties, including probably root exudate composition. In turn, this is likely to influence the functioning of its biotic interactions with PGPR, and we hypothesize that taking into consideration the genetic structure of cereals resulting from crop diversification will help understand differences in their association with PGPR partners. This project is tackling this issue with maize, and a type of PGPR belonging to Pseudomonas fluorescens and found worldwide in association to cereal roots. Maize originates from domestication of teosinte in Mexico, around 9,000 years ago. It was disseminated in Central America, North America, and then to Europe (from the 16th century on). During this process, maize diversified and adapted to distinct soil types, climatic conditions and cropping systems. It also met locally various bacterial populations, including PGPR from P. fluorescens. Maize dissemination and diversification resulted in the formation of 5 main genetic groups of maize, and the general objective of the SymbioMaize project is thus to determine whether membership of maize lines to these 5 genetic groups is a significant factor determining their ability to interact successfully with a P. fluorescens PGPR partner. This will be done using a collection of 15 genetically-characterized maize inbred lines and the well-studied PGPR model strain F113, which belongs to a type of P. fluorescens strains widely found in cropped soils. P. fluorescens F113 can stimulate the growth of maize, and its main phytostimulatory traits are 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity (AcdS) and the production of the root-branching signal 2,4-diacetylphloroglucinol (DAPG). First, root exudates of the 15 selected maize inbred lines will be characterized chemically. Second, the impact of the root exudates on the expression of F113’s genes acdS (ACC deaminase activity), and phlD (DAPG synthesis) will be characterized, in vitro and on roots under gnotobiotic conditions. Third, the response of maize lines to F113 inoculation will be determined based on (i) maize development and growth, and (ii) maize metabolite markers (benzoxazines) of the perception of the PGPR partner. Fourth, acdS and phlD mutants of P. fluorescens F113 will be used to evaluate whether their contribution to phytostimulation is higher in maize lines responding best to P. fluorescens F113 inoculation. Thus, the project will assess whether line membership to a particular maize genetic group resulting from crop diversification determines its root exudate profile and the ability of its exudates to trigger expression of PGPR genes, as well as its ability to respond to PGPR interaction and to benefit specifically from ACC deaminase activity and DAPG synthesis. Overall, it will enable a better understanding of the functioning of the maize-PGPR associative symbiosis, and of the signification of maize diversification for this interaction. It should also permit the identification of the maize genetic group(s) most effective at interacting with F113-like PGPR in European soils, and this might open new avenues for maize breeding in a context of sustainable agriculture. An interdisciplinary partnership has been assembled with expertise in bacterial ecology, plant-microbe interactions, plant metabolomics, and maize genetics to reach these goals.