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RESEARCH PRODUCT

The impact of the cultivation practices on arbuscular mycorrhizal symbiosis mechanism in a walnut tree - corn agroforestry system

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The English walnut (Juglans regia L.) is the main species cultivated for the production of edible nuts. Owing to a sparse canopy and a deep rooting system, walnut is an ideal species for alley cropping, an agroforestry practice able to enhance productivity through interplant facilitative mechanisms. Walnut agroforestry requires the large scale production of seedling rootstocks selected to provide the best anchorage, vigour, and tolerance of pathogens. Due to the heterozygosity of walnut, the characteristics of agronomical interest of the chosen cultivar are not inherited via seed propagation. In vitro plant tissue culture thus plays a key role in mass propagation of high-quality walnut rootstocks. Micropropagation of walnut explants needs an ex vitro acclimatization phase to repair the in vitro induced abnormalities, and further requires a post-acclimatization growth in greenhouse conditions when plantlets become photoautotrophic. However, poor survival and slow growth rates are common difficulties encountered in nurseries when establishing micropropagated walnut saplings. As many other fruit and nut bearing trees, walnut exhibits a high dependency on symbiotic soil-borne arbuscular mycorrhizal (AM) fungi for development due to a coarse root architecture that limits soil inorganic phosphate (Pi) uptake. In the context of rootstock production, the aim of this thesis was to investigate at different development stages the establishment of seven walnut rootstocks of economic interest, upon inoculation or not with an AM fungus and two contrasting Pi fertilization regimes. We demonstrated that biotization with the AM fungus Rhizophagus irregularis improves the development of micropropagated walnut rootstocks at two decisive stages, namely ex vitro and post-vitro acclimatization. Under low-Pi conditions, biotization increases seedling performance attributes including biomass, leaf number, stem height, photosynthesis efficiency and leaf nutrition (carbon, nitrogen, and Pi). We concluded that the AM fungus improves walnut Pi nutrition, which in turn leads to increased photosynthetic rates and improved plant growth. We also showed that these benefits are rootstock-dependent, indicating that walnut mycorrhizal dependency for Pi nutrition varies between cultivars, and that the symbiotic phosphate uptake pathway may be differentially recruited depending on the chosen rootstock. We then sought to identify in Juglans spp. the phosphate transporters inducible by AM colonization. Detection of putative orthologues of the AM-specific phosphate transporter MtPT4 of the model legume Medicago truncatula was performed with Orthofinder using the BLAST all-vs-all algorithm, which identified three putative orthologues in J. regia and J. microcarpa. We validated these candidates through their expression analysis upon inoculation or not with the AM fungus R. irregularis in Pi-deficient and Pi-replete conditions. To address the benefits of mycorrhizal walnuts in alley cropping, we experimentally simulated using compartmentalized microcoms a walnut agroforestry system in which young walnut hybrid roostocks RX1 (J. regia x J. microcarpa) were connected or not by a common mycelial network (CMN) to maize (Zea maize L.) plants grown under contrasting Pi supply levels. Within a few weeks, the inoculum reservoir formed by walnut saplings allowed the mycorrhization of maize roots, thereby giving access to the maize nutrient pool. We showed that hyphal connections under P deficient condition result for both plants in growth and nutritional benefits comparable to P sufficient condition without a CMN. We concluded that a CMN is able to alleviate Pi deficiency in co-cultivated walnut and maize plants, and may therefore contribute to limit the use of chemical fertilizers in agroforestry systems.