Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Pre-Symbiotic Signal Exchange Between the Host Plant Helianthemum Sessiliflorum and the Desert Truffle Terfezia Boudieri

Auteur : Turgeman Tidhar

Etablissement de soutenance : Ben-Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2013

Résumé partiel
The desert truffle Terfezia boudieri forms mycorrhizal association with its host plant Helianthemum sessiliflorum (Cistaceae), a small perennial desert shrub, but also with several other species of this family. Mycorrhizal associations enable the plant roots to better absorb nutrients and water from the soil by the external fungal hyphae. In return, the fungus acquires from the host plant photoassimilates. This type of mutualistic relationship exists in the majority of plant ecosystems and accounts for plant-growth and nutrients cycling. Successful establishment of mycorrhizal associations requires significant changes at the molecular and the physiological levels in both the host and the fungus. These changes start at the pre-symbiotic stage, prior to the physical contact between the partners and is achieved by chemical communication. Communication through a signal exchange at the pre-symbiotic stage was found to be highly important for the successful establishment of mycorrhiza. Helianthemum spp. and Terfezia spp. are distributed throughout the Negev desert, the Jordan valley and the Arava valley of Israel. These desert areas are characterized by extreme environmental conditions. . In order to survive in this harsh environment, both host and fungus stay in physiological dormancy most of the year. In the wet season, the host develops canopy and new fine roots system, and the fungus produces the primary hyphal network in order to penetrate into the root system. Establishment of this association in a relatively short period of time could be difficult due to low numbers of penetration sites (lateral roots) available along the main root. Moreover, the rate of root growth is six-fold higher than the mycelia growth rate and since mycelia are present only in the upper soil layers, the fast growing root tip may cross the mycelia layers evading inoculation. In this work we report on mechanisms that the fungus acquired to assure efficient inoculation. The first mechanism is host sensing by chemoattraction. We developed a bioassay to test chemoattraction and have partially characterized a putative active molecule. A 534.9 MW molecule is secreted from the roots and induces chemotropic growth of the hyphae towards the host. In a dual culture, the chemotrpic growth was accelerated when the sugar level in the media was reduced from 1% to 0.25% (1.41 and 2.94 fold, respectively).

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