Pre-symbiotic Interactions Between the Desert Truffle Terfezia boudieri and Its Host Plant Helianthemum sessiliflorum

Each of the two members of the mycorrhizal couple, the plant and the fungus, has developed mechanisms for signaling the other prior to the actual formation of the mycorrhizal association. The symbiotic pair Terfezia boudieri (Pezizaceae, a desert truffle) and Helianthemum sessiliflorum (Cistaceae) was studied with a view to determining how fungal secretion of auxin—a plant hormone—promotes establishment of mycorrhizas, on the one hand, and how plant chemoattractants affect fungal growth, on the other. In view of the known effects of auxin—inhibition of tap root growth, enhancement of secondary root development, bending of the direction of root growth towards the horizontal (negative chemotropism)—it seems that auxin secretion by the fungus is designed to keep the root tips (which are the fungal penetration sites) within its reach. Indeed, fungi grow about ten times more slowly than developing roots. Inhibition of root hair formation, another effect of fungal auxin, makes the plant dependent on the fungus for its nutrients. It is known that fungal oriented growth towards plant roots is very pronounced under limiting sugar availability and much weaker under high sugar concentrations. Our research has recently focused on isolation of potential plant chemoattractants; so far, one as yet unidentified compound with a molecular weight of about 535 has been found to possess chemotropic activity.