Hypaphorine from the ectomycorrhizal fungus Pisolithus tinctorius counteracts activities of indole-3-acetic acid and ethylene but not synthetic auxins in eucalypt seedlings

Very little is known about the molecules regulating the interaction between plants and ectomycorrhizal fungi during root colonization. The role of fun gal auxin in ectomycorrhiza has repeatedly been suggested and questioned, s uggesting that, if fungal auxin controls some steps of colonized root devel opment, its activity might be tightly controlled in time and in space by pl ant and/or fungal regulatory mechanisms. We demonstrate that fungal hypapho rine, the betaine of tryptophan, counteracts the activity of indole-3-aceti c acid (IAA) on eucalypt tap root elongation but does not affect the activi ty of the IAA analogs 2,4-D ((2,4-dichlorophenoxy)acetic acid) or NAA (1-na phthaleneacetic acid). These data suggest that IAA and hypaphorine interact during the very early steps of the IAA perception or signal transduction p athway. Furthermore, while seedling treatment with 1-amincocyclopropane-1-c arboxylic acid (ACC), the precursor of ethylene, results in formation of a hypocotyl apical hook, hypaphorine application as well as root colonization by Pisolithus tinctorius, a hypaphorine-accumulating ectomycorrhizal fungu s, stimulated hook opening. Hypaphorine counteraction with ACC is likely a consequence of hypaphorine interaction with IAA, In most plant-microbe inte ractions studied, the interactions result in increased auxin synthesis or a uxin accumulation in plant tissues. The P, tinctorius / eucalypt interactio n is intriguing because in this interaction the microbe down-regulates the auxin activity in the host plant. Hypaphorine might be the first specific I AA antagonist identified.