The auxin transport inhibitor 2,3,5‐triiodobenzoic acid (TIBA) inhibits the stimulation of in vitro lateral root formation and the colonization of the tap‐root cortex of Norway spruce (Picea abies) seedlings by the ectomycorrhizal fungus Laccaria bicolor

Abstract

Norway spruce (Picea abies (L.) Karst.) seedlings were inoculated with the ectomycorrhizal fungus Laccaria bicolor ((Marie) Orton), strain S238 N, in axenic conditions. The presence of the fungus slowed tap-root elongation by 26 % during the first 15 d after inoculation and then stimulated it by 136 %. In addition, it multiplied in vitro lateral root formation by 4n3, the epicotyl growth of the seedlings by 8n4 and the number of needles by 2. These effects were maintained when the fungus was separated from the roots by a cellophane membrane preventing symbiosis establishment, thus suggesting that the fungus acted by non-nutritional effects. We tested the hypothesis that IAA produced by L. bicolor S238 N would be responsible for the stimulation of fungal induced rhizogenesis. We showed in previous work that L. bicolor S238 N can synthesize IAA in pure culture. Exogenous IAA supplies (100 and 500 µ) reproduced the stimulating effect of the fungus on root branching but inhibited root elongation. The presence of 2,3,5-triiodobenzoic acid (TIBA) in the culture medium significantly depressed lateral root formation of inoculated seedlings. As TIBA had no significant effect on IAA released in the medium by L. bicolor S238 N, but counteracted the stimulation of lateral rhizogenesis induced by an exogenous supply of IAA, we suggest that TIBA inhibited the transport of fungal IAA in the root. Furthermore TIBA blocked the colonization of the main root cortex by L. bicolor S238 N and the formation of the Hartig net. These results specified the role of fungal IAA in the stimulation of lateral rhizogenesis and in ectomycorrhizal symbiosis establishment.

Biosynthesis and Secretion of Indole-3-Acetic Acid and Its Morphological Effects on Tricholoma vaccinum-Spruce Ectomycorrhiza

“…The phytohormone might act as a diffusible signal in the communication between the mycobiont and the plant (51). IAA transport mechanisms are known in plants, where TIBA acts as a polar auxin transport inhibitor and reverses the effects of IAA on spruce-Laccaria bicolor ectomycorrhiza (58). We observed similar TIBA effects although an ortholog to the plant transporters is not encoded within the T. vaccinum genome.…”

Fungal Auxin Antagonist Hypaphorine Competitively Inhibits Indole-3-acetic Acid-Dependent Superoxide Generation by Horseradish Peroxidase

“…A pathogenic fungus of rice, Pythium aphanidermatum is known to produce and accumulate IAA in host plants, that is 200 times higher than that in normal plants, and causes serious leaf-bent symptom (31). Recent studies have confirmed that fungal IAA controls major anatomical features and gene expression in ectomycorrhiza such as that of pine (32,33), spruce (34) and Eucalyptus (35,36). Other indolic compounds released from ectomycorrhizal fungi also play a role in symbiosis development (35,36).…”

The impact of ectomycorrhiza formation on monosaccharide transporter gene expression in poplar roots

“…Auxins have been made partially responsible for morphological and physiological changes of the host plant occuring during mycorrhiza formation (Gay et al, 1994;Rudawska & Kieliszewska-Rokicka, 1997;Karabaghli-Degron et al, 1998). It has been suggested that IAA excreted by the fungus is responsible for enhancing the infectivity of the fungus (Tranvan et al, 2000), changes in the plant cell wall network resulting in cell wall loosening (Duddrigde & Read, 1984), and facilitation of Hartig net establishment (Gay et al, 1995).…”