Felten J, Kohler A, Morin E, Bhalerao RP, Palme K, Martin F, Ditengou FA, Legue V
The ectomycorrhizal fungus Laccaria bicolor stimulates lateral root formation in poplar and Arabidopsis through auxin transport and signaling
Plant Physiology: 2008 151:1991-2005

The early phase of the interaction between tree roots and ectomycorrhizalfungi, prior to symbiosis establishment, is accompanied by astimulation of lateral root (LR) development. We aimed to identifygene networks that regulate LR development during the earlysignal exchanges between poplar (Populus tremula x Populus alba)and the ectomycorrhizal fungus Laccaria bicolor with a focuson auxin transport and signaling pathways. Our data demonstratedthat increased LR development in poplar and Arabidopsis (Arabidopsisthaliana) interacting with L. bicolor is not dependent on theability of the plant to form ectomycorrhizae. LR stimulationparalleled an increase in auxin accumulation at root apices.Blocking plant polar auxin transport with 1-naphthylphthalamicacid inhibited LR development and auxin accumulation. An oligoarray-basedtranscript profile of poplar roots exposed to molecules releasedby L. bicolor revealed the differential expression of 2,945genes, including several components of polar auxin transport(PtaPIN and PtaAUX genes), auxin conjugation (PtaGH3 genes),and auxin signaling (PtaIAA genes). Transcripts of PtaPIN9,the homolog of Arabidopsis AtPIN2, and several PtaIAAs accumulatedspecifically during the early interaction phase. Expressionof these rapidly induced genes was repressed by 1-naphthylphthalamicacid. Accordingly, LR stimulation upon contact with L. bicolorin Arabidopsis transgenic plants defective in homologs of thesegenes was decreased or absent. Furthermore, in Arabidopsis pin2,the root apical auxin increase during contact with the funguswas modified. We propose a model in which fungus-induced auxinaccumulation at the root apex stimulates LR formation througha mechanism involving PtaPIN9-dependent auxin redistributiontogether with PtaIAA-based auxin signaling.

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