Article révisé par les pairs
Résumé : This study investigates the impact of the alteration of the monolignol biosynthesis pathway on the establishment of thein vitro interaction of poplar roots either with a mutualistic ectomycorrhizal fungus or with a pathogenic root-knotnematode. Overall, the five studied transgenic lines downregulated for caffeoyl-CoA O-methyltransferase(CCoAOMT), caffeic acid O-methyltransferase (COMT), cinnamoyl-CoA reductase (CCR), cinnamyl alcoholdehydrogenase (CAD) or both COMT and CAD displayed a lower mycorrhizal colonisation percentage, indicating alower ability for establishing mutualistic interaction than the wild-type. The susceptibility to root-knot nematode infectionwas variable in the five lines, and the CAD-deficient line was found to be less susceptible than the wild-type. Wediscuss these phenotypic differences in the light of the large shifts in the metabolic profile and gene expression patternoccurring between roots of the CAD-deficient line and wild-type. A role of genes related to trehalose metabolism,phytohormones, and cell wall construction in the different mycorrhizal symbiosis efficiency and nematode sensitivitybetween these two lines is suggested. Overall, these results show that the alteration of plant metabolism caused by therepression of a single gene within phenylpropanoid pathway results in significant alterations, at the root level, in theresponse towards mutualistic and pathogenic associates. These changes may constrain plant fitness and biomass production,which are of economic importance for perennial industrial crops such as poplar.

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