How low levels of glucose and tryptophan may signal changes in development and symbiotic potential of ectomycorrhizal fungus Laccaria bicolor

Abstract: Symbiosis between tree species and ectomycorrhizal fungi is a fundamental relationship that promotes the overall health of boreal and temperate forests. Nutrient exchange between the two organisms is mutualistic in nature however establishment of this relationship is still not fully understood. Many molecules including phytohormones, sugars, and amino acids have been shown to be involved in this symbiotic process from establishment to prolonged interactions. Fungal auxin is known to be a key phytohormone in this interaction and it has been proposed that its production in ectomycorrhizal fungi is to aid in the fungi’s ability to establish symbiosis. Other molecules, however, have been proposed to work in conjunction with auxin to initiate these organismal interactions, questioning the role of fungal auxin as a master regulator. In this study, the effect of sugar and tryptophan feeding on growth and ectomycorrhiza formation was assessed. Free-living cultures of model ectomycorrhizal fungus Laccaria bicolor and lines altered (either overexpressing or RNAi-silenced) for expression of the auxin biosynthesis enzymes Aldehyde Dehydrogenase, Ald1 and Ald2 were utilized. A selection of these lines were grown in contact with common symbiotic tree species partner Populus. Fungal biomass, expression of auxin biosynthesis and transport-related genes, auxin production and the presence of ectomycorrhiza under different sugar and tryptophan concentrations was assessed. Eight out of 21 tested genetically modified L. bicolor lines secreted significantly different levels of indole-3-acetic acid as compared to the wild type when grown in standard glucose conditions and supplemented with tryptophan. Biomass production of wild type L. bicolor and transgenic lines was positively correlated to sugar concentrations in the medium, regardless of supplementation with tryptophan or genotype. Expression analysis for four of five auxin-related genes showed reduced expression for auxin-biosynthesis genes Ald1, Ald2 , and Ald3 and the putative auxin transport gene, ABCB5, in a selected line that was RNAi-silenced for Ald2. In addition, expression of Ald1, 2, 3 and ABCB5 in the WT fungus appeared to be up-regulated in low-sugar conditions as compared to standard sugar conditions supplemented with tryptophan. Bringing together these results allows for proposition of a first model to conclude how low glucose and tryptophan work together as signals to impact auxin pathways in the ectomycorrhizal fungus Laccaria bicolor to modulate fungal growth and affect its symbiotic potential.