Towards the Role of Fungal Auxin in Ectomycorrhizal Development.

Ectomycorrhizal (ECM) symbiosis are mutualistic interactions between tree roots and soil fungi. In exchange for plants sugars, fungi transfer nitrogen and phosphorous to the trees, thereby promoting tree growth. We aim at revealing the molecular mechanisms of ECM formation in the model system poplar/Laccaria bicolor. ECM formation leads to attenuated root growth, swollen root tips and a (yet not characterized) reorganization of the root apical meristem (RAM). During the colonization process, fungal hyphae adhere to and surround root tips to form a mantle. Subsequently, hyphae penetrate between root cells to form the nutrient exchange structure, called Hartig net, a process which requires cell wall remodeling. Auxin, a phytohormone also produced by the fungus, has been hypothesized to favor cell wall loosening during Hartig Net formation, and it may also be responsible for RAM modification. In order to better understand the role of fungal auxin in ECM formation, genes involved in both auxin biosynthesis and release in L. bicolor have been identified in silico. We have validated in vitro the activity of a tryptophan aminotransferase LbTam1 and two aldehyde dehydrogenases, LbAld1 and LbAld2, predicted to be involved in auxin biosynthesis. To reveal auxin efflux, the transport activity of putative ABCB and PIN/PILS-like transporters identified in silico will be addressed using a recently developed ratiometric auxin sensor system. Functional analysis of ECM formation with L. bicolor mutants with increased/decreased auxin production/release, will uncover the role of fungal auxin production for Hartig Net formation. In parallel we have generated transgenic Populus plants with GFP markers in different RAM tissues. These plants are used to decrypt how the RAM is altered during ECM formation. In combination with L. bicolor strains with increased or reduced auxin levels, they will reveal the effect of fungal auxin on RAM modification during ECM formation.