Linkage between plant sugar and fungal phosphate ex-port in ectomycorrhizal symbiosis

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The tight association between plant roots and certain soil fungi forming a novel organ, the ectomycorrhiza, is essential for the success of boreal and temperate forest ecosystems. A major function of ectomycorrhizal symbiosis is the bidirectional exchange of nutrients (e.g. phosphate) and carbohydrates between fungi and plants. However, superior plant nutrition is known to inhibit ectomycorrhiza formation. On the other hand, certain fungi seem to gain different amounts of carbohydrates per "unit of nutrients" delivered to the plant. Nevertheless, apart from mutualism, little is known about the molecular mechanisms regulating phosphate and carbohydrate interchange. While no fungal phosphate exporter have yet been characterized, plant sugar efflux carrier have been recently discovered. As certain member of this SWEET called gene family are induced in an ectomycorrhiza-dependent manner they will be used in this application to address molecular mechanisms of mutualistic phosphate and carbohydrate exchange. One target will be the discovery of promoter elements and transcription factors that allow a symbiosis specific gene regulation. Furthermore, transgenic plants that are a) defective in sugar export or b) compete with fungal hyphae for glucose uptake will be used to investigate the link between carbon and phosphate nutrition. Here, time resolved P/C imaging and determination of carbohydrate and phosphate fluxes in mycorrhizas of modulated plants will be performed. Whether the observed mechanism in poplar can be generalized will be investigated in Picea abies ectomycorrhizas. SWEET homologs will be identified and mycorrhizas formed between single trees and a number of different ectomycorrhizal fungi will be investigated for induction strength of gene expression. In combination with determination of the associated fungal partner this will allow a comparison of the impact of different fungal species on SWEET induction. As this analysis will be performed for trees grown in soils with different phosphate availability, this approach will help to link the variance of C/P ratios to SWEET gene induction. This experiment will therefore answer the question whether the induction strength of SWEET gene expression can be a key feature ectomycorrhizal fungal carbohydrate support. This would (in future research) allow a quick inspection of the ecological properties of different ectomycorrhizal fungi under natural conditions.