Dual role of the transcription factor PHR1 in Lotus japonicus, a model plant for root symbioses

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The objective of this proposal is to characterize the common molecular mechanisms underlying both the phosphate (Pi) starvation response and the arbuscular mycorrhizal (AM) symbiosis in the model legume Lotus japonicus. AM symbiosis facilitates Pi acquisition in plants in habitats of low Pi availability. In contrast, high Pi conditions suppress the development of a functional mycorrhiza. The underlying molecular mechanisms are largely unknown. Here a dual role of transcription factor PHR1 in the regulation of Pi starvation inducible (PSI) genes and the formation of the symbiosome, which is the site of bi-directional exchange of nutrients and metabolites, will be investigated. Firstly, the identification of PHR1- dependent and -independent PSI genes will be studied in phr1 mutants and in plants overexpressing PHR1. Next commonalities between AM-inducible/high Pi-repressible mycorrhizal gene expression and the repression/derepression mechanism underlying PSI gene regulation will be studied. While in the second project PHR1-dependent physiological responses and the regulation of AM-specific genes will be investigated, the third project will focus on a cis-acting regulatory module and its dependence on PHR1 activity. This module consists of two motifs, P1BS and CTTC, and is proposed to integrate AM-specific and Pi repressible transporter gene regulation. This work will help to elucidate the regulatory network which coordinately regulates AM symbiosome development in response to Pi availability.