Molecular and cellular mechanisms of Iysophosphatidylcholine signaling in plants

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Signaling in the arbuscular mycorrhiza (AM) has been proposed to trigger expression of plant phosphate transporter genes in a cell-autonomous way (Karandashov et al., 2004). We previously showed that root extracts of mycorrhizal plants, but not of non-mycorrhizal plants, contain a lipophilic signal capable of inducing the mycorrhiza-specific phosphate transporter genes StPT3 and StPT4 of potato (Solanum tuberosum L.) (Rausch et al., 2001; Drissner et al., submitted, see Annex 4). The same signal was active also in tomato (Lycopersicon esculentum L.), rice (Oryza sativa L.), and Arabidopsis thaliana suspension cultured cells, respectivley, and caused rapid extracellular alkalinisation in all three cell cultures. The active principle using alkalinisation in cell cultures as a bioassay was identified as the lysolipid lysophosphatidylcholine (LPC). The results highlighted the importance of lysophospholipids as signals in plants, and in particular in the AM symbiosis. Apparently, the signaling pathway to activate the mycorrhiza-specific phosphate transporters has its origin in the phospholipid phosphatidylcholine (PC), a major component of membranes of plants and, probably, also of the AM fungus. However, PC was not active in itself. It gained activity only after treatment with PLA2 (Drissner et al., submitted, see Annex 4) but not after treatment with phospholipase D, suggesting an important role of PLA2 in the establishment of the mycorrhizal Pi uptake pathway. In the proposed project, the function and localisation of PLA2 will be analyzed in Lotus japonicus. Moreover, it will be tested whether Arabidopsis thaliana exhibits a growth and/or transcriptional response to LPC. A. thaliana belongs to the approximately 20% of land plants which do not develop an AM symbiosis. This last part of the proposed project will indicate whether LPC could play a more general role as a regulatory signal in plant growth and development.