The conserved arbuscular mycorrhiza-specific transcription of the secretory lectin MtLec5 is mediated by a short upstream sequence containing specific protein binding sites

In Medicago truncatula a family of mycorrhiza-specific expressed lectins has been identified recently, but the function and regulation of these lectins during the arbuscular mycorrhiza symbiosis are still unknown. In order to characterize a first member of this protein family, MtLec5 was analyzed concerning its localization and regulation. Confocal laser scanning microscopy showed that MtLec5 is a secretory protein indicating a role as a vegetative storage protein, which is specifically expressed in mycorrhizal root systems. To study the molecular mechanisms leading to the mycorrhiza-specific transcription, deletion studies of pMtLec5 were done using reporter gene fusions. Potential cis-acting elements could be narrowed down to a 150 bp fragment that was located approximately at -300/-150 according to the transcription start, suggesting the binding of positive regulators to this area. Similar expression pattern of the reporter gene was found after transforming roots of the non-legume Nicotiana tabacum with the heterologous promoter-reporter fusions. This indicated that the observed mycorrhiza-specific transcriptional induction is not legume-specific. Electrophoretic mobility shift assays showed that several factors which were exclusively present in mycorrhizal roots bind within the 150 bp promoter area. This strengthens the hypothesis of positive regulators mediating the AM-specific gene expression.