Conservation of transcription factors in infected cells of nitrogen-fixing root nodules

Sammanfattning: Infected cells of nitrogen-fixing root nodules are the only plant cells that can stably internally accommodate a eubacterial symbiont. In the context of studying the differentiation of infected cells, we set about to analyse the conservation of infected cell-specific transcription factors using altogether four different promoters from genes showing infected cell-specific expression, or nodule-specific expression. The activity of promoter-reporter gene constructs was compared in transgenic hairy root systems of three different root nodule-forming plant species. The model species were two actinorhizal plants; the tree Casuarina glauca and the riparian weed Datisca glomerata, and the model legume Lotus japonicus. C. glauca and D. glomerata belong to different phylogenetic subgroups (Fagales vs. Cucurbitales) of actinorhizal plants showing strong differences with regard to nodule morphology, -metabolism and infection mechanism.The results indicate one type of infected cell-specific transcription factors previously shown to be conserved between legumes and C. glauca is also conserved in D. glomerata. However, another type of transcription factor specific to infection thread-containing cells of C. glauca and the model legume Medicago truncatula is not conserved in D. glomerata. These results indicate that in this group of actinorhizal plants, infection thread growth occurs via a different mechanism, a conclusion supported by cytological analysis.Furthermore, the results indicate that gene expression in infected cells of root nodules can be achieved by control mechanisms other than cell type-specific transcription factors, and that in this context, phytohormone regulation deserves further study.