Fig. 9

Schematic model depicting the induction of MtD27 expression by phosphate starvation and rhizobium LCO-induced signaling. Rhizobium LCOs activate expression of MtD27 through the common symbiosis signaling module consisting of MtDMI1, MtDMI2 and MtDMI3. Downstream of this module MtERN1 is required for full induction of MtD27. MtERN1 function is partly redundant, suggesting that MtERN1 might function in conjunction with or redundant to another unknown transcriptional regulator, indicated as X. This unknown factor might be MtERN2, a transcriptional regulator closely related to MtERN1 [48]. The pathway leading to activation of MtD27 expression by phosphate starvation remains unknown. The GRAS proteins MtNSP1 and MtNSP2 are required for expression of MtD27 during both phosphate starvation and following rhizobium LCO application. We propose that these proteins function in parallel to both signaling pathways, as previously already suggested for the LCO signaling cascade [7, 15]