The temporal regulation of MtCEP7 gene expression correlates with the symbiotic root competence window
CEP (C-terminally Encoded Peptides) signaling peptides are drivers of systemic pathways regulating Nitrogen (N) acquisition in different plants, from Arabidopsis to legumes, depending on mineral N availability (e.g. nitrate) and on the whole plant N demand. Recent studies in the Medicago truncatula model legume revealed how root-produced CEP peptides control the root competence for endosymbiosis with N fixing rhizobia soil bacteria through the activity of the CRA2 (Compact Root Architecture) CEP receptor in shoots. Among CEP genes, MtCEP7 was shown to be tightly linked to nodulation. The team SILEG at IPS2 recently reported in New Phytologist that the dynamic temporal regulation of MtCEP7 expression reflects the plant ability to maintain a different symbiotic root competence window depending on the symbiotic efficiency of the rhizobium strain, as well as to reinitiate a new window of root competence for nodulation.
MtCEP7 expression tightly correlates with the Medicago truncatula root competence to initiate new symbiotic nodules. Sinorhizobium/Ensifer bacteria symbiotic efficiency differs depending on species and strains, eg with the M. truncatula Jemalong A17 genotype, the WSM419 strain is very efficient, whereas the Sm1021 strain is less efficient, and the T173 strain is not able to allow nitrogen fixation to occur. Consequently, the number of nodule initiation events are restricted to a tight early temporal window for the WSM419 strain (upper graphs) compared to Sm1021 (upper left graph), whereas the T173 strain reinitiates a second wave of non-nitrogen fixing nodule initiation 10 days after the initial inoculation with the symbiotic bacteria (upper right graph). The expression of MtCEP7 remains higher over the longer Sm1021 root competence window for nodule initiation compared to the WSM419 efficient strain (lower left graph), and remains also higher over a 20-day kinetic for the non-nitrogen fixing T173 strain (lower right graph), correlating with the root ability of reinitiating a new wave of nodulation.
