Characterization of signaling pathways involving CDPKs and cytokinins in root nodule organogenesis in Medicago truncatula

GARGANTINI PR; RIZZO-GONZALEZ S; CHINCHILLA D; RAICES M; FRUGIER F; CRESPI M; ULLOA RM

Dijon, Francia

Congreso; The 5th European Conference on grain legumes and the 2nd International Conference of Legumes Genomics and Genetics; 2004

Nodule organogenesis is initiated by a molecular dialogue involving the host legume and the soil bacterium Rhizobium via flavonoids excreted by the plant and bacterial Nod factors. Nod factors induce a variety of responses including rapid calcium influx and calcium spiking in root hair cells, specific gene induction, alterations in epidermal cell morphology and cortical cell mitosis (2). Cytokinins and Nod factors show similar morphological responses in legume roots such as cortical cell division. On the other hand, changes in calcium concentration are transduced via calcium-binding modulator proteins that affect directly or indirectly the activity of a protein kinase enzyme. Calcium dependent protein kinases, CDPKs, are a unique class of serine/threonine protein kinases containing a regulatory calmodulin-like domain with calcium binding motifs. CDPKs have been reported to be involved in multiple signaling pathways and in the response to several environmental cues (3).We report the induction of expression of a specific CDPK isoform, MsCDPK1in roots inoculated with S. meliloti, the symbiotic partner of Medicago spp. Semiquantitative RT-PCR experiments revealed that MsCDPK1 was induced 3 d.a.i. (days after inoculation). These results were confirmed by quantitative real time PCR, which in addition showed a very early induction of MsCDPK1 mRNA in inoculated roots in correlation with calcium spiking events. Biochemical data revealed that CDPK activity was significantly enhanced in the particulate fraction of infected roots (4 d.a.i.). The N-terminal domain of MsCDPK1 has functional myristoylation and palmitoylation sites that are essential for the plasma membrane localization of the kinase as shown using translational MsCDPK1-GFP fusions. MsCDPK1 expression is also rapidly induced in roots treated with cytokinins (6hs). Our data suggest that MsCDPK1 could be an early component of the signal transduction pathways involving Nod-factors and cytokinins in legume roots.