ROLE OF KNOX TRANSCRIPTION FACTORS IN MEDICAGO TRUNCATULA SYMBIOTIC NODULE ORGANOGENESIS: THE SYMKNOX BILATERAL CNR/CNRS PROJECT

Legumes are able to develop symbiotic interactions with bacteria of the Rhizobia family to form nitrogen-fixing nodules from the infected roots. While L. japonicus forms N-fixing root nodules with determinate growth, M. truncatula forms indeterminate nodules which have a persistent meristem at their apices. Early molecular events of nodule initiation are well understood (Oldroyd et al., 2011) and involve activation of cytokinin signalling mediated by the MtCRE1 receptor (Gonzalez-Rizzo et al., 2006; Plet et al., 2011). Less is known about the molecular mechanisms governing the identity and maintenance of fixing nodule organs and their meristems. In seed plants, the role of KNOX homeodomain transcription factors of class 1 in shoot apical meristem formation and morphogenetic processes is well established (Hake et al., 2004; Hay and Tsiantis, 2010; Di Giacomo et al., 2013). Differently, KNOX of class 2 have been less characterized and their role in morphogenetic processes is still unclear. The genome of the model legume species M. truncatula harbors 11 KNOX genes (MtKNOXs). Gene expression studies revealed organ-specificity, possible cytokinin-dependent transcriptional activation of two MtKNOXs and expression of at least five MtKNOXs in roots (Di Giacomo et al., 2008). Due to KNOXs function in the morphogenetic processes underlying organ outgrowth, we wondered whether MtKNOXs expression in roots may subtend a role of KNOX in nodule organogenesis in response to rhizobia. This issue was investigated within the framework of the bilateral Scientific Cooperation between CNR (IT) and CNRS (France) "SYMKNOX".