Transport de l'auxine et développement du nodule actinorhizien chez l'arbre tropical Casuarina glauca

摘要

Actinorhizal plants belonging to 8 families of angiosperms can enter symbiosis with a soil actinomycete called Frankia. This interaction leads to the formation of nitrogen fixing nodules on the plant root system. The actinorhizal nodule is considered as a modified lateral root because i) it originates from divisions of pericycle cells situated in front of xylem poles, ii) its vasculature is central and its growth is indeterminate due to the presence of an apical meristem and iii) in some species such as Casuarina glauca a so-called “nodular root” is formed at the apex of each nodule lobe. Auxin, and more particularly auxin influx, is involved in lateral root formation. We identified auxin influx transporter genes in the actinorhizal plant C. glauca and studied the role of auxin influx transport during actinorhizal nodule formation.Two AUX-LAX genes encoding for auxin influx carriers have been identified in C. glauca. The expression patterns of CgAUX1 and CgLAX3 are highly conserved between C. glauca and Arabidopsis thaliana. Functional complementation of the Arabidopsis aux1 mutant revealed that CgAUX1 and AtAUX1 share equivalent functions. Our data suggest that functional divergence exists in the AUX-LAX family.We analysed the role of these genes during the actinorhizal symbiosis. Expression studies showed that CgAUX1 is expressed in all infected cells. Moreover, we confirmed that auxin influx transport is involved in the symbiotic process by taking advantage of an auxin influx transport inhibitor. We also observed that CgAUX1 is expressed in lateral root primordium but not in nodule primordium thus pinpointing some differences in the developmental program of these two organs.We then tried to identify the mechanisms acting downstream of auxin influx transport by studying the role of AtLAX3 in Arabidopsis. We showed that a set of cell wall remodeling genes are induced by auxin in a AtLAX3 dependent way during lateral root emergence. We next tried to identify cell wall remodeling genes that could be involved in the infection process in a CgAUX1 dependent way. Cg12 encodes for a subtilisin-like protease that is specifically expressed in Frankia infected cells and could be a target of CgAUX1 dependent auxin signaling.Our results suggest that auxin influx transport is involved in the infection process during actinorhizal nodule formation in C. glauca.