Investigation of Melampsora larici-populina effectors to understand Mlp-poplar pathosystem

Melampsora larici-populina (Mlp) is an obligate biotrophic rust fungus that infects poplar andlarch during its life cycle. Its genome encodes approximately 1184 candidate secreted effectorsincluding two sister effectors, Mlp72983 and 52166, with contrasting host expression: Mlp72983is larch-specific, whereas Mlp52166 is expressed during poplar infection. To test thefunctionality of these effectors, their sub-cellular localization was assessed in Nicotianabenthamiana, Arabidopsis thaliana and a poplar hybrid (clone 7171B4). In all three species,Mlp72983 accumulated in the nucleus and the chloroplast and nucleus, whereas Mlp52166showed nucleocytosolic localization. We performed a transcriptomic study in poplar andarabidopsis transgenics and found that each effector deregulates more than 700 and 200 genesin each species, respectively. In addition, we found that both effectors deregulate commonorthologs between the two species. The proteins encoded by the deregulated genes have role inDNA repair and plant growth and are present in the nucleus and chloroplast. Infection assaysusing a relative of Mlp, the rust Melampsora medusae f. sp. tremuloides (Mmt), againsttransgenics poplars expressing these effectors showed no difference in the spore count whencompared with control. However, the study requires to examine whether Mmt expresses theseeffectors which could impact on infection outcome. This study provides insights about the roleof Mlp72983 and 52166 in the Mlp-poplar pathosystem.