Melampsora larici-populina homologous effectors Mlp72983 and Mlp52166 display cell-type specific accumulation in Arabidopsis and Populus

Plants interact with microorganisms that can cause diseases and reduce crop productivity. The fungal pathogen Melampsora larici-populina (Mlp) causes the leaf rust disease of poplar trees by secreting proteins, termed effectors, into host tissues to promote pathogenesis. In this study, we functionally characterized two homologous Mlp candidate secreted effector proteins (CSEPs), Mlp72983 and Mlp52166. Confocal microscopy experiments revealed that Mlp72983 has cell type-specific differential localization. It accumulates in the guard cells’ chloroplast of the epidermis and in the nucleus of the spongy mesophyll of Arabidopsis thaliana and Populus alba x Populus tremula. Mlp52166 has a nucleocytosolic accumulation in the epidermal layer and has nuclear localization in the mesophyll layer of the two species. Transcriptomic experiments showed that Mlp72983 and Mlp52166 deregulate plant genes, when constitutively expressed in either Arabidopsis or poplar. The two CSEPs deregulate genes that encode histones, hydroxysteroid dehydrogenases and multidrug and toxic compound extrusion proteins, with roles in DNA repair, methylation, and xenobiotic detoxification. Inoculation assays showed that CSEPs overexpression in poplar transgenics did not enhance susceptibility to rust infection. Despite being closely related in sequence identity, Mlp72983 and Mlp52166 have cell-type specific subcellular localization and deregulate mostly unique sets of plant genes.