The late expressed effector Lep1 has a novel function for fungal development

In U. maydis most effector genes are expressed in waves during colonization, an early wave prior to penetration, a wave associated with establishment of biotrophy and a late wave associated with tumor development. We have functionally characterized the virulence-promoting Lep1 protein, a novel core effector which is highly expressed during tumor formation. lep1 mutants are unaltered during early infection stages but show attenuated hyphal aggregation, fail to undergo massive late proliferation and produce only few spores. Constitutive expression of lep1 can induce cell aggregation and the surface of filamentous colonies displayed enhanced hydrophobicity, suggesting that Lep1 is a novel surface-active protein. We provide immunological evidence that Lep1 is bound to the cell wall of biotrophic hyphae and demonstrate that Lep1 associates with the repellent protein Rep1 when constitutively expressed in hyphae. From the phenotypes of lep1, rep1 double mutants as well as from strains constitutively expressing either lep1 or rep1 we conclude that Lep1 acts as a novel kind of cell adhesin which functions together with other surface-active proteins to trigger proliferation of diploid hyphae as well as the induction of the morphological changes associated with spore formation.