Fungal effector SIB1 of Colletotrichum orbiculare has unique structural features and can suppress plant immunity in Nicotiana benthamiana

Functional screening of effector candidates using a transient expression assay in Nicotiana benthamiana identified two virulence-related effectors, named SIB1 and SIB2 (Suppression of Immunity in N. benthamiana), of an anthracnose fungus Colletotrichum orbiculare, which infects both cucurbits and N. benthamiana. Agrobacterium-mediated transient expression of SIB1 or SIB2 increased the susceptibility of N. benthamiana to C. orbiculare, which suggested these effectors can suppress immune responses in N. benthamiana. The presence of SIB1 and SIB2 homologs was found to be limited to the genus Colletotrichum. SIB1 suppressed both the generation of reactive oxygen species (ROS) triggered by the bacterial pathogen-associated molecular pattern (PAMP), flg22, and the cell death response triggered by the Phytophthora infestans INF1 elicitin in N. benthamiana. We determined the NMR-based structure of SIB1 to obtain its structural insights. The three-dimensional structure of SIB1 comprises five {beta}-strands, each containing three disulfide bonds. The overall conformation was found to be a cylindrical shape, such as the well-known antiparallel {beta}-barrel structure. However, the {beta}-strands were found to display a unique topology, one pair of these {beta}-strands formed a parallel {beta}-sheet. These results suggest that the effector SIB1 present in Colletotrichum fungi has unique structural features and can suppress PAMP-triggered immunity (PTI) in N. benthamiana.