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

Fungal plant pathogens secrete virulence-related proteins, called effectors, to establish host infection; however, the details are not fully understood yet. Functional screening of effector candidates using Agrobacterium -mediated transient expression assay in Nicotiana benthamiana identified two virulence-related effectors, named SIB1 and SIB2 ( S uppression of I mmunity in N. benthamiana ), of an anthracnose fungus Colletotrichum orbiculare , which infects both cucurbits and N. benthamiana . The 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 (i) the generation of reactive oxygen species triggered by two different pathogen-associated molecular patterns, chitin and flg22, and (ii) 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 β-strands, each containing three disulfide bonds. The overall conformation was found to be a cylindrical shape, such as the well-known antiparallel β-barrel structure. However, the β-strands were found to display a unique topology, one pair of these β-strands formed a parallel β-sheet. These results suggest that the effector SIB1 present in Colletotrichum fungi has unique structural features and can suppress pathogen-associated molecular pattern–triggered immunity in N. benthamiana .