Cell Death of Nicotiana benthamiana Is Induced by Secreted Protein NIS1 of Colletotrichum orbiculare and Is Suppressed by a Homologue of CgDN3

Colletotrichum orbiculare, the causal agent of cucumber anthracnose, infects Nicotiana benthamiana. Functional screening of C. orbiculare cDNAs in a virus vector-based plant expression system identified a novel secreted protein gene, NISI, whose product induces cell death in N. benthamiana. Putative homologues of NISI are present in selected members of fungi belonging to class Sordariomycetes, Dothideomycetes, or Orbiliomycetes. Green fluorescent protein based expression studies suggested that NISI is preferentially expressed in biotrophic invasive hyphae. NISI lacking signal peptide did not induce NIS1-triggered cell death (NCD), suggesting apoplastic recognition of NISI. NCD was prevented by virus-induced gene silencing of SGT1 and HSP90, indicating the dependency of NCD on SGT1 and HSP90. Deletion of NISI had little effect on the virulence of C. orbiculare against N. benthamiana, suggesting possible suppression of NCD by C. orbiculare at the post-invasive stage. The CgDN3 gene of C. gloeosporioides was previously identified as a secreted protein gene involved in suppression of hypersensitive-like response in Stylosanthes guianensis. Notably, we found that NCD was suppressed by the expression of a CgDN3 homologue of C. orbiculare. Our findings indicate that C. orbiculare expresses NISI at the postinvasive stage and suggest that NCD could be repressed via other effectors, including the CgDN3 homologue.