Abstract
Nicotiana benthamiana is a potential host to several plant pathogens, and immature leaves of N. benthamiana are susceptible to Phytophthora infestans. In contrast, mature leaves of N. benthamiana are weakly susceptible and show basal resistance to P. infestans. We screened a gene-silenced mature plant showing high resistance to P. infestans, designated as DS2 (Disease suppression 2). The deduced amino acid sequence of cDNA responsible for DS2 encoded a putative aminoacylase. Growth of P. infestans decreased in DS2 plants. Trypan blue staining revealed inhibited hyphae growth of P. infestans with an increased number of dead cells under the penetration site in DS2 plants. Consistent with growth inhibition of P. infestans, defense responses such as reactive oxygen generation and expression of a salicylic acid-dependent PR-1a increased markedly in DS2 plants compared with that of control plants. DS2 phenotype was compromised in NahG plants, suggesting DS2 phenotype depends on the salicylic acid signaling pathway. Accelerated defense response was observed in DS2 plants elicited by INF1 elicitin as well as by NbMEK2(DD), which is the constitutive active form of NbMEK2, and act as a downstream regulator of INF1 perception. On the other hand, INF1- and NbMEK2(DD)-induced defense responses were prevented by DS2-overexpressing transgenic tobacco. These results suggest that DS2 negatively regulates plant defense responses against P. infestans via NbMEK2 and SA-dependent signaling pathway in N. benthamiana.