Requirement of Jasmonate signaling for defense responses against Alternaria alternata and Phytophthora nicotiane in tobacco

Alternaria alternata and Phytophthora nicotianae are two major necrotrophic pathogens of tobacco. It is currently unknown whether they elicit the same suite of defense responses and whether jasmonate (JA) signaling plays an important role in the defense response of cultivated tobacco against these two pathogens. During transcriptome analysis, we found some conserved defense responses in the flue-cured tobacco cultivar K326 against A. alternata and P. nicotianae. Of the top 20 significantly enriched Kyoto Encyclopedia of Genes and Genomes pathways, eight were shared in 'K326' plants inoculated with A. alternata or P. nicotianae; these included the phenylpropanoid biosynthesis and the sesquiterpenoid biosynthesis pathways. Induction of expression of the genes feruloyl-CoA 6 '-hydroxylase 1 (F6'H1), 5-epi-aristolochene synthase (EAS), and 5-epi-aristolochene hydroxylase (EAH) for biosynthesis of the phytoalexins scopoletin (F6'H1) and capsidiol (EAS and EAH) was further confirmed by real-time polymerase chain reaction, and they are representative genes of phenylpropanoid and sesquiterpenoid pathways. Further analysis showed that the levels of JA and JA-Ile, and transcripts of allene oxide cyclase (AOC), the key enzyme gene for JA biosynthesis, were also significantly induced by both pathogens in 'K326.' Plants silenced for either AOC or Coronatine Insensitive 1 (COI1), the gene encoding the JA-Ile receptor, were highly susceptible to infection by both pathogens. Taken together, our results show that defense responses, especially the activation of the pathways for biosynthesis of the phytoalexins scopoletin and capsidiol, are conserved in N. attenuata and cultivated tobacco infected by A. alternata and P. nicotianae, and provided compelling evidence for the role of JA signaling in the defense against these two pathogens.