Expression profiles of genes for enzymes involved in capsidiol production inNicotiana benthamiana
In Solanaceae plants, the major phytoalexins produced during the induction of plant defense are sesquiterpenoids, such as capsidiol forNicotianaspecies and rishitin forSolanumspecies, which are produced via the mevalonate (MVA) pathway. Eight enzymes are involved in the production of farnesyl pyrophosphate (FPP), the common precursor of phytosterols for maintaining membrane integrity and sesquiterpenoid phytoalexins for plant defense. In this study, expression profiles ofN. benthamianagenes for the production of capsidiol during the induction of disease resistance were investigated. In the genome ofN. benthamiana, multiple copies of genes for each enzyme in the MVA pathway were identified, and the expression of some, but not all MVA genes, was significantly upregulated after inoculation withPhytophthora infestans, or treatment with the INF1 elicitor, a secretory protein ofP. infestans. For genes encoding enzymes involved in capsidiol production, 10 copies of 5-epi-aristolochene synthase (NbEAS) and six copies of 5-epi-aristolochene dihydroxylase (NbEAH) were identified, and all copies were significantly upregulated during the induction of disease resistance. Gene silencing of MAP kinase genesNbWIPK,NbSIPK,andNbNTF4compromised INF1-induced production of phytoalexins. Expression analysis of control andNbWIPK/SIPK/NTF4-silenced plants indicated that most of the MVA genes are not under the control of these MAP kinases. In contrast, the expression pattern ofNbWIPK/SIPK/NTF4and all copies ofNbEAHgenes showed significant correlation, suggesting that MAP kinases are critical regulators of transcriptional upregulation of specific genes for capsidiol production.