摘要
Biotic and abiotic stress lead to elevated levels of jasmonic acid （JA） and its derivatives and activation of the biosynthesis of nicotine and related pyridine alkaloids in cultivated tobacco （Nicotiana tabacum L.）. Among the JA- responsive genes is NtPMTla, encoding putrescine N-methyl transferase, a key regulatory enzyme in nicotine formation. We have characterized three genes （NtMYC2a, b, c） encoding basic helix-loop-helix （bHLH） transcription factors （TFs） whose expression is rapidly induced by JA and that specifically activate JA-inducible NtPMTla expression by binding a G-box motif within the NtPMTla promoter in in vivo and in vitro assays. Using split-YFP assays, we further show that, in the absence of JA, NtMYC2a and NtMYC2b are present as nuclear complexes with the NtJAZ1 repressor. RNA interference （RNAi）-mediated knockdown of NtMYC2a and NtMYC2b expression results in significant decreases in JA-inducible NtPMTla transcript levels, as well as reduced levels of transcripts encoding other enzymes involved in nicotine and minor alkaloid biosynthesis, in- cluding an 80-90% reduction in the level of transcripts encoding the putative nicotine synthase gene NtA662. In contrast, ectopic overexpression of NtMYC2a and NtMYC2b had no effect on NtPMTla expression in the presence or absence of exogenously added JA. These data suggest that NtMYC2a, b, c are required components of JA-inducible expression of multiple genes in the nicotine biosynthetic pathway and may act additively in the activation of JA responses.
Biotic and abiotic stress lead to elevated levels of jasmonic acid （JA） and its derivatives and activation of the biosynthesis of nicotine and related pyridine alkaloids in cultivated tobacco （Nicotiana tabacum L.）. Among the JA- responsive genes is NtPMTla, encoding putrescine N-methyl transferase, a key regulatory enzyme in nicotine formation. We have characterized three genes （NtMYC2a, b, c） encoding basic helix-loop-helix （bHLH） transcription factors （TFs） whose expression is rapidly induced by JA and that specifically activate JA-inducible NtPMTla expression by binding a G-box motif within the NtPMTla promoter in in vivo and in vitro assays. Using split-YFP assays, we further show that, in the absence of JA, NtMYC2a and NtMYC2b are present as nuclear complexes with the NtJAZ1 repressor. RNA interference （RNAi）-mediated knockdown of NtMYC2a and NtMYC2b expression results in significant decreases in JA-inducible NtPMTla transcript levels, as well as reduced levels of transcripts encoding other enzymes involved in nicotine and minor alkaloid biosynthesis, in- cluding an 80-90% reduction in the level of transcripts encoding the putative nicotine synthase gene NtA662. In contrast, ectopic overexpression of NtMYC2a and NtMYC2b had no effect on NtPMTla expression in the presence or absence of exogenously added JA. These data suggest that NtMYC2a, b, c are required components of JA-inducible expression of multiple genes in the nicotine biosynthetic pathway and may act additively in the activation of JA responses.
