Abstract
A large number of plants accumulate N-acylated polyamines (phenolamides [PAs]) in response to biotic and/or abiotic stress conditions. In the native tobacco (Nicotiana attenuata), the accumulation of two major PAs, caffeoylputrescine and dicaffeoylspermidine (DCS), after herbivore attack is known to be controlled by a key transcription factor, MYB8. Using a broadly targeted metabolomics approach, we show that a much larger spectrum of PAs composed of hydroxycinnamic acids and two polyamines, putrescine and spermidine, is regulated by this transcription factor. We cloned several novel MYB8-regulated genes, annotated as putative acyltransferases, and analyzed their function. One of the novel acyltransferases (AT1) is shown to encode a hydroxycinnamoyl-coenzyme A:putrescine acyltransferase responsible for caffeoylputrescine biosynthesis in tobacco. Another gene (acyltransferase DH29), specific for spermidine conjugation, mediates the initial acylation step in DCS formation. Although this enzyme was not able to perform the second acylation toward DCS biosynthesis, another acyltransferase gene, CV86, proposed to act on monoacylated spermidines, was isolated and partially characterized. The activation of MYB8 in response to herbivore attack and associated signals required the activity of LIPOXYGENASE3, a gene involved in jasmonic acid (JA) biosynthesis in N. attenuata. These new results allow us to reconstruct a complete branch in JA signaling that defends N. attenuata plants against herbivores: JA via MYB8's transcriptional control of AT1 and DH29 genes controls the entire branch of PA biosynthesis, which allows N. attenuata to mount a chemically diverse (and likely efficient) defense shield against herbivores.
Highlights
A large number of plants accumulate N-acylated polyamines in response to biotic and/or abiotic stress conditions
The silencing of the MYB8 TF resulted in a strong suppression of CP and DCS in N. attenuata (Kaur et al, 2010)
Peak matrices obtained from the analysis of wild-type and irMYB8 leaf extracts were first subjected to a principal component analysis (PCA) to examine the variance in our data set without referring to the class labels
Summary
A large number of plants accumulate N-acylated polyamines (phenolamides [PAs]) in response to biotic and/or abiotic stress conditions. One of the novel acyltransferases (AT1) is shown to encode a hydroxycinnamoyl-coenzyme A:putrescine acyltransferase responsible for caffeoylputrescine biosynthesis in tobacco Another gene (acyltransferase DH29), specific for spermidine conjugation, mediates the initial acylation step in DCS formation. When Kaur et al (2010) targeted a homolog of the NtMYBJS1 gene in N. attenuata (NaMYB8) to down-regulate the levels of CP and dicaffeoylspermidine (DCS), both metabolites appeared to be essential for N. attenuata’s defense against chewing herbivores While it highlighted the role of PAs in plant defense, the identity of the specific CP and DCS biosynthetic genes controlled by NaMYB8 TF remained unknown. The conventional use of sequence homology to identify substrates and enzyme activities of BAHD proteins often fails, because many BAHD enzymes with similar substrate specificities have evolved independently through a process of convergent evolution (Pichersky and Lewinsohn, 2011) This may include the case of CP and DCS secondary metabolites: they are very abundant in plants, their biosynthetic genes remained unresolved. Proteins from other groups can be involved in the biosynthesis of PAs in plants, such as structurally divergent tyramine hydroxycinnamoyltransferases (Bassard et al, 2010) that contain a GCN5-related N-acetyltransferase-like domain in their structure
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