Role of aromatic aldehyde synthase in wounding/herbivory response and flower scent production in different Arabidopsis ecotypes

Abstract

SUMMARYAromatic L-amino acid decarboxylases (AADCs) are key enzymes operating at the interface between primary and secondary metabolism. The Arabidopsis thaliana genome contains two genes, At2g20340 and At4g28680, encoding pyridoxal 5¢-phosphate-dependent AADCs with high homology to the recently identified Petunia hybrida phenylacetaldehyde synthase involved in floral scent production. The At4g28680 gene product was recently biochemically characterized as an L-tyrosine decarboxylase (AtTYDC), whereas the function of the other gene product remains unknown. The biochemical and functional characterization of the At2g20340 gene product revealed that it is an aromatic aldehyde synthase (AtAAS), which catalyzes the conversion of phenylalanine and 3,4-dihydroxy-L-phenylalanine to phenylacetaldehyde and dopaldehyde, respectively. AtAAS knock-down and transgenic AtAAS RNA interference (RNAi) lines show significant reduction in phenylacetaldehyde levels and an increase in phenylalanine, indicating that AtAAS is responsible for phenylacetaldehyde formation in planta. In A. thaliana ecotype Columbia (Col-0), AtAAS expression was highest in leaves, and was induced by methyl jasmonate treatment and wounding. Pieris rapae larvae feeding on Col-0 leaves resulted in increased phenylacetaldehyde emission, suggesting that the emitted aldehyde has a defensive activity against attacking herbivores. In the ecotypes Sei-0 and Di-G, which emit phenylacetaldehyde as a predominant flower volatile, the highest expression of AtAAS was found in flowers and RNAi AtAAS silencing led to a reduction of phenylacetaldehyde formation in this organ. In contrast to ecotype Col-0, no phenylacetaldehyde accumulation was observed in Sei-0 upon wounding, suggesting that AtAAS and subsequently phenylacetaldehyde contribute to pollinator attraction in this ecotype.

An alternative pathway contributes to phenylalanine biosynthesis in plants via a cytosolic tyrosine:phenylpyruvate aminotransferase

“…Concentration of purified DNA fragment was determined with the NanoDrop 1000 spectrophotometer (Thermo Scientific, West Palm Beach, FL, USA). Several dilutions were prepared from 4 ng ml À 1 to 6.4 pg ml À 1 and used to obtain standard curves in qRT-PCR with gene-specific primers (Supplementary Table S6) 38 . Individual qRT-PCR reactions contained 5 ml of the SYBR Green PCR master mix (Applied Biosystems, Foster City, CA, USA), 3 ml of 50-fold diluted cDNA, and 1 ml of 5 mM forward and reverse primers.…”

“…To investigate whether the cytosolic pool of tyrosine can influence phenylalanine levels in vivo, the level of tyrosine was depleted in petunia flowers of PhADT1xPhPPA-AT RNAi via transient overexpression of the cytosolic Arabidopsis tyrosine decarboxylase (AtTYDC), which exclusively catalyses decarboxylation of L-tyrosine to tyramine 37,38 . AtTYDC overexpression resulted in 50% reduction of the tyrosine pool and a 9-fold increase in tyramine levels with a concomitant 50% decrease in phenylalanine levels (Fig.…”

Aromatic amino acid decarboxylase is involved in volatile phenylacetaldehyde production in loquat (<i>Eriobotrya japonica</i>) flowers

“…Our phylogenetic analyses indicate that EjAADC1 falls into the same clade as RhAADC, whereas EjAADC2 is closely related to an Arabidopsis clade that contains proteins biochemically characterized as possessing AAS and tyrosine decarboxylase activities (Gutensohn et al 2011;Lehmann and Pollmann 2009) (Figure 2A). Although Gutensohn et al (2011) reported that AtAAS contributes phenylacetaldehyde formation in planta, AtAAS shows the higher enzymatic activity toward L-Dopa rather than Phe. This suggests that the catalytic property of AtAAS is different from that of PhPAAS and RhAADC.…”

“…Similarly, rose flowers produce phenylacetaldehyde from Phe using the phenylacetaldehyde synthase RhAADC, but also have an alternative pathway leading to phenylacetaldehyde via phenylpyruvate formation (Hirata et al 2016). In Arabidopsis, aromatic aldehyde synthase (AAS; AtAAS), which has high homology to AADCs, catalyzes the decarboxylation of Phe into phenylacetaldehyde (Gutensohn et al 2011). Recently, AASs have been identified in Medicago truncatula and Cicer arietinum, and these enzymes have decarboxylation activities against several hydrophobic amino acids (phenylalanine, methionine, tryptophan, and leucine), although it is not yet known whether they produce phenylacetaldehyde in planta (Torrens-Spence et al 2014).…”

“…Since the first isolation of PAAS genes in Petunia hybrida cv. Mitchell, several other genes responsible for phenylacetaldehyde biosynthesis have been characterized at the biochemical and molecular levels (Gutensohn et al 2011).…”