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

Post anthesis, loquat flowers emit volatile benzenoids, including phenylacetaldehyde, phenylethyl alcohol, and 2-phenethyl benzoate. Previous studies have shown that pyridoxal phosphate-dependent aromatic L-amino acid decarboxylase (AADC) produces phenylacetaldehyde from L-phenylalanine. Here, two AADC genes (EjAADC1 and EjAADC2) were isolated from loquat (Eriobotrya japonica) flowers. The EjAADC1 and EjAADC2 proteins showed approximately 72% and 55% identity, respectively, to a rose AADC homolog that has phenylacetaldehyde synthase activity. Transcript analyses indicated that EjAADC1 was specifically expressed in petals, with the highest level of expression in fully opened flowers; the petals showed high levels of volatile benzenoids, including phenylacetaldehyde. In contrast, EjAADC2 was expressed at a lower level than EjAADC1 in all tested tissues, including leaves and developing flowers. Functional characterization of a recombinant EjAADC1 protein expressed in Escherichia coli showed that it catalyzes the formation of phenylacetaldehyde from L-phenylalanine in a pyridoxal phosphate-dependent manner. Our results suggest that EjAADC1 is mainly responsible for the biosynthesis of volatile benzenoids in loquat flowers.