Cytosolic aromatic aldehyde dehydrogenase provides benzoic acid for xanthone biosynthesis in Hypericum.

Benzoic acid is a building block of a multitude of well-known plant natural products, such as paclitaxel and cocaine. Its simple chemical structure contrasts with its complex biosynthesis. Hypericum species are rich in polyprenylated benzoic acid-derived xanthones, which have received attention due to their biological impact on human health. The upstream biosynthetic sequence leading to xanthones is still incomplete. To supply benzoic acid for xanthone biosynthesis, Hypericum calycinum cell cultures use the CoA-dependent non-β-oxidative pathway, which starts with peroxisomal cinnamate CoA-ligase (HcCNL). Here, we use the xanthone-producing cell cultures to identify the transcript for benzaldehyde dehydrogenase (HcBD), a pivotal player in the non-β-oxidative pathways. In addition to benzaldehyde, the enzyme efficiently catalyzes the oxidation of trans-cinnamaldehyde in vitro. The enzymatic activity is strictly dependent on the presence of NAD ⁺ as co-factor. HcBD is localized to the cytosol upon ectopic expression of reporter fusion constructs. HcBD oxidizes benzaldehyde, which moves across the peroxisome membrane, to form benzoic acid. Increases in the HcCNL and HcBD transcript levels precede the elicitor-induced xanthone accumulation. The current work addresses a crucial step in the yet incompletely understood CoA-dependent non-β-oxidative route of benzoic acid biosynthesis. Addressing this step may offer a new biotechnological tool to enhance product formation in biofactories.
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