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Theme 3: Biosynthesis of 4-hydroxybenzoic acid in hairy root cultures of Daucus carota

In contrast to 3-hydroxybenzoic acid which is derived directly from the shikimic pathway, 4-hydroxybenzoic acid is formed via the general phenylpropanoid pathway. Our research provided evidence that 4-hydroxybenzoic acid accumulates in the cytosol of Daucus carota hairy roots as un-conjugated form, which later incorporated in the cell wall as an alkaline-labile form. An apparent absence of any 4-hydroxybenzoic acid-glucoside supported this hypothesis. The biosynthesis of 4-hydroxybenzoic acid is far from complete, and therefore in a DST-funded project, we aim to provide an insight on the complete enzymatic route to 4-hydroxybenzoic acid formation in chitosan-elicited hairy root cultures of D. carota. Feeding experiments with precursors and/or inhibitors along with in vitro conversions by cell-free extracts suggested a CoA-independent and non-ß-oxidative route of 4-hydroxybenzoic acid biosynthesis. The 4-hydroxybenzoic acid is formed via4-hydroxycinnamic acid (4-coumaric acid), as demonstrated in methyl-jasmonate-treated hairy roots of D. carota. We also reported partial characterization of an unusual hydroxybenzaldehyde synthase, an enzyme which catalyzes the penultimate step of p-hydroxybenzoic acid (C6-C1) biosynthesis by cleaving a C2 fragment from p-coumaric acid (C6-C3) without involvement of any cofactors, apart from stimulation by thiol reagents.

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