A new enzymatic activity from elicitor-treated pear cell cultures converting trans-cinnamic acid to benzaldehyde

Cell cultures of Asian pear (Pyrus pyrifolia) are known to produce benzoate-derived biphenyl phytoalexins upon elicitor treatment. Although the downstream pathway for biphenyl phytoalexin biosynthesis is almost known, the upstream route of benzoic acid biosynthesis in pear has not been completely elucidated. In the present work, we report benzaldehyde synthase (BS) activity from yeast extract-treated cell suspension cultures of P. pyrifolia. BS catalyzes the in vitro conversion of trans-cinnamic acid to benzaldehyde using a non-oxidative C₂ -side chain cleavage mechanism. The enzyme activity was strictly dependent on the presence of a reducing agent, dithiothreitol being preferred. C₂ -side chain shortening of the cinnamic acid backbone resembled the mechanisms catalyzed by 4-hydroxybenzaldehyde synthase (HBS) activity in Vanilla planifolia and salicylaldehyde synthase (SAS) activity in tobacco and apple cell cultures. A basal BS activity was also observed in the non-elicited cell cultures. Upon yeast extract-treatment, a 13-fold increase in BS activity was observed when compared to the non-treated control cells. Moreover, feeding of the cell cultures with trans-cinnamic acid, the substrate for BS, resulted in an enhanced level of noraucuparin, a biphenyl phytoalexin. Comparable accumulation of noraucuparin was observed upon feeding of benzaldehyde, the BS product. The preferred substrate for BS was found to be trans-cinnamic acid, for which the apparent K_m and V_max values were 0.5 mM and 50.7 pkat mg^-1 protein, respectively. Our observations indicate the contribution of BS to benzoic acid biosynthesis in Asian pear via the CoA-independent and non-β-oxidative route.