Permalink : http://hdl.handle.net/2115/32917

KEYWORDS : bacterial 4-hydroxycinnamate decarboxylase;decarboxylation of phenylpropanoic acids;phenolics of plant origin;rhizosphere bacteria;root epiphytes

We isolated 332 rhizoplane bacteria from 74 plant samples consisting of 38 different plant species that belong to 20 different families. The bacterial isolates were screened for their decarboxylation activities on common phenolics of plant origin. Screening tests using Klebsiella oxytoca JCM 1665 as a positive control involved substrate-specificity assay and 4-hydroxycinnamate decarboxylase (4-HCD) induction assay. Most isolates (57%) were observed to have 4-HCD activity on both (E)-3, 4-dihydroxycinnamic acid (synonym of (E)-caffeic acid, CFA) and (E)-4-hydroxycinnamic acid (4-HCA). Substrate-specificity of CFAdecarboxylative rhizobacteria was tested on 24 positive isolates,and they generally accepted (E)-ferulic acid (FRA) as well as CFA. However, some bacteria could not decarboxylate 4-HCA. Neither 6-hydroxynaphthoic acid (6-HNA) nor (E)-2-hydroxycinnamic acid (2-HCA) was accepted as the decarboxylative products. These results suggested that 4-HCDs in bacteria have wide range of enzymatic variation to show alternative substrate-specificity. Some few rhizobacterial isolates (2%) undergo peroxidase- or tyrosinase-associating melanin production. This study has suggested that metabolic properties of many eubacteria inhabiting the rhizosphere environments are dependent on polyphenolic secondary metabolites accumulated in the plant roots. It was thus found that phenylpropanoid acid-decarboxylative bacteria are widely distributed in the terrestrial ecosystem.