In a previous study we reported that administered ferulic acid (FA) was incorporated into guaiacyl and syringyl lignin in poplar callus (Populus alba L.) but administered sinapic acid (SA) was not. It is possible that SA may induce or enhance the biosynthesis of syringyl lignin. In this study, enzyme activities in lignin biosynthesis were assayed in FA- or SA-treated callus. The activity of 4-coumarate:coenzyme A ligase (4CL) toward FA in FA-treated callus was not affected, and its activity toward caffeic and sinapic acids was quite low. In addition, phenylalanine ammonia-lyase (PAL) activity. cinnamate:4-hydroxylase (C4H) transcription in the upstream steps of lignin biosynthesis, also was not affected. 4CL from SA-treated callus exhibited no detectable activity toward SA, suggesting that SA is not a precursor for syringyl lignin. However, all upstream steps of lignin biosynthesis (PAL activity, C4H transcription, 4CL activity toward FA) exhibited increased activity. These results suggest that SA may induce or enhance enzyme activities involved in syringyl lignin biosynthesis in poplar callus.
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