Multi-site modulation of flux during monolignol formation in loblolly pine(Pinus taeda)

Loblolly pine (Pinus taeda L.) cell suspension cultures secrete monolignols when placed in 8% sucrose/20 mM KI solution, and these were used to identi fy phenylpropanoid pathway flux-modulating steps. When cells were provided with increasing amounts of either phenylalanine (Phe) or cinnamic acid, cel lular concentrations of immediate downstream products (cinnamic and p-couma ric acids, respectively) increased, whereas caffeic and ferulic acid pool s izes were essentially unaffected. Increasing Phe concentrations resulted in increased amounts of p-coumaryl alcohol relative to coniferyl alcohol. How ever, exogenously supplied cinnamic, p-coumaric, caffeic, and ferulic acids resulted only in increases in their intercellular concentrations, but not that of downstream cinnamyl aldehydes and monolignols. Supplying p-coumaryl and coniferyl aldehydes up to 40,000-320,000-fold above the detection limi ts resulted in rapid, quantitative conversion into the monolignols. Only at nonphysiological concentrations was transient accumulation of intracellula r aldehydes observed. These results indicate that cinnamic and p-coumaric a cid hydroxylations assume important regulatory positions in phenylpropanoid metabolism, whereas cinnamyl aldehyde reduction does not serve as a contro l point. (C) 1999 Academic Press.