QUANTITATIVE RELATIONSHIP BETWEEN PHENYLALANINE AMMONIA-LYASE LEVELS AND PHENYLPROPANOID ACCUMULATION IN TRANSGENIC TOBACCO IDENTIFIES A RATE-DETERMINING STEP IN NATURAL PRODUCT SYNTHESIS

Phenylalanine ammonia lyase (PAL) catalyzes the first step in phenylpr opanoid synthesis. The role of PAL in pathway regulation was investiga ted by measurement of product accumulation as a function of enzyme act ivity in a collection of near-isogenic transgenic tobacco plants exhib iting a range of PAL levels from wild type to 0.2% of wild type. In le af tissue, PAL level is the dominant factor regulating accumulation of the major product chlorogenic acid and overall nux into the pathway. In stems, PAL at wild-type levels contributes, together with downstrea m steps, in the regulation of lignin deposition and becomes the domina nt, rate-determining step at levels 3- to 4-foId below wild type. The metabolic impact of elevated PAL levels was investigated in transgenic leaf callus that overexpressed PAL. Accumulation of the flavonoid rut in, the major product in wild-type callus, was not increased, but seve ral other products accumulated to similarly high levels. These data in dicate that PAL is a key step in the regulation of overall flux into t he pathway and, hence, accumulation of major phenylpropanoid products, with the regulatory architecture of the pathway poised so that downst ream steps control partitioning into different branch pathways.