Developmentally regulated patterns of expression directed by poplar PAL promoters in transgenic tobacco and poplar

Phenylalanine ammonia-lyase (PAL) catalyzes the first step in phenylpropano id metabolism and plays a central role in the biosynthesis of phenylpropano id compounds. We have previously cloned two PAL genes, PAL1 and PAL2, from a Populus trichocarpa x P. deltoides F-1 hybrid. Here, we describe the prop erties of PAL1 and PAL2 promoters and their expression patterns in transgen ic tobacco and poplar. The promoters were 75% identical in the regions sequ enced, and each contained two copies of AC-rich putative cis acting element s that matched a consensus plant myb transcription factor binding site sequ ence. In transgenic tobacco, PAL1-GUS and PAL2-GUS fusions directed similar patterns of expression in developing primary xylem of leaves, stems, and o ther organs, and in secondary xylem of stems. Contrary to previously docume nted patterns of PAL1/2 expression in poplar, no expression of either fusio n was detected in epidermal or subepidermal cell layers of young tobacco le aves or stems. In poplar, the PAL2-GUS fusion directed the highest levels o f expression in roots and young leaves and stems. In young leaves and stems , high GUS activity was detected in epidermal or subepidermal cells as well as in primary xylem and phloem fibers. GUS activity was low in woody stems , and was weak or absent in developing secondary xylem. The patterns of PAL 2-GUS expression in poplar are very similar to those of PAL1/2 mRNA accumul ation in poplar. However, the distinct patterns of expression directed by t he PAL2 promoter in poplar and tobacco show that PAL2-GUS expression in tob acco does not accurately reflect all aspects of PAL2 expression in poplar.