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

Phenylalanine ammonia-lyase (PAL) catalyzes the first step in phenylpropanoid metabolism and plays a central role in the biosynthesis of phenylpropanoid compounds. We have previously cloned two PAL genes, PALI and PAL2, from a Populus trichocarpa x P. deltoides F1 hybrid. Here, we describe the properties of PALI and PAL2 promoters and their expression patterns in transgenic tobacco and poplar. The promoters were 75% identical in the regions sequenced, and each contained two copies of AC-rich putative cis-acting elements that matched a consensus plant myb transcription factor binding site sequence. In transgenic tobacco, PALI-GUS and PAL2-GUS fusions directed similar patterns of expression in developing primary xylem of leaves, stems, and other organs, and in secondary xylem of stems. Contrary to previously documented patterns of PAL1/2 expression in poplar, no expression of either fusion was detected in epidermal or subepidermal cell layers of young tobacco leaves or stems. In poplar, the PAL2-GUS fusion directed the highest levels of 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 PAL2-GUS expression in poplar are very similar to those of PAL1/2 mRNA accumulation in poplar. However, the distinct patterns of expression directed by the PAL2 promoter in poplar and tobacco show that PAL2-GUS expression in tobacco does not accurately reflect all aspects of PAL2 expression in poplar.