XYLEM-SPECIFIC GENE-EXPRESSION IN LOBLOLLY-PINE

Two genes preferentially expressed in differentiating xylem of lobloll y pine (Pinus taeda L.) were cloned from cDNA and genomic libraries an d designated PtX3H6 and PtX14A9. Transcripts of PtX3H6 and PtX14A9 are very abundant in differentiating xylem, less abundant in needles, and very low or nondetectable in embryos and megagametophytes. PtX3H6 con tains a putative signal peptide, a threonine-rich region, a proline-ri ch region, and a hydrophobic tail. Repeats of Pro-Pro-Pro-Val-X-X are similar to repeats found in proline-rich cell wall proteins. The amino acid compositions of PtX3H6 and PtX14A9 are similar to those of arabi nogalactan proteins (AGPs). PtX14A9 contains an 8 amino acid sequence similar to amino terminal sequences of ryegrass, carrot and rose AGPs. Upstream sequences have been determined from genomic clones encoding PtX3H6 and PtX14A9. A 7 bp sequence found in the 5' flanking regions o f both genes has previously been shown to be involved in the vascular- specific expression of GRP 1.8, a glycine-rich protein found in bean. The sequence is also present upstream of another glycine-rich protein from bean, GRP 1.0, and may be partially responsible for the xylem-spe cific expression of pTx3H6 and PtX14A9.