Sequences upstream and downstream of two xylem-specific pine genes influence their expression.

The identification of regulatory elements conferring high levels of expression in differentiating pine xylem will be valuable for genetic engineering of wood properties and will contribute to our understanding of gene regulation in this important group of forest trees. We examined the roles of both upstream and downstream elements in regulating the expression of two genes with preferential expression in developing xylem of loblolly pine. Gene constructs containing a PtX3H6, PtX14A9, or CaMV 35S promoter, the uidA gene encoding beta-glucuronidase, and a PtX3H6, PtX14A9, or NOS terminator were used to transform tobacco and hybrid poplar. When combined with the NOS terminator, neither pine promoter conferred xylem-specific expression in tobacco. When combined with the PtX3H6 promoter, an element at the 3' end of PtX3H6 reduced GUS expression resulting in preferential expression in vascular tissues. This silencing effect was not observed when the pine terminator was tested in conjunction with the CaMV 35S promoter. The PtX14A9 terminator did not increase tissue specificity. In leaves of transgenic poplar, both pine promoters conferred preferential GUS expression in veins when combined with the NOS terminator. The PtX3H6 terminator greatly decreased expression in leaves and stems when combined with the PtX3H6 promoter but only slightly altered expression when combined with the CaMV 35S promoter. An element at the 3' end of PtX14A9 increased GUS expression in veins when used in conjunction with either the PtX14A9 or CaMV35S promoter.