| During the course of studying vascular development of quaking aspen ( Populus tremuloides), nine cDNA clones were isolated from vascular tissues. Among these, four cDNAs exhibited xylem-preferential expression based on Northern blot analysis. They encode 3-phosphoglycerate kinase (PGK), H-protein (HP) of the glycine decarboxylase complex, ADP-ribosylation factor (ARF) and α-tubulin (TUA), respectively. In situ hybridization revealed that the transcripts of PtHP, PtARF1 and PtTUA1 were primarily localized in developing xylem cells and phloem fibers, suggesting an association with secondary cell wall thickening and/or lignification. The involvement of PtHP in lignification was supported by the presence of collapsed xylem vessels and lignified pith cells in transgenic aspen with a constitutively expressed PtHP transgene. PtARF1 and PtTUA1, based on their putative protein functions, may take part in secondary cell wall biogenesis by transporting wall materials to the plasma membrane and directing cellulose microfibril deposition, respectively. The proposition for specific involvement of PtARF1 and PtTUA1 in secondary wall biogenesis was supported in part by the cloning of two other related cDNA clones, designated PtARF2 and PtTUA2, whose transcripts were widely distributed throughout the plant body, but were not abundant in secondary vascular tissues.; On the other hand, three cDNAs originated from mRNA differential display as phloem up-regulated clones. One of these cDNAs, PtCyP, encodes a protein similar to cyclophilins (CyPs), whereas the other two, PtP12G-1 and PtP15A, did not show significant homology with known-function genes. These three cDNAs exhibited similar seasonal expression patterns. While being preferentially expressed in phloem during most of the growing season, their transcript levels in xylem increased dramatically in late August and early September, leading to a late-season switch toward xylem-preferential expression. Possible roles of these cDNAs in vascular development are discussed and some future research directions are offered. |
