ARABINOGALACTAN-PROTEINS AND XYLEM DEVELOPMENT

Texas A&M University, Dept. of Forest Science and Crop Biotechnology Center, College Station, TX 77843-2123 USA

We have cloned and characterized the genes encoding three arabinogalactan-proteins (AGPs) or AGP-like proteins from xylem of loblolly pine. AGPs are a group of glycoproteins or proteoglycans with many possible functions including possible roles in cell-cell communications and cellular interactions during development. The most recently cloned of the three genes, PtAGP3, is the predominant AGP extracted from differentiating xylem using a low salt buffer. N-terminal sequence was obtained from the purified protein and used to identify a cDNA clone in the NCSU loblolly pine EST collection. The protein contains a hydrophobic signal peptide, a cleaved hydrophilic region, a hydroxyproline-/proline-, serine-, alanine-rich region typical of many AGPs and a hydrophobic putative transmembrane domain. PtX3H6 and PtX14A9 were cloned from a xylem cDNA library by differential screening for xylem-specific genes. Analyses of transgenic tobacco plants expressing PtX3H6 with a c-myc epitope have demonstrated PtX3H6 encodes an AGP tightly associated with the cell wall. PtX14A9 is also tightly associated with the cell wall and is extractable only after treatment with cellulase. A poplar homolog was isolated by differential screening of a xylem cDNA library and cDNAs or ESTs with similarity to PtX14A9 have been identified in Genbank. The presence of a gene known to be preferentially and abundantly expressed in xylem of a conifer (pine) and a woody dicot (poplar), as well as being expressed in herbaceous dicots (arabidopsis and bean) and a monocot (rice) suggests this protein may play a very important role in xylem development.