Characterization of a cDNA encoding a putative extensin from developing barley grains (Hordeum vulgare L.)

A partial cDNA clone Hvex1 from Hordeum vulgare L. encoding a putative hydroxyproline-rich protein of the extensin family was isolated in an experiment designed to identify transcripts differentially expressed in the coenocytic endosperm and the surrounding sporophytic tissues during the early stages of grain development. The amino acid sequence derived from the Hvex1 cDNA has a high proportion of Pro, Lys and Thr residues, and a pi of approximately 11. However, the deduced Hvex1 polypeptide has an unusual structure unlike those of known monocot extensins and consists of four domains with repeats of the sequences KPP, PKPAPPTY(K/S)P, SPPAYKPAPKV, and (H/Y)KPPTPTPPA, respectively, and a fifth domain with a single SPPPP motif. In situ hybridization and Northern analyses reveal that Hvex1 transcripts are expressed in the nucellus, the nucellar epidermis and the nucellar projection of developing barley grains. In addition to nucellar tissues, the Hvex1 transcript is also detected in the vascular tissue of the pericarp, scutellum of the developing embryo, stigma, and root tips. The Hvex1 transcript is encoded by a single gene located near the centromere of barley chromosome 2.