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

Abstract

and the diploid polar nucleus of the central cell (Lopes and Larkins, 1993). Throughout its life, the central cell A partial cDNA clone Hvex1 from Hordeum vulgare L. and later the endosperm is embedded in nucellar tissues encoding a putative hydroxyproline-rich protein of the (Norstog, 1974; Cass et al., 1985; Engell, 1989, 1994). extensin family was isolated in an experiment designed Based on data from mutant studies in plants, as well as to identify transcripts differentially expressed in the from studies of embryonic systems such as Drosophila, coenocytic endosperm and the surrounding sporo- there are good reasons to believe that endosperm developphytic tissues during the early stages of grain develop- ment is influenced by the surrounding sporophytic tissues ment. The amino acid sequence derived from the (Driever et al., 1989). During grain development, the Hvex1 cDNA has a high proportion of Pro, Lys and Thr nucellus undergoes several distinct developmental phases. residues, and a pI of approximately 11. However, the Following the maturation of the embryo sac, starting a deduced Hvex1 polypeptide has an unusual struc- few days before fertilization and lasting until approxiture unlike those of known monocot extensins mately 5 d after pollination (DAP), the main body of and consists of four domains with repeats of the nucellus undergoes autolysis (Norstog, 1974). After comsequences KPP, PKPAPPTY(K/S)P, SPPAYKPAPKV, pletion of nucellar parenchyma cell degradation, around and (H/Y)KPPTPTPPA, respectively, and a fifth domain 4 DAP, the peripheral cell layer develops into the nucellar with a single SPPPP motif. In situ hybridization and epidermis. At later developmental stages, the nucellar cell Northern analyses reveal that Hvex1 transcripts are walls thicken and the outer walls are covered by a expressed in the nucellus, the nucellar epidermis and cuticular layer, possibly serving to protect the embryo sac the nucellar projection of developing barley grains. In from invading pathogens and osmotic stress (Cochrane addition to nucellar tissues, the Hvex1 transcript is and DuVus, 1979). Concomitant with the development also detected in the vascular tissue of the pericarp,