Characterization of the expression of a wheat cystatin gene during caryopsis development

A cDNA coding for phytocystatin, a protease inhibitor, was isolated from wheat embryos by differential display RT-PCR and the corresponding full-length cDNA (named WC5 for wheat cystatin gene 5) subsequently obtained by RACE. The deduced primary sequence of the protein suggests the presence of a 28 amino acid N-terminal signal sequence and a 100 amino acid mature protein containing the three consensus motifs known to interact with the active site of cysteine peptidases. Northern and western analysis revealed a spatio-temporal pattern of the cystatin gene expression during caryopse development. In the embryo, WC5 was only expressed during early embryogenesis whereas, in seed covering layers, WC5 expression was restricted to the maturation stage of grain development. In addition, immunolocalization experiments showed that cystatin accumulated in the aleurone layer of the maturating seed and in the parenchymal tissues of the embryo scutellum. A recombinant form of the wheat cystatin was shown to be able to inhibit peptidase activities present in whole seed protein extracts. In addition, immunological techniques allowed us to identify two putative target peptidases. The possible roles of the cystatin protein are discussed in relation with tissular localization and putative peptidase targets during seed maturation.