The promoter of the asi gene directs expression in the maternal tissue of the seed in transgenic barley

The bifunctional α-amylase/subtilisin inhibitor (BASI) is an abundant protein in barley seeds, proposed to play multiple and apparently diverse roles in regulation of starch hydrolysis and in seed defence against pathogens. In the Triticeae, the protein has evolved the ability to specifically inhibit the main group of α-amylases expressed during germination of barley and encoded by the amy1 gene family found only in the Triticeae. The expression of the asi gene that encodes BASI has been reported to be controlled by the hormones abscisic acid (ABA) and gibberellic acid (GA). Despite many studies at the gene and protein level, the function of this gene in the plant remains unclear. In this study, the 5'-flanking region (1033 bp, 1033-asi promoter) and the 3'-flanking region (655 bp) of the asi gene were isolated and characterised. The 1033-asi promoter sequence showed homology to a number of ciselements that play a role in ABA and GA regulated expression of other genes. With a green fluorescent protein gene (gfp) as reporter, the 1033-asi promoter was studied for spatial, temporal and hormonal control of gene expression. The 1033-asi promoter and its deletions direct transient gfp expression in the pericarp and at low levels in mature aleurone cells, and this expression is not regulated by ABA or GA. In transgenic barley plants, the 1033-asi promoter directed tissue-specific expression of the gfp gene in developing grain and germinating grain but not in roots or leaves. In developing grain, expression of gfp was observed specifically in the pericarp, the vascular tissue, the nucellar projection cells and the endosperm transfer cells and the hormones ABA or GA did not regulate this expression. In mature germinating grain gfp expression was observed in the embryo but not in aleurone or starchy endosperm. However, GA induced gfp expression in the aleurone of mature imbibed seeds from which the embryo had been removed. Expression in maternal rather than endosperm tissues of the grain suggests that earlier widespread assumptions that the protein is expressed largely in the endosperm may have been largely based on analysis of mixed grain tissues. This novel pattern of expression suggests that both activities of the protein may be primarily involved in seed defence in the peripheral tissues of the seed.