Endosperm-specific activity of a zein gene promoter in transgenic tobacco plants.

Transgenic tobacco plants containing maize gene (Z4) encoding a 23-kd zein protein, which is normally synthesized in the endosperm of maize seeds, were obtained using a modified Ti plasmid vector. Although a polyadenylated transcript homologous to the Z4 gene was present in the seeds of some of these transgenic plants, zein protein could not be detected in any of the plants tested (35 total). To simplify the analysis of the tissue specificity of the Z4 promoter (Z4(pro)) in different organs of transformed tobacco plants, additional transgenic plants containing the chimeric genes Z4(pro)-CAT and Z4(pro)-GUS were produced. Very weak seed-specific CAT activity was observed in one out of ten Z4(pro)-CAT-transformed plants. When the more sensitive GUS assay system was used to evaluate Z4(pro) activity in tobacco, it could be shown in all 11 transgenic plants obtained that GUS activity was restricted to the endosperm tissue of transgenic tobacco seeds. To study the synthesis and stability of the zein proteins in different organs of transgenic tobacco plants, the Z4 protein-coding region and also a cDNA clone (A30) encoding a 19-kd zein protein were placed under the control of the 35S promoter (35S(pro)) of cauliflower mosaic virus. Undegraded zein proteins of both size classes were detected in roots, leaves and endosperm tissue, but not embryos, of mature seeds from 35S(pro)-zein-transformed plants. The zein proteins did not appear to be broken down during tobacco seed germination; synthesis of zeins began in green cotyledons.