TISSUE-SPECIFICITY AND GENETIC-CONTROL OF THE BETA-GLU NULL PHENOTYPE IN MAIZE

Sporophytic maize beta-glucosidase (beta-D-glucoside glucohydrolase; EC 3.2.1.21) is a dimeric enzyme expressed in several seedling and mature plant tissues. The enzyme shows a high degree of polymorphism and null individuals are frequent. The expression pattern of the enzyme was analysed spectrophotometrically and by zymogram analysis in different tissues of several maize genotypes (normal, nulls, F(1)s and backcrosses). It is shown that the null phenotype is tissue specific, since the so called 'null' genotypes have a normal glucosidase activity in all tissues tested except the coleoptile. Coleoptilar extracts from F(1)s between a null and normal lines also lack stainable beta-glucosidase activity. It is hypothesized that the null phenotype is due to a dominant regulatory element that affects the correct assembly of the enzyme monomers into an active dimeric molecule. Analysis of the backcross generations to the null and to the normal parent indicated that the regulatory locus is independent of the structural locus Glu1. In one backcross a highly distorted segregation yielding a large excess of null phenotypes was observed. The genetic basis of this phenomenon remains unclear, although the presence of gametophytic factors or of paramutation as possible causes are excluded.