Globulin-1 gene expression in regenerable Zea mays (maize) callus

Since maize callus cultures regenerate plants via somatic embryogenesis, one might expect to find similar proteins in both zygotic embryos and tissue cultures. The 63-kD globulin protein designated GLB1, the expression of which is regulated by abscisic acid (ABA), is one such protein. When maize Type I regenerable callus was exposed for 24 h to 0.1 mM ABA or a water stress induced by 0.53 M mannitol, GLB1 was produced as determined by Western analysis. This protein was not detected in ABA or mannitol-treated regenerable cultured tissue of a null genotype or in tissues not exposed to ABA or water stress. Exposure to ABA in the culture medium increased the callus ABA levels greatly but a mannitol-induced water stress had only a small effect on ABA levels. Regenerable callus exposed to 0.1 mM ABA also produced mRNA that hybridized on a Northern blot with a globulin-1 gene (Glb1) probe. When both Type I and Type 11 regenerable cultured tissues were exposed to regeneration medium without ABA or mannitol, several GLB1 antibody immunoreactive proteins were produced. These proteins were not detected in regenerated plants nor in non-regenerable callus treated with ABA. These results suggest that: (1) at least for expression of Glb1, somatic embryogenesis is similar to zygotic embryogenesis, (2) there may be a regulatory role for auxin in the processing of Glb1-encoded polypeptides since fewer are seen when dicamba is present in the medium, (3) ABA has a role in somatic embryogenesis, and (4) regenerability of a maize callus culture may be assessed by treating the cultured tissue with 0.1 mM ABA to determine if GLB1 proteins are induced.