--Renato Paiva and Alan L. Kriz

Studies on gene regulation during precocious germination of the viviparous mutants may provide important insights to processes of embryo development, maturation, and germination. GLB1 and GLB2, the major storage proteins of maize embryos which are respectively encoded by the Glb1 and Glb2 genes, accumulate to high levels during embryo development and are subsequently degraded with the initiation of seed germination. We previously reported that levels of Glb1 transcripts in embryos of viviparous mutants differ from those of embryos undergoing normal germination, indicating that vp/vp embryos do not completely switch from development/maturation processes to germination processes (Kriz et al., Plant Physiol. 92:538, 1990). These studies also indicated that Glb1 expression is positively regulated by ABA, and that homozygous vp1 embryos are completely lacking in Glb1 products, suggesting that a functional Vp1 gene product is essential for Glb1 expression. This is consistent with studies by McCarty et al. (Plant Cell 1:523, 1989) which indicate that the Vp1 gene product is involved in ABA-mediated signal transduction processes in the developing embryo.

To further investigate the effect of ABA on the expression of Glb1 and Glb2, we performed a series of "ABA-rescue" experiments of vp embryos. Sib normal and mutant embryos were obtained from 22 DAP ears segregating for each of the vp mutants and maintained for 4 days on growth medium in the presence or absence of 10 µM ABA. For all genotypes, absence of ABA in the medium resulted in germination while the presence of ABA suppressed germination except for the ABA-insensitive vp1/vp1 embryos. Levels of Glb1 and Glb2 transcripts were subsequently evaluated by northern blot analysis. Results of this analysis for Glb1 are shown in Figure 1; essentially identi-cal results were obtained for Glb2. In the absence of ABA, globulin transcripts are absent except for Vp7 embryos, which apparently represents a special case. With the exception of vp1/vp1 embryos, globulin transcripts are present in embryos of all genotypes maintained on medium supplemented with ABA. We therefore conclude that ABA plays a major role in regulation of the globulin genes, and that a functional Vp1 gene product is required for globulin gene expression during embryo development.

Figure 1. Northern blot analysis of Glb1 transcripts in embryos after 4 days on Murashige-Skoog medium. Sib normal and vp embryos were collected at 22 DAP (except for vp8, which was collected at 30 DAP) and maintained on Murashige-Skoog medium for 4 d in the absence (-) or presence (+) of 10 µM ABA. 

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