--Alan L. Kriz, Jana S. Holt and Renato Paiva
The availability of viviparous mutants in maize provides an excellent opportunity for the study of molecular mechanisms involved in the control of embryo maturation. Embryos homozygous for the recessive allele at any of the six vp loci apparently fail to undergo normal maturation but rather germinate precociously on the ear. As a first step in determining if gene expression in vp embryos is switched from a pattern of development/maturation to one of germination, we are investigating the expression of genes encoding seed globulins during precocious and normal germination. These genes are ideal markers for this type of study: both the Mr 63,000 product of the Glb1 gene and the Mr 45,000 product of the Glb2 gene accumulate to high levels during embryo development and are rapidly degraded during the early stages of seed germination. For these experiments, RNAs and proteins were extracted from developmentally staged (23 to 31 DAP) normal and precociously germinating sib embryos homozygous for the different vp alleles. Northern blot and immunoblot analyses were performed by using as probe either a Glb1-specific cDNA clone (see above note by Belanger et al.) or globulin-specific antisera, respectively.
The results of these experiments are presented in Table 1. Each of the vp mutants exhibits characteristics of normal germinating embryos in that the Glb2 protein is absent. With respect to Glb1 expression, however, each of the mutants has a distinctive phenotype. For example, vp7 exhibits the pattern of normally germinating kernels in that Glb1 transcript levels are drastically reduced and the protein is absent; on the other hand, vp2 and vp9 both exhibit characteristics of normal embryo development (high levels of Glb1 transcript) and normal germination (reduced levels, presumably through degradation, of Glb1 protein).
Table 1. Levels of Glb gene products in precociously germinating
*Amount relative to normal sib embryos
These studies will be extended this summer to characterize globulin
gene expression in vp mutants prior to the onset of precocious germination.
This work may provide some insight as to the molecular basis of embryo
maturation in maize.
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