THE PLANT CELL, Vol 5, Issue 11 1599-1609, Copyright © 1993 by American Society of Plant Biologists
opaque2 Modifiers Act Post-Transcriptionally and in a Polar Manner on [gamma]-Zein Gene Expression in Maize Endosperm
E. Or, S. K. Boyer and B. A. Larkins
Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721
The opaque2 (o2) modifier genes convert the soft endosperm of an o2 mutant
to a hard, vitreous phenotype. The primary biochemical change associated
with the expression of these genes is a two- to threefold increase in
synthesis of the 27-kD [gamma]-zein storage protein. To investigate the
mechanism of modifier gene activity, we examined the level of [gamma]-zein
mRNA and protein synthesis during the early stages of endosperm development
in normal, o2, and modified o2 genotypes. Although the o2 mutation was
found to reduce expression of the 27-kD [gamma]-zein genes, the activity of
o2 modifier genes dramatically increased the level of both [gamma]-zein
protein and mRNAs as early as 16 days after pollination. At this stage,
transcription of [gamma]-zein genes is reduced by ~50% in both o2 and
modified o2 genotypes compared to wild type. Thus, it appears that the
modifiers regulate [gamma]-zein synthesis through a post-transcriptional
mechanism. Analysis of transcripts from the two nearly identical genes (A
and B) encoding the 27-kD [gamma]-zein protein showed differences in the
mRNA ratios in different genotypes. In modified o2 mutants, accumulation of
A over B transcript was greatly enhanced during endosperm development.
Somatic recombination at this locus was found to reduce the number of B
genes in the endosperm, but this could not account for the preferential
accumulation of the A transcript. Our results suggest that a product of the
o2 modifier genes increases the translation or stability of the A gene
mRNA, leading to enhanced synthesis of 27-kD [gamma]-zein protein.
