Expression of o2 modifiers in normal maize genotypes
--Glóverson L. Moro, Maurício A. Lopes, Bruce Hamaker and Brian Larkins

The effects of opaque2 (o2) modifier genes are well described in Quality Protein Maize (QPM). However, all the available information describes the effects of these genes in o2 backgrounds. Very little is known about the kind of biochemical and physical changes o2 modifier genes cause in wild-type (O2) endosperm. In order to address these questions, we used a backcross scheme to develop genotypes containing o2 modifiers and the O2 allele. The W64A line was used as source of the normal Opaque2 allele and Pool 34 QPM was used as recurrent parent and source of modifier genes. After each round of backcrossing, we visually selected against the o2 allele. Selected BC5 plants went through three generations of selfing to eliminate the o2 mutation. Ultimately, the presence of the O2 allele in BC5-S3 plants was confirmed by Southern analysis with an O2 cDNA clone. The 13 BC5-S3 plants homozygous for the O2 allele were selfed and their kernels (BC5-S4) were analyzed.

As observed in QPMs, modified normal kernels show a high content of  g-zein, but this is not necessarily followed by an increase in kernel density or hardness. There was no statistical difference in kernel density among the different inbreds and the two original parents (Pool 34 QPM and W64A). Hardness, as measured by the Stenvert test, showed a broad range of variation and was weakly correlated with  g-zein content (r = 0.40, P = 0.098). However, based on the range of Stenvert values (19.5 to 28.4), all the inbreds tested would be considered to have a very hard endosperm. Therefore, it seems that there is a threshold of density and hardness in normal endosperm above which the effects of  g-zein are not evident. 

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