University of California

Physiological genetics of dominant dwarfs

--Rodney G. Winkler and Michael Freeling

D8 is a dominant gibberellin (GA) insensitive, anther-eared maize (Zea mays L.) dwarf mutant proposed to be involved in GA reception. In addition to D8, five other dominant, anther-eared maize dwarf mutants of varying severity are known. We show, by mapping and similarity of phenotype, that all six are likely alternative alleles at the D8 locus. We define the phenotypic differences in GA response by three criteria: (a) ability to respond to exogenous GA, (b) ability to dwarf when GA synthesis is blocked, and (c) ability to respond to exogenous GA when endogenous GA synthesis is blocked. Although each allele specifies a different plant stature, from very mild to very severe dwarfism, each allele showed the same response to all three tests. Each allele is apparently saturated in its ability to respond to endogenous GA, but is able to dwarf when GA synthesis is blocked, and each allele can be returned to its "normal" dwarf phenotype when exogenous GA is applied. However, each allele sets a different maximum limit on growth. The fact that six independent alleles affect both GA responsiveness and dwarfing suggests that the wild type function of D8 is growth retardation and that GA responsiveness and dwarfing are linked, but in a complex manner. Dominant alleles are particularly valuable in dissecting the structure and function of gene products. Molecular analysis of these six alleles should provide insight into the D8 locus and the GA reception pathway.

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