The gai mutation of Arabidopsis confers a dwarf phenotype resembling that of mutants defective in gibberellin (GA) biosynthesis. However, gai mutant plants differ from GA biosynthesis mutants because they fail to respond to exogenous GAs and accumulate endogenous GA species to higher (rather than lower) levels than found in wild-type controls. The gai mutation, therefore, identifies a gene that modulates the response of plant cells to GA. We have mapped gai with respect to visible and

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... n fragment length polymorphism (RFLP) markers from chromosome 1. To observe the phenotype exhibited by individuals potentially lacking wild-type (GAl) function, we have also isolated nove1 irradiationinduced derivative alleles of gai. When homozygous, these alleles confer a revertant phenotype that is indistinguishable from the wild type. gai is a semidominant mutation that exerts its effects either because it is a gain-of-function mutation or because it is a lossof-function or reduced-function mutation. The genetic and physiological properties of the derivative alleks am considered with reference to these alternative modes of dominance of gai. Because these alleles are potential deletion or rearrangement mutations, together with the closely linked RFLP markers identified in the linkage mapping experiments, they provide useful resources for the isolation of the gai locus via a map-based cloning approach.