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Potato (Solanum tuberosum) homeobox 1 (POTH1) is a class I homeobox gene isolated from an early-stage tuber cDNA library. The RNA expression pattern of POTH1, unlike that of most other class I knotted-like homeobox genes, is widespread in the cells of both indeterminate and differentiated tissues. Using in situ hybridization, POTH1 transcripts were detected in meristematic cells, leaf primordia, and the vascular procambium of the young stem. Overexpression of POTH1 produced dwarf plants with altered leaf morphology. Leaves were reduced in size and displayed a "mouse-ear" phenotype. The mid-vein was less prominent, resulting in a palmate venation pattern. The overall plant height of overexpression lines was reduced due to a decrease in internode length. Levels of intermediates in the gibberellin (GA) biosynthetic pathway were altered, and the bioactive GA, GA(1), was reduced by one-half in sense mutants. Accumulation of mRNA for GA 20-oxidase1, a key biosynthetic enzyme, decreased in overexpression lines. In vitro tuberization was enhanced under both short- and long-day photoperiods in several POTH1 overexpression lines. Sense lines produced more tubers at a faster rate than controls. These results imply that POTH1 mediates the development of potato by acting as a negative regulator of GA biosynthesis.