GLABRA2 (GL2)/ATHB-10 encodes a homeodomain protein that belongs to the homeodomain-leucine zipper family. Mutant studies have revealed that this gene is involved in trichome, root-hair and seed-coat development. We used reverse genetics to investigate the role of GL2 in trichome development. A transgene consisting of a GL2-coding fragment preceded by the cauliflower mosaic virus 35S promoter (35S::GL2) did not complement defects in the gl2-1 mutant. In the wild-type genetic background, 35S::GL2 caused gl2-mutant-like and scarcely viable phenotypes, suggesting that ectopic overexpression of GL2 interrupts endogenous GL2 function in trichome development and is toxic to plants. On the other hand, another GL2 transgene containing the GL2 promoter (pGL2::GL2) complemented the gl2-1 mutation. Entopically additive expression of GL2 by introduction of pGL2::GL2 in the wild-type genetic background noticably increased the number of trichomes and induced production of adjacent trichomes. Consistent with this result, gl2-1/+ heterozygous leaves, whose GL2 expression was expected to decrease, had fewer trichomes than +/+ leaves. These results indicate that GL2 quantitatively regulates the frequency of trichome initiation and is involved in determining trichome spacing.