Most genes identified that control Arabidopsis trichome initiation and formation are transcription factors or regulatory components in transcriptional networks and include GLABROUS 1 (GL 1), GLABRA2 (GL2), GLABRA3 (GL3) and TRANSPARENT TESTA GLABRA1 (TTG1).We report here that an importin β like protein, SENSITIVE TO ABA AND DROUGHT2 (SAD2), is required for trichome initiation.Mutations in SAD2 disrupted trichome initiation resulting in reduced trichome number, but had no effect on trichome development or root hair number and development.Expression levels of GL1, MYB23, GL2 and TTG1 were reduced in shoots of sad2 mutants while expression levels of GL3 and ENHANCER OF GLABRA3 (EGL3) were enhanced.Over-expression of GL3 increased trichome numbers in wild type but not in sad2 mutants, indicating that the function of the GL3 protein is altered in the sad2 mutants.In contrast, over-expression of GFP-GL1 decreased trichome number in both wild type and sad2.