SAD2 in Arabidopsis functions in trichome initiation through mediating GL3 function and regulating GL1, TTG1 and GL2 expression

Most genes identified that control Arabidopsis trichome initiation and formation are transcription factors or regulatory components in transcriptional networks and include GLABROUS1 (GL1), GLABRA2 (GL2), GLABRA3 (GL3) and TRANSPARENT TESTA GLABRA1 (TTG1). Herein, we report that an importin beta-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. Overexpression 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, overexpression of GFP-GL1 decreased trichome number in both wild type and sad2. Double mutant analysis of gl1 sad2 and gl3 sad2 indicated that SAD2 functions genetically, at least in part, in the same pathway with these two genes. Co-immunoprecipitation indicated that the sad2 mutation does not disrupt formation of the TTG1-GL3-GL1 complex. Analysis of GFP fusions of GL1, GL2, GL3 and TTG1 suggested that these proteins are most likely not direct cargo of SAD2. Our data suggest that SAD2 is involved in trichome initiation by regulating these nuclear genes.