Control of GL2 expression in Arabidopsis leaves and trichomes.

More than twenty genes are required for the correct initiation, spacing, and morphogenesis of trichomes in Arabidopsis. The initial selection of trichome precursors requires the activity of both the GLABROUS1 (GL1) and TRANSPARENT TESTA GLABROUS (TTG) genes. The GLABRA2 (GL2) gene is required for subsequent phases of trichome morphogenesis such as cell expansion, branching, and maturation of the trichome cell wall. Previous studies have shown that GL2 is a member of the homeodomain class of transcription factors. Here we report a detailed analysis of GL2 expression in the shoot using anti-GL2 antibodies and the GUS reporter gene fused to the GL2 promoter. The GL2 expression profile in the shoot is complex, and involves spatial and temporal variation in developing leaves and trichomes. Two separate promoter domains that are expressed in trichomes were identified. GL2, like GL1, is expressed in developing trichomes and in cells surrounding trichomes during early stages of trichome development. Unlike GL1, GL2 expression persists in mature trichomes. It was found that while GL1 and TTG were not required for the initiation of GL2 expression in the non-trichome cells, the presence of a functional GL1 or TTG gene was able to increase GL2 expression in these cells compared to ttg gl1 plants. The hypothesis that GL1 regulates aspects of GL2 expression is consistent with epistatic analysis of gl1 and gl2 and the expression patterns of GL1 and GL2. In support of this hypothesis, it was found that ectopic expression of GL1 in the presence of ectopic expression of the maize R gene, which can bypass the requirement for TTG, can ectopically activate GL2 transcription.