Promoter of a cotton fibre MYB gene functional in trichomes of Arabidopsis and glandular trichomes of tobacco

Cotton fibres are unicellular seed trichomes. Our previous study suggested that the cotton R2R3 MYB transcript factor GaMYB2 is a functional homologue of the Arabidopsis trichome regulator GLABRA1 (GL1). Here, the GaMYB2 promoter activity is reported in cotton (Gossypium hirsutum), tobacco (Nicotiana tabacum), and Arabidopsis plants. A 2062 bp promoter of GaMYB2 was isolated from G. arboreum, and fused to a beta-glucuronidase (GUS) reporter gene. In cotton, the GaMYB2 promoter exhibited activities in developing fibre cells and trichomes of other aerial organs, including leaves, stems and bracts. In Arabidopsis the promoter was specific to trichomes. Different from Arabidopsis and cotton that have unicellular non-glandular simple trichomes, tobacco plants contain more than one type of trichome, including multicellular simple and glandular secreting trichomes (GSTs). Interestingly, in tobacco plants the GaMYB2 promoter directed GUS expression exclusively in glandular cells of GSTs. A series of 5'-deletions revealed that a 360 bp fragment upstream to the translation initiation codon was sufficient to drive gene expression. A putative cis-element of the T/G-box was located at -233 to -214; a yeast one-hybrid assay showed that Arabidopsis bHLH protein GLABRA3 (GL3), also a trichome regulator, and GhDEL65, a GL3-like cotton protein, had high binding activities to the T/G-box motif. Overexpression of GL3 or GhDEL65 enhanced the GaMYB2 promoter activity in transgenic Arabidopsis plants. A comparison of GaMYB2 promoter specificities in trichomes of different plant species with different types of trichomes provides a tool for further dissection of plant trichome structure and development.