Cotton seed hair development share many similarities with Arabidopsis leaf trichome development , which is mediated by a “trichome activation complex”. Leaf trichome initiation in Arabidopsis thalianais promoted by the positive transcription regulators GLABROUS1 (GL1), TRANSPARENT TESTA GLABRA1 (TTG1), GLABRA3 (GL3), and ENHANCER of GL3 (EGL3) that are counteracted by the negative regulators TRIPTYCHON (TRY), CAPRICE (CPC), and ENHANCER of TRY and CPC1 (ETC1, 2, and 3) that encode single MYB-domain protein families [1, 4, 6, 12, 14]. GLABROUS2 (GL2) functions downstream of the GL1/TTG/GL3 complex and also plays a role in leaf trichome development [4, 13]. Some MYB factors such as AtMYB5 and AtMYB23 have minor effects on trichome initiation but regulate mucilage biosynthesis and seed coat development . Moreover, trichome genes such as TTG1 and GL2 affect mucilage biosynthesis and columella cell formation [17, 19], suggesting a role of these trichome genes in seed coat development .
Figure 3. Production of ectopic trichomes and seed hair in Arabidopsis plants overexpressing two cotton fiber-related genes. (A) Siliques of Arabidopsis plants overexpressing a plasmid vector (left) or two cotton genes. (B) A scanning electron micrograph (SEM) showing ectopic trichomes produced inside of anArabidopsis silique. (C). Transgenic seed with a single hair. (D) A SEM image showing two seed hairs on a seed.
Several studies using cotton fiber-related genes have demonstrated a close relationship between cotton seed fibers and Arabidopsis leaf trichomes. Gossypium arboreum MYB2 (GaMYB2) encoding a putative homolog of GL1 MYB transcription factor complements the trichomeless gl1 mutant and induces occasional hair formation in A. thaliana seeds . GaHOX1, a homeobox gene, encodes a HD-ZIP IV transcription factor, and is a functional homologue of the A. thaliana GL2 gene . Two WD-repeat genes from Gossypium hirsutum (GhTTG1) restore trichome formation in A. thaliana ttg1mutant plants and complement anthocyanin defects in Matthiola incana ttg1 mutants . Moreover, microrarray and gene expression analyses have uncovered many cotton fiber-related genes, including those encoding MYB transcription factors and phytohormonal regulators [7, 18]. For example, differential expression of six MYB genes is observed in allotetraploid cotton (G. hirsutum L.) . Several MYB and RDL genes are expressed in fiber initials through microarray analysis . GhMYB25 regulates early fiber and trichome development in cotton . The data collectively suggest that Arabidopsis and cotton use similar transcription factors for the development of leaf trichomes and seed hairs. However, the mechanisms responsible for the differentiation of branched trichomes in vegetative tissues (leaves) and unbranched hairs in reproductive organs (seeds) may not be the same, and many seed plants including Arabidopsis do not produce seed hairs.
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