Abstract
Trichomes are specialized epidermal cells located in aerial parts of plants that function in plant defense against biotic and abiotic stresses. The simple unicellular trichomes of Arabidopsis serve as an excellent model to study the molecular mechanism of cell differentiation and pattern formation in plants. Loss-of-function mutations in Arabidopsis thaliana have suggested that the core genes GL1 (which encodes a MYB transcription factor) and TTG1 (which encodes a WD40 repeat-containing protein) are important for the initiation and spacing of leaf trichomes, while for normal trichome initiation, the genes GL3, and EGL3 (which encode a bHLH protein) are needed. However, the positive regulatory genes involved in multicellular trichrome development in cucumber remain unclear. This review focuses on the phenotype of mutants (csgl3, tril, tbh, mict, and csgl1) with disturbed trichomes in cucumber and then infers which gene(s) play key roles in trichome initiation and development in those mutants. Evidence indicates that MICT, TBH, and CsGL1 are allelic with alternative splicing. CsGL3 and TRIL are allelic and override the effect of TBH, MICT, and CsGL1 on the regulation of multicellular trichome development; and affect trichome initiation. CsGL3, TRIL, MICT, TBH, and CsGL1 encode HD-Zip proteins with different subfamilies. Genetic and molecular analyses have revealed that CsGL3, TRIL, MICT, TBH, and CsGL1 are responsible for the differentiation of epidermal cells and the development of trichomes. Based on current knowledge, a positive regulator pathway model for trichome development in cucumber was proposed and compared to a model in Arabidopsis. These data suggest that trichome development in cucumber may differ from that in Arabidopsis.
Highlights
Plant trichomes are highly specialized epidermal protrusions that are located on the surfaces of leaves, stems, petioles, sepals, seed coats, and other aerial organs
In the model plant Arabidopsis thaliana, Study on Trichome in Cucumber extensive studies have been performed on unicellular trichome development, especially on leaves (Hülskamp, 2004; Ishida et al, 2008; Pesch and Hülskamp, 2009)
The crucial positive transcription factors belong to three protein classes: one WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) (Galway et al, 1994; Walker et al, 1999); four basic helix-loop-helix proteins: GLABRA3 (GL3)m ENHANCER OF GLABRA3 (EGL3) (Payne et al, 2000; Zhang et al, 2003), TRANSPARENT TESTA (TT8) (Zhang et al, 2003), and MYC-1 (Zhao et al, 2012); and three R2-R3 type-MYB transcription factors: GLABRA1 (GL1), MYB23 and MYB5 (Oppenheimer et al, 1991; Li et al, 2009; Tominaga-Wada et al, 2012)
Summary
Plant trichomes are highly specialized epidermal protrusions that are located on the surfaces of leaves, stems, petioles, sepals, seed coats, and other aerial organs. We begin this review by focusing on cucumber trichomerelated mutants and the role that they play in trichome formation in plants with multicellular trichomes Several mutants, such as trichome-less (tril) (Wang et al, 2016), glabrous 3 (csgl3) (Pan et al, 2015; Cui et al, 2016), tiny branched hair (tbh) (Chen et al, 2014), micro-trichome (mict) (Zhao et al, 2015a), and glabrous 1 (csgl1) (Li et al, 2015), inhibit trichome development via different mechanisms, but all of these mutants cause a reduction in one type of spine. The possible mechanisms involved in modulating trichome development in cucumber mutants are summarized
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