Abstract
TRANSPARENT TESTA GLABRA1 (TTG1) is a WD40 repeat protein. The phenotypes caused by loss-of-function of TTG1 were observed about half a century ago, but the TTG1 gene was identified only about twenty years ago. Since then, TTG1 has been found to be a plant-specific regulator with multiple roles and multiple functional mechanisms. TTG1 is involved in the regulation of cell fate determination, secondary metabolisms, accumulation of seed storage reserves, plant responses to biotic and abiotic stresses, and flowering time in plants. In some processes, TTG1 may directly or indirectly regulate the expression of downstream target genes via forming transcription activator complexes with R2R3 MYB and bHLH transcription factors. Whereas in other processes, TTG1 may function alone or interact with other proteins to regulate downstream target genes. On the other hand, the studies on the regulation of TTG1 are very limited. So far, only the B3-domain family transcription factor FUSCA3 (FUS3) has been found to regulate the expression of TTG1, phosphorylation of TTG1 affects its interaction with bHLH transcription factor TT2, and TTG1 proteins can be targeted for degradation by the 26S proteasome. Here, we provide an overview of TTG1, including the identification of TTG1, the functions of TTG1, the possible function mechanisms of TTG1, and the regulation of TTG1. We also proposed potential research directions that may shed new light on the regulation and functional mechanisms of TTG1 in plants.
Highlights
The phenotypes of Arabidopsis ttg mutants were first reported about 50 years ago [1,2]
It has been proposed that TESTA GLABRA1 (TTG1) can interact with different R2R3 MYB and basic helix-loop-helix transcription factors to form multiple MYB-bHLH-WD40 (MBW) activator complexes to regulate the expression of downstream genes, thereby regulating cell fate determination including trichome initiation and root hair formation, and secondary metabolism including flavonoid biosynthesis and seed coat mucilage production [24,25,26,27,28,29]
TTG1 in other plants may use different mechanisms to regulate epidermal cell fate determination, as molecular and genetic analysis show that even though Cucumis sativus TTG1 (CsTTG1) acts in parallel to CsGL1 in regulating fruit trichome initiation, CsTTG1 can directly interact with CsGL1, whereas in Arabidopsis, TTG1 directly interacts GL3 but not GL1 [76]
Summary
The phenotypes of Arabidopsis ttg (transparent testa, glabra) mutants were first reported about 50 years ago [1,2]. It was proposed that the TRANSPARENT TESTA GLABRA (TTG) locus has pleiotropic roles in the regulation of trichome initiation, anthocyanin biosynthesis, and seed coat mucilage biosynthesis in Arabidopsis [3]. It has been proposed that TTG1 can interact with different R2R3 MYB and basic helix-loop-helix (bHLH) transcription factors to form multiple MYB-bHLH-WD40 (MBW) activator complexes to regulate the expression of downstream genes, thereby regulating cell fate determination including trichome initiation and root hair formation, and secondary metabolism including flavonoid biosynthesis and seed coat mucilage production [24,25,26,27,28,29]. We provide here an overview of TTG1, including the summary of its functions and possible functional mechanisms in regulating epidermal cell fate determination and secondary metabolism with an emphasis on recent progress, but with a specific focus on the identification history of TTG1, its functions that may not require the formation of the MBW complexes, the functions of TTG1 orthologs in other plants, and perspectives on potential future research directions
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