The first trihelix gene to be discovered was the GT-1 transcription factor in pea (Green et al., 1987). This TF was found to bind to two photosensitive “GT” sequences conserved in the promoters of the light-induced pea rbcS family members, namely box II (-151 to -138; GTGTGGTTAATATG) and box III (-125 to -114; ATCATTTTCACT) (Cuozzo-Davis et al., 1990). The GT-1 protein was found to contain a helix–loop–helix–loop–helix structural domain (GT1, cd12203) and this region closely resembles the myb domain, but with longer helices. GT-2 is a plant transcriptional activator from rice that contains two separate, but similar, trihelix DNA-binding domains (Dehesh et al., 1990). These proteins are members of the larger SANT/myb superfamily. SANT is named after 'SWI3, ADA2, N-CoR and TFIIIB', which are several factors that share this domain (Maréchal et al., 1999). Trihelix family members are usually divided into 5 or 6 subfamilies (GT-1, GT-2, SH4, GTγ, SIP1, and GTδ) (Liu et al., 2020, Li et al., 2021).

Although gene members across the entire trihelix family participate in plant developmental programmes and light response, some GT factors are also involved in the basic resistance of plants to abiotic stresses, especially salt tolerance (Qin et al., 2015, Yang et al., 2023). Genomic studies have revealed 30 members of this family in Arabidopsis (Kaplan-Levy et al., 2012)

In maize, the ZmThx20 gene is required for kernel development. A natural mutation that causes abnormal kernel development, and displays a shrunken kernel phenotype was identified (named "shrunken 2008” (sh2008). In this mutant the starch grains are loose and have a less proteinaceous matrix surrounding them. The underlying gene was coned and found to encode a GT-1 type trihelix family member (Li et al., 2021). Another maize THX family member named ZmGT-3b has been implicated in the coordination of metabolism during growth-defense trade-off by optimizing the temporal and spatial expression of photosynthesis- and defense-related genes (Zhang et al., 2021).

Last updated June 2023 by John Gray

References:

Green PJ, Kay SA, Chua NH. Sequence-specific interactions of a pea nuclear factor with light-responsive elements upstream of the rbcS-3A gene. EMBO J. 1987 Sep;6(9):2543-9. doi: 10.1002/j.1460-2075.1987.tb02542.x. PMID: 3678200; PMCID: PMC553672.

Dehesh K, Bruce WB, Quail PH. A trans-acting factor that binds to a GT-motif in a phytochrome gene promoter. Science. 1990 Dec 7;250(4986):1397-9. doi: 10.1126/science.2255908. PMID: 2255908.

Cuozzo-Davis M, Yong MH, Gilmartin PM, Goyvaerts E, Kuhlemeier C, Sarokin L, Chua NH. Minimal sequence requirements for the regulated expression of rbcS-3A from Pisum sativum in transgenic tobacco plants. Photochem Photobiol. 1990 Jul;52(1):43-50. doi: 10.1111/j.1751-1097.1990.tb01753.x. PMID: 2399285.

Qin Y, Ma X, Yu G, Wang Q, Wang L, Kong L, Kim W, Wang HW. Evolutionary history of trihelix family and their functional diversification. DNA Res. 2014 Oct;21(5):499-510. doi: 10.1093/dnares/dsu016. Epub 2014 May 25. PMID: 24864043; PMCID: PMC4195496.

Maréchal E, Hiratsuka K, Delgado J, Nairn A, Qin J, Chait BT, Chua NH. Modulation of GT-1 DNA-binding activity by calcium-dependent phosphorylation. Plant Mol Biol. 1999 Jun;40(3):373-86. doi: 10.1023/a:1006131330930. PMID: 10437822.

Li P, Li Z, Xie G, Zhang J. Trihelix Transcription Factor ZmThx20 Is Required for Kernel Development in Maize. Int J Mol Sci. 2021 Nov 9;22(22):12137. doi: 10.3390/ijms222212137. PMID: 34830019; PMCID: PMC8624104.

Zhang Q, Zhong T, E L, Xu M, Dai W, Sun S, Ye J. GT Factor ZmGT-3b Is Associated With Regulation of Photosynthesis and Defense Response to Fusarium graminearum Infection in Maize Seedling. Front Plant Sci. 2021 Nov 18;12:724133. doi: 10.3389/fpls.2021.724133. PMID: 34868109; PMCID: PMC8638620.

Liu X, Zhang H, Ma L, Wang Z, Wang K. Genome-Wide Identification and Expression Profiling Analysis of the Trihelix Gene Family Under Abiotic Stresses in Medicago truncatula. Genes (Basel). 2020 Nov 23;11(11):1389. doi: 10.3390/genes11111389. PMID: 33238556; PMCID: PMC7709032.

Li K, Duan L, Zhang Y, Shi M, Chen S, Yang M, Ding Y, Peng Y, Dong Y, Yang H, Li Z, Zhang L, Fan Y, Ren M. Genome-wide identification and expression profile analysis of trihelix transcription factor family genes in response to abiotic stress in sorghum [Sorghum bicolor (L.) Moench]. BMC Genomics. 2021 Oct 14;22(1):738. doi: 10.1186/s12864-021-08000-7. PMID: 34649496; PMCID: PMC8515681.

Kaplan-Levy RN, Brewer PB, Quon T, Smyth DR. The trihelix family of transcription factors--light, stress and development. Trends Plant Sci. 2012 Mar;17(3):163-71. doi: 10.1016/j.tplants.2011.12.002. Epub 2012 Jan 10. PMID: 22236699.

Yang J, Tang Z, Yang W, Huang Q, Wang Y, Huang M, Wei H, Liu G, Lian B, Chen Y, Zhang J. Genome-wide characterization and identification of Trihelix transcription factors and expression profiling in response to abiotic stresses in Chinese Willow (Salix matsudana Koidz). Front Plant Sci. 2023 Mar 3;14:1125519. doi: 10.3389/fpls.2023.1125519. PMID: 36938039; PMCID: PMC10020544.