Trichomes are specialized single epidermal cell layer, often branched and distributed on aerial parts of the plant like leaves, sepals, petioles, stems and leaves margin. The GA dependent trichome initiation stage is positively regulated by two genes namely GL1 (GLABROUS1) and TTG (TRANSPARENT TESTA GLABRA) in Arabidopsis, which are necessary to initiate three successive endoreduplications in trichome precursor cells (Hülskamp et al., 1994). Coexpression of both GL1 and R, a maize homolog of TTG is needed to restore the trichome initiation process in gl1-3 mutant. Based on mutation study, GL1 is involved only in trichome formation, whereas TTG plays a role in seed coat pigmentation and mucilage, anthocyanin production and trichome formation (Koornneef et al., 1982 ,Koornneef., 1981). GL1 and R belong to the MYB and bHLH family of transcription factors (Oppenheimer et al.,1991, Ludwig et al., 1989) respectively. In Arabidopsis, GeBP (GL1 enhancer binding protein) localizes to the nucleus and binds to the 152 bp GL1 central enhancer region thus regulating its expression. The conserved central region with unknown function and C-terminal putative leucine zipper motif in GeBPs and its homologs are needed for its activity in yeast, however unlike GL1, GeBP function is independent of GA signaling. Also, GA responsive meristematic tissue specific KNAT1 genes expression positively correlates with GeBP expression, thus implying its possible role in leaf initiation (Curaba et al., 2003)

Maize harbors at least 19 GeBP genes, highest among 9 gramineae crops with 7 duplication gene pairs distributed unevenly across the genome as a result of WGD/segmental duplication. Out of 15 conserved motifs, motifs 1, 2 and 3 are present in all the 19 GeBPs implying their gene regulatory role. Expression profiles of 13 rice GeBPs revealed that all are expressed at different levels during different developmental stages with highest expression in leaf, palea, lemma. Interestingly, OsGeBP1 was positively regulated by GAs, cytokinin and metal stress like ZnCl2, CdCl2 and CuCl2 but negatively regulated by auxin, thus highlighting GeBPs role in both hormonal and stress responsive pathways (Huang et al., 2021). In Arabidopsis,  four founding members GeBP and GeBP like proteins (GPL) 1, 2 and 3 out of total 21 share an unconventional C- terminal leucine zipper motif involved in homo and heterodimerization and play a redundant role in cytokinin pathway regulation. A triple mutant of closely related gebp, gebp1, gebp2 genes showed reduced sensitivity to exogenous cytokinin and increase in transcript level of type-A ARR cytokinin response genes involved in negative feedback regulation in cytokinin signaling, thereby leading to cytokinin insensitivity. Thus the role of GPL TFs  in cytokinin hormone signaling is to antagonize the negative feedback regulation of ARR genes to trigger cytokinin response (Chevalier et al., 2008). The plasticity of the roots during plant development is responsible for adaptation to the changing environment contributed by change in the root architecture. In response to Cd metal stress, root avoidance is observed in Arabidopsis due to inhibition of root growth by GeBP-like 4 (GPL4) transcription factor expressed in root tip. GPL4 inhibits root growth by modulating reactive oxygen species concentration in exposed plants (Khare et al., 2016). A putative type IIIa membrane protein encoded by CONSTITUTIVE EXPRESSOR OF PATHOGENESIS RELATED GENES5 (CPR5) in Arabidopsis Functions in cell proliferation, cell expansion, pathogen response and cell death. Based on  transcriptomic analysis of quadruple gebp, gebp1, 2, 3 mutant and one overexpressing cpr5 like phenotypes, it was found the GeBP/GPL regulates subset of CPR5 pathway genes involved in cell wall metabolism and stress response. However, CPR5 does not regulate GeBP/GPL due to its unchanged transcript level in cpr5 mutant. Also, GeBP/GPL act as a repressor of pathogen response genes (PR) implying GeBP/GPL role in cell defense and cell death pathway (Perazza et al., 2011)

Last updated June 2023 by Ankita Abnave

References:

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Huang J, Zhang Q, He Y, Liu W, Xu Y, Liu K, Xian F, Li J, Hu J. Genome-Wide Identification, Expansion Mechanism and Expression Profiling Analysis of GLABROUS1 Enhancer-Binding Protein (GeBP) Gene Family in Gramineae Crops. Int J Mol Sci. 2021 Aug 15;22(16):8758. doi: 10.3390/ijms22168758. PMID: 34445464; PMCID: PMC8395763.

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Chevalier F, Perazza D, Laporte F, Le Hénanff G, Hornitschek P, Bonneville JM, Herzog M, Vachon G. GeBP and GeBP-like proteins are noncanonical leucine-zipper transcription factors that regulate cytokinin response in Arabidopsis. Plant Physiol. 2008 Mar;146(3):1142-54. doi: 10.1104/pp.107.110270. Epub 2007 Dec 27. PMID: 18162594; PMCID: PMC2259040.