The first transcription factor TF known to interact with
The first transcription factor (TF) known to interact with one of the cis-acting GAREs is GAMYB protein, the expression of which is induced by GA in the aleurone Cy3 NHS ester of barley (HvGAMYB) (Gubler and Jacobsen, 1992). GAMYB is an important regulator of endosperm functions during seed development and germination (Yan et al., 2014). The binding of this TF to the GARE activates the expression of a number of hydrolase genes required for the mobilization of storage materials during seed germination (Gubler et al., 1995, Gubler et al., 1999). HvGAMYB-responsive promoters are present in the genes encoding low- and high-pI α- amylases, the EII (1-3, 1-4-β)-glucanase, the cathepsin B-like protease and EP-B cysteine protease (Cejudo et al., 1992, Cercos et al., 1999, Gubler et al., 1995, Gubler et al., 1999). Additionally, GAMYB has been shown to be involved in GA-mediated PCD in the aleurone cells of both cereals and Arabidopsis (Alonso-Peral et al., 2010, Guo and Ho, 2008). The GAMYB of rice seeds (OsGAMYB) also induces the expression of a different set of gibberellin-responsive genes in the aleurone layer, which indicates that other TFs may be required for the endosperm- specific gibberellin response (Tsuji et al., 2006). The rice gamyb mutant and Arabidopsis triple mutant myb33myb65myb101 show defects in GA-induced respones in the aleurone layer of the endosperm (Alonso-Peral et al., 2010, Kaneko et al., 2004, Yan et al., 2014). Another class of transcription factors, the members of which are also expressed during the growth of cereal seeds, is the DOF (DNA-binding One Zinc Finger) family. The members of this class are characterized by a zinc-finger domain that binds to the prolamin box located in the promoters of genes encoding cereal endosperm proteins (Yan et al., 2014). In addition, these TFs are well known in cereal seeds for the regulatory roles that they play during seed maturation and germination. Barley has two DOF factors that have been characterized, BPBF (Barley Prolamine-box Binding Factor) and SAD (Scutellum and Aleurone-expressed DOF), which are involved in gene regulation during seed development and germination (Mena et al., 2002, Mena et al., 1998). These factors bind to the pyrimidine box of the GARC. BPBF was initially recognized as an activator of hordein gene expression during endosperm development but was found to also be a transcriptional repressor of genes in the aleurone layer upon germination (Mena et al., 2002, Mena et al., 1998). SAD does not respond to gibberellins but acts as a transcriptional activator of the cathepsin B-like cysteine protease gene (Al21) by recognizing the pyrimidine box motifs in its promoter. Therefore, two TFs, BPBF and SAD, act oppositely in regulating the expression of a hydrolase gene in the barley aleurone layer. This regulation of gene expression could be explained by SAD possibly competing with BPBF for binding to the same cis-motifs. Moreover, Diaz et al. (2002) and Isabel-LaMoneda et al. (2003) demonstrated that SAD and BPBF both interact with GAMYB protein. Analysis of the interactions between different TFs that bind the promoter of the barley cathepsin B-like cysteine protease gene (Al21) showed that during the early germination phase when the ABA:GA ratio is high, transcription of Al21 is repressed by ABA through HvDOF19. In turn, during the late germination phase when the ABA:GA ratio is lower, the repression of the transcription of this cysteine protease gene can be overcome by the increased expression of the positive effectors GAMYB and HvDOF23/SAD (Moreno-Risueno et al., 2007). Therefore, the role of TFs as repressors or activators of gene expression depends on their interaction with other TFs (Ma, 2005). BdDOF24 of Brachypodium distachyon acts as repressor or activator of the cathepsin B-like protease (BdCathB) expressed during germination. The dual function of this factor depends on its interaction with BdGAMYB (González-Calle et al., 2014). BdDOF24 together with BdGAMYB significantly increased activity of the reported gene (GUS), driven by BdCathB promotor, compare to alone BdGAMYB (González-Calle et al., 2014). Furthermore, a seed-specific transcriptional regulator VP1 in barley (HvVP1), through the interaction with other transcription factors, regulates key stages of seed maturation and germination (Abraham et al., 2016). It has been observed that HvVP1 reduced the positive interaction between two factors: GAMYB and BPBF and decreased the binding capability of GAMYB and BPBF to the cis-elements of hordein and α-amylase promotors (Abraham et al., 2016).