GAMYB and BPBF Transcriptional Factors in the Control of Gene Expression During Development of Barley Endosperm

Abstract

During cereal seed development, the starchy endosperm cells accumulate proteins, mainly prolamines, as well as several antimicrobial peptides and inhibitors of heterologous hydrolases implicated in plant defence (Shewry et al., 1995; Carbonero et ah, 1999). The genes encoding the storage proteins are coordinately expressed in the developing endosperm where they are under spatial and temporal transcriptional control, involving cz's-acting motifs in their promoters and fnms-acting transcriptional factors (TFs). A number of consensus sequences in gene promoters have been shown to be involved in conferring endosperm specificity in cereals (Albani et ah, 1997; Marzabal et ah, 1998; Mena et ah, 1998; Vicente-Carbajosa et ah, 1998; Onate et ah, 1999; Carbonero et ah, 2000; Wu et ah, 2000; Diaz et ah, 2002). A conserved ds-acting motif found in most storage protein gene promoters of seeds is the endosperm-box, a bipartite motif located -300 bp upstream of the translation initiation codon that contains two distinct binding sites: the GCN4-like motif (GLM) motif (5'-ATGAG/CTCAT-3') that resembles the yeast GCN4 motif and the prolamine box (PB: 5'-TGTAAAG-3')- There is a third motif (5'-AACA/TA-3')/ which plays a role in the expression of genes encoding glutelins in rice (Suzuki et ah, 1998). These three types of motifs are the target sites of different TFs. In barley, the GLM is recognized by bZIP proteins of the Opaque2 family (BLZ1 and BLZ2) that through dimer formation, as either homo- or heterodimers (Vicente-Carbajosa et ah, 1998; Onate et ah, 1999), fraws-activate the expression of endosperm-specific genes such as the Hor2 and Itrl genes encoding the B-hordeins and a trypsin inhibitor, respectively. The prolamine box is recognized by a TF of the DOF class, DNA binding with One Finger (BPBF), that activates transcription of a native Horl promoter in co-bombarded barley endosperms through binding to the prolamine box (PB) (Mena et al, 1998; Carbonero et al., 2000). We describe here how the 5'-AACA/TA-3' motif interacts with a TF of the MYB class, which was previously shown to have a role in the activation of gibberellin (GA)-induced genes upon germination in barley aleurone cells (Gubler et al., 1995,1999).