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 al ., 1999). The genes encoding the storage proteins are
coordinately expressed in the developing endosperm where they are under
spatial and temporal transcriptional control, involving cis-acting motifs in
their promoters and trans-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 al ., 1997; Marzabal et
al ., 1998; Mena et al ., 1998; Vicente-Carbajosa et al ., 1998; Oñate et al ., 1999;
Carbonero et al ., 2000; Wu et al ., 2000; Díaz et al ., 2002). A conserved cis-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 al ., 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 al ., 1998; Oñate et al ., 1999), trans-activate the
expression of endosperm-specific genes such as the Hor2 and Itr1 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 Hor2 promoter in co-bombarded barley endosperms through binding to the prolamine box (PB)
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).