HvMCB1, a R1MYB transcription factor from barley with antagonistic regulatory functions during seed development and germination

Potassium transport systems in the moss Physcomitrella patens: pphak1 plants reveal the complexity of potassium uptake

Potassium uptake is one of the most basic processes of plant physiology. However, a comprehensive description is lacking. At a cellular level fungi have provided a helpful but imperfect plant model, which we aim to improve using Physcomitrella patens. Blast searches in expressed sequence tag databases demonstrated that Physcomitrella expresses the same families of K(+) and Na(+) transport systems as flowering plants. We cloned two inward rectifier channels, PpAKT1-2, and four HAK-type transporters (PpHAK1-4). In both types of transport system, phylogenetic analyses revealed that despite their high sequence conservation they could not be included in Arabidopsis or rice (Oryza sativa) clusters. Both inward rectifier channels and one HAK transporter (PpHAK1) were expressed in yeast. PpAKT1 and activated mutants of PpAKT2 and PpHAK1 showed clear functions that were similar to those of homologous systems of flowering plants. A pphak1 null mutant line of Physcomitrella failed to deplete K(+) below 10 mum. Moreover, in a non-K(+)-limiting medium in which wild-type plants grew only as protonema, pphak1-1 plants produced leafy gametophores and contained 60% more K(+). We found that Physcomitrella takes up K(+) through several systems. PpHAK1 is the dominant system in plants that underwent K(+) starvation for long periods but an as-yet unidentified system, which is non-selective for K(+), Rb(+), and Cs(+), dominates in many other conditions. Finally, we discuss that, similar to PpHAK1, one of the functions of AtHAK5 may be to control cellular K(+) content and that a non-selective as-yet unidentified system also exists in Arabidopsis.

The HvDOF19 transcription factor mediates the abscisic acid-dependent repression of hydrolase genes in germinating barley aleurone

Upon germination of seed cereals, mobilization of the reserves stored in the endosperm is regulated by the phytohormones gibberellins (GA) and abscisic acid (ABA). In barley, the cis regulatory elements and the trans-acting factors mediating the ABA response of hydrolase genes remain elusive. Two new barley genes, HvDof17 and HvDof19, encoding transcription factors (TFs) of the DNA binding with one finger (DOF) class have been characterized and their role upon germination investigated. HvDOF19 binds in a specific manner to the pyrimidine box within the GARC of a thiol-protease gene (Al21), and mediates the ABA repression of this gene in the barley aleurone. Silencing of HvDof19 in transient expression assays diminishes the inhibitory effect of ABA upon expression of the Al21 gene promoter. Transcripts from HvDof17 and HvDof19 accumulate early in germinating aleurones with a peak at 16 h after seed imbibition (hai), whereas the mRNAs of the GA-induced activator GAMYB remain little expressed. At 48 hai, mRNA content of both genes is comparatively insignificant compared with that of GAMYB, which reaches a maximum. Both TFs repress, in transient expression assays, the GA- and the GAMYB-mediated activation of this thiol-protease gene (Al21). In addition, HvDOF17 and HvDOF19 interact with GAMYB in plant cell nuclei, and HvDOF17, but not HvDOF19, interferes with the DNA binding of GAMYB to its target site in the promoter of the Al21 gene. A regulatory model of hydrolase gene expression upon germination is proposed.

Combined networks regulating seed maturation

Seed maturation is an important phase of seed development during which embryo growth ceases, storage products accumulate, the protective tegument differentiates and tolerance to desiccation develops, leading to seed dormancy. The spatial and temporal regulation of all these processes requires the concerted action of several signaling pathways that integrate information from genetic programs, and both hormonal and metabolic signals. Recent genetic studies have identified some of the interactions that occur between four master regulators in Arabidopsis, increasing our knowledge of the control of the transcriptional program involved in seed maturation. Moreover, several recent breakthroughs have led to a better understanding of the role of abscisic acid signal modulation and the importance of metabolic regulation in the maternal to filial switch leading to the maturation phase.

Molecular dissection of the interaction between the transcriptional activator ZmMRP-1 and the promoter of BETL-1

The interaction between the transfer cell specific transcriptional activator ZmMRP-1 and the promoter of the transfer cell specific gene BETL-1 constitutes an exceptionally robust system. Reporter constructs containing the BETL-1 promoter are virtually silent in a variety of cell types, from maize leaves to yeast. The introduction of ZmMRP-1 in co-transformation assays leads to the transactivation of the reporter construct by up to two orders of magnitude. In this work we have investigated the molecular basis of this interaction. We found that the BETL-1 promoter includes four potential targets for ZmMRP-1 binding, consisting of a 12 bp motif containing two repeats. Co-transformation assays and electrophoretic mobility shift experiments identified the sequence TATCTCTATCTC as the preferred one for the interaction with the transcription factor. Identification of similar sequences in other transfer cell specific promoters lead us to propose as a transfer cell box a sequence related to those identified in the BETL-1 promoter, positioned 50-100 bp upstream the TATA box.

Ternary complex formation between HvMYBS3 and other factors involved in transcriptional control in barley seeds

The SHAQKYF R1MYB transcription factor (TF) HvMYBS3 from barley is an activator of gene expression both during endosperm development and in aleurone cells upon seed germination. Its mRNA was detected as early as 10 days after flowering in developing barley endosperm, with a peak at 18 days, and in aleurone cells at 8 h after water imbibition, as shown by Northern blot and in situ hybridization analyses. The HvMYBS3 protein expressed in bacteria binds to oligonucleotides containing a GATA core derived from the promoters of: (i) the developing endosperm gene Itr1 (5'-GATAAGATA-3') encoding trypsin inhibitor BTI-CMe, and (ii) the post-germinating aleurone gene Amy6.4 (5'-TATCCAC-3'/5'-GTGGATA-3') encoding a high-pI alpha-amylase. Transient expression experiments in co-bombarded developing endosperms and in barley aleurone layers demonstrated that HvMYBS3 trans-activated transcription both from Itr1 and Amy6.4 promoters, in contrast with a previously reported seed-expressed R1MYB, HvMCB1, which was an activator of Itr1 and a transcriptional repressor of the Amy6.4 gene. In the yeast three-hybrid system, the HvMYBS3 protein formed a ternary complex with BPBF and BLZ2, two important seed TFs. However, no binary interactions could be detected between HvMYBS3 and BLZ2 or between HvMYBS3 and BPBF.