The prolamin box (P-box) is a highly conserved 7-bp sequence element (5-TGTAAAG-3) found in the promoters of many cereal seed storage protein genes. Nuclear factors from maize endosperm specifically interact with the P-box present in maize prolamin genes (zeins). The presence of the P-box in all zein gene promoters suggests that interactions between endosperm DNA binding proteins and the P-box may play an important role in the coordinate activation of zein gene expression during endosperm development. We have cloned an endosperm-specific maize cDNA, named prolaminbox binding factor (PBF), that encodes a member of the recently described Dof class of plant Cys 2 -Cys 2 zinc-finger DNA binding proteins. When tested in gel shift assays, PBF exhibits the same sequence-specific binding to the P-box as factors present in maize endosperm nuclei. Additionally, PBF interacts in vitro with the basic leucine zipper protein Opaque2, a known transcriptional activator of zein gene expression whose target site lies 20 bp downstream of the P-box in the 22-kDa zein gene promoter. The isolation of the PBF gene provides an essential tool to further investigate the functional role of the highly conserved P-box in regulating cereal storage protein gene expression.

We have searched the Arabidopsis and rice (Oryza sativa) genomes for homologs of LRX1, an Arabidopsis gene encoding a novel type of cell wall protein containing a leucine-rich repeat (LRR) and an extensin domain. Eleven and eight LRX (LRR/EXTENSIN) genes have been identified in these two plant species, respectively. The LRX gene family encodes proteins characterized by a short N-terminal domain, a domain with 10 LRRs, a cysteine-rich motif, and a variable C-terminal extensin-like domain. Phylogenetic analysis performed on the conserved domains indicates the existence of two major clades of LRX proteins that arose before the eudicot/monocot divergence and then diversified independently in each lineage. In Arabidopsis, gene expression studies by northern hybridization and promoter::uidA fusions showed that the two phylogenetic clades represent a specialization into "reproductive" and "vegetative" LRXs. The four Arabidopsis genes of the "reproductive" clade are specifically expressed in pollen, whereas the seven "vegetative" genes are predominantly expressed in various sporophytic tissues. This separation into two expression classes is also supported by previous studies on maize (Zea mays) and tomato (Lycopersicon esculentum) LRX homologs and by information on available rice ESTs. The strong conservation of the amino acids responsible for the putative recognition specificity of the LRR domain throughout the family suggests that the LRX proteins interact with similar ligands.With the completion of the Arabidopsis genome sequence, it became clear that many Arabidopsis genes are members of multigene families. Although this had already been suggested by the analysis of expressed sequence tag (EST) databases and by classical gene searches, the availability of the full gene set of a plant provides the unique opportunity to get a complete inventory of all the members of a gene family. Among the 25,500 genes predicted in the Arabidopsis genome, 65% are members of a multigene family and 37% belong to families of more than five members (Arabidopsis Genome Initiative, 2000). Although the predicted total gene number of Arabidopsis is significantly larger than that of other sequenced multicellular eukaryotes such as Caenorhabditis elegans (19,000; C. elegans Sequencing Consortium, 1998) or Drosophila melanogaster (13,600; Adams et al., 2000), the absolute number of gene families and singletons (11,601 in Arabidopsis) is comparable in all these organisms (Arabidopsis Genome Initiative, 2000). This indicates that frequent gene duplications and consequently large gene families are a distinctive feature of the Arabidopsis genome, and possibly of all plant genomes. With the recent publication of a high-quality draft from two different subspecies, rice (Oryza sativa) is the second plant whose genome can be comprehensively investigated. Depending on the stringency applied in gene prediction, the rice genome contains between 32,277 and 61,668 genes (Goff et al., 2002;Yu et al., 2002). With 77% of the genes distributed in about 15,000 m...

Rapid Alkalinization Factors (RALFs) are plant peptides that rapidly increase the pH of plant suspension cell culture medium and inhibit root growth. A pollen-specific tomato (Solanum lycopersicum) RALF (SlPRALF) has been identified. The SlPRALF gene encodes a preproprotein that appears to be processed and released from the pollen tube as an active peptide. A synthetic SlPRALF peptide based on the putative active peptide did not affect pollen hydration or viability but inhibited the elongation of normal pollen tubes in an in vitro growth system. Inhibitory effects of SlPRALF were detectable at concentrations as low as 10 nm, and complete inhibition was observed at 1 μ  m peptide. At least 10-fold higher levels of alkSlPRALF, which lacks disulfide bonds, were required to see similar effects. A greater effect of peptide was observed in low-pH-buffered medium. Inhibition of pollen tube elongation was reversible if peptide was removed within 15 min of exposure. Addition of 100 nm SlPRALF to actively growing pollen tubes inhibited further elongation until tubes were 40 to 60 μm in length, after which pollen tubes became resistant to the peptide. The onset of resistance correlated with the timing of the exit of the male germ unit from the pollen grain into the tube. Thus, exogenous SlPRALF acts as a negative regulator of pollen tube elongation within a specific developmental window.