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Transcription factors of the plant-specific homeodomain leucine zipper IV (HD-ZIP IV) family have been found from moss to higher plants, and several family members have been associated with epidermis-related expression and/or function. In maize (Zea mays), four of the five characterized HD-ZIP IV family members are expressed specifically in the epidermis, one contributes to trichome development, and target genes of another one are involved in cuticle biosynthesis. Assessing the phylogeny, synteny, gene structure, expression, and regulation of the entire family in maize, 12 novel ZmHDZIV genes were identified in the recently sequenced maize genome. Among the 17 genes, eight form homeologous pairs duplicated after the split of maize and sorghum (Sorghum bicolor), whereas a fifth duplication is shared with sorghum. All 17 ZmHDZIV genes appear to be derived from a basic module containing seven introns in the coding region. With one possible exception, all 17 ZmHDZIV genes are expressed and show preferential expression in immature reproductive organs. Fourteen of 15 ZmHDZIV genes with detectable expression in laser-dissected tissues exhibit a moderate to very strong expression preference for the epidermis, suggesting that at least in maize, the majority of HD-ZIP IV family members may have epidermis-related functions. Thirteen ZmHDZIV genes carry conserved motifs of 19 and 21 nucleotides in their 3' untranslated region. The strong evolutionary conservation and the size of the conserved motifs in the 3' untranslated region suggest that the expression of HD-ZIP IV genes may be regulated by small RNAs. more...

WRINKLED1 (WRI1), a key regulator of seed oil biosynthesis in Arabidopsis (Arabidopsis thaliana), was duplicated during the genome amplification of the cereal ancestor genome 90 million years ago. Both maize (Zea mays) coorthologs ZmWri1a and ZmWri1b show a strong transcriptional induction during the early filling stage of the embryo and complement the reduced fatty acid content of Arabidopsis wri1-4 seeds, suggesting conservation of molecular function. Overexpression of ZmWri1a not only increases the fatty acid content of the mature maize grain but also the content of certain amino acids, of several compounds involved in amino acid biosynthesis, and of two intermediates of the tricarboxylic acid cycle. Transcriptomic experiments identified 18 putative target genes of this transcription factor, 12 of which contain in their upstream regions an AW box, the cis-element bound by AtWRI1. In addition to functions related to late glycolysis and fatty acid biosynthesis in plastids, the target genes also have functions related to coenzyme A biosynthesis in mitochondria and the production of glycerol backbones for triacylglycerol biosynthesis in the cytoplasm. Interestingly, the higher seed oil content in ZmWri1a overexpression lines is not accompanied by a reduction in starch, thus opening possibilities for the use of the transgenic maize lines in breeding programs. more...

Transcription factors of the homeodomain-leucine zipper IV (HD-ZIP IV) family play crucial roles in epidermis-related processes. To gain further insight into the molecular function of OUTER CELL LAYER1 (OCL1), 14 target genes up- or down-regulated in transgenic maize (Zea mays) plants overexpressing OCL1 were identified. The 14 genes all showed partial coexpression with OCL1 in maize organs, and several of them shared preferential expression in the epidermis with OCL1. They encoded proteins involved in lipid metabolism, defense, envelope-related functions, or cuticle biosynthesis and include ZmWBC11a (for white brown complex 11a), an ortholog of AtWBC11 involved in the transport of wax and cutin molecules. In support of the annotations, OCL1-overexpressing plants showed quantitative and qualitative changes of cuticular wax compounds in comparison with wild-type plants. An increase in C24 to C28 alcohols was correlated with the transcriptional up-regulation of ZmFAR1, coding for a fatty acyl-coenzyme A reductase. Transcriptional activation of ZmWBC11a by OCL1 was likely direct, since transactivation in transiently transformed maize kernels was abolished by a deletion of the activation domain in OCL1 or mutations in the L1 box, a cis-element bound by HD-ZIP IV transcription factors. Our data demonstrate that, in addition to AP2/EREBP and MYB-type transcription factors, members of the HD-ZIP IV family contribute to the transcriptional regulation of genes involved in cuticle biosynthesis. more...

Small GTP-binding proteins regulate diverse processes in eukaryotic cells such as signal transduction, cell proliferation, cytoskeletal organization, and intracellular membrane trafficking. These proteins function as molecular switches that cycle between "active" and "inactive" states, and this cycle is linked to the binding and hydrolysis of GTP. The Arabidopsis genome contains 93 genes that encode small GTP-binding protein homologs. Phylogenetic analysis of these genes shows that plants contain Rab, Rho, Arf, and Ran GTPases, but no Ras GTPases. We have assembled complete lists of these small GTPases families, as well as accessory proteins that control their activity, and review what is known of the functions of individual members of these families in Arabidopsis. We also discuss the possible roles of these GTPases in relation to their similarity to orthologs with known functions and localizations in yeast and/or animal systems. more...