Molecular cloning and partial characterization of a tobacco cDNA encoding a small bZIP protein

PhOBF1, a petunia ocs element binding factor, plays an important role in antiviral RNA silencing

Virus-induced gene silencing (VIGS) is a common reverse genetics strategy for characterizing the function of genes in plants. The detailed mechanism governing RNA silencing efficiency triggered by viruses is largely unclear. Here, we reveal that a petunia (Petunia hybrida) ocs element binding factor, PhOBF1, one of the basic leucine zipper (bZIP) transcription factors, was up-regulated by Tobacco rattle virus (TRV) infection. Simultaneous silencing of PhOBF1 and a reporter gene, phytoene desaturase (PDS) or chalcone synthase (CHS), by TRV-based VIGS led to a failure of the development of leaf photobleaching or the white-corollas phenotype. PhOBF1 silencing caused down-regulation of RNA silencing-related genes, including RNA-dependent RNA polymerases (RDRs), Dicer-like RNase III enzymes (DCLs), and Argonautes (AGOs). After inoculation with the TRV-PhPDS, PhOBF1-RNAi lines exhibited a substantially impaired PDS silencing efficiency, whereas overexpression of PhOBF1 resulted in a recovery of the silencing phenotype (photobleaching) in systemic leaves. A compromised resistance to TRV and Tobacco mosaic virus was found in PhOBF1-RNAi lines, while PhOBF1-overexpressing lines displayed an enhanced resistance to their infections. Compared with wild-type plants, PhOBF1-silenced plants accumulated lower levels of free salicylic acid (SA), salicylic acid glucoside, and phenylalanine, contrarily to higher levels of those in plants overexpressing PhOBF1. Furthermore, transcripts of a number of genes associated with the shikimate and phenylpropanoid pathways were decreased or increased in PhOBF1-RNAi or PhOBF1-overexpressing lines, respectively. Taken together, the data suggest that PhOBF1 regulates TRV-induced RNA silencing efficiency through modulation of RDRs, DCLs, and AGOs mediated by the SA biosynthesis pathway.

Deregulation of sucrose-controlled translation of a bZIP-type transcription factor results in sucrose accumulation in leaves

Sucrose is known to repress the translation of Arabidopsis thaliana AtbZIP11 transcript which encodes a protein belonging to the group of S (S - stands for small) basic region-leucine zipper (bZIP)-type transcription factor. This repression is called sucrose-induced repression of translation (SIRT). It is mediated through the sucrose-controlled upstream open reading frame (SC-uORF) found in the AtbZIP11 transcript. The SIRT is reported for 4 other genes belonging to the group of S bZIP in Arabidopsis. Tobacco tbz17 is phylogenetically closely related to AtbZIP11 and carries a putative SC-uORF in its 5′-leader region. Here we demonstrate that tbz17 exhibits SIRT mediated by its SC-uORF in a manner similar to genes belonging to the S bZIP group of the Arabidopsis genus. Furthermore, constitutive transgenic expression of tbz17 lacking its 5′-leader region containing the SC-uORF leads to production of tobacco plants with thicker leaves composed of enlarged cells with 3–4 times higher sucrose content compared to wild type plants. Our finding provides a novel strategy to generate plants with high sucrose content.

Transcriptional and metabolic profiles of stress-induced, embryogenic tobacco microspores

Higher plant microspores, when subjected to various stress treatments in vitro, are able to reprogram their regular gametophytic development towards the sporophytic pathway to form haploid embryos and plants. Suppression subtractive hybridization (SSH) and metabolic profiling were used to characterize this developmental switch. Following differential reverse Northern hybridizations 90 distinct up-regulated sequences were identified in stressed, embryogenic microspores (accessible at www.univie.ac.at/ntsm). Sequence analyses allowed the classification of these genes into functional clusters such as metabolism, chromosome remodeling, signaling, transcription and translation, while the putative functions of half of the sequences remained unknown. A comparison of metabolic profiles of non-stressed and stressed microspores using gas chromatography/mass spectrometry (GC/MS) identified 70 compounds, partly displaying significant changes in metabolite levels, e.g., highly elevated levels of isocitrate and isomaltose in stressed microspores compared to non-stressed microspores. The formation of embryogenic microspores is discussed on the basis of the identified transcriptional and metabolic profiles.

In vivo binding of hot pepper bZIP transcription factor CabZIP1 to the G-box region of pathogenesis-related protein 1 promoter

We find that salicylic acid and ethephon treatment in hot pepper increases the expression of a putative basic/leucine zipper (bZIP) transcription factor gene, CabZIP1. CabZIP1 mRNA is expressed ubiquitously in various organs. The green fluorescent protein-fused transcription factor, CabZIP1::GFP, can be specifically localized to the nucleus, an action that is consistent with the presence of a nuclear localization signal in its protein sequence. Transient overexpression of the CabZIP1 transcription factor results in an increase in PR-1 transcripts level in Nicotiana benthamiana leaves. Using chromatin immunoprecipitation, we demonstrate that CabZIP1 binds to the G-box elements in native promoter of the hot pepper pathogenesis-related protein 1 (CaPR-1) gene in vivo. Taken together, our results suggest that CabZIP1 plays a role as a transcriptional regulator of the CaPR-1 gene.

Functional dissection of a small anaerobically induced bZIP transcription factor from tomato

A small anaerobically induced tomato transcription factor was isolated from a subtractive library. This factor, designated ABZ1 (anaerobic basic leucine zipper), is anaerobically induced in fruits, leaves and roots and encodes a nuclear localized protein. ABZ1 shares close structural and sequence homology with the S-family of small basic leucine zipper (bZIP) transcription factors that are implicated in stress response. Nuclear localization of ABZ1 is mediated by the basic region and occurs under normoxic conditions. ABZ1 binds to G-box-like target sites as a dimer. Binding can be abolished by heterodimerization with a truncated protein retaining the leucine zipper but lacking the DNA binding domain. The protein binds in a sequence specific manner to the CaMV 35S promoter which is down regulated when ABZ1 is coexpressed. This correlates with the anaerobic down regulation of the 35S promoter in tomato and tobacco. These results may suggest that small bZIP proteins are involved in the negative regulation of gene expression under anaerobic conditions.

Phosphatidylcholine biosynthesis at low temperature: differential expression of CTP:phosphorylcholine cytidylyltransferase isogenes in Arabidopsis thaliana

We cloned the gene and a cDNA for a second CTP: phosphorylcholine cytidylyltransferase (CCT, EC 2.7.7.15) annotated in chromosome 4 by the Arabidopsis genome project, and designated the gene AtCCT2 to discriminate it from the isogene AtCCT1 in chromosome 2. When Arabidopsis plants were chilled at 2 degrees C for 12 h, the level of AtCCT2 transcripts in the rosettes increased about 6-fold over that before 2 degrees C treatment. By contrast, no significant change occurred in the level of AtCCT1 transcripts during 7 d of 2 degrees C treatment. Immunoblot analysis revealed that the level of AtCCT2 in the rosettes chilled at 2 degrees C increased, and that the level of AtCCT1 showed minor changes, when compared with those before cold treatment. Total CCT activity measured at 2 degrees C increased in plants subjected to 2 degrees C treatment, and this increase was sufficient to account for lipid changes induced by the 2 degrees C treatment. We therefore concluded that Arabidopsis utilizes two distinct CCT isozymes for CDP-choline synthesis during cold acclimation. Our findings are important in understanding the physiological functions of CCT isozymes in Arabidopsis and will also stimulate efforts to understand the physiological significance of phosphatidylcholine at low temperatures.

BZI-1 specifically heterodimerises with the tobacco bZIP transcription factors BZI-2, BZI-3/TBZF and BZI-4, and is functionally involved in flower development

The tobacco (Nicotiana tabacum) protein BZI-1 is closely related to the plant bZIP transcription factors CPRF2, G/HBF-1 and OHP1. Using the C-terminal part of BZI-1, which includes the bZIP domain, as a bait in a yeast two-hybrid screen, three BZI-1 interacting bZIP transcription factors, referred to as BZI-2, BZI-3/TBZF and BZI-4, were isolated. The observed interactions are due to the leucine zipper dimerisation domain and have been found to be specific, in so far as other bZIP transcription factors do not interact with BZI-1. The formation of heterodimers is favoured to homodimerisation. Furthermore, physical protein-protein interaction was confirmed by in vitro binding studies. Expression analysis reveals that BZI-2 mRNA is predominantly located in the stems and parts of the flower. BZI-3/TBZF expression has been observed predominantly in flowers and, to a lesser extent, in the vegetative parts of the plant. In particular, BZI-4 is transcribed specifically in the stamen, the petals and the pistils of the tobacco flower. Since BZI-1 is expressed ubiquitously in tobacco plants, co-localisation with BZI-2, BZI-3/TBZF or BZI-4 might influence BZI-1 heterodimerisation and consequently target gene selection. Analysis of transgenic plants displaying changes in BZI-1 protein level revealed that BZI-1 regulates BZI-4 expression. Moreover, a reduction in functional BZI-1 protein resulted in flowers having a reduced size; in particular, the stamen and the petals are affected. Consequently, BZI-1 homo- or heterodimers are involved in the control of the size of the organs of flowers. Based on these data, we discuss a model postulating BZI transcription factor heterodimerisation as a mechanism determining target gene selection and regulating processes involved in plant development.

Specific association of transcripts of tbzF and tbz17, tobacco genes encoding basic region leucine zipper-type transcriptional activators, with guard cells of senescing leaves and/or flowers

Induction by low temperature is a common feature of the lip19 subfamily members of the basic region leucine zipper gene family in plants. Here, we characterize two tobacco (Nicotiana tabacum) genes, tbzF and tbz17, belonging to the lip19 subfamily, whose gene products, TBZF and TBZ17, show 73% identity and are located in nuclei. They preferentially bind to DNA fragments spanning A-box/G-box and C-box/G-box hybrid motifs and show transactivation activity in cobombarded tobacco BY-2 cells, indicating they function as transcriptional activators. Transcripts of tbzF were detected at a high level in senescing leaves and flowers. In contrast, tbz17 transcripts could be shown to accumulate in aged leaves but not in flowers. In situ hybridization analysis revealed transcripts of tbzF and tbz17 to be predominantly located in guard cells and vascular tissues of senescing leaves. These results suggest that TBZF and TBZ17 are both involved in controlling gene transcription related to functions of guard cells in senescing leaves and that TBZF bifunctionally acts in floral development.