The promoter of a cytosolic glutamine synthetase gene from the conifer Pinus sylvestris is active in cotyledons of germinating seeds and light-regulated in transgenic Arabidopsis thaliana

Differential responses of the promoters from nearly identical paralogs of loblolly pine (Pinus taeda L.) ACC oxidase to biotic and abiotic stresses in transgenic Arabidopsis thaliana

Promoters from an ACC oxidase gene (PtACO1) and its nearly identical paralog (NIP) (PtACO2) of loblolly pine (Pinus taeda L.) were recovered from genomic DNA using PCR amplification. Transgenic Arabidopsis plants harboring genetic constructs from which beta-glucuronidase (GUS) expression was driven by the full-length (pACO1:GUS, pACO2:GUS) or truncated (pACO1-1.2:GUS, pACO2-1.2:GUS) loblolly pine ACC oxidase gene promoters displayed distinctive patterns of expression for the different promoter constructs. Both full-length promoter constructs, but not those using truncated promoters, responded to indole-3-acetic acid (IAA) and wounding. Both pACO1:GUS and pACO1-1.2:GUS responded to pathogen attack, while neither version of the pACO2 promoter responded to infection. In the inflorescence stalks, the full-length pACO1 promoter construct, but not the truncated pACO1-1.2:GUS or either pACO2 construct, responded to bending stress. When flowering transgenic Arabidopsis plants were placed in a horizontal position for 48 h, expression from pACO2:GUS, but not the other constructs, was induced on the underside of shoots undergoing gravitropic reorientation. The expression pattern for the pACO2:GUS construct in transgenic Arabidopsis was consistent with what might be expected for a gene promoter involved in the compression wood response in loblolly pine. Although near complete sequence identity between PtACO1 and PtACO2 transcripts prevented quantitation of specific gene products, the promoter expression analyses presented in this study provide strong evidence that the two ACC oxidase genes are likely differentially expressed and responded to different external stimuli in pine. These results are discussed with respect to the potential functional differences between these two genes in loblolly pine.

The importance of cytosolic glutamine synthetase in nitrogen assimilation and recycling

Glutamine synthetase assimilates ammonium into amino acids, thus it is a key enzyme for nitrogen metabolism. The cytosolic isoenzymes of glutamine synthetase assimilate ammonium derived from primary nitrogen uptake and from various internal nitrogen recycling pathways. In this way, cytosolic glutamine synthetase is crucial for the remobilization of protein-derived nitrogen. Cytosolic glutamine synthetase is encoded by a small family of genes that are well conserved across plant species. Members of the cytosolic glutamine synthetase gene family are regulated in response to plant nitrogen status, as well as to environmental cues, such as nitrogen availability and biotic/abiotic stresses. The complex regulation of cytosolic glutamine synthetase at the transcriptional to post-translational levels is key to the establishment of a specific physiological role for each isoenzyme. The diverse physiological roles of cytosolic glutamine synthetase isoenzymes are important in relation to current agricultural and ecological issues.

Differential regulation of two glutamine synthetase genes by a single Dof transcription factor

The PpDof5 transcription factor from maritime pine (Pinus pinaster) is a regulator of the expression of glutamine synthetase (GS) genes in photosynthetic and non-photosynthetic tissues. PpDof5 mRNA is detected almost ubiquitously during pine development with low levels of gene expression in green tissues and much higher levels in roots and lignified shoots. The PpDof5 protein expressed in bacteria binds to oligonucleotide probes containing the AAAG core sequence derived from the promoters of GS1a and GS1b genes. Transient expression experiments in agroinfiltrated tobacco leaves and in pine protoplasts demonstrated that PpDof5 is able to trans-regulate differentially the transcription of both GS1a and GS1b. PpDof5 activated transcription of the GS1b promoter and, in contrast, behaved as a transcriptional repressor of the GS1a promoter. These results support a regulatory mechanism for the transcriptional control of the spatial distribution of cytosolic GS isoforms in pine. Considering the precise expression patterns of GS1 genes required to fulfil the ammonium assimilation requirements during tree development, we hypothesize that PpDof5 could have a key role in the control of ammonium assimilation for glutamine biosynthesis in conifers. A regulatory model of GS1 gene expression in pine is proposed.

Isolation and functional analysis of the 5'-flanking region of carrot top1beta gene coding for the beta isoform of DNA topoisomerase I

We have isolated and functionally characterized the promoter region of the top1beta gene encoding carrot (Daucus carota) DNA topoisomerase Ibeta. The major transcription start site was mapped by primer extension analysis 164 nt upstream the ATG translation start codon. Sequence analysis of the 5'-upstream region of the gene revealed the presence of a canonical TATA-like box at position -35 bp and several cis-acting sequences, including a (CT)n element in the leader region of the gene, a myb-related motif and the Dof element NtBBF-1, which correlate with the inducible expression pattern of this gene. Functional reporter analysis of the top1beta 5'-flanking region was performed in both carrot and Arabidopsis thaliana transfected protoplasts. The region at -719 to +161 was sufficient to confer high expression level in both species. The transient expression assay in protoplasts induced to stop dividing confirmed that the promoter, whose activity is low in quiescent cells, is activated when protoplasts are induced to proliferate by exogenous application of growth factors.

Molecular and enzymatic analysis of ammonium assimilation in woody plants

Ammonium is assimilated into amino acids through the sequential action of glutamine synthetase (GS) and glutamate synthase (GOGAT) enzymes. This metabolic pathway is driven by energy, reducing power and requires the net supply of 2-oxoglutarate that can be provided by the reaction catalysed by isocitrate dehydrogenase (IDH). Most studies on the biochemistry and molecular biology of N-assimilating enzymes have been carried out on annual plant species and the available information on woody models is far more limited. This is in spite of their economic and ecological importance and the fact that nitrogen is a common limiting factor for tree growth. GS, GOGAT and IDH enzymes have been purified from several woody species and their kinetic and molecular properties determined. A number of cDNA clones have also been isolated and characterized. Although the enzymes are remarkably well conserved along the evolutionary scale, major differences have been found in their compartmentation within the cell between angiosperms and conifers, suggesting possible adaptations to specific functional roles. The analysis of the gene expression patterns in a variety of biological situations such as changes in N nutrition, development, biotic or abiotic stresses and senescence, suggest that cytosolic GS plays a central and pivotal role in ammonium assimilation and metabolism in woody plants. The modification of N assimilation efficiency has been recently approached in trees by overexpression of a cytosolic pine GS in poplar. The results obtained, suggest that an increase in cytosolic GS might lead to a global effect on the synthesis of nitrogenous compounds in the leaves, with enhanced vegetative growth of transgenic trees. All these data suggest that manipulation of cytosolic GS may have consequences for plant growth and biomass production.