Induction of a gene encoding an oleosin homologue in cultured citrus cells exposed to salt stress

The unknown soldier in citrus plants: polyamines-based defensive mechanisms against biotic and abiotic stresses and their relationship with other stress-associated metabolites

Citrus plants are challenged by a broad diversity of abiotic and biotic stresses, which definitely alter their growth, development, and productivity. In order to survive the various stressful conditions, citrus plants relay on multi-layered adaptive strategies, among which is the accumulation of stress-associated metabolites that play vital and complex roles in citrus defensive responses. These metabolites included amino acids, organic acids, fatty acids, phytohormones, polyamines (PAs), and other secondary metabolites. However, the contribution of PAs pathways in citrus defense responses is poorly understood. In this review article, we will discuss the recent metabolic, genetic, and molecular evidence illustrating the potential roles of PAs in citrus defensive responses against biotic and abiotic stressors. We believe that PAs-based defensive role, against biotic and abiotic stress in citrus, is involving the interaction with other stress-associated metabolites, particularly phytohormones. The knowledge gained so far about PAs-based defensive responses in citrus underpins our need for further genetic manipulation of PAs biosynthetic genes to produce transgenic citrus plants with modulated PAs content that may enhance the tolerance of citrus plants against stressful conditions. In addition, it provides valuable information for the potential use of PAs or their synthetic analogs and their emergence as a promising approach to practical applications in citriculture to enhance stress tolerance in citrus plants.

Genome-Wide Analysis of Oleosin Gene Family in 22 Tree Species: An Accelerator for Metabolic Engineering of BioFuel Crops and Agrigenomics Industrial Applications?

Trees contribute to enormous plant oil reserves because many trees contain 50%-80% of oil (triacylglycerols, TAGs) in the fruits and kernels. TAGs accumulate in subcellular structures called oil bodies/droplets, in which TAGs are covered by low-molecular-mass hydrophobic proteins called oleosins (OLEs). The OLEs/TAGs ratio determines the size and shape of intracellular oil bodies. There is a lack of comprehensive sequence analysis and structural information of OLEs among diverse trees. The objectives of this study were to identify OLEs from 22 tree species (e.g., tung tree, tea-oil tree, castor bean), perform genome-wide analysis of OLEs, classify OLEs, identify conserved sequence motifs and amino acid residues, and predict secondary and three-dimensional structures in tree OLEs and OLE subfamilies. Data mining identified 65 OLEs with perfect conservation of the "proline knot" motif (PX5SPX3P) from 19 trees. These OLEs contained >40% hydrophobic amino acid residues. They displayed similar properties and amino acid composition. Genome-wide phylogenetic analysis and multiple sequence alignment demonstrated that these proteins could be classified into five OLE subfamilies. There were distinct patterns of sequence conservation among the OLE subfamilies and within individual tree species. Computational modeling indicated that OLEs were composed of at least three α-helixes connected with short coils without any β-strand and that they exhibited distinct 3D structures and ligand binding sites. These analyses provide fundamental information in the similarity and specificity of diverse OLE isoforms within the same subfamily and among the different species, which should facilitate studying the structure-function relationship and identify critical amino acid residues in OLEs for metabolic engineering of tree TAGs.

Identification, classification and differential expression of oleosin genes in tung tree (Vernicia fordii)

Triacylglycerols (TAG) are the major molecules of energy storage in eukaryotes. TAG are packed in subcellular structures called oil bodies or lipid droplets. Oleosins (OLE) are the major proteins in plant oil bodies. Multiple isoforms of OLE are present in plants such as tung tree (Vernicia fordii), whose seeds are rich in novel TAG with a wide range of industrial applications. The objectives of this study were to identify OLE genes, classify OLE proteins and analyze OLE gene expression in tung trees. We identified five tung tree OLE genes coding for small hydrophobic proteins. Genome-wide phylogenetic analysis and multiple sequence alignment demonstrated that the five tung OLE genes represented the five OLE subfamilies and all contained the "proline knot" motif (PX5SPX3P) shared among 65 OLE from 19 tree species, including the sequenced genomes of Prunus persica (peach), Populus trichocarpa (poplar), Ricinus communis (castor bean), Theobroma cacao (cacao) and Vitis vinifera (grapevine). Tung OLE1, OLE2 and OLE3 belong to the S type and OLE4 and OLE5 belong to the SM type of Arabidopsis OLE. TaqMan and SYBR Green qPCR methods were used to study the differential expression of OLE genes in tung tree tissues. Expression results demonstrated that 1) All five OLE genes were expressed in developing tung seeds, leaves and flowers; 2) OLE mRNA levels were much higher in seeds than leaves or flowers; 3) OLE1, OLE2 and OLE3 genes were expressed in tung seeds at much higher levels than OLE4 and OLE5 genes; 4) OLE mRNA levels rapidly increased during seed development; and 5) OLE gene expression was well-coordinated with tung oil accumulation in the seeds. These results suggest that tung OLE genes 1-3 probably play major roles in tung oil accumulation and/or oil body development. Therefore, they might be preferred targets for tung oil engineering in transgenic plants.

Transcript profiles uncover temporal and stress-induced changes of metabolic pathways in germinating sugar beet seeds

BACKGROUND

With a cultivation area of 1.75 Mio ha and sugar yield of 16.7 Mio tons in 2006, sugar beet is a crop of great economic importance in Europe. The productivity of sugar beet is determined significantly by seed vigour and field emergence potential; however, little is known about the molecular mechanisms underlying these traits. Both traits exhibit large variations within sugar beet germplasm that have been difficult to ascribe to either environmental or genetic causes. Among potential targets for trait improvement, an enhancement of stress tolerance is considered because of the high negative influence of environmental stresses on trait parameters. Extending our knowledge of genetic and molecular determinants of sugar beet germination, stress response and adaptation mechanisms would facilitate the detection of new targets for breeding crop with an enhanced field emergence potential.

RESULTS

To gain insight into the sugar beet germination we initiated an analysis of gene expression in a well emerging sugar beet hybrid showing high germination potential under various environmental conditions. A total of 2,784 ESTs representing 2,251 'unigenes' was generated from dry mature and germinating seeds. Analysis of the temporal expression of these genes during germination under non-stress conditions uncovered drastic transcriptional changes accompanying a shift from quiescent to metabolically active stages of the plant life cycle. Assay of germination under stressful conditions revealed 157 genes showing significantly different expression patterns in response to stress. As deduced from transcriptome data, stress adaptation mechanisms included an alteration in reserve mobilization pathways, an accumulation of the osmoprotectant glycine betaine, late embryogenesis abundant proteins and detoxification enzymes. The observed transcriptional changes are supposed to be regulated by ABA-dependent signal transduction pathway.

CONCLUSION

This study provides an important step toward the understanding of main events and metabolic pathways during germination in sugar beet. The reported alterations of gene expression in response to stress shed light on sugar beet stress adaptation mechanisms. Some of the identified stress-responsive genes provide a new potential source for improvement of sugar beet stress tolerance during germination and field emergence.

Oleosin gene family of Coffea canephora: quantitative expression analysis of five oleosin genes in developing and germinating coffee grain

Coffee grains have an oil content between 10% and 16%, with these values associated with Coffea canephora (robusta) and C. arabica (arabica), respectively. As the majority of the oil stored in oil seeds is contained in specific structures called oil bodies, we were interested in determining whether there are any differences in the expression of the main oil body proteins, the oleosins, between the robusta and arabica varieties. Here, we present the isolation, characterization and quantitative expression analysis of six cDNAs representing five genes of the coffee oleosin family (CcOLE-1 to CcOLE-5) and one gene of the steroleosin family (CcSTO-1). Each coffee oleosin cDNA encodes for the signature structure for oleosins, a long hydrophobic central sequence containing a proline KNOT motif. Sequence analysis also indicates that the C-terminal domain of CcOLE-1, CcOLE-3 and CcOLE-5 contain an 18-residue sequence typical of H-form oleosins. Quantitative RT-PCR showed that the transcripts of all five oleosins were predominantly expressed during grain maturation in robusta and arabica grain, with CcOLE-1 and CcOLE-2 being more highly expressed. While the relative expression levels of the five oleosins were similar for robusta and arabica, significant differences in the absolute levels of expression were found between the two species. Quantitative analysis of oleosin transcripts in germinating arabica grain generally showed that the levels of these transcripts were lower in the grain after drying, and then further decreased during germination, except for a small spike of expression for CcOLE-2 early in germination. In contrast, the levels of CcSTO-1 transcripts remained relatively constant during germination, in agreement with suggestions that this protein is actively involved in the process of oil body turnover. Finally, we discuss the implications of the coffee oleosin expression data presented relative to the predicted roles for the different coffee oleosins during development and germination.