Oleosins in the gametophytes of Pinus and Brassica and their phylogenetic relationship with those in the sporophytes of various species

Genome-wide identification and functional analysis of oleosin genes in Brassica napus L

BACKGROUND

Rapeseed is the third largest oil seed crop in the world. The seeds of this plant store lipids in oil bodies, and oleosin is the most important structural protein in oil bodies. However, the function of oleosin in oil crops has received little attention.

RESULTS

In the present study, 48 oleosin sequences from the Brassica napus genome were identified and divided into four lineages (T, U, SH, SL). Synteny analysis revealed that most of the oleosin genes were conserved, and all of these genes experienced purifying selection during evolution. Three and four important oleosin genes from Arabidopsis and B. napus, respectively, were cloned and analyzed for function in Arabidopsis. Overexpression of these oleosin genes in Arabidopsis increased the seed oil content slightly, except for BnaOLE3. Further analysis revealed that the average oil body size of the transgenic seeds was slightly larger than that of the wild type (WT), except for BnaOLE1. The fatty acid profiles showed that the linoleic acid content (13.3% at most) increased and the peanut acid content (11% at most) decreased in the transgenic lines. In addition, the seed size and thousand-seed weight (TSW) also increased in the transgenic lines, which could lead to increased total lipid production.

CONCLUSION

We identified oleosin genes in the B. napus genome, and overexpression of oleosin in Arabidopsis seeds increased the seed weight and linoleic acid content (13.3% at most).

A Combination of Histological, Physiological, and Proteomic Approaches Shed Light on Seed Desiccation Tolerance of the Basal Angiosperm Amborella trichopoda

Desiccation tolerance allows plant seeds to remain viable in a dry state for years and even centuries. To reveal potential evolutionary processes of this trait, we have conducted a shotgun proteomic analysis of isolated embryo and endosperm from mature seeds of Amborella trichopoda, an understory shrub endemic to New Caledonia that is considered to be the basal extant angiosperm. The present analysis led to the characterization of 415 and 69 proteins from the isolated embryo and endosperm tissues, respectively. The role of these proteins is discussed in terms of protein evolution and physiological properties of the rudimentary, underdeveloped, Amborella embryos, notably considering that the acquisition of desiccation tolerance corresponds to the final developmental stage of mature seeds possessing large embryos.

Evolution of oleosin in land plants

Oleosins form a steric barrier surface on lipid droplets in cytoplasm, preventing them from contacting and coalescing with adjacent droplets. Oleosin genes have been detected in numerous plant species. However, the presence of oleosin genes in the most basally diverging lineage of land plants, liverworts, has not been reported previously. Thus we explored whether liverworts have an oleosin gene. In Marchantia polymorpha L., a thalloid liverwort, one predicted sequence was found that could encode oleosin, possessing the hallmark of oleosin, a proline knot (-PX5SPX3P-) motif. The phylogeny of the oleosin gene family in land plants was reconstructed based on both nucleotide and amino acid sequences of oleosins, from 31 representative species covering almost all the main lineages of land plants. Based on our phylogenetic trees, oleosin genes were classified into three groups: M-oleosins (defined here as a novel group distinct from the two previously known groups), low molecular weight isoform (L-oleosin), and high molecular weight isoform (H-oleosin), according to their amino-acid organization, phylogenetic relationships, expression tissues, and immunological characteristics. In liverworts, mosses, lycophytes, and gymnosperms, only M-oleosins have been described. In angiosperms, however, while this isoform remains and is highly expressed in the gametophyte pollen tube, two other isoforms also occur, L-oleosins and H-oleosins. Phylogenetic analyses suggest that the M-oleosin isoform is the precursor to the ancestor of L-oleosins and H-oleosins. The later two isoforms evolved by successive gene duplications in ancestral angiosperms. At the genomic level, most oleosins possess no introns. If introns are present, in both the L-isoform and the M-isoform a single intron inserts behind the central region, while in the H-isoform, a single intron is located at the 5'-terminus. This study fills a major gap in understanding functional gene evolution of oleosin in land plants, shedding new light on evolutionary transitions of lipid storage strategies.

Refurbishing the plasmodesmal chamber: a role for lipid bodies?

Lipid bodies (LBs) are universal constituents of both animal and plant cells. They are produced by specialized membrane domains at the tubular endoplasmic reticulum (ER), and consist of a core of neutral lipids and a surrounding monolayer of phospholipid with embedded amphipathic proteins. Although originally regarded as simple depots for lipids, they have recently emerged as organelles that interact with other cellular constituents, exchanging lipids, proteins and signaling molecules, and shuttling them between various intracellular destinations, including the plasmamembrane (PM). Recent data showed that in plants LBs can deliver a subset of 1,3-β-glucanases to the plasmodesmal (PD) channel. We hypothesize that this may represent a more general mechanism, which complements the delivery of glycosylphosphatidylinositol (GPI)-anchored proteins to the PD exterior via the secretory pathway. We propose that LBs may contribute to the maintenance of the PD chamber and the delivery of regulatory molecules as well as proteins destined for transport to adjacent cells. In addition, we speculate that LBs deliver their cargo through interaction with membrane domains in the cytofacial side of the PM.

Plant lipid bodies and cell-cell signaling: a new role for an old organelle?

Plant lipid droplets are found in seeds and in post-embryonic tissues. Lipid droplets in seeds have been intensively studied, but those in post-embryonic tissues are less well characterised. Although known by a variety of names, here we will refer to all of them as lipid bodies (LBs). LBs are unique spherical organelles which bud off from the endoplasmic reticulum, and are composed of a single phospholipid (PL) layer enclosing a core of triacylglycerides. The PL monolayer is coated with oleosin, a structural protein that stabilizes the LB, restricts its size, and prevents fusion with adjacent LBs. Oleosin is uniquely present at LBs and is regarded as a LB marker. Although initially viewed as simple stores for energy and carbon, the emerging view is that LBs also function in cytoplasmic signalling, with the minor LB proteins caleosin and steroleosin in a prominent role. Apart from seeds, a variety of vegetative and floral structures contain LBs. Recently, it was found that numerous LBs emerge in the shoot apex of perennial plants during seasonal growth arrest and bud formation. They appear to function in dormancy release by reconstituting cell-cell signalling paths in the apex. As apices and orthodox seeds proceed through comparable cycles of dormancy and dehydration, the question arises to what degree LBs in apices share functions with those in seeds. We here review what is known about LBs, particularly in seeds, and speculate about possible unique functions of LBs in post-embryonic tissues in general and in apices in particular.

Stable Oil Bodies Sheltered by a Unique Oleosin in Lily Pollen

Stable oil bodies were purified from mature lily (Lilium longiflorum Thunb.) pollen. The integrity of pollen oil bodies was maintained via electronegative repulsion and steric hindrance possibly provided by their surface proteins. Immunodetection revealed that a major protein of 18 kDa was exclusively present in pollen oil bodies and massively accumulated in late stages of pollen maturation. According to mass spectrometric analyses, this oil body protein possessed a tryptic fragment of 13 residues matching that of a theoretical rice oleosin. A complete cDNA fragment encoding this putative oleosin was obtained by PCR cloning with primers derived from its known 13-residue sequence. Sequence analysis as well as immunological non-cross-reactivity suggests that this pollen oleosin represents a distinct class in comparison with oleosins found in seed oil bodies and tapetum. In pollen cells observed by electron microscopy, oil bodies were presumably surrounded by tubular membrane structures, and encapsulated in the vacuoles after germination. It seems that pollen oil bodies are mobilized via a different route from that of glyoxysomal mobilization of seed oil bodies after germination.

Oleosin gene expression in olive

To study the spatial and temporal regulation of oleosin gene expression during olive drupe development, a cDNA (OeOLE) was isolated from embryos. The deduced amino acid sequence of 165 amino acid residues exhibits a long central hydrophobic stretch, including the conserved "proline knot" motif. Phylogenetic analysis grouped OeOLE into the class of high (H) molecular weight oleosins. Southern blotting indicated that the gene is represented by 1-2 copies in the olive genome. Transcript analysis revealed that OeOLE is expressed solely in seeds. A similar bell-shaped pattern of expression was observed in both embryos and endosperms. Transcript accumulation starts at late heart embryo stage, reaches maximum levels at mid-torpedo stage and thereafter declines, coinciding the stages of most oil accumulation in those tissues.

Intracellular lipid particles of eukaryotic cells

In this review article we describe characterization of intracellular lipid particles of three different eukaryotic species, namely mammalian cells, plants and yeast. Lipid particles of all types of cells share a general structure. A hydrophobic core of neutral lipids is surrounded by a membrane monolayer of phospholipids which contains a minor amount of proteins. Whereas lipid particles from mammalian cells and plants harbor specific classes of polypeptides, mainly perilipins and oleosins, respectively, yeast lipid particles contain a more complex set of enzymes which are involved in lipid biosynthesis. Function of lipid particles as storage compartment and metabolic organelle, and their interaction with other subcellular fractions are discussed. Furthermore, models for the biogenesis of lipid particles are presented and compared among the different species.

The transcripts encoding two oleosin isoforms are both present in the aleurone and in the embryo of barley (Hordeum vulgare L.) seeds

Two transcripts (Ole-1 and Ole-2) encoding two oleosin isoforms homologous to the 18 and 16 kDa oleosins of maize, respectively, have been isolated from developing barley embryos and aleurone layers where lipid bodies are highly abundant organelles. For each of the isoforms the aleurone and embryo transcripts are identical, indicating that the same genes are expressed in both tissues. The temporal accumulation of the two transcripts during seed development is similar. At a low frequency, lipid bodies are found also in starchy endosperm cells of barley. Accordingly, a low transcript level is observed for both oleosins during starchy endosperm development.