Genetic dissection of the co‐expression of genes encoding the two isoforms of oleosins in the oil bodies of maize kernel

Abstract

SummaryOleosins are abundant structural proteins on the surface of intracellular oil bodies in seeds. In many maize (Zea mays L.) inbreds, there are three oleosins, OLE18, OLE17, and OLE16, termed according to their apparent molecular weight, which are present in the proportional amounts of about 1:1:2 in isolated oil bodies. In some inbreds, OLE18 and OLE17 occur as molecular weight variants with a molecular weight difference of 1000 or less. In inbreds CM555 and FR2, OLE18 and OLE17, respectively, are absent; the respective genes olel8and olel7in the inbreds are present, but are transcriptionally inactive. The F 1 of CM555 × FR2 possesses both OLE18 and OLE17, as expected from the inheritance of ole18 and ole17 genes. In all inbreds examined, including CM555, FR2, and their F 1 hybrids, and in both the diploidic embryos and triploidic aleurone layers, the quantity of OLE18 and/or OLE17 equals that of OLE16. Since OLE18 and OLE17 are close members of the high-molecular weight (H) oleosin isoform whereas OLE16 belongs to the low-molecular weight (L) oleosin isoform, the results indicate the presence of equal amounts of the H and L isoforms in the oil bodies. The equality suggests the occurrence of a hetero-dimer, or -multimer, of the two isoforms. To probe the control mechanism for such a hetero-mer formation, CM555, FR2, and their F 1 hybrids, as well as Mo17 (a common inbred) have been used to analyze oleosin gene copy number, nuclear run-off transcription activity, transcript amount, and oleosin protein content. It has been determined that equal levels of H and L oleosin isoforms accumulate in oil bodies irrespective of the dosage of functional H and L genes. Therefore, the control of equality is not the consequence of the competition among the active H isoform genes for transcriptional machineries, but occurs at the post-transcriptional level, presumably at translation and/or through oleosin interaction (formation of heteromer) and stability.

Bioinformatics Reveal Five Lineages of Oleosins and the Mechanism of Lineage Evolution Related to Structure/Function from Green Algae to Seed Plants

“…This ratio suggests that, in vivo, SL oleosins and SH oleosins are at a 1:1 ratio of a dimer or a multimer, assuming the absence of differential posttranscriptional controls. This suggestion is consistent with earlier observations that, in both the diploid embryos and the triploid aleurone layer of diverse Z. mays lines, despite their being products of recent and intensive breeding, the ratio of SL and SH oleosin transcripts as well as oleosin proteins in isolated LDs is always approximately 1:1 (Lee et al, 1995). Having SL and SH oleosin dimeric or multimeric complexes would also explain the retention of SL and SH oleosin genes and their approximately 1:1 transcript:protein levels throughout the evolution of angiosperms.…”

The Accumulation of Oleosins Determines the Size of Seed Oilbodies inArabidopsis

“…In KnockOLEO1 and the other OLEO1-suppressed lines, accumulation of OLEO2 and OLEO4 isoforms was similar in both transgenic and wild-type seeds. There is no apparent increase in accumulation of these peptides as it occurs in maize when one isoform is missing (Lee et al, 1995). It is unlikely, based on divergent sequence, that the Antisense, Hairpin, and Loop constructs interfere with the transcript of the other isoforms.…”

“…In fact, the maize lines FR2 and CM555 are depleted in one polypeptide isoform. However, this is compensated for by higher accumulation of another isoform maintaining the overall amount of oleosins (Lee et al, 1995). Such circumstantial evidence has pointed to an important role for oleosins in seeds.…”

Role of the Proline Knot Motif in Oleosin Endoplasmic Reticulum Topology and Oil Body Targeting

“…One reason for instability of the proline knot variant may be its inability to form a heterodimer with an endogenous oleosin isoform. Such a possibility was highlighted by Lee et al (1995).…”

“…The proline knot motif may exert more subtle effects on the secondary structure, which is important for downstream targeting events, as discussed below. It is unlikely that this topology is preferred because of interaction with oleosin isoforms (Lee et al, 1995), for although this could occur in the in vivo experiments, no complementary isoforms are present in the canine microsome experiments. The hydrophobic domain must contain a critical feature that supports the cytosolic orientation of both flanking domains.…”