Wildtype sunflower typically produces 12-24% oleic acid (18:1) and 70-82%
linoleic acid (18:2). High-oleic sunflower, by contrast, produces up to 80-94% oleic
acid. The monounsaturated oleic acid has a greater oxidative stability than the
polyunsaturated linoleic acid, predominant in wildtype sunflower, and has greater
nutritional benefits than polyunsaturated and saturated fatty acids. High oleic
sunflower lines are based on high-oleic acid germplasm, which originated from an
induced mutation (Oh). Oi is necessary but often not sufficient for producing the high
oleic phenotype, presumably because additional quantitative trait loci (QTL) segregate
in some genetic backgrounds. The seed specific oleate desaturase (FAD2-1), which
displays greatly reduced transcript levels in high oleic lines, has been established as
the principal candidate gene affecting oleic acid content in sunflower kernels and
cosegregates with Ol.
In this research, we demonstrated that the FAD2-1 gene is tandemly duplicated
in mutant high oleic sunflower lines, with the two copies being separated by 3.1 kb.
We showed that reduced transcript levels of FAD2-1 in developing kernels of high
oleic lines were caused by the RNA interference (RNAi) pathway. Further evidence
was obtained when sense and antisense transcripts of FAD2-1 were detected in mutant
lines; wildtype lines only produced transcripts in the sense direction. Bi-directional transcripts formed double-stranded RNA, which serve as a trigger for RNAi
In addition, we compared the transcript levels of 48 glycerolipid biosynthetic
genes in the developing kernels of four low and four high oleic sunflower lines using
microarrays. The analyses revealed that the lipid transfer protein was the only
additional gene besides FAD2-] with differing transcript levels in each of the four low
versus high oleic comparisons. The microarray experiments also revealed no
significant differences in transcript levels for genes directly involved in oleic acid
synthesis for three of the four comparisons. Only one comparison showed differing
transcript levels for about a quarter of the genes in the glycerolipid biosynthesis.
Sequence-based molecular markers were developed for eleven candidate genes
involved in the oleic acid biosynthesis and used to identify candidate genes for the
QTL underlying oleic acid content in sunflower seeds. A population of 262
recombinant inbred lines segregating for oleic acid content was analyzed and revealed
that the oleic acid phenotype is caused by the intralocus and interlocus effects of
several genes. These findings emphasize the complexity of the phenotype and indicate a limitation in the applicability of marker-assisted selection for high oleic acid content.
Overall, our understanding of the molecular mechanisms underlying the high oleic
acid phenotype in sunflower increased.
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