Oil Composition Variations - Genetics, Genomics and Breeding of Crop Plants - page 274

sunflower. However, our molecular tools show that it remains as an insertion
of approximately 1 kb instead of the 8 kb, but the fragment is not detectable
due to its yet unknown sequence, however, it lengthens the 8.0 kb Hind III
fragment to 9.0 kb.
Gene silencing in plants is environmentally susceptible and the variation
of oleic acid content in commercial hybrids is not surprising. But we can
predict that oleic acid variation is not due to the oleate desaturase but
probably to a silencing mechanism that affects the final transcript stability.
8.2.3.2.6 Other Molecular and Biochemical Studies on the
Pervenets Mutation
Martínez-Rivas et al. (2001) have characterized three oleate desaturase
cDNAs. Two genes are constitutively expressed and one is embryo specific.
The latter probably corresponded to MOD whereas the two others
correspond to other loci. Only the MOD gene is affected by the Pervenets
mutation, and consequently most of the lipid pattern is normal in HO
sunflower as shown by Lagravère (1999) and Lagravère et al. (2000). To
reconcile the results from Martínez-Rivas et al. (2001) with those of Lacombe
et al. (2001) on the number of MOD genes in sunflower, we propose that the
5.85 kb Eco RI fragment carrying the MOD gene could be duplicated at three
loci. The MOD locus expressed in the embryo is not affected by the Pervenets
mutation as we have proposed that it is silenced.
Lagravère et al. (2004) proposed a new hypothesis based on biochemical
differences found between traditional and HO sunflowers. They suggest
that two different oleate desaturases could function in sunflower seeds to
explain the remaining linoleic acid in HO sunflower.
Merrien et al. (2005) have studied environmental factors that modify
the oleic acid content in HO sunflower oil. They measured variations in the
coldest temperature during the time of oil deposition and desaturation.
Temperature may also affect the intensity of gene silencing on fatty acid
composition. More studies are, therefore, required to unravel whether
breeding HO sunflower has to be done by targeting either the oleate
desaturase or the silencing suppressors. However, in this review we show
that most data favor the second hypothesis.
Schuppert et al. (2006) have studied in detail the MOD region using RT-
PCR strategy. They have found most of the features already presented for
the Pervenets mutation organization. However, some details seem different
and in their map, the 5.85 kb Eco RI fragment is by 4 kb. An explanation of
the difference is that they did not clone the genomic fragment and they
concatenated the sequences of the PCR products. Because of the other loci
revealed by Martínez-Rivas et al. (2001), Schuppert et al. (2006) may have
concatenated fragments from different loci. There are many sequence repeats
in the MOD region and they may have shifted a sequence to the wrong