Oil Composition Variations - Genetics, Genomics and Breeding of Crop Plants - page 278

(> 85%) have been developed. The first studies have concluded that the trait
in both lines was determined by recessive alleles at the Tph2 locus. Bulked
segregant analysis identified two simple sequence repeat (SSR) markers on
LG 8 linked to Tph2 . A large linkage group was constructed by genotyping
additional markers. Tph2 mapped closely to linked PCR-based markers.
The location of the Tph2 gene on the sunflower genetic map will be useful for
marker-assisted selection and further characterization of tocopherol
biosynthesis in sunflower seeds.
Tang et al. (2006) have shown that the Ty3/gypsy-like retrotransposon
knockout of a 2-methyl-6-phytyl-1,4-benzoquinone methyltransferase
produces novel tocopherol (vitamin E) profiles in sunflower. The m ( Tph1 )
mutation partially disrupted the synthesis of -tocopherol in sunflower

seeds and disrupted a methyltransferase activity necessary for the synthesis
of -tocopherol. It corresponded to a nonlethal knockout mutation of

MT-1 caused by the insertion of a 5.2-kb Ty3/gypsy-like retrotransposon in
exon 1. MT-1 and m cosegregated and mapped to LG 1. MT-1 was not
transcribed in mutant homozygotes ( m/m ). They isolated two 2-methyl-6-
phytyl-1,4-benzoquinone/2-methyl-6-solanyl-1,4-benzoquinone
methyltransferase (MPBQ/MSBQ-MT) paralogs from sunflower (MT-1 and
MT-2) and uncovered a cryptic codominant mutation ( d ).The m locus was
epistatic to the d locus, that is, the d locus had no effect in m [+] m [+] and m [+]
m individuals, but significantly increased

- and

-tocopherol percentages in m/m
individuals. MT-2 and d cosegregated, MT-2 alleles isolated from mutant
homozygotes ( d d ) carried a 30-bp insertion at the start of the 5'-UTR, and
MT-2 was more strongly transcribed in seeds and leaves of wild type ( d [+]/
d [+]) than mutant ( d/d ) homozygotes (transcripts were 2.2- to 5.0-fold more
abundant in the former than the latter). The double mutant ( m/m//d/d ) was
nonlethal and produced 24-45%

- and 55-74% -tocopherol.

8.4 Phytosterols

The sterol biosynthesis pathway produces a large set of phytosterols
(sitosterol, campesterol), which are structurally similar to cholesterol and
act in the intestine to lower cholesterol absorption. Campesterol also serves
as a precursor to the brassinosteroid class of phytohormones. In plants,
phytosterols have a structural role in membrane fluidity ( Table 8-1 ). All
their functions are not clear; however, in Arabidopsis , mutants have helped
reveal a role for sterols in plant embryogenesis (Schrick et al. 2002).
Crude sunflower seed oil contains sterols up to 300 mg/100 g of oil, in
which -sitosterol is predominant (60%) and is followed by stigmasterol

and campesterol (10% each). A natural variation is observed according to
seed lots, but it is still not possible to explain if the variation is due to the
genetic background or to environmental factors (Philips et al. 2005).