Gene Cloning and Characterization - Genetics, Genomics and Breeding of Crop Plants - page 200

family (Slabaugh et al. 2003). These RGC paralogs showed differences in
intron length between 90 to >800 nucleotides. Twenty-four HaRGC1 loci
were mapped to a 2-4 cM region of LG 8. Nine haplotypes (based on these
24 RGAs) were identified among elite inbred lines and were correlated to
known downy mildew resistance specificities (Slabaugh et al. 2003).
In a large-scale approach, Radwan et al. (2008) mined the sunflower EST
database and used comparative genomics to develop a deeper understanding
of the diversity and distribution of NBS-LRR homologs in the sunflower
genome. Collectively, 630 NBS-LRR homologs were identified, 88 by mining a
database of 284,241 sunflower ESTs and 542 by sequencing 1,248 genomic
DNA clones isolated from common and wild sunflower species. These genomic
sequences were obtained by using degenerate primers against four conserved
NBS amino acid sequences. DNA markers were developed from 196 unique
NBS-LRR sequences and facilitated the mapping of 167 NBS-LRR loci. The
latter were distributed throughout the sunflower genome in 44 clusters and
singletons (Radwan et al. 2008). The largest and most complex cluster of
NBS-LRR loci discovered so far in sunflower is located on linkage group 8 of
the public sunflower map. It spans a segment of 36 cM in three subclusters
containing 54 NBS-LRR loci. This region also harbors the Pl 1 and other downy
mildew R-genes. The second largest NBS-LRR cluster was identified on the
lower end of LG 13, with 27 loci in two subclusters and a few singletons
spanning a segment of 31 cM. This is the segment that harbors downy mildew
( Pl 5 and Pl 8 ) and black rust ( RADV ) R-genes (Lawson et al. 1998; Radwan et
al. 2003; Slabaugh et al. 2003; Yu et al. 2003). This cluster harbors TIR- and
non-TIR-NBS-LRR encoding genes. TIR sequences are identified by the
presence of the RNBS-A TIR motif (FLENIRExSKKHGLEHLQKKLLSKLL)
and aspartic acid (D) as the last amino acid residue in the Kin-2 motif, whereas
non-TIR sequences are identified by the presence of the RNBS-A non-TIR
motif (FDLxAWVCVSQxF) and tryptophan (W) as the last amino acid residue
in the Kin-2 motif (Meyers et al. 1999; Pan et al. 2000). Using multiple sets of
primers, NBS fragments were amplified from genomic DNA of three species of
the Asteraceae family (Plocik et al. 2004): Helianthus annuus (sunflower), Lactuca
sativa (lettuce), and Cichorium intybus (chicory). Analysis suggest that
Asteraceae species share distinct families of R-genes, composed of genes related
to both coiled-coiled (CC) and toll-interleukin-receptor (TIR) homology domain
containing NBS-LRR resistance genes.
6.3.5 Quality Traits in Sunflower
6.3.5.1 Cloning Genes of Fatty Acid Biosynthesis
Conventional sunflower seed oil contains about 11% of saturated fatty acids
(palmitic and stearic acid), a moderate level of monounsaturated fatty acids
(oleic acid), and a high concentration of polyunsaturated fatty acids (linoleic