Mapping and Tagging of Simply Inherited Traits - Genetics, Genomics and Breeding of Crop Plants - page 128

of the salicylic pathway (Zélicourt et al. 2007). In the study of Zélicourt et al.
(2007), a recombinant peptide corresponding to the Ha-DEF1 defensin
domain was produced in an E. coli expression system. Ha-DEF1 induced
necrotic symptoms on Orobanche seedlings that should be correlated with
the necrosis occurring in Orobanche tubercles attached to the root of a resistant
sunflower. Further studies have to show whether there is a link between one
of the broomrape resistant genes and HaDef1 .
4.3 Herbicide Resistance
Sulfonylurea (SU) and imidazolinone (IMI) herbicides have been widely
used to control sunflowers in corn, soybean, and other crop rotations and
have been selected for herbicide resistance in wild sunflowers (Al-Khatib et
al. 1998, 1999; White et al. 2002, 2003). SU and IMI herbicides are specific
inhibitors of acetohydroxyacid synthase (AHAS, EC 2.2.1.6). Species differ
in herbicide susceptibility and can develop resistance to different classes of
AHAS inhibitors. With few exceptions, resistances to AHAS-inhibiting
herbicides, in otherwise susceptible species, are caused by point mutations
in genes encoding AHAS that reduce the sensitivity of the enzyme to herbicide
inhibition. Genes for resistance to AHAS-inhibiting herbicides in sunflower
have been introgressed from resistant wild populations into elite inbred
lines to develop and deploy herbicide resistant cultivars and hybrids (for
references see Kolkmann et al. 2004). By searching the sunflower EST database
( http://compgenomics.ucdavis.edu , Kozik et al. 2002 ) 11 sunflower AHAS ESTs
were found. When the DNA sequences of various amplicons were aligned,
three paralogous AHAS genes were discovered and named AHAS1 , AHAS2 ,
and AHAS3 . SNP markers were developed for AHAS1 and AHAS3 and SSCP
markers were developed for a six base-pair INDEL in AHAS2 and a G/A
SNP in AHAS3 . Additionally, an SSR marker was developed for AHAS1
based on the poly-Thr repeat in the putative transit peptide of AHAS1 . AHAS1
and AHAS3 were genotyped and genetically mapped in population RHA280
x RHA801 and AHAS2 was mapped in NMS373 x ANN1811. The three loci
mapped to LG 2 ( AHAS3 ), LG 6 ( AHAS2 ), and LG 9 ( AHAS1 ) (Kolkmann et
al. 2004) of the public sunflower map. The loci were flanked by mapped SSR
or INDEL markers (Tang et al. 2002; Yu et al. 2003). In the study of Kolkman
et al. (2004), DNA polymorphisms were not found between herbicide-
susceptible and -resistant inbred lines in the AHAS2 and AHAS3 coding
sequences but two mutations in the sunflower AHAS1 gene were identified,
an Ala205Val mutation and a Pro197Leu mutation, conferring resistance to
AHAS-inhibiting herbicides. Pro197 and Ala205 are conserved amino acids
in AHAS enzymes in numerous species (Tranel and Wright 2002). The
mutation of Pro197 is one of the most common mutations found in plants
resistant to AHAS-inhibiting herbicides but mutation of Ala205 in inhibitor-