Polyphenol oxidase (PPO) catalyses undesirable darkening in wheat products
such as Asian noodles. Genetic variation for PPO activity is characterized in bread
wheat. Australian wheat breeding programmes recognize that reduced PPO activity
is an important quality target. Despite this interest from breeders, no varieties
possessing extremely low and null PPO activity exist. The development of null PPO
wheat varieties is dependant on an understanding of the genetic control of the null
phenotype. Knowledge of these factors will accelerate efforts to develop them.
The inheritance of PPO activity was investigated in two populations that
were derived from hybrids between a null PPO genotype and Australian wheat
varieties Lang and QAlBis. Observed genetic ratios were consistent with two and
three gene control, respectively in these populations.
QTL mapping was performed in the QALBis x VAW08-A17 population. The
Diversity Array Technology (DArT) approach was employed to genotype the
QALBis x VAW08-A17 population. Three highly significant QTLs that control PPO
activity were identified on chromosomes 2AL, 2BS and 2DL. Close associations
between PPO activity and DArT marker loci wPt-7024, wPt-0094 and wPt-2544
were observed, respectively. Collectively, these loci explained 74% of the observed
variation in PPO activity across seasons. Significant QTLs on chromosomes 1B and
3B were also identified that together explained an additional 17% of variation in
The relationship between PPO activity and yellow alkaline noodles (YAN)
colour stability parameters was investigated in a DM5637*B8 x H45 doubled haploid population. PPO activity and changes in YAN brightness (ΔL* 0-24h) and
yellowness (Δb* 0-24h) in both seasons were analysed.
Quantitative trait analyses of PPO activity, flour yellowness (b*) and YAN
colour stability was also conducted in this population. QTL mapping of variation in
PPO activity in the DM5637*B8 x H45 DH population identified a highly significant
QTL on chromosome 2AL, which explained 52% of the observed variation across
seasons. Regression analysis identified that wPt-7024 was highly significantly
associated with PPO activity in this population. A highly significant association
between this marker and PPO was also identified in the QALBis x VAW08-A17
population. Collectively, the three identified QTLs (on chromosomes 2AL, 7A and
7B) explained 71% of variation in PPO activity across seasons.
A highly significant (P<0.001) QTL on chromosome 2B along with
significant (P<0.01) QTLs on the chromosomes 1A, 3B, 4B and 5B were found to
control flour yellowness. The QTLs on 2B, 4B and 5B were detected in both seasons
analysed and accounted for 90% of variation in flour b* across seasons.
The study on YAN colour stability located two highly significant (P<0.001)
QTLs and two significant (P<0.01) QTLs that controlled the change in brightness of
yellow alkaline noodle. The 2A QTL accounted for 64% of observed variation across
seasons. It was in the same location as the PPO QTL and shared a common closest
marker wPt-7024. Only one significant QTL for YAN a* (0-24h) was identified. It
accounted for 12% of variation across seasons and was only detected in one season.
One highly significant (P<0.001) QTL and two significant (P<0.01) QTLs were
identified that controlled the change in yellowness of yellow alkaline noodle. The 2A
QTL accounted for 68% of observed variation across seasons. The location of this
QTL corresponded with that of 2A QTLs for PPO activity and L* of YAN in this study. Furthermore, wPt-7024 was also identified as the marker with the most
significant association with L*. The identification of a correlation between the
characters and a common location of a highly significant QTL for each of these
characters indicates that it is likely that PPO activity is directly responsible for a
large proportion of the changes in brightness and yellowness of YAN.
QTLs for L* and b* of YAN were detected in a common location on
chromosome 1A. However, no corresponding QTL was identified that controls PPO
activity, highlighting the complexity of the relationship between these traits.
Resistance to three rust pathogens (Puccinia graminis, Puccinia striiformis,
and Puccinia triticina) was also investigated in the DM5637*B8 x H45 DH
population because they are major yield limiting diseases in wheat. Disease response
data at the seedling stage were converted to genotypic scores for rust genes
Sr24/Lr24, Sr36, Lr13 and Yr7 to construct a genetic linkage map. No recombination
was observed between rust resistance genes Sr36, Lr13 and Yr7 in this DH
population. Therefore, these genes mapped in the same position on chromosome 2B.
The Lr24/Sr24 locus was incorporated into the chromosome 3D map. Interval
mapping analysis identified QTLs on chromosomes 2B, 3B, 4B and 5B that control
adult plant resistance (APR) to stripe rust. Two QTLs on chromosomes 2B and 3D
were identified that controlled APR to leaf rust in this DH population.