Kolumbina Mrva

A QTL located on chromosome 4A associated with dormancy in white- and red-grained wheats of diverse origin

Improved resistance to preharvest sprouting in modern bread wheat (Triticum aestivum. L.) can be achieved via the introgression of grain dormancy and would reduce both the incidence and severity of damage due to unfavourable weather at harvest. The dormancy phenotype is strongly influenced by environmental factors making selection difficult and time consuming and this trait an obvious candidate for marker assisted selection. A highly significant Quantitative Trait Locus (QTL) associated with grain dormancy and located on chromosome 4A was identified in three bread wheat genotypes, two white- and one red-grained, of diverse origin. Flanking SSR markers on either side of the putative dormancy gene were identified and validated in an additional population involving one of the dormant genotypes. Genotypes containing the 4A QTL varied in dormancy phenotype from dormant to intermediate dormant. Based on a comparison between dormant red- and white-grained genotypes, together with a white-grained mutant derived from the red-grained genotype, it is concluded that the 4A QTL is a critical component of dormancy; associated with at least an intermediate dormancy on its own and a dormant phenotype when combined with the R gene in the red-grained genotype and as yet unidentified gene(s) in the white-grained genotypes. These additional genes appeared to be different in AUS1408 and SW95-50213.

Mapping quantitative trait loci associated with variation in grain dormancy in Australian wheat

Preharvest sprouting is a problem in many regions of the world, resulting in downgrading of quality, substantial economic losses to wheat growers, and difficulties for grain handling and marketing agencies. Improvements in tolerance from the introduction of better grain dormancy at, or near, harvest-ripeness would be expected to have a significant impact on the incidence and severity of sprouting. Intermediate levels of dormancy in older Australian wheats, such as Halberd, and a small number of current cultivars could be used in the short term while more extreme dormancy is being introgressed into locally adapted germplasm. A doubled haploid population derived from Cranbrook (extremely non-dormant, very susceptible to sprouting) x Halberd (intermediate dormancy, moderately tolerant to preharvest sprouting) was grown in replicated experiments and ripe grain harvested for assessment of dormancy, measured as a germination index. Consistent differences were observed between the parents in both experiments. For the bulk of the progeny, the germination index fell within a range defined by Cranbrook at the upper and Halberd at the lower end. Significant quantitative trait loci, all contributed by the very susceptible parent, that explained 11%, 9%, and 9% of the phenotypic variation were identified on chromosome arms 2AL, 2DL, and 4AL, respectively. These QTLs offer the opportunity to develop molecular markers for grain dormancy and to develop a better understanding of the mechanisms involved in this trait.

Wheat grain preharvest sprouting and late maturity alpha-amylase.

Preharvest sprouting (PHS) and late maturity α-amylase (LMA) are the two major causes of unacceptably high levels of α-amylase in ripe wheat grain. High α-amylase activity in harvested grain results in substantially lower prices for wheat growers and at least in the case of PHS, is associated with adverse effects on the quality of a range of end-products and loss of viability during storage. The high levels of α-amylase are reflected in low falling number, the internationally accepted measure for grain receival and trade. Given the significant losses that can occur, elimination of these defects remains a major focus for wheat breeding programs in many parts of the world. In addition, the genetic, biochemical and molecular mechanisms involved in the control of PHS and LMA as well as the interactions with environmental factors have attracted a sustained research interest. PHS and LMA are independent, genetically controlled traits that are strongly influenced by the environment, where the effects of particular environmental factors vary substantially depending on the stage of grain development and ripening. This review is a summary and an assessment of results of recent research on these important grain quality defects.

Dormancy in white-grained wheat: Progress towards identification of genes and molecular markers

Preharvest sprouting limits the consistent production of high quality wheat in many regions of the world. Improvements in tolerance from the introduction of better grain dormancy at, or near, harvest-ripeness would be expected to have a significant impact on the incidence and severity of sprouting. Genetic and molecular investigations have provided new evidence for the presence of dormancy genes on chromosome 3D of white-grained genotypes AUS1408 and N72.72 (T. sphaerococcumderivative), and identified a potential molecular marker for one of the dormancy genes in AUS1408. Analysis of a doubled haploid population derived from Cranbrook (extremely non-dormant, very susceptible to sprouting) × Halberd (intermediate dormancy, moderately tolerant to preharvest sprouting) identified three significant quantitative trait loci on chromosome arms 2AL, 2DL, and 4AL, all contributed by the very susceptible parent.

Expression of late maturity α-amylase in wheat containing gibberellic acid insensitivity genes

SummaryTwo wheat cultivars, Spica and Lerma 52, which consistently produce high levels of α-amylase during the later stages of grain development (late maturity α-amylase), were crossed with a set of four near-isogenic lines carrying the tall (rht) allele or one of the dwarfing genes Rht1, Rht2 or Rht3 (GA-insensitive alleles). The F1 and F2 populations were developed and analysed for grain α-amylase and plant height. The Rht3 gene exhibited the strongest influence on plant height and strongly inhibited new α-amylase synthesis during the later part of grain ripening. By comparison, Rht1 and Rht2 had a less pronounced effect but still significantly reduced the expression of late maturity α-amylase. These observations suggest that gibberellic acid is involved either directly or indirectly in this phenomenon. The implications of the effect of dwarfing genes on expression of late maturity α-amylase are discussed in relation to cultivar improvement and to the identification and control of high α-amylase germplasm.

Genetic, Hormonal, and Physiological Analysis of Late Maturity α-Amylase in Wheat

Transitory expression of high pI amylases during grain development occurs as a result of an altered hormonal environment. Late maturity α-amylase (LMA) is a genetic defect that is commonly found in bread wheat (Triticum aestivum) cultivars and can result in commercially unacceptably high levels of α-amylase in harvest-ripe grain in the absence of rain or preharvest sprouting. This defect represents a serious problem for wheat farmers, and apart from the circumstantial evidence that gibberellins are somehow involved in the expression of LMA, the mechanisms or genes underlying LMA are unknown. In this work, we use a doubled haploid population segregating for constitutive LMA to physiologically analyze the appearance of LMA during grain development and to profile the transcriptomic and hormonal changes associated with this phenomenon. Our results show that LMA is a consequence of a very narrow and transitory peak of expression of genes encoding high-isoelectric point α-amylase during grain development and that the LMA phenotype seems to be a partial or incomplete gibberellin response emerging from a strongly altered hormonal environment.

Regulation of high pI α-amylase synthesis in wheat aleurone by a gene(s) located on chromosome 6B

Synthesis of high and low pI α-amylase in germinating wheat grains and GA3-treated de-embryonated grains of Chinese Spring (CS) and its ditelosomic derivative (CS dit 6BS) were compared and related to high pI α-amylase production in grains affected by late maturity α-amylase (LMA). In de-embryonated grains of CS dit 6BS (lacking the long arm of chromosome 6B) treated with GA3, synthesis of high pI α-amylase isozymes controlled by Amy-1 genes on chromosomes 6A and 6D was dramatically reduced compared to Chinese Spring. The results suggest the presence of a gene(s) on the long arm of chromosome 6B, which is (are) required for GA-induced α-amylase synthesis in the aleurone. Similarly, in wheat grains affected by LMA, high pI α- Amy genes on the group 6 chromosomes are activated apparently by a single gene, tentatively located on chromosome 6B. Both genes may be part of the GA-induced amylase synthesis pathway in the aleurone. By contrast, synthesis of the high pI α-amylase isozymes in the early stages of germination of CS dit 6BS grains was very similar to CS. This contrast between GA3-treated de-embryonated grains and germinating grains could be explained by control of α-amylase synthesis in scutellum in the early stages of germination being different to that in GA3-treated aleurone.

Genetic variation for quality traits in synthetic wheat germplasm

Synthetic hexaploid wheats offer breeders ready access to potentially novel genetic variation associated with the D genome of Aegilops tauschii. In order to assess the application of this germplasm to wheat quality improvement, collections of primary and derived synthetic hexaploid wheat lines were surveyed for traits that determine colour and colour stability in Asian noodles and the frequency of a genetic defect know as late maturity α-amylase (LMA). The range of variation was then compared with bread wheat and durum wheat cultivar collections. Primary synthetics contained substantial genetic variation for quality traits associated with colour and colour stability of Asian noodles including near-zero extremes for polyphenol oxidase and lipoxygenase. These extremes represent a significant advantage compared with current bread wheat cultivars and are similar to the best durum wheats. While alternative strategies for reducing polyphenol oxidase and lipoxygenase are available, the synthetics nevertheless provide a useful resource for wheat breeders attempting to develop improved wheat cultivars for the Asian noodle market. Unfortunately, however, most primary synthetics were prone to late maturity α-amylase and mature grain contained unacceptably high levels of α-amylase. Elimination of this genetic defect, or selection within breeding populations for low or non-LMA, is both time consuming and labour intensive and presents a significant obstacle to exploitation of variation for other traits. As proof of concept, near-zero polyphenol oxidase (PPO) lines, free from LMA, were recovered from backcross populations involving a high LMA primary synthetic.

Screening methods and identification of QTLs associated with late maturity α-amylase in wheat

The synthesis of high levels of germination-type (high pI isozymes) α-amylase was induced in wheat genotypes prone to late maturityα-amylase (LMA) following the exposure of detached tillers to cool temperature during grain development. The detached tiller method was successfully applied to a range of genotypes and to a doubled haploid (DH) population derived from the cross Cranbrook (LMA genotype) × Halberd (low amylase). The number of grains in ripe, treated tillers that contained high pI α-amylase isozymes was measured using an ELISA antibody kit highly specific for high pI isozymes. Quantitative trait loci (QTL) controlling the expression of LMA in wheat were detected in DH population Cranbrook × Halberd. The DH population and parents were sown in 2 replicated sowings at the same location with sowing times differing by 2 weeks. QTL analyses were conducted separately for each sowing, but results from both sowings were consistent and indicated a highly significant (it p > 0.01) QTL on the long arm of chromosome7B, with Cranbrook contributing the higher value allele. A second QTL with less significant effect was found on the long arm of chromosome 3B, on the basis of data from the first sowing.

Inheritance of late maturity α-amylase in wheat

SummaryTwo wheat cultivars that consistently show high levels of grain α-amylase at harvest ripeness, in the absence of preharvest sprouting, were crossed with a control, low α-amylase cultivar, and F1, F2 and BC1 populations were developed. Grain of these populations was analysed for α-amylase activity at harvest ripeness. Distribution and segregation patterns were consistent with control at a single locus with high α-amylase the recessive allele. This mode of inheritance would make it extremely difficult to differentiate homozygous low α-amylase lines from heterozygotes (low α-amylase phenotype but carriers of high α-amylase) and has important implications for wheat breeders. High α-amylase, termed late maturity α-amylase, was not linked with the awned inhibitor gene, B2, located on the long arm of chromsome 6B.