Improvement of sprouting control in wheat; roles of embryo cavity waxes and grain colour genes
About this project
Pre-harvest sprouting presents the greatest single obstacle to increased incorporation of home-grown wheat into high value, such as bread flour. It is generally agreed that the solution to the problems of sprouting must be solved by breeding greater control of dormancy into new varieties but establishment of the dormancy status of new varieties is difficult and may take several years. Improvement thus comes slowly and inclusion of sprout resistance characters from one generation to the next cannot be guaranteed. In the absence of adequately reliable tests for resistance to sprouting the identification of markers which distinguish dormant from non-dormant types was an important objective of this project. Markers of two types have been examined: an established marker, grain colour, has been scrutinised in order to make it more readily and reliably usable by breeders, and the significance of a novel embryo cavity wax, indicated to be associated with sprout resistance in an earlier study, has been critically evaluated.
In relation to genes controlling grain colour, the aims of this project were to measure the effects of individual R genes and to identify molecular markers for the R genes and to identify the R genes present in U.K. breeding stock.
The dormancy-enhancing effects of genes for red grain colour were demonstrated in nearisogenic lines carrying five different R genes in a white spring wheat background. These lines provided compelling evidence that the R genes are directly responsible for the greater sprout-resistance of red-grained (as opposed to white) varieties and are not simply acting as genetic markers for other dormancy factors. Additional observations suggested that this effect depends on the presence of a continuous layer of red/brown pigment in the seed coat. Different R genes each have a similar effect on grain dormancy, delaying germination by a factor which diminishes with time after harvest ripeness.
Genetic markers for each of the R genes are described for the first time, with genetic maps showing the gene order and recombination frequencies between diagnostic DNA sequences and the R2 gene on the long arm of chromosome 3A, R3 on 3B, and Rl on 3D. These genetic maps support growing evidence that the chromosomes of different cereal crops share a common ancestral structure, with similar genes in similar arrangements.
The number, and in most cases the identities of R genes carried by 109 wheat varieties and breeding lines were analyzed with respect to their frequencies in the U.K. gene pool. Although U.K. varieties were virtually all red-grained, they showed no evidence of effective selection against any of the three r (white-grained) alleles, probably because these are masked by the presence of dominant R alleles. This survey included the first description of varieties carrying R3 in the absence of other genes for grain colour, allowing genetic mapping of R3 without confounding effects from duplicate R genes on other chromosomes.
Sprout-resistance scores were shown to vary between different U.K. varieties in confirmation of the acknowledged resistance of red- as opposed to white-grained types, however variation within the red-grained group is independent of R dosage and is not associated with any particular R gene more than the others. The presence and practical importance of grain dormancy genes independent of the red pigment effect is inferred, and evidence for simple additive gene effects on grain dormancy in crosses between Rl R2 R3 varieties is presented.
It is concluded that adequate sprout-resistance depends upon a combination of traits. Red grain colour is one of the necessary factors, but very little is known about the other components. In order to understand the mechanisms of resistance, and to discriminate conveniently between resistant and susceptible wheats, marker traits associated with different aspects of resistance to pre-harvest deterioration in Hagberg Falling Number were investigated. Potentially useful marker traits including embryo cavity waxes, the rate of water penetration into different grain tissues during imbibition, and the production of germinative-type alpha-amylase enzymes during ripening were evaluated.
Variation for alpha-amylase content in ripe, ungerminated grains is described among recombinant lines from a low-amylase x high-amylase cross, in which the high-amylase character appears to be controlled by two or more recessive genes. Evidence is described supporting the involvement of a gene on the long arm of chromosome 3D, either the rl gene or a different, linked gene.
In relation to embryo cavity waxes, the objective of the project was to investigate their composition and their relation to grain colour/dormancy differences, and to investigate mechanisms underlying variation for resistance to sprouting. Success in relation to this objective has been severely limited by technical difficulties associated with handling extremely small quantities of very complex materials. While broad generalisations concerning the composition of embryo cavity waxes can now be made, it has not been possible to distinguish components or relationships among components that are indicative of a particular dormancy condition. The same problems of scale and complexity also proved too great for conclusions to be reached about the physical properties of the waxes. At an early stage in the investigation, the surface activity of embryo cavity extracts promised to provide the basis of a rapid test by which sprout resistance might be recognised. However, excessive variation obscured any systematic differences that might be present.
When the pursuit of waxes as markers was abandoned, attention was focused on another possible marker for alpha-amylase activity. Unlike colour and the hypothesised role of embryo cavity waxes, grain size has more significance in relation to the enzymes that are produced in endosperm in the late stages of grain maturation, than to those produced during germination, although there is some evidence for a connection here also. Evidence from several sources suggests that, in varieties with large grains, higher levels of activity than those found in small grained varieties are likely. The observation that this was the case came originally from coordination of data recorded elsewhere but it has been substantiated in this project. Also in this project, the unusual (and unlikely) enzyme source has been identified as the aleurone tissue in the crease region of the grain. The relationships between grain size and both late maturity and germination alpha-amylases are considered worthy of further investigation.
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