C.A. Howitt, CSIRO Plant Industry, Canberra, ACT, Australia; K.R. Gale, Environment Protection Branch, Policy Coordination & Environment Protection Division, Department of the Environment & Heritage, GPO Box 787, Canberra, ACT 2601, Australia; A. Juhász, Agricultural Research Institute of the Hungarian Academy of Sciences, Martonvásár, Brunszvik 2, H-2462, Hungary
Gliadin and Glutenin: The Unique Balance of Wheat Quality
Pages 333-361
DOI: https://doi.org/10.1094/9781891127519.015
ISBN: 978-1-891127-51-9
Abstract
Wheat is the one of the most important food crops grown world wide, with over 600 millions tonnes grown each year. Much of this is consumed by humans in the form of bread, noodles, and pasta. Cultivars of bread wheat (Triticum aestivum L.) and durum wheat (T. turgidum var. durum) currently used are the result of extensive selection by breeders to meet both quality and agronomic requirements for the wide range of products for which they are utilized and the diverse environments under which they are grown. As the world's population grows, the challenge for breeders is to increase yield while at the same time improving grain quality for end-product manufacture.
Until recently, improvement of traits through breeding relied solely on physical measurement of the trait(s) of interest and selection of the best performed lines, which could then be used as breeding stock. With simple traits such as plant height and grain number, this is relatively simple and quick to do. However with quality traits, which require assessment of an end product, only a limited numbers of lines can be analyzed in a comprehensive manner, even when using small-scale dough-testing equipment. These techniques are often time consuming and impose a major constraint on the breeders’ resources. In an effort to overcome this limitation, ways to rapidly assess large numbers of lines for markers linked to the trait of interest have been developed. For a small number of quality traits, diagnostic antibody markers are available, and these have been converted into enzyme-linked immunosorbent assays (ELISA) which allow simple high-throughput screening. Currently, antibody-based tests are available for the presence of wheat/rye chromosome translocations, previously used to introduce disease-resistance alleles into wheat (Andrews et al 1996; Skerritt et al 1996). Tests are also available for the detection of lines which are null for homoeoalleles of granule-bound starch synthase 1 (GBSS1), which causes increased flour-swelling volumes that are preferable for Udon-noodle wheats (Graybosch et al 1998; Gale et al 2001) and for the prediction of grain hardness (Partridge et al 2002). A test for the prediction of starch quality by measurement of amylase levels expressed in the grain has also been developed (Verity et al 1999; Skerritt and Heywood 2000).
With the advent of molecular biology it has become possible to develop DNA-based molecular markers for traits of interest. These markers, which are an indirect indicator of the trait, can be scored relatively simply in a high-throughput manner and can be multiplexed, allowing multiple traits to be scored in a single assay, thus improving the efficiency of selection strategies. These techniques are based around the detection of sequence variations between varieties or accessions of wheat. Two types of markers are available, the first are diagnostic (also known as “perfect”), where the marker is directly associated with the gene that influences the trait, thus having absolute linkage with the trait being selected. They also do not require independent validation for each parental line used in a breeding program. See Gale (2005) for a recent comprehensive review on diagnostic markers associated with wheat quality.
The second type is linked markers, and they are generally associated with quantitative trait loci (QTLs) identified, in a mapping population, for the trait of interest. They are not directly associated with the gene underpinning the trait, and thus they only have a probability of being co-inherited with the trait of interest. Where the sequence variation is located in a region of the genome closely linked to a trait of interest, the polymorphism can be used to predict the presence or absence of the trait (Langridge et al 2001). As linked markers are identifed within a specific population, it is required that they be validated in any other population used in a breeding program. The strength of the prediction by the linked marker will depend upon two factors: the closeness of the genetic linkage between the sequence variation and the target locus, and secondly the precise, well-defined measurement of the trait of interest. In the case of most of the traits related to wheat-flour quality, this second requirement has become the critical issue in attempts to utilize the potential advantages that molecular markers could offer in wheat breeding. Therefore, in parallel with the intensive research effort in developing markers, there is a concentrated effort in cereal science to provide better measurements for different aspects of quality attributes, such as dough strength, extensibility or water absorption (Gras et al 2001; Bekes et al 2002).
Since the first demonstration of the application of DNA markers in wheat, to analyze polymorphisms in γ-gliadins of durum (D'Ovidio et al 1990), there has been extensive research to develop markers for many quality traits. As a result, DNA markers are now the markers of choice for the majority of marker-assisted breeding applications (Gupta et al 1999; Eagles et al 2001). As the results from DNA markers, for specific loci, are independent of environmental effects and can be performed at an early stage of selection in breeding, they offer significant advantages over the direct assessment of phenotype. In this chapter, we will give an overview of the different aspects of QTL analysis of quality traits for wheat through a brief introduction to molecular genetics, cereal chemistry and statistical methods developed and applied recently in this area. The major focus will be on diagnostic markers for specific genes that influence the expression of quality traits and QTL for which linked markers can be developed. It is not, however, intended to be a comprehensive review of all QTL identified to date that influence quality traits in wheat.
