Pyramiding of mutated genes contributing to crop quality & resistance to stress affecting quality


Background:

Crop quality improvement is gaining unprecedented importance in both developed and developing countries. Products with improved quality give the farmer added value and a competitive market advantage, which in turn will result in improved human welfare and increased farm income. Thus, the improvement of quality characters in crop plants has great potential to alleviate problems caused by poverty and malnutrition through both direct (food quality and quantity) and indirect effects (income stability, etc) that affect farmer's social and economic status. There are many aspects of 'crop quality', which can be defined differently according to, for example, crop species, geographical region, and intended use of crop or crop product. Some attributes that define crop quality include: nutritional value (amino acid composition, protein content, micronutrients, vitamins, secondary metabolites, nutraceuticals, etc.), consumer preference (flavour, texture, colour, grain size/shape), pre- and post-harvest and industrial/technological characteristics (fibre traits, sucrose content, storage quality, sprouting, oil content, starches, processing, bread-making).
Biotic and abiotic stresses have a negative impact on product quality. Indeed, in developing countries with low input agricultural systems or reduced access to plant protection compounds, such stresses can have devastating effects on crop quality. Resistances to such 'quality-affecting stresses' should be essential components of breeding programmes aimed at crop quality improvement. Therefore, quality traits are usually complex, controlled by the action of several genes, and are also subject to environmental influences. The complex genetics of quality traits has led to their being difficult to improve through conventional plant breeding. Furthermore the complexity in assessing some quality characters aggravates the difficulties in improving crop quality. Breeding activities that rely on use of genotypic rather than phenotypic selection have the potential to overcome these limitations. For example, traits controlled by recessive alleles, environmentally-sensitive characters, traits that are expensive or difficult to score, or expressed late in development will benefit greatly from application of genotypic selection.

The advent of molecular marker techniques now makes it possible to 'tag' alleles conferring desirable quality traits. For many crop plants large numbers of molecular markers of different types (RFLP, SSR, AFLP etc) are now available. These vary widely in cost of development and deployment and the choice of markers to be used will depend largely on the breeder's objectives and the available facilities. Markers based on use of PCR will predominantly be used owing to their many advantages over other methods (little DNA required per sample, speed, cost, ease of use, etc) and co-dominant markers, such as SSRs, offer considerable advantages in determining allelic composition in heterozygous material. Development of markers tightly linked to quality traits or resistance to quality-affecting stresses should enable breeders to select on a genotypic basis.
Marker assisted selection (MAS) offers great promise for both the selection of individual quality traits and the 'pyramiding' of a number of important quality characters simultaneously into the same improved genotype. Breeding programmes could benefit from the implementation of MAS for gene pyramiding in terms of time saving and reduction of cost as compared to conventional breeding. MAS should be considered as complementary to conventional breeding, in that it can be used to replace certain phenotypic assessments, rather than as an alternative to selection based on phenotype. Despite their great potential, DNA-based markers have not yet been widely implemented in breeding for quality, particularly in developing countries, due to economic constrains and cost effectiveness.
Improvement of quality traits depends on the availability of sufficient variability for the targeted traits. In the past the level of variability in some quality traits has been increased using mutagenesis. More than 2300 registered mutant varieties have been developed using induced mutations. However, the full potential of induced mutations has not been realized in plant breeding for quality traits because of the difficulty in screening large mutagenized populations for infrequent mutations generating desirable crop quality alleles. The exploitation of mutated genes has been restricted to those that have easily identifiable phenotypes (e.g. waxy mutation in rice, barley and other crops). Recent development of screening procedures for quality traits in mutagenised populations will allow the more efficient identification of novel and useful variants. Such screening procedures and the development of markers linked to the induced mutant alleles will facilitate their incorporation into breeding programmes.

Overall objectives:

The objectives of this CRP will focus on the development and transfer between participants of methodologies and technologies for the identification and tagging of mutated genes contributing to important crop quality characters, and their pyramiding to develop improved breeding material using molecular marker assisted selection.

Specific objectives:

  1. Development and establishment of efficient methodologies for the induction of mutants and screening of crop germplasm with diverse and desirable quality characters, including nutritional characters, and tolerance to stresses affecting quality in crop plants.
  2. Development of molecular markers for tagging genes for quality characters and enhanced tolerance to stresses in induced mutants or other germplasm.
  3. Use of molecular markers to pyramid loci determining desired quality characters and enhanced tolerance to stresses with the aim of developing improved crop varieties.
  4. Genetic dissection of quality and stress resistance characters using molecular markers to establish the number and the linkage relationships of genes responsible for such traits.
  5. Transfer of knowledge and technologies between participants to accelerate breeding programs for the improvement of quality, nutritional, and stress tolerance traits in crops.
  6. Develop improved germplasm with enhanced quality traits.

Project Officer:

Q.Y. Shu