Introduction of new genetic diversity amphiploids and alien introgression lines

The introduction of new genetic diversity into the gene pools of crop species, which have undergone a narrowing of their genetic base during domestication, is essential for crop improvement. Crop wild relatives have provided plant breeders with potentially useful genetic resources for tolerance to abiotic stress for over a century (Prescott-Allen and Prescott-Allen, 1986). As plant breeders have demanded more diversity in germplasm the progenitors of crops and closely related species have been increasingly utilized. Trends since the mid-1980s show an increased use of wild species, with over 100 traits being transferred to crop species during the last 20 years (Hajjar and Hodgkin, 2007). Additionally, while pest and disease resistance is the predominant target of wild introgressions, due to being controlled by fewer genes and easier screening within breeding programmes, the incorporation of abiotic stress tolerance is increasing (Hajjar and Hodgkin, 2007). However, only a handful of examples of wild relatives contributing genetic resistance to abiotic stresses in crops have reached the stage of cultivar release, even though many wild relatives with potential have been described (Hajjar and Hodgkin, 2007; Colmer and Flowers, 2008; Flowers and Colmer, 2008).

Despite the low number of released culti-vars for salt, waterlogging and inundation tolerance, there exists a large resource of potential germplasm for increasing the genetic base of crop plants. For example, salt tolerance during late vegetative stages has been reported among wild species of tomato (Lycopersicon pennelli and Lycopersicon peru-vianum) (Tal and Shannon, 1983). Colmer et al. (2006) also list 38 species as possible sources of salt tolerance in the Triticeae, with examples from the Triticum, Aegilops, Elytrigia, Elymus, Thinopyrum, Leymus and Hordeum species. Further to this, when Munns et al. (2000) screened 54 Triticum turgidum tetraploids comprising the subspecies durum, turgidum, polonicum, turanicum and carthlicum, they identified large and useful genetic variation for improving the salt tolerance of durum wheat. From this study, Line 149, derived from a cross between a Triticum monococcum (accession C68-101) and a durum cultivar, 'Marrocos' (The, 1973), was selected with a very low Na+ uptake. Genetic studies of the low Na+ phenotype led to the mapping of two quantitative trait loci (QTLs), designated Nax1 and Nax2. Molecular markers closely linked to the loci are being used to select low Na+ progeny in a durum and bread wheat breeding programme (Lindsay et al, 2004; Byrt et al, 2007). Another notable example of the successful introduction of new genetic diversity is the use of the highly salt tolerant landrace 'Kharchia'. Salt tolerance from 'Kharchia 65' was hybridized with a high-yielding wheat variety ('WL 71 I') to develop India's first systematically bred salt-tolerant wheat cultivar ('KRL I-4') at the Central Soil Salinity Research Institute, Karnal (Singh and Chatrath, 2001). Likewise, the tolerance of rice to submergence is improved through the introgression of the Sub1 locus. The Sub1 locus is derived from the landrace 'FR13A', and accounts for 70% of the phenotypic variation in submergence tolerance (Xu and Mackill, 1996). Through marker assisted selection (MAS), the Sub1 locus has been introgressed into mega-varieties and is currently undergoing advanced stages of field evaluation. The new varieties promise to be more widely adopted by farmers due to improved yield and quality characteristics.

The development of amphiploids and alien introgression lines is one approach that has been used to generate additional genetic variation in crop species, but great care must be taken in the choice of parents in the development of such hybrids. Reading the literature we sense that the development and testing of amphiploids so far has largely been opportunistic rather than strategic. However, the work of Professor Tim Colmer and his colleagues stands as an exception in this area. This group identified Hordeum marinum as a source of genes for salt and waterlogging tolerance that could be transferred into bread wheat (Colmer et al., 2005). Systematic assessments were made of a range of accessions of H. marinum for tolerance to salinity (Garthwaite et al., 2005), waterlogging (Garthwaite et al., 2003, 2008) and the interaction between these two stresses (Malik et al., 2009), and some of these lines have now been incorporated into amphiploids with wheat (Islam et al, 2007).

However, perhaps even this work could develop further. We suggest that it may not be enough simply to create amphiploids using the natural variation within species. Perhaps the creation of better adapted amphiploids should be preceded by the breeding of better wild grass partners as a preliminary step. The case for this can be argued using the data of Malik et al. (2009). These workers assessed the impacts of the imposition of 200 mM NaCl with or without hypoxia on the growth and ion relations of eight accessions of H. marinum subsp. gussoneanum and nine accessions of H. mari-num subsp. marinum.

In all accessions, increasing the salinity in the nutrient solution from 0 to 200 mM NaCl increased the concentration of Na+ in the youngest fully expanded leaf and decreased relative growth rate (RGR) of the shoot, but there was no correlation between these two characters (Fig. 6.5a). However, some of the individual accessions were of interest. Salinity caused only slight decreases in the RGR of three accessions (H522, H823 and H826; decreases of 14-15% of controls), and two of these (H522 and H826) had quite large differences in the change in Na+ concentration in the youngest fully expanded leaf associated with salinity. Clearly, all three accessions have traits of interest with respect to salinity tolerance, but H826 also has a considerable ability to tolerate Na+ in the leaves. The imposition of hypoxia in addition to salinity (Fig. 6.5b) caused an additional decrease in shoot RGR (compared to aerated non-saline controls) and this was generally associated with further change in the concentration of Na+ due to hypoxia. With this combination of stresses accession H87 was of interest; with the imposition of hypoxia the Na+ concentration in the youngest leaf of this plant actually decreased and this plant had only a slight (7%) further decrease in RGR. Data published by the authors showed that H87 had a very tight barrier to ROL that was induced by the combination of salinity and hypoxia (Malik et al, 2009).

What this analysis shows is that none of the accessions appears to have the combined traits for tolerance to salinity and hypoxia that we might require in the ideal parent for an amphiploid; accession H87 had poor prospects under saline aerated conditions and accessions H522 and H826 (particularly) had poor prospects with the imposition of hypoxia under saline conditions (Fig. 6.5b). Clearly, a crossing programme that co-locates genes associated with strong salt tolerance (accessions H522, H823 and H826) with genes associated with strong tolerance to hypoxia (H87) might yield a better parent for incorporation into an amphiploid.

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Fig. 6.5. Effects of salinity and hypoxia on the relative growth rate (RGR) and Na+ relations in the youngest fully expanded leaf of accessions of Hordeum marinum (data from Malik et al., 2009). (a) Relationship between decrease in RGR and change in Na+ due to the imposition of salinity under aerated conditions. (b) Relationship between the further decrease in RGR and the further change in Na+ due to the imposition of hypoxia to plants under saline conditions. Open symbols = subsp. marinum; closed symbols = subsp. gussoneanum. H-numbers are accessions referred to in the text. DM, dry mass.

Alternatively, H823 would appear to be a reasonable compromise as a candidate wild grass parent.

There are significant limitations to the successful introgression of favourable genes from wild species, including plant responses to complex interactions between saline, waterlogged and inundated environments, difficulties with interspecific crossability and the retention of undesirable agronomic traits. The importance of crop adaptation to combined salinity and waterlogging stress (Barrett-Lennard, 2003) emphasizes the need for crop adaptation to both stresses, as culti-vars bred for only one abiotic constraint may have limited success in farmers' fields. Since the late 1980s there have been major advances in hybridization methodologies, molecular technologies and breeding strategies. This has reduced the limitations associated with interspecific crossability, and has enabled more efficient alien introgression with reduced undesirable 'linkage drag'. Technological advances, combined with a greater understanding of the complex physiological mechanisms underlying salt, waterlogging and inundation tolerance, have enabled an increased incorporation of distantly related taxa and will continue to provide vital genetic diversity for improvements in crop yields in hostile environments.

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