Exploration of genetic resources to boost physiological and molecular breeding

While conventional plant breeding has already achieved significant progress in stress breeding as outlined, three main approaches can be employed to widen gene pools: (i) introgression of traits from genetic resources with compatible genomes, such as landraces; (ii) wide crosses involving interspecific or intergeneric hybridization; and (iii) genetic transformation. To date genetic resources have been used mainly to introduce resistance to biotic stresses (Dwivedi et al., 2008), while relatively few wild crop relatives have been exploited for adaptation to abiotic stress (Hajjar and Hodgkin, 2007). In fact, the majority of accessions in germ-plasm collections remain uncharacterized in terms of their potential to improve yield under abiotic stress; current challenges are to identify elite sources of traits among genetic resources, estimate potential yield gains associated with trait expression in good agronomic backgrounds, and define potentially complementary traits that if introgressed into a common genetic background are likely to result in cumulative gene action for yield (Reynolds et al., 2009). Hexaploid wheat has been a useful model for alien introgressions and impacts include increased yield in a range of environments including drought (Trethowan and Mujeeb-Kazi, 2008). Another avenue currently being explored is the use of Leymus racemosus to introgress genes for root exudation of nitrification inhibitors (Subbarao et al., 2007); the potential impact on reducing potent greenhouse gas emissions is enormous and could significantly mitigate global warming if successfully adopted on a global scale. A vast reserve of genetic potential in closely related crop species has yet to be evaluated, and as understanding of the physiological and genetic basis of stress adaptation improves, it will become easier to apply molecular marker technology to mine genetic resource collections for potentially useful alleles.

Transgenic technology effectively removes taxonomic barriers altogether but although much data has been collected under controlled environments for candidate genes that improve survival of both model and crop species under abiotic stress (Umezawa et al., 2006), more candidate genes need to be tested in a range of relevant field environments (Nelson et al., 2007) if impacts are to be achieved. Candidate genes, such as those associated with functional proteins and especially upstream regulation, could affect any of the drivers of yield under stress (see Fig. 5.1 for examples) depending on at what stage of development and in which tissue they are expressed. One well-studied candidate gene is DREB1A; stress-regulated expression of this gene with the rd29A promoter produced plants with increased tolerance to freezing, salt and drought stresses, without producing changes in the normal phenotype of the transformed plants (Chandler and Robertson, 1994). The gene has been associated with improved root growth under water stress in well-controlled phenotyping studies in groundnut (Vadez et al., 2007). Another approach that may be useful in tackling climate change is to replace genes that are especially heat susceptible in temperate crops with their analogues from tropical species like rice or maize; a good example would be soluble starch synthase which is a rate limiting step to grain filling in wheat at high temperature (Hurkman et al., 2003).

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