ReviewDrought tolerance improvement in crop plants: An integrated view from breeding to genomics
Introduction
Drought is by far the most important environmental stress in agriculture and many efforts have been made to improve crop productivity under water-limiting conditions. While natural selection has favoured mechanisms for adaptation and survival, breeding activity has directed selection towards increasing the economic yield of cultivated species. More than 80 years of breeding activities have led to some yield increase in drought environments for many crop plants. Meanwhile, fundamental research has provided significant gains in the understanding of the physiological and molecular responses of plants to water deficits, but there is still a large gap between yields in optimal and stress conditions. Minimizing the ‘yield gap’ and increasing yield stability under different stress conditions are of strategic importance in guaranteeing food for the future.
The evolution of crops since their domestication has been driven by the selection of desired traits recognized at the phenotypic level. Nevertheless, direct selection for grain yield under water-stressed conditions has been hampered by low heritability, polygenic control, epistasis, significant genotype-by-environment (G × E) interaction and quantitative trait loci (QTLs)-by-environment (QTL × E) interaction (Piepho, 2000). The complexity of drought tolerance mechanisms explains the slow progress in yield improvement in drought-prone environments. In recent years, crop physiology and genomics have led to new insights in drought tolerance providing breeders with new knowledge and tools for plant improvement (Tuberosa and Salvi, 2006). This article aims to provide an overview of the breeding progress in drought tolerance, and highlight future perspectives in plant breeding that could result from the integration of the recent advances in physiology and genomics.
Section snippets
Breeding progress in favourable versus drought-prone environments
Increases in yield potential achieved by plant breeding during the last Century have been well documented for numerous crops. Frequently, genetic gain has been studied by comparing in the same field trial the yield of cultivars characterized by different years of release. For most crops a linear relation between yield and year of release was found, the slope of which gives an estimate of the genetic improvement. Comparison of the different cultivars then enabled identification of the main
Drought tolerance assessment
A crucial aspect in all studies dedicated to drought tolerance is the assessment of the degree of drought tolerance of different genotypes. In many studies the identification of tolerant and susceptible cultivars is based on few physiological measures related to drought response. The difficulty in identifying a physiological parameter as a reliable indicator of yield in dry conditions has suggested that yield performance over a range of environments should be used as the main indicator for
Physiological bases for yield under drought
The physiologically relevant integrators of drought effects are the water content and the water potential of plant tissues (Jones, 2007). They in turn depend on the relative fluxes of water through the plant within the soil-plant-atmosphere continuum. Thus, apart from the resistances and water storage capacities of the plant, it is the gradient of water vapour pressure from leaf to air, and the soil water content and potential that impose conditions of drought on the plant. Once a drop in water
Molecular markers to dissect drought tolerance-related traits
Molecular markers can be used to explore germplasm through segregation and association mapping to identify useful alleles in both cultivated varieties and wild relatives. Although association mapping is intrinsically more powerful than ‘classical’ genetic linkage mapping because it scrutinizes the results of thousands of generations of recombination and selection (Syvänen, 2005), most of the data available up to date on drought tolerance are based on segregation mapping and QTL analysis. Many
Genes and metabolites conferring drought tolerance
New chances to further improve yield and/or yield stability under limiting conditions come from the last 10 years’ progress in the identification of the genetic determinants of the physiological responses related to stress tolerance. Adaptation of plants to drought and to the consequent cellular dehydration induces an active plant molecular response. This response significantly improves the tolerance to negative constraints and it is to a great extent under transcriptional control. Many
Field phenotyping
A comprehensive and careful field evaluation of mapping populations and transgenic plants is urgently needed in order to provide reliable information on the effectiveness of QTLs, candidate genes and transgenes. Due to the multigenic nature of drought tolerance, the introduction of a single gene or QTL into an elite germplasm may result in a subtle phenotypic effect or yield increase. Capacity for precise phenotyping under reliable conditions probably represents the most limiting factor for the
Future directions
When phenotypic selection was the only tool available to improve yield under drought, the improvements in crop yield observed were likely due to an increase in yield potential through the unconscious pyramiding of yield-related traits or loci. Research in the last three decades has opened up three main approaches: (i) plant physiology provided new tools to understand the complex network of drought-related traits and several drought-related traits useful to improve selection efficiency have been
Acknowledgements
The authors wish to thank an anonymous referee who provided a number of stimulating comments useful for the improvement of this review. This work was supported by grants from the Italian Ministry of Agriculture (Progetto FRUMISIS), the Italian Ministry of Science (Progetto AGROGEN) and by EU INCOA3 Project MABDE No. ICA3-2002-10073.
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Present address: University of Modena and Reggio Emilia, Department of Agricultural Sciences, Via J.F. Kennedy 17, I-42100 Reggio Emilia, Italy.