Genome-wide Approaches to Investigate and Improve Maize Response to Drought

Genome-wide approaches offer new, unprecedented opportunities to identify, clone, and manipulate the plethora of genes affecting drought tolerance in model species and crops. Compared to conventional breeding approaches, the dissection of the genetic basis of quantitative traits into their single components (i.e., quantitative trait loci [QTLs]) provides a more direct access to valuable genetic diversity of the morpho-physiological processes regulating the adaptive response to drought. This, in turn, enables us to utilize marker-assisted selection (MAS) for enhancing crops' performance. However, despite the impressive progress in molecular techniques and the large number of QTLs shown to influence yield in drought-stressed crops, the overall impact of MAS on the release of drought-tolerant cultivars has so far been marginal. It is foreseeable that QTL cloning will be facilitated by sequence information and the profiling of the transcriptome, proteome, and metabolome, all of which will improve the identification of plausible candidate genes. The cloning of major QTLs will offer additional opportunities for a more effective exploitation of the allelic richness present in germplasm collections. Allele mining in germplasm and mutant collections through forward- and reverse-genetics approaches, coupled with marker-assisted backcrossing and/or genetic engineering, will further expand the possibilities to introgress novel genetic variation in elite materials. New QTL-based modeling approaches, while improving our capacity to understand the genetic and molecular bases of genotype × environment interaction at varying water regimes, will contribute to singling out the most promising “molecular” ideotypes. This notwithstanding, a sizeable impact of MAS and other genomics approaches on the release of cultivars more resilient to drought will only be possible through (i) a deeper integration with conventional breeding methodologies, (ii) the capacity to accurately phenotype on a large scale, and (iii) a sound multidisciplinary knowledge of the biochemical and physiological processes determining crops' yield and its stability under a broad range of water regimes.