Improving drought tolerance in maize: a view from industry

Significant yield losses in maize (Zea mays L.) from drought are expected to increase with global climate change as temperatures rise and rainfall distribution changes in key traditional production areas. The success of conventional crop improvement over the past 50 years for drought tolerance forms a baseline against which new genetic methods must be compared. Selection based on performance in multi-environment trials (MET) has increased grain yield under drought through increased yield potential and kernel set, rapid silk exertion, and reduced barrenness, though at a lower rate than under optimal conditions. Knowledge of the physiology of drought tolerance has been used to dissect the trait into a series of key processes. This has been complemented by genetic dissection through the identification of QTL associated with these same traits. Both have been used to identify suitable organ- and temporal-specific promoters and structural genes. Phenotyping capacity has not kept pace with the exponential increase in genotypic knowledge, and large-scale managed stress environments (MSE) are now considered essential to further progress. These environments provide ideal settings for conducting massively parallel transcript profiling studies, and for validating candidate regions and genes. Genetic and crop physiological models of key processes are now being used to confirm the value of traits for target environments, and to suggest efficient breeding strategies. Studies of gene to phenotype relationships suggest that most putative drought tolerance QTL identified thus far are likely to have limited utility for applied breeding because of their dependency on genetic background or their sensitivity to the environment, coupled with a general lack of understanding of the biophysical bases of these context dependencies. Furthermore, the sample of weather conditions encountered during progeny selection within the multi environment testing of conventional breeding programs can profoundly affect allele frequency in breeding populations and the stress tolerance of elite commercial products. We conclude that while gains in kernels per plant can be made by exploiting native genetic variation among elite breeding lines, improvements in functional stay-green or in root distribution and function may require additional genetic variation from outside the species. Genomic tools and the use of model plants are considered indispensable tools in this search for new ways of optimizing maize yield under stress. (C) 2004 Elsevier B.V. All rights reserved