Molecular breeding for drought and heat stress in maize: Revisiting the progress and achievements

Abiotic stresses are a potential threat to global food security commonly induced by moisture and temperature stresses, key reason for maize yield losses. Maize is the third most important global crop after rice and wheat; however, being rainfed crop often suffers from water stress (drought). The rising acreage under spring maize cultivation is highly sensitive to heat stress. The prediction models indicate toward further increases in yield losses due to drought and heat stress in the future. Hence, appropriate breeding strategies will be required in maize for developing drought or/and heat-tolerant cultivars. Some success has been achieved with conventional breeding but at slower rate due to the complex (polygenic) nature of stress tolerance. However, molecular breeding (use of molecular markers in conventional breeding) is an effective approach for attaining rapid genetic gains. Quantitative trait loci (QTL) mapping and association mapping have been extensively used for mapping the genomic regions for drought or/and heat stress tolerance-associated traits in maize. However, the availability of limited major QTLs/genes and the introgression studies for drought and heat stress tolerance in maize are discouraging. However, the initiation of attempts to use genomic selection (GS) or prediction for drought and heat stress tolerance in maize, as well as the integration of molecular breeding with phenomics, physiology, and genome editing, should be more rewarding in the future. This book chapter attempts to revisit the progress in identification of genomic regions through QTL mapping strategies, association mapping, and GS and achievements.