Recognizing the hidden half in wheat: root system attributes associated with drought tolerance.

Improving drought tolerance in wheat is crucial for maintaining productivity and food security. Roots are responsible for the water uptake from soil, and a number of root traits are associated with drought tolerance. Studies have revealed many QTLs and genes controlling root development in plants. However, the genetic dissection of root traits in response to drought stress in wheat is still unclear. Here, we review crop root traits associated with drought stress, key genes governing root development in plants, and QTLs and genes regulating root system architecture (RSA) under water limited conditions in wheat. Deep roots, optimal root length density and xylem diameter, and increased root surface area are root traits contributing to the drought tolerance. Given the diverse environments where wheat is grown, the balance among root and shoot traits, as well as individual and population performance are discussed. The known functions of key genes provide information for the genetic dissection of root development of wheat in multiple conditions, and will be beneficial for molecular marker development, marker-assisted selection and genetic improvement in drought tolerance breeding.