Are crop deep roots always beneficial for combating drought: A review of root structure and function, regulation and phenotyping

Improving crop drought resistance is crucial for mitigating the impacts of drought stress on crop production worldwide. For a long time, deep roots have been considered one of the most effective ways to facilitate full utilization of subsoil water when topsoil water is not available under drought conditions. However, a deeper root system is not always associated with a higher drought resistance. Therefore, gaining a comprehensive understanding of deep roots would be beneficial for improving drought resistance. In this review, we first systematically outline the deep root functions influencing crop drought resistance. We propose that a deeper root system with low metabolic costs can benefit drought resistance, particularly when subsoil water exists in water-limited environments. Then, we overview the methods, including crop breeding and field managements, to increase the drought resistance by regulating deep roots. Third, we review the methods for phenotyping deep roots directly and indirectly. High-throughput deep root phenotyping methods in the field are highly needed, and electromagnetic technologies are promising to satisfy this need in the future. Looking forward, we provide some perspectives on studying deep root systems. •Deep root functions influencing crop drought resistance were outlined.•The methods to increase the drought resistance by regulating deep roots were reviewed.•The methods for phenotyping deep roots directly and indirectly were presented.•Perspectives on studying deep root systems were provided.