Plants modulate root growth in response to changes in the local environment,
guided by intrinsic developmental genetic programs. The hormone Abscisic Acid (ABA)
mediates responses to different environmental factors, such as the presence of nitrate in the
soil, water stress and salt, shaping the structure of the root system by regulating the
production of lateral roots as well as controlling root elongation by modulating cell division
and elongation. Curiously, ABA controls different aspects of root architecture in different
plant species, perhaps providing some insight into the great diversity of root architecture in
different plants, both from different taxa and from different environments. ABA is an ancient
signaling pathway, acquired well before the diversification of land plants. Nonetheless, how
this ancient signaling module is implemented or interacts within a larger signaling network
appears to vary in different species. This review will examine the role of ABA in the control
of root architecture, focusing on the regulation of lateral root formation in three plant species,
Arabidopsis thaliana, Medicago truncatula and Oryza sativa. We will consider how the
implementation of the ABA signaling module might be a target of natural selection, to help
contribute to the diversity of root architecture in nature.