Lateral root development in Arabidopsis: fifty shades of auxin

• We review the roles of the phytohormone auxin during lateral root formation in the model plant Arabidopsis thaliana.
• Different auxin ‘signalling modules’ act sequentially to control the various steps of lateral root formation from founder cell priming to initiation, patterning, and emergence.
• Auxin acts as a common integrator to many endogenous and environmental signals regulating lateral root formation.

The developmental plasticity of the root system represents a key adaptive trait enabling plants to cope with abiotic stresses such as drought and is therefore important in the current context of global changes. Root branching through lateral root formation is an important component of the adaptability of the root system to its environment. Our understanding of the mechanisms controlling lateral root development has progressed tremendously in recent years through research in the model plant Arabidopsis thaliana (Arabidopsis). These studies have revealed that the phytohormone auxin acts as a common integrator to many endogenous and environmental signals regulating lateral root formation. Here, we review what has been learnt about the myriad roles of auxin during lateral root formation in Arabidopsis.