The regulation of plant growth responds to many stimuli.These responses allow environmental adaptation,thereby increasing fitness.In many cases,the relay of information about a plants environment is through plant hormones.These messengers integrate environmental information into developmental pathways to determine plant shape.This review will use,as an example,auxin in the root ofArabidopsis thaliana to illustrate the complex nature of hormonal signal processing and transduction.It will then make the case that the application of a systems-biology approach is necessary,if the relationship between a plants environment and its growth/developmental responses is to be properly understood.