The plant hormone auxin is vital for all aspects of plant growth and development. Auxin perception is mediated by the TIR1 and the AFB1 through AFB5 family of auxin receptors. The TIR1/AFB1-3 proteins have many overlapping functions throughout plant development as shown by higher order mutant analysis. The conservation of two of the more distantly related members of the family, AFB4 and AFB5 suggests that they maintain a distinct function. The work described in this dissertation provides insight into the specialized roles of these two receptors during Arabidopsis thaliana seedling development. Mutant analysis reveals that the AFB4 clade is the major target of the picolinate herbicides in Arabidopsis. AFB4 and AFB5 exhibit a unique affinity for this compound that is not shared among the other family members. Furthermore, AFB4 appears to be a negative regulator of auxin signaling in the seedling as depicted by extensive phenotypic analysis of the afb4-2 mutant and its affect on auxin response. The discovery of the long hypocotyl phenotype in afb4-2 led to a series of hypocotyl transcriptome experiments in afb4-2 and afb5-5 to uncover the downstream targets of AFB4 and the genes involved in auxin regulation of hypocotyl elongation. Results from this analysis reveal a mechanism for auxin regulated hypocotyl growth independent of the PIF4/5 pathway as well as a role for AFB4 in the circadian regulation of hypocotyl growt
