The roles of auxin response factors in patterning processes during Arabidopsis development

The plant hormone auxin controls a large number of developmental processes in plants such as embryo and vascular patterning and the initiation and positioning of lateral organs. Auxin regulates these processes, in part, through the action of the Auxin Response Factors (ARFs) family of transcription factors which consists of 23 members. Due to the large size of the ARF family, mutations in only a subset of ARFs are associated with visible defects indicating widespread functional redundancy.;Finally, experimental manipulation of auxin biosynthesis and inhibition of auxin transport was used to investigate the role of auxin in embryo patterning. Manipulation of auxin homeostasis, using the bacterial auxin biosynthesis gene IAAH and exogenous application of auxin, was monitored indirectly by the expression of the auxin-dependent reporter DR5. Enhanced auxin biosynthesis caused an increase in DR5 expression in cells that normally express the marker, and although no embryo patterning defects were observed, severe effects were observed in post-embryonic vascular patterning. However, chemical inhibition of polar auxin transport resulted in altered auxin distribution, severe embryo defects and altered vascular patterning. These results suggest that although embryos are highly flexible in accommodating changes in auxin levels, polar auxin transport activity is necessary for proper patterning of the embryo.;The relatedness of the ARF family members has been determined based on sequence comparisons. Five phylogenetically related ARFs (ARF5/MONOPTEROS, ARF6, ARF7/NONPHOTOTROPIC HYPOCOTYL4, ARF8 and ARF19) contain glutamine-rich middle domains and are proposed to act as transcriptional activators of auxin-responsive genes. To address the issue of redundancy between the five glutamine-rich ARFs, multiple arf mutant combinations were generated. Although single arf mutations interfere with distinct auxin-regulated processes, generation of multiple arf mutant combinations in ARF5, ARF6, ARF7, ARF8 and ARF19 showed that they have overlapping functions. Furthermore, to determine whether one ARF can substitute for another ARF irrespective of its relative expression profile, ARF5 was overexpressed in arf7; arf19 and arf6; arf8 double mutant backgrounds. Interestingly, ubiquitous expression of ARF5 is sufficient to complement all known functions of ARF7 and ARF19 but not those of ARF6 and ARF8, indicating the functional divergence between individual ARFs.