Genetic analysis of the Arabidopsis TIR1/AFB auxin receptors reveals both overlapping and specialized functions

The TIR1/AFB auxin co-receptors mediate diverse responses to the plant hormone auxin. The Arabidopsis genome encodes six TIR1/AFB proteins representing three of the four clades that were established prior to angiosperm radiation. To determine the role of these proteins in plant development we performed an extensive genetic analysis involving the generation and characterization of all possible multiply mutant lines. We find that loss of all six TIR1/AFB proteins results in defects in embryogenesis as early as the 8-cell stage, and possibly earlier. Mutant embryos progress but exhibit frequent cell division errors followed by proliferation of the suspensor, and eventually seed abortion. Despite this dramatic phenotype, a single wild-type allele ofTIR1orAFB2is sufficient to support growth throughout plant development. Further, gametophytic expression of theTIR1/AFBgenes is not essential for development of the male or female gametophyte. Our analysis reveals extensive functional overlap between even the most distantly relatedTIR1/AFBgenes except forAFB1. Surprisingly, the AFB1 protein has a specialized function in rapid auxin-dependent inhibition of root growth and early phase of root gravitropism. This activity may be related to a difference in subcellular localization compared to the other members of the family.