Auxin sensitivities of all Arabidopsis Aux/IAAs for degradation in the presence of every TIR1/AFB.

Auxin plays a key role in regulation of almost all processes of plant growth and development. Different physiological processes are regulated by different ranges of auxin concentrations; however, the underlying mechanisms creating these differences are largely unknown. The first step of auxin signaling is auxin-dependent interaction of an auxin receptor with transcriptional co-repressors (Aux/IAA), which leads to Aux/IAA degradation. Arabidopsis has six homologous auxin receptors (TIR1 and five AFBs), 29 Aux/IAA proteins and two types of active auxins, IAA and phenylacetic acid (PAA). Therefore, a large number of possible combinations between these three factors may contribute to the creation of complex auxin responses. Using a yeast heterologous reconstitution system, we investigated auxin-dependent degradation of all Arabidopsis Aux/IAAs in combination with every TIR or AFB receptor component. We found that TIR1 and AFB2 were effective in mediating Aux/IAA degradation. We confirmed that the Aux/IAA domain II, which binds TIR1, is essential for degradation. IAA and other natural auxins, 4-chloroindole-3-acetic acid (4-Cl-IAA) and PAA, induced Aux/IAA degradation; and IAA and 4-Cl-IAA had higher activity than PAA. Effective auxin concentrations for Aux/IAA degradation depended on both Aux/IAAs and TIR1 or AFB2 receptors, which is consistent with the Aux/IAA-TIR1/AFB co-receptor concept.