UC San Diego

The plant hormone auxin is vital for all aspects of plant development and growth. Auxin mediates the interaction between the F-box proteins TIR1/AFB with the Aux/IAA transcription repressors and further induces the degradation of Aux/IAAs through the ubiquitin-proteasome pathway. To learn more about the interaction between these proteins, a mutant screen was performed using the LexA yeast-two hybrid system. Two classes of TIR1 mutations were identified that increase the interaction with Aux/ IAAs upon auxin treatment. The first type of TIR1 mutations, E12K and E15K, can stabilize the TIR1 protein level in vivo. The tir1 E12K and E15K transgenes exhibit severe defects throughout plant growth. The mutations, E12K and E15K, are located in the first helix (H1) of F- box domain of TIR1. The study shows that the H1 F-box domain plays a critical role in the negative regulation of the TIR1 protein level and is responsible for the difference between TIR1 and AFB1 accumulation in Arabidopsis. The second type of TIR1 mutations, D170E and M473L, increase the affinity between TIR1 and the degron motif of Aux/IAAs and further enhance the SCF function. This results in faster degradation of Aux/IAAs and increased transcription of auxin responsive genes in the plant. The pTIR1:tir1 D170E/M473L-Myc transgene exhibits diverse developmental defects during plant growth and displays a significantly auxin-hypersensitive phenotype