Acceleration of Aux/IAA proteolysis is specific for auxin and independent of AXR1

Summary Aux/IAA proteins are short‐lived transcriptional regulators involved in auxin signaling. Using Aux/IAA luciferase (LUC) fusion proteins expressed in Arabidopsis thaliana , we previously showed that rapid degradation of these proteins requires conserved Aux/IAA domain II and that exogenous auxin accelerates their degradation. To further examine auxin‐mediated increases in proteolysis, the degradation of two other LUC fusion proteins, a non‐cleavable ubiquitin LUC fusion (UB1–72::LUC) and SAUR15::LUC was determined in vivo in seedlings. Their half‐lives were 20 ± 4 and 104 ± 10 min, respectively. SAUR15::LUC half‐life was not affected by pre‐incubation with 2,4‐D. Auxin did not have an equivalent effect on UB(1–72)::LUC steady‐state levels as compared to PsIAA6:LUC. LUC fused to an Aux/IAA domain II degraded more rapidly following auxin application, demonstrating that this region is sufficient for auxin‐mediated acceleration of proteolysis. Hormonal cross‐talk at the level of Aux/IAA proteolysis was examined. 1‐aminocyclopropane‐1‐carboxylic acid (ACC), benzyladenine (BA), abscisic acid (ABA), and brassinolide (BL) did not affect the degradation rate of IAA1::LUC, and gibberellic acid (GA 3 ) and salicylic acid (SA) did not specifically affect the steady‐state levels of Aux/IAA::LUC proteins. An Aux/IAA::LUC transgene was crossed into the auxin resistant‐1 ( axr1–12 ) background. In axr1–12 , the half‐life of PsIAA6(1–73)::LUC increased 4.5‐fold, but proteolysis still accelerated in response to exogenous auxin. These data suggest that auxin is the only phytohormone that accelerates Aux/IAA proteolysis, and that this acceleration is specific for Aux/IAA proteins. In addition, AXR1 plays an important role in rapid basal proteolysis of Aux/IAA proteins, but is not required for auxin‐mediated acceleration of their degradation.