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Zenser N, Dreher KA, Edwards SR, Callis J: Acceleration of Aux/IAA proteolysis is specific for auxin and independent of AXR1. Plant J. 2003 Aug;35(3):285-94.
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 (GA3) 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. |
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