Auxin modulates the degradation rate of Aux/IAA proteins.
Aux/IAA gene family members were first identified by their rapid transcriptional increase in response to auxin. Auxin/indole-3-acetic acid protein (Aux/IAA) luciferase (LUC) fusions expressed in Arabidopsis under control of a non-auxin-responsive promoter were used to monitor the effect of auxin on protein abundance independent of transcriptional regulation by auxin. After 2 hr in the presence of 1 microM exogenous dichlorophenoxyacetic acid (2,4D), a synthetic auxin, the levels of pea IAA6 (PSIAA6) and Arabidopsis IAA1 LUC activity were 35% and 67%, respectively, of mock-treated genetically identical seedlings, whereas the activity of LUC alone from equivalently treated seedlings remained unaltered. The steady-state level of an Aux/IAA fusion protein lacking domain II, one of the conserved domains found in all Aux/IAA proteins, was not reduced in the presence of auxin. Higher levels of exogenous auxin were required to affect the steady-state level of the PSIAA6LUC fusion with a point mutation in domain II. A 13-aa consensus sequence from domain II fused to LUC created an auxin-responsive fusion protein. The change in steady-state levels in response to auxin is extremely rapid, with a decrease in LUC activity detectable by 2 min after auxin application. Direct half-life measurements show that the decrease caused by exogenous auxin is due to the decrease in fusion protein half-life. These results suggest that auxin rapidly modulates the degradation rate of Aux/IAA proteins, with higher levels of auxin increasing the proteolytic rate of Aux/IAA family members.
Zenser
N
N
Section of Molecular and Cellular Biology, Graduate Group in Plant Biology, University of California, 1 Shields Avenue, Davis, CA 95616, USA.
Ellsmore
A
A
Leasure
C
C
Callis
J
J
D017360:Arabidopsis / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D003513:Cycloheximide / Q000494:pharmacology; D007210:Indoleacetic Acids / Q000378:metabolism* / Q000494:pharmacology*; D007700:Kinetics; D008156:Luciferases / Q000378:metabolism*; D010937:Plant Growth Regulators; D010940:Plant Proteins / Q000378:metabolism; D030821:Plants, Genetically Modified; D011110:Polymorphism, Genetic; D012333:RNA, Messenger / Q000187:drug effects / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*
D007210:Indoleacetic Acids; D010937:Plant Growth Regulators; D010940:Plant Proteins; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; C046131:auxin receptor, plant; C030737:indoleacetic acid; D003513:Cycloheximide; D008156:Luciferases