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Plant and Cell Physiology Advance Access published online on April 16, 2008
Plant and Cell Physiology, doi:10.1093/pcp/pcn060
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Published by Oxford University Press on behalf of the Japanese Society of Plant Physiologists (JSPP) 2008

Cytokinin inhibits the proteasome-mediated degradation of carbonylated proteins in Arabidopsis leaves

Vanita Jain1,4, Werner Kaiser2 and Steven C. Huber1,3,*

1Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 USA
2Julius-von-Sachs-Institut für Biowissenschaften, Lehrstuhl für Molekulare Pflanzenphysiologie und Biophysik, Julius-von-Sachs-Platz 2, 97082 Würzburg, Germany
3United States Department of Agriculture-Agricultural Research Service, Photosynthesis Research Unit, and Department of Crop Sciences, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA

*Corresponding Author: Prof. Steven C. Huber, United States Department of Agriculture-Agricultural Research Service, Photosynthesis Research Unit, University of Illinois, Departments of Plant Biology and Crop Sciences, 1201 W. Gregory Drive, 197 ERML, Urbana, IL 61801 USA. E-mail: schuber1{at}uiuc.edu, FAX: 217 244 4419, Tel: 217 265 0909


  Abstract

Under normal conditions, plants contain numerous carbonylated proteins, which are thought to be indicative of oxidative stress damage. Conditions that promote formation of reactive oxygen species (ROS) enhance protein carbonylation, and protein degradation is required to reverse the damage. However, it is not clear how the degradation of carbonylated proteins is controlled in planta. In this report, we show that detached Arabidopsis leaves rapidly and selectively degrade carbonylated proteins when kept in the dark. The loss of carbonylated proteins corresponded to a loss of soluble protein and accumulation of free amino acids. Degradation of carbonylated proteins and the loss of soluble protein was blocked by MG132 but not 3-methyladenine suggesting that the 26S proteasome pathway rather than the autophagic pathway was involved. Consistent with this, rpn10 and rpn12 mutants, which are defective in proteasome function, had increased (rather than decreased) levels of carbonylated proteins when detached in the dark. Feeding metabolites (amino acids and sucrose) to detached leaves of wild type Arabidopsis in the dark had little or no effect on the loss of carbonylated proteins whereas providing soybean xylem sap via the transpiration stream effectively prevented degradation. The effect of xylem sap was mimicked by feeding 10 µM kinetin. We postulate that disruption of cytokinin flux to detached leaves triggers the selective degradation of carbonylated proteins via the proteasome pathway. The results may have implications for the control of protein mobilization in response to changes in N-availability.

Keywords: Arabidopsis - Autophagy - Cytokinin - Protein carbonylation - Protein degradation - Proteasomes

4Permanent address: Division of Plant Physiology, Indian Agricultural Research Institute, New Delhi, India 1100012.

(Received January 29, 2008; Accepted April 4, 2008)
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