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JXB Advance Access published online on July 7, 2009
Journal of Experimental Botany, doi:10.1093/jxb/erp221
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© 2009 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)

RESEARCH PAPER

RACK1 is a negative regulator of ABA responses in Arabidopsis

Jianjun Guo1, Junbi Wang1,2, Li Xi1, Wei-Dong Huang2, Jiansheng Liang3 and Jin-Gui Chen1,*

1Department of Botany, University of British Columbia, 6270 University Boulevard, Vancouver, BC, V6T 1Z4 Canada
2College of Food Science and Nutritional Engineering, China Agricultural University, Beijing 100083, China
3College of Bioscience and Biotechnology, Yangzhou University, Yangzhou 225009, China

* To whom correspondence should be addressed: E-mail: jingui{at}interchange.ubc.ca

Receptor for Activated C Kinase 1 (RACK1) is viewed as a versatile scaffold protein in mammals. The protein sequence of RACK1 is highly conserved in eukaryotes. However, the function of RACK1 in plants remains poorly understood. Accumulating evidence suggested that RACK1 may be involved in hormone responses, but the precise role of RACK1 in any hormone signalling pathway remains elusive. Molecular and genetic evidence that Arabidopsis RACK1 is a negative regulator of ABA responses is provided here. It is shown that three RACK1 genes act redundantly to regulate ABA responses in seed germination, cotyledon greening and root growth, because rack1a single and double mutants are hypersensitive to ABA in each of these processes. On the other hand, plants overexpressing RACK1A displayed ABA insensitivity. Consistent with their proposed roles in seed germination and early seedling development, all three RACK1 genes were expressed in imbibed, germinating and germinated seeds. It was found that the ABA-responsive marker genes, RD29B and RAB18, were up-regulated in rack1a mutants. Furthermore, the expression of all three RACK1 genes themselves was down-regulated by ABA. Consistent with the view that RACK1 negatively regulates ABA responses, rack1a mutants lose water significantly more slowly from the rosettes and are hypersensitive to high concentrations of NaCl during seed germination. In addition, the expression of some putative RACK1-interacting, ABA-, or abiotic stress-regulated genes was mis-regulated in rack1a rack1b double mutants in response to ABA. Taken together, these findings provide compelling evidence that RACK1 is a critical, negative regulator of ABA responses.

Key words: ABA, drought, early seedling development, RACK1, salt, seed germination

Received 24 April 2009; Revised 11 June 2009 Accepted 22 June 2009


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