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Published on July 27, 2007; 10.1104/pp.107.103374

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Received June 4, 2007
Accepted July 19, 2007

AtNUDT7, a Negative Regulator of Basal Immunity in Arabidopsis, Modulates Two Distinct Defense Response Pathways and Is Involved in Maintaining Redox Homeostasis

Xiaochun Ge , Guo-Jing Li , Sheng-Bing Wang , Huifen Zhu , Tong Zhu , Xun Wang , and Yiji Xia *

Donald Danforth Plant Science Center, 975 North Warson Road, St. Louis, MO 63132; State Key Laboratory of Genetic Engineering, Department of Biochemistry and Molecular Biology, Fudan University, Shanghai, 200433, China; Syngenta Biotechnology Inc., 3054 Cornwallis Road, Research Triangle Park, North Carolina 27709

* Corresponding author; email: yxia{at}danforthcenter.org.

Plants have evolved complicated regulatory systems to control immune responses. Both positive and negative signaling pathways interplay to coordinate development of a resistance response with the appropriate amplitude and duration. AtNUDT7, a Nudix-domain containing protein in Arabidopsis that hydrolyzes nucleotide derivatives, was found to be a negative regulator of the basal defense response and its loss-of-function mutation results in enhanced resistance to infection by Pseudomonas syringae. The nudt7 mutation does not cause a strong constitutive disease resistance phenotype but it leads to a heightened defense response, including accelerated activation of defense-related genes that can be triggered by pathogenic and non-pathogenic microorganisms. The nudt7 mutation enhances two distinct defense response pathways: one independent of and the other dependent on NPR1 and salicylic acid (SA) accumulation. In-vitro enzymatic assays revealed that ADP-ribose (ADPR) and NADH are preferred substrates of NUDT7, and the hydrolysis activity of NUDT7 is essential for its biological function and is sensitive to inhibition by Ca++. Further analyses indicate that ADPR is not likely the physiological substrate of NUDT7. However, the nudt7 mutation leads to perturbation of cellular redox homeostasis and a higher level of NADH in pathogen-challenged leaves. The study suggests that the alteration in cellular anti-oxidant status caused by the nudt7 mutation primes the cells for the amplified defense response and NUDT7 functions to modulate the defense response to prevent excessive stimulation.






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