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Published on June 1, 2007; 10.1104/pp.107.099705

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Received March 16, 2007
Accepted May 19, 2007

Chemical Genetic Identification of Glutamine Phosphoribosylpyrophosphate Amidotransferase as the Target for a Novel Bleaching Herbicide in Arabidopsis

Terence A. Walsh *, Teresa Bauer , Roben Neal , Ann Owens Merlo , Paul R. Schmitzer , Glenn R. Hicks , Mary Honma , Wendy Matsumura , Karen Wolff , and John P. Davies

Dow AgroSciences, Discovery Research, 9330 Zionsville Road, Indianapolis, IN 46268; Exelixis, 170 Harbor Way, South San Francisco, CA 94083-0511; Exelixis Plant Sciences, 16160 SW Upper Boones Ferry Road, Portland, OR 97224

* Corresponding author; email: tawalsh{at}dow.com.

A novel phenyltriazole acetic acid compound (DAS734) produced bleaching of new growth on a variety of dicotyledonous weeds and was a potent inhibitor of Arabidopsis (Arabidopsis thaliana) seedling growth. The phytotoxic effects of DAS734 on Arabidopsis were completely alleviated by addition of adenine to the growth media. A screen of ethylmethanesulfonate-mutagenized Arabidopsis seedlings recovered seven lines with resistance levels to DAS734 ranging from five to 125-fold. Genetic tests determined that all the resistance mutations were dominant and allelic. One mutation was mapped to an interval on chromosome 4 containing At4g34740, which encodes an isoform of glutamine phosphoribosylamidotransferase (AtGPRAT2), the first enzyme of the purine biosynthetic pathway. Sequencing of At4g34740 from the resistant lines showed that all seven contained mutations producing changes in the encoded polypeptide sequence. Two lines with the highest level of resistance (125-fold) contained the mutation Arg264Lys. The wild-type and mutant AtGPRAT2 enzymes were cloned and functionally overexpressed in Escherichia coli. Assays of the recombinant enzyme showed that DAS734 was a potent slow-binding inhibitor of the wild-type enzyme (I50 ~0.2 µM) whereas the mutant enzyme R264K was not significantly inhibited by 200 µM DAS734. Another GPRAT isoform in Arabidopsis, AtGPRAT3, was also inhibited by DAS734. This combination of chemical, genetic and biochemical evidence indicates that the phytotoxicity of DAS734 arises from direct inhibition of GPRAT and establishes its utility as a new and specific chemical genetic probe of plant purine biosynthesis. The effects of this novel GPRAT inhibitor are compared to the phenotypes of known AtGPRAT genetic mutants.




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