First published online January 16, 2004; 10.1105/tpc.019372
The Plant Cell 16:353-366 (2004)
© 2004 American Society of Plant Biologists
Divergent Roles in Arabidopsis thaliana Development and Defense of Two Homologous Genes, ABERRANT GROWTH AND DEATH2 and AGD2-LIKE DEFENSE RESPONSE PROTEIN1, Encoding Novel Aminotransferases
Jong Tae Song,
Hua Lu and
Jean T. Greenberg1
Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois 60637
1 To whom correspondence should be addressed. E-mail jgreenbe{at}midway.uchicago.edu; fax 773-702-9270.
The disease-resistant Arabidopsis thaliana aberrant growth and death2 (agd2-1) mutant has elevated levels of the defense signal salicylic acid (SA), altered leaf morphology, and mild dwarfism. AGD2 and its close homolog ALD1 (for AGD2-LIKE DEFENSE RESPONSE PROTEIN1) encode aminotransferases that act on an overlapping set of amino acids in vitro. However, kinetic parameters indicate that AGD2 and ALD1 may drive the aminotransferase reaction in opposite directions. ALD1-deficient mutants have the opposite phenotypes from agd2-1, showing reduced SA production and increased disease susceptibility. Furthermore, ALD1 transcript levels are elevated in agd2-1 and are induced in the wild type by bacterial pathogen infection. ALD1 is responsible for some of the elevated SA content and a majority of the disease resistance and dwarfism of agd2-1. A complete knockout of AGD2 renders embryos inviable. We suggest that AGD2 synthesizes an important amino acid–derived molecule that promotes development and suppresses defenses, whereas ALD1 generates a related amino acid–derived molecule important for activating defense signaling.
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