ARS | Publication request: A previously unknown oxalyl-CoA synthetase is important for oxalate catabolism in Arabidopsis

Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2012
Publication Date: March 1, 2012
Citation: Foster, J., Kim, H.U., Nakata, P.A., Browse, J. 2012. A previously unknown oxalyl-CoA synthetase is important for oxalate catabolism in Arabidopsis. The Plant Cell. 24(3):1217-1229.

Interpretive Summary: Plant scientists have been avidly working to discover new strategies to increase crop resistance to oxalate-secreting fungal pathogens. These fungal pathogens produce oxalate as a plant toxin that is required by the fungus for plant infection. Such fungi are responsible for major crop losses each year. In this study we report the discovery of oxalyl-CoA synthetase which is an enzyme that catalyzes the first step in a previously uncharacterized pathway of oxalate degradation and provide evidence that overexpression of this enzyme in plants increases the plant's resistance to these fungal pathogens. Biochemical analysis showed that oxalyl-CoA synthetase was specific for oxalate degradation and that plants lacking a functional copy of this enzyme were unable to degrade oxalate. Biological analysis revealed that the plants lacking a functional copy of the enzyme were more susceptible to these oxalate-secreting fungal pathogens while plants engineered to over-express this enzyme showed an increase capacity to degrade oxalate and were more resistant to these phytopathogens. Thus, the identification and isolation of this enzyme is an important advancement in our understanding of oxalate metabolism. It also provides us with a new strategy that may help protect crop plants from oxalate-secreting phytopathogens which would have a beneficial impact on crop yields.

Technical Abstract: Oxalate is produced by several catabolic pathways in plants. The best characterized pathway for subsequent oxalate degradation is via oxalate oxidase, but some species, such as Arabidopsis thaliana, have no oxalate oxidase activity. Previously, an alternative pathway was proposed in which oxalyl-CoA synthetase (EC 6.2.1.8) catalyzes the first step, but no gene encoding this function has been found. Here we identify ACYL-ACTIVATING ENZYME3 (AAE3; At3g48990) from Arabidopsis as a gene encoding oxalyl-CoA synthetase. Recombinant AAE3 protein has high activity against oxalate, with Km=149.0 +/- 12.7 uM and Vmax=11.4 +/- 1.0 umoles/min/mg protein, but no detectable activity against other organic acids tested. Allelic aae3 mutants lacked oxalyl-CoA synthetase activity and were unable to degrade oxalate into CO2. Seeds of mutants accumulated oxalate to levels three-fold higher than wild-type, resulting in the formation of oxalate crystals. Crystal formation was associated with seed-coat defects and substantially reduced germination of mutant seeds. Leaves of mutants were damaged by exogenous oxalate and more susceptible than wild-type to infection by the fungus, Sclerotinia sclerotiorum, that produces oxalate as a phytotoxin to aid infection. Our results demonstrate that, in Arabidopsis, oxalyl-coA synthetase encoded by AAE3 is required for oxalate degradation, for normal seed development, and for defense against an oxalate-producing fungal pathogen.