An oxalyl-CoA dependent pathway of oxalate catabolism plays a role in regulating calcium oxalate crystal accumulation and defending against oxalate-secreting phytopathogens in Medicago truncatula

Authors: FOSTER, JUSTIN - Children'S Nutrition Research Center (CNRC); LUO, BIN - Children'S Nutrition Research Center (CNRC); Nakata, Paul

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2016
Publication Date: 2/22/2016
Publication URL: http://handle.nal.usda.gov/10113/62640
Citation: Foster, J., Luo, B., Nakata, P.A. 2016. An oxalyl-CoA dependent pathway of oxalate catabolism plays a role in regulating calcium oxalate crystal accumulation and defending against oxalate-secreting phytopathogens in Medicago truncatula. PLoS One. 11(2):e0149850.

Interpretive Summary: Plant scientists have been avidly working to discover new strategies to increase the nutritional quality and production of plant foods. Increasing the ability of edible plants to degrade oxalate has the potential to improve both calcium availability and resistance to oxalate-secreting fungal pathogens. Oxalate-secreting 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. Oxalate present in plant foods is also a known antinutrient in that it binds calcium in a form that renders it unavailable for nutritional absorption by humans and other animals. In this study we report the discovery of oxalyl-CoA synthetase in the calcium oxalate accumulating model legume, Medicago truncatula. Oxalyl-CoA synthetase is an enzyme that catalyzes the first step in a previously uncharacterized pathway of oxalate degradation. 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 and bound more of its tissue calcium in the formed of calcium oxalate crystal. Plants engineered to overexpress this enzyme showed an increase capacity to degrade oxalate and had less tissue oxalate and more resistance to oxalate-secreting 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 potential new strategy to aid our efforts to improve the nutritional quality and production of plant foods.

Technical Abstract: Considering the widespread occurrence of oxalate in nature and its broad impact on a host of organisms, it is surprising that so little is known about the turnover of this important acid. In plants, oxalate oxidase is the most well studied enzyme capable of degrading oxalate, but not all plants possess this activity. Recently, an Acyl Activating Enzyme 3 (AAE3), encoding an oxalyl-CoA synthetase, was identified in Arabidopsis. AAE3 has been proposed to catalyze the first step in an alternative pathway of oxalate degradation. Whether this enzyme and proposed pathway is important to other plants is unknown. Here, we identify the Medicago truncatula AAE3 (MtAAE3) and show that it encodes an oxalyl-CoA synthetase activity exhibiting high activity against oxalate with a Km = 81 +/- 9 uM and Vmax = 19 +/- 0.9 umoles min(-1) mg protein(-1). GFP-MtAAE3 localization suggested that this enzyme functions within the cytosol of the cell. Mtaae3 knock-down line showed a reduction in its ability to degrade oxalate into CO2. This reduction in the capacity to degrade oxalate resulted in the accumulation of druse crystals of calcium oxalate in the Mtaae3 knock-down line and an increased susceptibility to oxalate-secreting phytopathogens such as Sclerotinia sclerotiorum. Taken together, these results suggest that AAE3 dependent turnover of oxalate is important to different plants and functions in the regulation of tissue calcium oxalate crystal accumulation and in defense against oxalate-secreting phytopathogens.