The Genome of Ricinus Communis Encodes a Single Glycolate Oxidase with Different Functions in Photosynthetic and Heterotrophic Organs

The biochemical characterization of glycolate oxidase in Ricinus communis hints to different physiological functions of the enzyme depending on the organ in which it is active. Enzymatic activities of the photorespiratory pathway are not restricted to green tissues but are present also in heterotrophic organs. High glycolate oxidase (GOX) activity was detected in the endosperm of Ricinus communis. Phylogenetic analysis of the Ricinus l-2-hydroxy acid oxidase (Rc(l)-2-HAOX) family indicated that Rc(l)-2-HAOX1 to Rc(l)-2-HAOX3 cluster with the group containing streptophyte long-chain 2-hydroxy acid oxidases, whereas Rc(l)-2-HAOX4 clusters with the group containing streptophyte GOX. Rc(l)-2-HAOX4 is the closest relative to the photorespiratory GOX genes of Arabidopsis. We obtained Rc(l)-2-HAOX4 as a recombinant protein and analyze its kinetic properties in comparison to the Arabidopsis photorespiratory GOX. We also analyzed the expression of all Rc(l)-2-HAOXs and conducted metabolite profiling of different Ricinus organs. Phylogenetic analysis indicates that Rc(l)-2-HAOX4 is the only GOX encoded in the Ricinus genome (RcGOX). RcGOX has properties resembling those of the photorespiratory GOX of Arabidopsis. We found that glycolate, the substrate of GOX, is highly abundant in non-green tissues, such as roots, embryo of germinating seeds and dry seeds. We propose that RcGOX fulfills different physiological functions depending on the organ in which it is active. In autotrophic organs it oxidizes glycolate into glyoxylate as part of the photorespiratory pathway. In fast growing heterotrophic organs, it is most probably involved in the production of serine to feed the folate pathway for special demands of those tissues.