1. Introduction
After transformation, transgenic plants are identified by means of selection markers. In traditional plant selection systems, selectable marker genes encode herbicide tolerance (e.g., paraquat, BASTA, etc.) or resistance to antibiotics (e.g., kanamycin, ampicillin, hygromycin, etc.), most of which may pose potential danger to human health or the environment [1]. Here, we present a novel, alternative selection marker system that is based on the metabolism of d-lactate. d-Lactate results from the glyoxalase system, which catabolizes methylglyoxal (MG), a compound formed as a byproduct of glycolysis in all type of cells through non-enzymatic phosphate elimination from triose phosphates (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate; [2]) (Fig. 1). The glyoxalase system comprises (i) glyoxalase I, an enzyme that catalyzes the formation of S-d-lactoylglutathione from the hemithioacetal formed non-enzymatically from MG and glutathione and (ii) glyoxalase II, which catalyzes the hydrolysis of S-d-lactoylglutathione to regenerate glutathione and liberate d-lactate [3] and [4]. d-Lactate is further converted to pyruvate by d-lactate dehydrogenase (d-LDH) (Fig. 1). The Arabidopsis thalianad-LDH is encoded by a single gene (At5g06580), localizes to mitochondria, and uses cytochrome c as electron acceptor [5]. Moreover, this enzyme shows a high specificity for d-lactate with a catalytic efficiency 200- and 2000-fold higher than that for l-lactate and glycolate, respectively [5].