Analysis of potential redundancy among Arabidopsis 6-phosphogluconolactonase (PGL) isoforms in peroxisomes.

Recent work revealed that PGD2, an Arabidopsis 6-phosphogluconate dehydrogenase (6-PGD), catalyzing the third step of the oxidative pentose-phosphate pathway (OPPP) in peroxisomes, is essential during fertilization. Earlier studies on the second step catalyzed by PGL3, a dually targeted Arabidopsis 6-phosphogluconolactonase (6-PGL), reported importance of OPPP reactions in plastids but irrelevance in peroxisomes. Assuming redundancy of 6-PGL activity in peroxisomes, we examined the sequences of other higher plant enzymes. In tomato, two 6-PGL isoforms with the strong PTS1 motif SKL exist. However, their analysis revealed problems regarding peroxisomal targeting: reporter-PGL detection in peroxisomes required construct modification, which was also applied to the Arabidopsis isoforms. Relative contribution of PGL3 versus PGL5 during fertilization was assessed by mutant crosses. Reduced transmission ratios were found for pgl3 1 (T-DNA-eliminated PTS1) and also for knock-out allele pgl5 2. The prominent role of PGL3 showed as compromised growth of pgl3-1 seedlings on sucrose and higher activity of mutant PGL3-1 versus PGL5 using purified recombinant proteins. Evidence for PTS1-independent uptake was found for PGL3-1 and other Arabidopsis PGL isoforms, indicating that peroxisome import may be supported by a piggybacking mechanism. Thus, multiple redundancy at the level of the second OPPP step in peroxisomes explains occurrence of pgl3 1 mutant plants.