Function of plastidial pyruvate kinases in seeds of Arabidopsis thaliana

Authors


  • After this paper had been provisionally accepted, C. Andre and coworkers published a manuscript entitled ‘A heteromeric plastidic pyruvate kinase complex involved in seed oil biosynthesis in Arabidopsis’ (http://www.plantcell.org/cgi/doi/10.1105/tpc.106.048629) using a different nomenclature for PKp genes. The PKp1 gene (this paper; At3g22960) corresponds to PKp-α in Andre et al. (2007), whereas PKp2 and PKp3 (At5g52920 and At1g32440, respectively) refer to PKp-β1 and PKp-β2, respectively. The pkp1 mutant studied by Andre and coworkers, which exhibits a mutation of the At5g52920 (PKp-β1) gene, thus corresponds to the pkp2-2 allele presented in this paper.

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Summary

Pyruvate kinase (PK) catalyses the irreversible synthesis of pyruvate and ATP, which are both used in multiple biochemical pathways. These compounds are essential for sustained fatty acid production in the plastids of maturing Arabidopsis embryos. Using a real-time quantitative reverse transcriptase (RT)-PCR approach, the three genes encoding putative plastidial PKs (PKps) in Arabidopsis, namely PKp1 (At3g22960), PKp2 (At5g52920) and PKp3 (At1g32440), were shown to be ubiquitously expressed. However, only PKp1 and PKp2 exhibited significant expression in maturing seeds. The activity of PKp1 and PKp2 promoters was consistent with this pattern, and the study of the PKp1:GFP and PKp2:GFP fusion proteins confirmed the plastidial localization of these enzymes. To further investigate the function of these two PKp isoforms in seeds comprehensive functional analyses were carried out, including the cytological, biochemical and molecular characterization of two pkp1 and two pkp2 alleles, together with a pkp1pkp2 double mutant. The results obtained outlined the importance of these PKps for fatty acid synthesis and embryo development. Mutant seeds were depleted of oil, their fatty acid content was drastically modified, embryo elongation was retarded and, finally, seed germination was also affected. Together, these results provide interesting insights concerning the carbon fluxes leading to oil synthesis in maturing Arabidopsis seeds. The regulation of this metabolic network by the WRINKLED1 transcription factor is discussed, and emphasizes the role of plastidial metabolism and the importance of its tight regulation.

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