Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2006
Publication Date: 11/2/2006
Citation: Lohmann, A., Schottler, M.A., Block, R., Cahoon, E.B., Dormann, P. 2006. The final methylation step of phylloquinone (vitamin K1) synthesis is crucial for maximal photosynthetic activity in Arabidopsis. Journal of Biological Chemistry. Available at: www.jbc.org/cgi/doi/10.1074/jbc.M609412200. Interpretive Summary: Vitamin K1 or phylloquinone is a component of the light reactions of photosynthesis in plant cells. Vitamin K1 is also an essential nutrient in human and livestock diets and is required for blood clotting and for the deposition of calcium in bones. Plants are the primary nutritional source of vitamin K1. Despite its importance in photosynthesis and its dietary requirement, the biosynthesis of vitamin K1 in plants is not well characterized. In this report, we describe the identification of the gene for the last step in the biosynthesis of vitamin K1 in plants. We also show that disruption of this gene results in the production of an aberrant form of vitamin K1 and reduces the ability of plants to tolerate high light. Information from these studies will be useful to biochemists, geneticists, and plant breeders who are studying the role of photosynthesis in plant productivity and are attempting to develop plants with improved nutritional value. This research may ultimately contribute to the development of higher yielding crops for U.S. farmers and more nutritious foods for consumers.
Technical Abstract: Phylloquinone (vitamin K1) is synthesized in cyanobacteria and in chloroplasts of higher plants where it serves as electron carrier of photosystem I. The last step of phylloquinone synthesis in cyanobacteria is the methylation of 2-phytyl-1,4-naphthoquinone by the menG gene product. The gene encoding 2-phytyl-1,4-naphthoquinone methyltransferase has not been identified in higher plants. Here, we report that the uncharacterized Arabidopsis gene At1g23360 which shows sequence similarity to menG from Synechocystis functionally complements the Synechocystis menG mutant. An Arabidopsis mutant, AtmenG, carrying a T-DNA insertion in the gene At1g23360 was totally devoid of phylloquinone, but contained an increased amount of 2-phytyl-1,4-naphthoquinone. The replacement of phylloquinone with 2-phytyl-1,4-naphthoquinone in AtmenG had only minor effects on growth and photosynthesis under normal light conditions. However upon exposure to high-light, AtmenG plants showed a reduction in the number of P700 reaction centers and in chlorophyll content as well as a decrease in PSII quantum yield. Therefore, the loss of the methyl group in phylloquinone causes degradation of PSI complexes, resulting in over-reduction of PSII and decrease of PSII quantum yield under photo-oxidative conditions.