Deficiency in Phylloquinone (Vitamin _K1) Methylation Affects Prenyl Quinone Distribution, Photosystem I Abundance, and Anthocyanin Accumulation in the <i>Arabidopsis AtmenG</i> Mutant

Phylloquinone (vitamin _K1) is synthesized in cyanobacteria and in chloroplasts of 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 <i>menG</i> gene product. Here, we report that the uncharacterized <i>Arabidopsis</i> gene At1g23360, which shows sequence similarity to <i>menG</i>, functionally complements the <i>Synechocystis menG</i> mutant. An <i>Arabidopsis</i> mutant, <i>AtmenG</i>, carrying a T-DNA insertion in the gene At1g23360 is devoid of phylloquinone, but contains an increased amount of 2-phytyl-1,4-naphthoquinone. Phylloquinone and 2-phytyl-1,4-naphthoquinone in thylakoid membranes of wild type and <i>AtmenG</i>, respectively, predominantly localize to photosystem I, whereas excess amounts of prenyl quinones are stored in plastoglobules. Photosystem I reaction centers are decreased in <i>AtmenG</i> plants under high light, as revealed by immunoblot and spectroscopic measurements. Anthocyanin accumulation and chalcone synthase (CHS1) transcription are affected during high light exposure, indicating that alterations in photosynthesis in <i>AtmenG</i> affect gene expression in the nucleus. Photosystem II quantum yield is decreased under high light. Therefore, the loss of phylloquinone methylation affects photosystem I stability or turnover, and the limitation in functional photosystem I complexes results in overreduction of photosystem II under high light.