Dual subcellular localization and non-photosynthetic functions of phylloquinone in parasitic and non-parasitic plants
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Phylloquinone (PhQ) is a group of lipid-soluble naphthoquinone derivatives produced by photosynthetic organisms to support photosystem I electron transport. Involvement of PhQ in non-photosynthetic plasma membrane redox activities of plants has been reported but is not well characterized due to challenges in preventing chloroplast contamination. This research aimed to understand the non-canonical function(s) and subcellular localization of PhQ biosynthesis using a photosynthesis-free study system, and to leverage the gained knowledge to assist the investigation in photosynthetic species. Non-photosynthetic holoparasites offer a photosynthesis-free system to explore the non-canonical function of PhQ. However, available transcriptome assemblies were not of sufficient quality to study the PhQ biosynthetic pathway. To overcome the limitation, a Parallelized Local Assembly of Sequences (PLAS) pipeline was developed that showed improved performance over other de novo assembly algorithms. PLAS successfully reconstructed full-length transcripts for the entire PhQ biosynthetic pathway genes for the holoparasite Phelipanche aegyptiaca and two of its photosynthetic relatives. Careful inspection of the sequences revealed that the terminal two enzymes of the PhQ pathway have been redirected to the plasma membrane in the holoparasite, but remain plastid-targeted in the photosynthetic parasites. Comparative gene coexpression network analyses reveal an association of PhQ with plasma membrane redox activities in the holoparasite. Plasma membrane PhQ biosynthesis was also predicted to exist as a minor route in multiple photoautotrophic species, indicating that the association between PhQ and the plasma membrane is evolutionarily conserved. Despite the insight from the parasitic plant system, investigation in photoautotrophic plants remains challenging, even when using heterotrophic tissues. The results from gene expression analyses revealed a dominant role of PhQ in photosynthesis, regardless of tissue. However, multiple lines of evidence indicated a large degree of plasticity of the PhQ biosynthetic pathway through lineage-dependent gene duplication, retention, and functional divergence among higher plants. This work was the first to investigate the plasma membrane biosynthesis of PhQ and its non-photosynthetic function in a photosynthesis-free system. Results from this work open new opportunities for future investigations to confirm the function of PhQ in parasitic plants and to characterize the PhQ pathway gene duplication in photoautotrophic plants.