Unravelling the Function of the Unusual Antioxidants Ergothioneine and Ovothiol in Plants and Photosynthetic Protists

Ergothioneine and ovothiol are histidine derived thiols, predicted to provide antioxidant and cytoprotective roles. They have been identified in a small number of photosynthetic organisms. However, their biosynthesis and function in vivo has not been extensively explored. Metabolite profiling combined with genomic surveys indicated that the specificity of the SAM-methyltransferase is an important determinant of the biosynthetic pathway present, where the streptophyta exclusively synthesize ergothioneine, and green and red algae produce both ergothioneine and ovothiol. It has also demonstrated differences in the biosynthetic pathways between major lineages of the Archaeplastida and highlighted the loss of this pathway in the angiosperms. Knockout of the mpegt1 gene in the liverwort Marchantia polymorpha Tak-1 resulted in a reduction of ergothioneine and high light-induced stress showed an increase in the rate of development in the mutant strains. Knockout of the SAM-methyltransferase domain in the diatom Phaeodactylum tricornutum resulted in a loss of ovothiol B, but an accumulation of ovothiol A, an ovothiol B like compound, and ascorbate. P. tricornutum ovothiol mutant strains showed higher specific growth rates under high light compared to the wild type. This study has provided the first steps in the characterisation of these sulphur-containing histidine derivatives and developed tools including metabolite profiling techniques. It has also resulted in the generation of knockout mutants in two model organisms. The mutant strains have shown the involvement of histidine derived thiols in response to high light and provides two new systems which can be utilised to allow their wide roles and cellular functions to be more fully understood.