Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae

Eukaryotic microalgae have been classified into several biological divisions and have evolutionarily acquired diverse morphologies, metabolisms, and life cycles. They are naturally exposed to environmental stresses that cause oxidative damage due to reactive oxygen species accumulation. To cope with environmental stresses, microalgae contain various antioxidants, including carotenoids, ascorbate (AsA), and glutathione (GSH). Carotenoids are hydrophobic pigments required for light harvesting, photoprotection, and phototaxis. AsA constitutes the AsA-GSH cycle together with GSH and is responsible for photooxidative stress defense. GSH contributes not only to ROS scavenging, but also to heavy metal detoxification and thiol-based redox regulation. The evolutionary diversity of microalgae influences the composition and biosynthetic pathways of these antioxidants. For example, α-carotene and its derivatives are specific to Chlorophyta, whereas diadinoxanthin and fucoxanthin are found in Heterokontophyta, Haptophyta, and Dinophyta. It has been suggested that AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and discussed the diversity of microalgal antioxidants, including recent findings.

Thioredoxin and Glutaredoxin Systems Required for Oxidative Stress Resistance, Fungicide Sensitivity, and Virulence ofAlternaria alternata

ABSTRACTThis study determined the function of thioredoxin and glutaredoxin systems in the phytopathogenic fungusAlternaria alternatavia analyzing mutants obtained from the targeted deletion of genes encoding thioredoxin peroxidase (Tsa1), thioredoxin reductase (Trr1), and glutathione reductase (Glr1).Trr1andGlr1, but notTsa1, are required for growth and conidiation. The reduced growth and conidiation seen in theTrr1orGlr1deletion mutant can be restored by glutathione. Deletion mutants showing growth inhibition by oxidants are defective for H2O2detoxification and induce smaller lesions on citrus leaves.Trr1andGlr1, but notTsa1, also contribute to NaCl resistance.Glr1is required for sorbitol resistance and is responsible for resistance to mancozeb and boscalid but not chlorothalonil fungicides, a novel phenotype that has not been reported in fungi.Trr1is required for resistance to boscalid and chlorothalonil fungicides but confers susceptibility to mancozeb. TheTsa1deletion mutant displays wild-type sensitivity to the tested fungicides. The expression ofTsa1andTrr1is regulated by the oxidative stress responsive regulators Yap1, Hog1, and Skn7. The expression ofTsa1, but notTrr1, is also regulated indirectly by the NADPH oxidase. The results indicate that the capability to resist oxidative stress is required for virulence ofA. alternata.IMPORTANCEThe thioredoxin and glutaredoxin systems are important thiol antioxidant systems in cells, and knowledge of these two systems in the plant-pathogenic fungusA. alternatais useful for finding new strategies to reduce the virulence of this pathogen. In this study, we demonstrated that thiol antioxidant system-related genes (Tsa1,Trr1, andGlr1) are required for H2O2detoxification and virulence inA. alternata. Moreover, deletion ofTrr1results in hypersensitivity to the fungicides chlorothalonil and boscalid, andGlr1deletion mutants are highly sensitive to mancozeb, which is the fungicide mostly used in citrus fields. Therefore, our findings demonstrate that the ability to detoxify reactive oxygen species (ROS) plays a critical role in pathogenesis on citrus and provide novel insights into the physiological functions of thiol-containing systems in fungicide sensitivity forA. alternata.