Downregulation of mitochondrial alternative oxidase affects chloroplast function, redox status and stress response in a marine diatom

• Diatom dominance in contemporary aquatic environments indicates that they have developed unique and effective mechanisms to cope with the rapid and considerable fluctuations that characterize these environments. In view of their evolutionary history from a secondary endosymbiosis, inter-organellar regulation of biochemical activities may be of particular relevance. Diatom mitochondrial alternative oxidase (AOX) is believed to play a significant role in supplying chloroplasts with ATP produced in the mitochondria. • Using the model diatom Phaeodactylum tricornutum we generated AOX knockdown lines, and followed sensitivity to stressors, photosynthesis and transcriptome and metabolome profiles of wild-type and knockdown lines. • We show here that expression of the AOX gene is upregulated by various stresses including H₂O₂, heat, high light illumination, and iron or nitrogen limitation. AOX knockdown results in hypersensitivity to stress. Knockdown lines also show significantly reduced photosynthetic rates and their chloroplasts are more oxidized. Comparisons of transcriptome and metabolome profiles suggest a strong impact of AOX activity on gene expression, which is carried through to the level of the metabolome. • Our data provide evidence for the involvement of mitochondrial AOX in processes central to the cell biology of diatoms, revealing that cross-talk between mitochondria and chloroplasts is crucial for maintaining sensitivity to changing environments.