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

Abstract

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 H O , 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.

Cross‐compartment metabolic coupling enables flexible photoprotective mechanisms in the diatomPhaeodactylum tricornutum

“…However, this comparison also suggests that the model underestimated nonphotochemical dissipation of captured light energy at high light. Additionally, the photoprotective function suggested by the simulations are consistent with decreased photosynthetic rates observed in AOX_m knockout lines (Murik et al ., ).…”

PhaeoNet: A Holistic RNAseq-Based Portrait of Transcriptional Coordination in the Model Diatom Phaeodactylum tricornutum

“…Previously, intricate metabolic connections have been observed between P. tricornutum chloroplasts and mitochondria, which are distinctive to those found in plants (Prihoda et al, 2012;Bailleul et al, 2015;Broddrick et al, 2019;Murik et al, 2019). We wished to determine which of these connections were visible within our data, noting multiple, transcriptionally independent connections between the predicted proteomes of chloroplasts and mitochondria in PhaeoNet data (highlighted in Supplementary Figure 6).…”

“…Finally, we noted the presence of genes encoding chloroplast-targeted plastoquinol terminal oxidase (Phatr3_J4283) and mitochondria-targeted alternative oxidase (Phatr3_EG02359), which are both associated with the photoprotective removal of excess metabolic reducing potential in the skyblue merged module ( Bailleul et al, 2015 ; Murik et al, 2019 ). This merged module, as discussed above, contains genes encoding three successive enzymes associated with the TCA cycle (Phatr3_J40430 encoding α-ketoglutaryl dehydrogenase; Phatr3_J42015 encoding succinyl-CoA synthetase and Phatr3_J41812 encoding succinate dehydrogenase; Kroth et al, 2008 ), along with methylmalonyl-CoA mutase (Phatr3_J51830), which may allow excess succinyl-CoA to be diverted into lipid synthesis via propionyl-CoA ( Helliwell et al, 2011 ; Valenzuela et al, 2012 ).…”

Genome-scale metabolic model of the diatomThalassiosira pseudonanahighlights the importance of nitrogen and sulfur metabolism in redox balance

“…Both cyanobacteria and the green alga Chlamydomonas accumulate fermentation products under dark anaerobic conditions [109,110], likely due to inhibition of mitochondrial respiration [111]. The diatom P. tricornutum appears to increase rates of mitochondrial respiration under stressful conditions (high light intensity, iron or nitrate limitation, or supraoptimal temperatures) as suggested by upregulation of alternative oxidase expression [112]. While T. pseudonana displays here the potential to secrete fermentation products, fermentation is likely minimal during normal culture conditions.…”