Singlet oxygen-induced signalling depends on the metabolic status of the Chlamydomonas reinhardtii cell

Using a mutant screen, we identified trehalose 6-phosphate phosphatase 1 (TSPP1) as a functional enzyme dephosphorylating trehalose 6-phosphate (Tre6P) to trehalose in Chlamydomonas reinhardtii. The tspp1 knock-out results in reprogramming of the cell metabolism via altered transcriptome. As a secondary effect, tspp1 also shows impairment in (1)O(2)-induced chloroplast retrograde signalling. From transcriptomic analysis and metabolite profiling, we conclude that accumulation or deficiency of certain metabolites directly affect (1)O(2)-signalling. (1)O(2)-inducible GLUTATHIONE PEROXIDASE 5 (GPX5) gene expression is suppressed by increased content of fumarate and 2-oxoglutarate, intermediates in the tricarboxylic acid cycle (TCA cycle) in mitochondria and dicarboxylate metabolism in the cytosol, but also myo-inositol, involved in inositol phosphate metabolism and phosphatidylinositol signalling system. Application of another TCA cycle intermediate, aconitate, recovers (1)O(2)-signalling and GPX5 expression in otherwise aconitate-deficient tspp1. Genes encoding known essential components of chloroplast-to-nucleus (1)O(2)-signalling, PSBP2, MBS, and SAK1, show decreased transcript levels in tspp1, which also can be rescued by exogenous application of aconitate. We demonstrate that chloroplast retrograde signalling involving (1)O(2) depends on mitochondrial and cytosolic processes and that the metabolic status of the cell determines the response to (1)O(2).