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 O-1(2)-induced chloroplast retrograde signalling. From transcriptomic analysis and metabolite profiling, we conclude that accumulation or deficiency of certain metabolites directly affect O-1(2)-signalling. O-1(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 O-1(2)-signalling and GPX5 expression in otherwise aconitate-deficient tspp1. Genes encoding known essential components of chloroplast-to-nucleus O-1(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 O-1(2) depends on mitochondrial and cytosolic processes and that the metabolic status of the cell determines the response to O-1(2). A combination of genetic, gene expression and metabolic analyses demonstrated that the chloroplast retrograde signalling, triggered by singlet oxygen, depends on complex interactions between chloroplast, mitochondria and cytosol.

Automated summary

Using a mutant screen, the functional enzyme trehalose 6-phosphate phosphatase 1 (TSPP1) was identified as responsible for dephosphorylating trehalose 6-phosphate (Tre6P) in Chlamydomonas reinhardtii. Knocking out the tspp1 gene results in reprogramming of cell metabolism and impairment of O-1(2)-induced chloroplast retrograde signalling. Transcriptomic analysis and metabolite profiling indicate that the accumulation or deficiency of certain metabolites directly affect the O-1(2)-signalling process.