PSR1 is a global transcriptional regulator of phosphorus deficiency responses and carbon storage metabolism in Chlamydomonas reinhardtii.

Many eukaryotic microalgae modify their metabolism in response to nutrient stresses such as P starvation, which substantially induces storage metabolite biosynthesis, but the genetic mechanisms regulating this response are poorly understood. Here we show that P starvation-induced lipid and starch accumulation is inhibited in a Chlamydomonas reinhardtii mutant lacking the transcription factor PSR1. Transcriptomic analysis identified specific metabolism transcripts that are induced by P starvation but mis-regulated in the psr1 mutant. These include transcripts for starch and triacylglycerol synthesis but also transcripts for photosynthesis-, redox- and stress signalling-related proteins. To further examine the role of PSR1 in regulating lipid and starch metabolism, PSR1 complementation lines in the psr1 strain, and PSR1 over-expression lines in a cell wall-deficient strain, were generated. PSR1 expression in the psr1 lines was shown to be functional due to rescue of the psr1 phenotype. PSR1 over-expression lines exhibited increased starch content and number of starch granules per cell, which correlated with higher expression of specific starch metabolism genes but reduced neutral lipid content. Furthermore, this phenotype was consistent in the presence and absence of acetate. Together, these results identify a key transcriptional regulator in global metabolism, and demonstrate transcriptional engineering in microalgae to modulate starch biosynthesis.