Downregulation of a putative plastid PDC E1α subunit impairs photosynthetic activity and triacylglycerol accumulation in nitrogen-starved photoautotrophic Chlamydomonas reinhardtii

The chloroplast pyruvate dehydrogenase complex (cpPDC) catalyses the oxidative decarboxylation of pyruvate forming acetyl-CoA, an immediate primer for the initial reactions of de novo fatty acid (FA) synthesis. Little is known about the source of acetyl-CoA in the chloroplasts of photosynthetic microalgae, which are capable of producing high amounts of the storage lipid triacylglycerol (TAG) under conditions of nutrient stresses. We generated Chlamydomonas reinhardtii CC-1618 mutants with decreased expression of the PDC2_E1 alpha gene, encoding the putative chloroplast pyruvate dehydrogenase subunit E1 alpha, using artificial microRNA. A comparative study on the effects of PDC2_E1 alpha silencing on FAs and TAG production in C. reinhardtii, grown photoautotrophically and mixotrophically, with and without a nitrogen source in the nutrient medium, was carried out. Reduced expression of PDC2_E1 alpha led to a severely hampered photoautotrophic growth phenotype with drastic impairment in TAG accumulation under nitrogen deprivation. In the presence of acetate, downregulation of PDC2_E1 alpha exerted little to no effect on TAG production and photosynthetic activity. In contrast, under photoautotrophic conditions, especially in the absence of a nitrogen source, a dramatic decline in photosynthetic oxygen evolution and photosystem II quantum yield against a background of the apparent over-reduction of the photosynthetic electron chain was recorded. Our results suggest an essential role of cpPDC in the supply of carbon precursors for de novo FA synthesis in microalgae under conditions of photoautotrophy. A shortage of this supply is detrimental to the nitrogen-starvation-induced synthesis of storage TAG, an important carbon and energy sink in stressed Chlamydomonas cells, thereby impairing the acclimation ability of the microalga.