Inhibitors of photosystem II increase the expression of chloroplast diacylglycerol acyltransferase-3 and promote triacylglycerol accumulation in Chlamydomonas reinhardtii

Abstract

Considerable progress has been made towards the understanding of triacylglycerol (TAG) accumulation in algae. A key aspect is finding conditions that trigger TAG production without reducing cell division. We identified a soluble diacylglycerol acyltransferase (DGAT), exclusive to the green lineage and moderately related to plant DGAT3, with heterologous DGAT activity. We demonstrated that DGAT3 localizes to the chloroplast in the model green alga Chlamydomonas reinhardtii and that its expression is induced by light in the presence of acetate, consistent with TAG accumulation. The light-dependent expression of DGAT3 and the presence of a 2Fe-2S iron-sulfur cluster-binding domain in its sequence indicates that this protein could accept electrons, directly or indirectly, from the photosynthetic machinery. The aim of this study was to investigate the relationship between DGAT3 expression and photosynthetic electron transport. With that purpose, we incubated C. reinhardtii wild type cc-125 cells with two photosystem II (PSII) inhibitors, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone (DBMIB), and one photosystem I (PSI) inhibitor, N,N′-dimethyl-4 4′-bipyridinium dichloride (paraquat). Both, Dgat3 mRNAs and TAGs increased in DCMU and DBMIB-treated cells as early as 15 minutes after initiating the experiment, whereas no significant variations were observed in paraquat-treated cells in the course of a 3 h incubation period. Our results suggest that DGAT3 expression and TAG biosynthesis increase when PSII is over-reduced in order to avoid photodamage. This occurs naturally in situations in which light absorption is higher than the rate of photosynthesis (e.g. during illumination at high light intensity) or artificially in chemically-altered PSII centers. Currently, this hypothesis is being evaluated in mutants that have deficiencies in the function of PSI, PSII or the cytb6f complex.