Abstract
BackgroundPhotosynthetic microalgae are considered a viable and sustainable resource for biofuel feedstocks, because they can produce higher biomass per land area than plants and can be grown on non-arable land. Among many microalgae considered for biofuel production, Nannochloropsis oceanica (CCMP1779) is particularly promising, because following nutrient deprivation it produces very high amounts of triacylglycerols (TAG). The committed step in TAG synthesis is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT). Remarkably, a total of 13 putative DGAT-encoding genes have been previously identified in CCMP1779 but most have not yet been studied in detail.ResultsBased on their expression profile, six out of 12 type-2 DGAT-encoding genes (NoDGTT1-NoDGTT6) were chosen for their possible role in TAG biosynthesis and the respective cDNAs were expressed in a TAG synthesis-deficient mutant of yeast. Yeast expressing NoDGTT5 accumulated TAG to the highest level. Over-expression of NoDGTT5 in CCMP1779 grown in N-replete medium resulted in levels of TAG normally observed only after N deprivation. Reduced growth rates accompanied NoDGTT5 over-expression in CCMP1779. Constitutive expression of NoDGTT5 in Arabidopsis thaliana was accompanied by increased TAG content in seeds and leaves. A broad substrate specificity for NoDGTT5 was revealed, with preference for unsaturated acyl groups. Furthermore, NoDGTT5 was able to successfully rescue the Arabidopsis tag1-1 mutant by restoring the TAG content in seeds.ConclusionsTaken together, our results identified NoDGTT5 as the most promising gene for the engineering of TAG synthesis in multiple hosts among the 13 DGAT-encoding genes of N. oceanica CCMP1779. Consequently, this study demonstrates the potential of NoDGTT5 as a tool for enhancing the energy density in biomass by increasing TAG content in transgenic crops used for biofuel production.
Highlights
Photosynthetic microalgae are considered a viable and sustainable resource for biofuel feedstocks, because they can produce higher biomass per land area than plants and can be grown on non-arable land
Transcriptional profiling of N. oceanica CCMP1779 diacylglycerol acyltransferase (DGAT) genes The genome of N. oceanica CCMP1779 encodes one putative type 1 DGAT (NoDGAT1) and 12 putative type 2 DGAT genes [19]
The highest fold change in expression among all the analyzed DGAT genes was observed for NoDGTT5, with a 20-fold increase at its maximum
Summary
Photosynthetic microalgae are considered a viable and sustainable resource for biofuel feedstocks, because they can produce higher biomass per land area than plants and can be grown on non-arable land. Microalgae have experienced a renaissance as a potential resource for biofuel production because many of them have the ability to store as much as 20–50% of their dry weight as lipids. Two different types of membrane-bound DGAT enzymes are directly involved in TAG formation—type 1 (DGAT) and type 2 (or DGTT) Both have similar function, but their genes evolved separately [5]. Nannochloropsis species are small unicellular heterokont algae living in marine, fresh, or brackish water Their lipid content is especially high following nitrogen (N) deprivation [10,11,12,13]
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