A diacylglycerol acyltransferase (DGAT1) gene from Tropaeolum majus encodes a DGAT1 polypeptide that regulates the production of triacylglycerols from diacylglycerols. Transformation of cells with the Tropaeolum majus DGAT1 (TmDGAT1) gene, or variants thereof, leads to changes in DGAT1 activity and alterations in oil and/or fatty acid content and/or seed weight. In particular, oil content, seed weight, erucic acid and/or trierucin content may be increased.