In angiosperm seeds, the relative proportions of the two zygotic tissues vary considerably from species to species. In many field-grown oilseed species, and in those of the model species Arabidopsis thaliana, the embryo predominates, and studies of lipid metabolism in whole seeds reflect embryonic metabolism. Metabolism in the endosperm has long been ignored in these species, where this tissue is reduced in size in the mature seed. As a result of recent methodological developments that allow us to follow up on the accumulation of transcripts and metabolites in different areas of these seeds, it has become clear that the lipid metabolism of the endosperm is often different from that of the embryo. However, as the differences between the two zygotic tissues are variations on the same theme rather than strict divergences, there is a lack of genetic tools to study either tissue specifically. To remedy this, we have identified and characterized a promoter sequence in A. thaliana that is specifically active in the seed endosperm during the maturation phase: the At3g29190 (TPS15) gene promoter. We have then shown that it is possible to use this promoter sequence to modulate fatty acid metabolism specifically in the endosperm, either by activating or repressing the expression of target genes in this tissue. This tool and the transgenic lines that can be generated will contribute to a better understanding of the specific features of lipid metabolism in oilseed endosperm and its physiological implications for the seed.