Spatial and Temporal Mapping of Key Lipid Species in Brassica napus Seeds

The regulation of lipid synthesis in oil seeds is still not fully understood. Oilseed rape (Brassica napus) is the third most productive vegetable oil crop on the global market; therefore, increasing our understanding of lipid accumulation in oilseed rape seeds is of great economic, as well as intellectual, importance. Matrix-assisted laser/desorption ionization-mass spectrometry imaging (MALDI-MSI) is a technique that allows the mapping of metabolites directly onto intact biological tissues, giving a spatial context to metabolism. We have used MALDI-MSI to study the spatial distribution of two major lipid species, triacylglycerols and phosphatidylcholines. A dramatic, heterogenous landscape of molecular species was revealed, demonstrating significantly different lipid compositions between the various tissue types within the seed. The embryonic axis was found to be particularly enriched in palmitic acid, while the seed coat/aleurone layer accumulated vaccenic, linoleic, and α-linoleic acids. Furthermore, the lipid composition of the inner and outer cotyledons differed from each other, a remarkable discovery given the supposed identical functionality of these two tissues. Triacylglycerol and phosphatidylcholine molecular species distribution was analyzed through a developmental time series covering early seed lipid accumulation to seed maturity. The spatial patterning of lipid molecular species did not vary significantly during seed development. Data gathered using MALDI-MSI was verified through gas chromatography analysis of dissected seeds. The distinct lipid distribution profiles observed imply differential regulation of lipid metabolism between the different tissue types of the seed. Further understanding of this differential regulation will enhance efforts to improve oilseed rape productivity and quality.