Isolation and characterization of lipid in phloem sap of canola

Phloem isolated from canola (Brassica napus L.) stems was found to contain phospholipid, diacylglycerol, triacylglycerol, steryl and wax esters, and comparatively high concentrations of unesterified fatty acids. Indeed, the composition of phloem lipid was markedly different from that of microsomal membranes and cytosol isolated from both leaves and stems. Specifically, phloem lipid consisted predominantly of unesterified fatty acids and was enriched in medium-chain fatty acids, in particular, lauric, myristic and pentadecanoic acids. This unique composition also distinguished phloem lipid from that of well-characterized cytosolic lipid particles such as oil bodies found in plant cells. Moreover, levels of medium-chain fatty acids in the phloem increased when canola plants were stressed by exposure to sublethal doses of ultraviolet irradiation. Phloem levels of lauric acid, for example, increased by 11-fold upon treatment with sublethal ultraviolet irradiation. Spherical lipid particles were discernible in isolated phloem sap by electron microscopy, suggesting that the lipid in phloem is in the form of lipid particles. The presence of lipid in phloem may be reflective of long-distance lipid transport in plants, primarily in the form of free fatty acids.