Role of lipid bodies and lipid-body proteins in seeds and other tissues

Lipid bodies are present in most plant cells and not just in storage tissues, such as seeds, as is often assumed. There is a growing number of proteins that are associated with lipid bodies in different plant tissues. In seeds, the most common class of lipid-body protein is the oleosins, which are found on the surfaces on lipid bodies in all desiccation tolerant plants but are absent from lipid bodies in the seeds of desiccation-sensitive plants, e.g. cocoa or neem. The absence of oleosins in these seeds is not problematic during dehydration but leads to a fatal coalescence of lipid bodies after germination. A class of anther-specific proteins has a precursor form that contains an oleosin-like hydrophobic domain and is initially localised on lipid bodies in the tapetum. However, the oleosin-like domain is cleaved off after release into the anther locule and the mature proteins, which are the major protein components of the pollen coat, contain no oleosin-like sequences. Therefore these proteins, which have been named «pollenins», use their oleosin-like domain as a novel form of cleavable targeting sequence. A third type of lipid-body protein is caleosin, which also has endoplasmic reticulum associated isoforms and may be involved in lipid-body biogenesis and membrane trafficking. Analysis of the expression patterns of oleosin and caleosin genes/proteins revealed an unexpected localisation in young root tips, as confirmed by expressing promoter-GUS constructs in transgenic plants. Several other classes of storage product-related genes hitherto believed to seed-specific are also transiently expressed in root tips following seed germination. We discuss the implications of these findings for root development and the use of «seed-specific», gene promoters in general.