Subcellular Localization Studies Indicate That Lipoxygenases 1 to 6 Are Not Involved in Lipid Mobilization during Soybean Germination

Soybean (Glycine max) lipoxygenase (LOX) has been proposed to be involved in reserve lipid mobilization during germination. Here, subcellular fractionation studies show that LOX1, -2, -3, -4, -5, and -6 isozymes were associated with the soluble fraction but not with purified oil bodies. The purified oil bodies contained small amounts of LOX1 (<0.01% total activity), which apparently is an artifact of the purification process. Immunogold labeling indicated that, in cotyledon parenchyma cells of LOX wild-type seeds that had soaked and germinated for 4 d, the majority of LOX protein was present in the cytoplasm. In 4-d-germinated cotyledons of a LOX1/2/3 triple null mutant (L0), a small amount of label was found in the cytoplasm. In epidermal cells, LOX appeared in vacuoles of both wild-type and L0 germinated seeds. No LOXs cross-reacting with seed LOX antibodies were found to be associated with the cell wall, plasma membrane, oil bodies, or mitochondria. Lipid analysis showed that degradation rates of total lipids and triacylglycerols between the wild type and L0 were not significantly different. These results suggest that LOX1, -2, -3, -4, -5, and -6 are not directly involved in reserve lipid mobilization during soybean germination.