Oleosomes (lipid bodies) in nitrogen‐fixing peanut nodules

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Abstract
Nitrogen‐fixing peanut root nodules have oleosomes (lipid bodies) in the infected cells. The oleosomes have been characterized and compared with their counterparts in the seed tissue. Eighty per cent of the nodule oleosomes were found to be of smaller size (0.16–1.0 μm diameter [dia.]) while in the seeds the larger size (2.0–5.5 μm dia.) dominated. The larger oleosomes were exclusively found in the uninfected three layers of cortical cells adjacent to the infected zone. Morphometric analyses have revealed significantly higher numbers of oleosomes covering about 8% of the infected cell area in immature (white) nodules, whereas the mature/old (pink) nodules had lesser numbers occupying about 4–5% of the cell area. The decrease in the amount of oleosomes possibly reflects their utilization in mature/old nodules which effectively fix nitrogen. The oleosomes were distinctly stained by p‐phenylenediamine (pPD) at both light and electron microscopic levels. An electron‐dense rim was observed around the nodule oleosomes; where lipolytic activity was also demonstrated using cytochemical methods. The rim was absent in the seed oleosomes. The defatted oleosomes were found to be surrounded by a ‘half unit membrane’ and a non‐extractable rim of possible pro‐teinaceous substance. Gas chromatographic analyses of the lipid from the isolated oleosomes indicated the presence of higher amounts of saturated fatty acids in the nodule oleosomes than the ones in the seed. The study indicates that the nodule oleosomes differ from seed oleosomes, with respect to the presence of (1) an electron‐dense rim showing (2) lipolytic activity and (3) higher amounts of saturated fatty acids. Nodule oleosomes seem to be transient storage organelles to be metabolized, while in the seed they are meant for long‐term storage.