Oleosomes (lipid bodies) in N₂-fixing peanut root nodules

Subramanian, Jayaram (1991) Oleosomes (lipid bodies) in N₂-fixing peanut root nodules. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Nitrogen-fixing peanut root nodules have oleosomes (lipid bodies) in infected and uninfected cells. The oleosomes were characterized and compared with their counterparts in the seed tissue. Measurements made from scanning electron micrographs of isolated oleosomes revealed that eighty percent of the nodule oleosomes were of smaller size (0.16-1.0 µm in diameter [dia.]) while in the seeds the larger size (1.0-5.5 µm dia.) dominated. The larger oleosomes in the nodules were exclusively found in the uninfected three layers of cortical cells adjacent to the infected zone. Morphometric analyses have shown 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 number 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. Oleosomes are osmiophilic and spherical in structure. 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 proteinaceous 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 in the seed. This study indicates that the nodule oleosomes differ from the seed oleosomes, with respect to the presence of (1) an electron-dense rim (2) showing 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.

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