Oleosomes in some nitrogen-fixing root nodules

Khetmalas, Madhukar B. (1996) Oleosomes in some nitrogen-fixing root nodules. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The high energy-demanding process of nitrogen fixation in symbiotic root nodules is generally supported by a supply of carbon compounds derived from current photosynthate of the host plant. However, in Arachis hypogaea L (peanut) nodules, which have oleosomes (lipid bodies) in the infected cells, the lipid catabolism may supplement the energy supply in case of photosynthate stress. The present investigation was undertaken to further study oleosomic metabolism in Arachis hypogaea and four other legumes: A. pintoi L., A. duranensis L., A. batizocoi L. and Lathyrus maritimus L. (Bigel) (beach pea) nodules where oleosomes are present. -- The oleosomes of A. hypogaea root nodules contained diacylglycerol (DAG), triacylglycerol (TAG), phospholipids (PL) and oleosins. The oleosomes varied in size, electron density and in the width of a less electron-dense peripheral layer. Four oleosin bands having molecular weights 66.0 KD, 61.1 KD, 56.3 KD and 10.0 KD could be resolved by polyacrylamide gel electrophoresis. -- The development of symbiosis and oleosome distribution was studied in three wild species of Arachis i.e. A. pintoi, A. duranensis and A. batizocoi. Oleosomes were present in the infected cells of A. pintoi during the infection process and before establishment of symbiosis. In A. duranensis and A. batizocoi oleosomes persisted during symbiosis in mature nodules. A.pintoi mature nodules were devoid of oleosomes in infected cells, but reappeared during senescence. Another interesting feature in this species was the reversion of spherical bacteroids into rod-forms within the confines of the senescent nodule tissue. -- Studies on the distributional pattern of oleosomes in the root nodules of naturally growing L. maritimus (beach pea) revealed that the pre-winter nodules were filled with large numbers of oleosomes and amyloplasts in uninfected interstitial and parenchyma cells. These storage organelles could not be seen in the cells of nodule sampled during post- winter periods before aerial shoots emerged. The results indicate that either the oleosomes are catabolized slowly during the winter months, to allow the nodules to survive the extreme cold temperatures or they are rapidly mobilized just before the growing season. The olesomes in beach pea nodules seem to serve as storage organelles in the uninfected and parenchyma cells and not directly related to nitrogen fixation per se. The overwintered nodules are capable of resuming nitrogen fixation due to the presence of persistent infection threads with rhiozobia and many rod-shaped Rhizobium among the senescent infected cells.

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