Role of lipid bodies in N2-fixing root nodules of peanut (Arachis hypogaea L.)

Siddique, Abu-Baker M. (1991) Role of lipid bodies in N2-fixing root nodules of peanut (Arachis hypogaea L.). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The peanut plant (Arachis hypogaea L.) , unlike other legumes, can sustain nitrogen fixation when prolonged periods of darkness or detoping curtail the supply of photosynthate to the nodule. This ability to withstand photosynthate stress is attributed to the presence of lipid bodies in infected nodule cells. In both dark-treated and detopped plants, the lipid bodies show a gradual decrease in numbers, which suggests their utilization as a source of energy and carbon for nitrogen fixation. Lipolytic activity can be localized in the lipid bodies and the existence of the B-oxidation pathway and glyoxylate cycle is shown by the release of ¹⁴CO₂ from lino-leoyl Coenzyme A by the nodule homogenate. In addition, the biochemical assay and cytochemical localization of catalase and malate synthase also confirm lipid catabolism in the nodule. -- Catalase from cultured Bradyrhizobium sp. 32H1, root nodules and seeds of peanut is active in a wide range of pH, having two pH optima. The enzyme activity is associated with both the bacteroids and the host cytosol. In isolated nodule bacteroids the presence of catalase is restricted to the bacteroid surface only, whereas in the rhizobia grown in culture, the activity remains inside the cells. Triazole-sensitive DAB reaction revealed microbodies in the host cytoplasm, often lying close to the peribacteroid membrane. DAB-positive dense bodies are also found on the bacteroid surface at the host-symbiont interface. Ineffective nodules of peanut induced by two nod+fix+ strains of Bradyrhizobium sp. were compared with the ones induced by nod+fix+ strain NC92. Both fix strains (639 and 7091) induce small nodules lacking leghemoglobin and nitrogen-fixing activity. Ultrastructural observations revealed that the nodules of 639 have an enlarged peribacteroid space and lack persistence of nodule function. The 7091-induced nodules showed an impediment to bacteroid release and differentiation. In the ineffective nodules, larger numbers of lipid bodies were found to accumulate, compared to the effective NC92 nodules. The large lipid accumulation in the absence of nitrogen fixation serves as further evidence to confirm that the nodule lipid bodies are utilized as a supplementary source of carbon and energy for nitrogen fixation in peanut root nodules.

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