Involvement of microbodies in symbiotic nitrogen-fixing Arachis hypogaea L. (peanut) root nodules

Studies were undertaken to examine the ultrastructure and enzyme activities of peanut, Arachis hypogaea L. Var. Jumbo virginia root nodules induced by the nitrogen-fixing rhizobial strain, Bradyrhizobium sp. 32H1. Root nodules at different stages of development were assayed for their nitrogen-fixing ability using the acetylene reduction technique. Highest values for nitrogen fixation were obtained in 35 days old nodules. The bacteroids and the host cytosol/organelle fractions of root nodules were assayed for catalase activity. Catalase activity was present in all fractions, being highest in the bacteroids. The in vitro grown 32H1 were also assayed for catalase activity. There was negligible or no catalase activity in intact bacteria, but the supernatant from broken bacteria showed considerable catalase activity. -- Peanut root nodule anatomy differs considerably from the nodules of other nitrogen-fixing legumes. The central infected zone is devoid of any uninfected cells except for the rays of cortical cells running through the infected cells, which divide them into several masses. Sections fixed in a mixture of paraformaldehyde and glutaraldehyde were processed for ultrastructural studies. The 3,3'diaminobenzidine (DAB) reaction for localization of catalase was used. Controls were run by adding 3-amino-1,2,4-triazole (AT), potassium cyanide (KCN) in the incubation mixture and also by pre-boiling sections of incubating the sections without DAB and hydrogen peroxide. -- The characteristic ultrastructural arrangement and differentiation of rhizobia into large spherical bacteroids was observed within the host cells. Besides other organelles, lipid bodies were present in abundance and in close association with the bacteroids. Often they were seen attached to the peribacteroidal membrane envelope. Microbodies were observed in both uninfected and infected cells at all stages of nodule development unlike other nitrogen-fixing legumes where they are only found in uninfected cells. The microbodies in peanut root nodules were in close association and often in contact with the peribacteroidal membrane envelope and the bacteroids. Occasionally their membranes were seen fusing with the peribacteroidal membrane envelope. Microbodies were found to be DAB positive and the osmiophilic electron dense reaction product was found within its matrix. The presence of abundant lipid bodies, their physical contact with the peribacteroidal membrane envelope and with the bacteroids along with the microbodies and high catalase activity suggest that the lipids may be utilized as a carbon source during symbiosis through β-oxidation pathway. -- Besides the lipid bodies and the microbodies another osmiophilic DAB positive structure called the dense body was observed associated with the bacteroids. It resembled the central core of the microbody, lacked a membrane and was present at the interface between the peribacteroidal membrane envelope and the outer bacteroidal membrane and also within the matrix of the bacteroids. The DAB positive reaction shown by the dense bodies and their association with the bacteroids indicate that there is a possible role for them in the breakdown of hydrogen peroxide, produced during lipid utilization