Localization of oleosomes in the root nodules of alfalfa (Medicago sativa)

Abstract

Histological studies of rhizobia-induced nodules and spontaneous pseudonodules of alfalfa were undertaken to reveal the distribution pattern of oleosomes. Alfalfa (Medicago sativa L. cv. 'Algonquin') seedlings were inoculated with Rhizobium meliloti (strain Balsac), and grown in controlled conditions. Seedlings were also grown aseptically to obtain spontaneous pseudonodules. Nodule slices were fixed in a paraformaldehyde-glutaraldehyde mixture and processed for embedding in Spurr's medium after OsO4 treatment and en bloc staining, with p-phenylenediamine (pPD) in 70% ethanol, for oleosomes. As a control, hexane extraction of lipids was employed. In pseudonodules, oleosomes were present in the vascular parenchyma, the meristem region and in the five to seven layers of cortical cells adjacent to the vascular tissue. The allocation of energy stored as triacylglycerides, in the form of oleosomes, and starch indicates that pseudonodules are a sink for carbon, possibly functioning as storage organs. In mature nodules, oleosomes were distinguished clearly in nodule parenchyma but were absent in infected cells. Young nodules were devoid of oleosomes. The prevalence of oleosomes in the mature nodules may indicate that the triacylglycerides are stored for overwintering, as alfalfa nodules are known to be perennial. The oleosomes in root nodules of alfalfa do not appear to be directly associated with nitrogen fixation per se.Key words: oleosomes, root nodules, Medicago sativa, Rhizobium, pseudonodules.