Asymbiotic association of Rhizobium with pea epicotyls treated with a plant hormone

Regulation of the expression of leghaemoglobin genes in effective and ineffective root nodules of soybean

The expression of leghaemoglobin genes in effective and ineffective (unable to fix nitrogen) root nodules of soybean developed by Rhizobium japonicum strains 61A76, 61A24 and SM5 was measured by using a cDNA probe or a cloned leghaemoglobin sequence and in vitro translation of Lb-mRNA. Hybridization of the poly(A)-containing nodule polysomal RNA from 3-week-old nodules with a kinetically purified Lb-cDNA or with plasmid (pLbl) containing a leghaemoglobin sequence showed that Lb-mRNA is present in ineffective nodules formed by strains SM5 and 61A24 at reduced levels. Of the two major classes of electrophoretically distinguishable leghaemoglobins in soybean, LbS was not synthesized in 3-week-old strain 61A24-induced nodules while both sorts of leghaemoglobin were synthesized and accumulated in ineffective nodules formed by strain SM5 of Rhizobium. Ineffective nodules formed by strain 61A24 are green inside and do not appear to accumulate leghaemoglobin as measured by the haemochromogen assay, although low levels of apoleghaemoglobin were detected using leghaemoglobin antibodies. SM5-induced nodules were found to have about half as much as leghaemoglobin of the of effective (61A76-induced) nodules. This study demonstrates that while the appearance of leghaemoglobin is independent of nitrogenase activity in bacteroids, its synthesis is influenced to different degrees both by a mutation (SM5) and incompatibility (61A24) of Rhizobium. The primary regulation appears to be at the level of transcription or processing of mRNA since ineffective nodules contain Lb-mRNA approximately in proportion to the amount of apoleghaemoglobin present in these nodules.

Development of nodules of Glycine max infected with an ineffective strain of Rhizobium japonicum

Bacteroids in ineffective (nitrogenase negative) nodules of Glycine max, infected with Rhizobium japonicum 61-A-24, as compared to those in effective nodules are characterized by reduced specific activities of alanine dehydrogenase to 15%, of 3-hydroxybutyrate dehydrogenase to 50%, and an increase of glutamine synthetase to 400%. In the plant cytoplasm of ineffective nodules, glutamine synthetase activity is reduced to 10-30%, glutamate dehydrogenase to 50-70%, and the aspartate aminotransferase and alanine aminotransferase are enhanced to 120-200%, depending on the age of the nodules. The total pool of soluble amino acids is reduced to 52 μmol per g nodule fresh weight, as compared to 186 μmol in effective nodules, with a replacement of asparagine (42 mol% of the amino acids) by an unknown amino compound. This compound is absent in nitrogenase, repressed and derepressed, free-living Rhizobium japonicum cells and in the uninfected root tissue. In nitrogenase derepressed, as compared to the repressed free-living cells of Rhizobium japonicum 61-A-101, arginine shows the most obvious change with a reduction to less than one tenth. The ultrastructure of the ineffective nodule is different from the effective organ even in the early stages. The membrane envelopes of the infection vacuoles are decomposing in heavily infected cells within 18 to 20 d after infection. In lightly infected cells very large vacuoles develop with only a few bacteroids inside. No close associations of cristae-rich mitochondria with amyloplasts are observed as in effective nodules. The uninfected cells keep their large starch granules even 40 d after infection. Some poly-β-hydroxybutyrate accumulation in the bacteroids is observed but only in the early stages, and it is almost absent in old nodules (40 d). At this age the infected cells are obviously compressed by uninfected cells, whereas in effective nodules with nitrogenase activity and leghaemoglobin formation, the infected cells have a much higher osmotic pressure than the neighbouring uninfected cells.

A Nodule-Specific Plant Protein (Nodulin-35) from Soybean

Nodulin-35, a 35,000-molecular-weight protein, is present in soybean root nodules developed by different strains of Rhizobium japonicum, irrespective of their effectiveness in fixing atmospheric nitrogen. This protein is not detected in uninfected plants and bacteroids or in free-living Rhizobium and appears to be synthesized by the plant during the formation of root nodules.

Isolation and metabolism of Vigna unguiculata root nodule protoplasts

Axenic cultures of bacteroid-containing protoplasts were isolated from root nodules of Vigna unguiculata L. Walp. Dimensions of the protoplasts were 35 to 135 μm long x 35 to 95 μm wide. Yields were about 30 to 50 mg dry weight per gram fresh weight of nodules. About 5x10(8) protoplasts packed into 1 ml of basal medium under the influence of gravity. When incubated in hypertonic, nitrogen-free media, freshly isolated protoplasts began to reduce acetylene to ethylene after a lag period of 24 to 48 h. Various additions to the basal medium showed that the system possessed functional glycolytic and tricarboxylic acid pathways. Endogenous application of various intermediary metabolites stimulated both acetylene reduction and respiration, though not often equally. As acetylene reduction, but not respiration, was inhibitable by both asparagine and glutamine, the system appears suitable for the study of mechanisms controlling symbiotic nitrogen fixation.