Fate of Nodule-Specific Polysaccharide Produced by Bradyrhizobium japonicum Bacteroids

Abstract

A polysaccharide produced by 5radyrhizobium japonicum bacteroids i n nodules (NPS) on soybean (Glycine max [LI Merr.) roots is different in composition and structure from the extracellular polysaccharide produced in culture by this organism. lsogenic strains either capable or incapable of NPS synthesis supported similar rates of plant growth and nitrogenase activity, indicating that polysaccharide deposition was not detrimental. The possibility that NPS may have some protective or nutritional role for bacteroids was considered. Analysis of disintegrating nodules over periods of 1 to 3 months indicated greater recovery of viable bacteria from NPS+ nodules prior t o the breakdown of NPS. During and after the breakdown of NPS, the decline in viable bacteria was similar for NPS+ and NPS-strains. Bacteroid destruction in senescing nodules may be accelerated by exposure t o proteolytic enzymes in host cytoplasm; however, highly purified NPS had no significant effect on the i n vitro activity of partially purified proteases, so protection of bacteroids via this mechanism is unlikely. 5. japonicum USDA 438 did not utilize NPS as a carbon source for growth in liquid culture. I n vitro assays of NPS depolymerase activity in cultured bacteria and bacteroids were negative using a variety of strains, all of which contained extracellular polysaccharide depolymerase. It seems highly unlikely that 5. japonicum can utilize the polysaccharide it synthesizes in nodules, and NPS breakdown in senescing nodules is probably caused by saprophytic fungi.Certain strains of Bradyrkizobium japonicum and Bradyrkizobium elkanii (formerly B. japonicum Group 11; Kuykendall et al., 1992) synthesize large quantities of polysaccharide in soybean nodules . The NPS accumulates in the symbiosomes of infected cells and is a major feature seen in micrographs of infected cells of soybean nodules after 50 or 60 d of growth

Soil Biology of the Rhizobiaceae

“…(1993) reported no significant decline in soybean rhizobia for up to 4 years after the last crop. It has been suggested that such ability to survive over long periods of time in soil might be helped to some degree by the protective action of a nodule-specific polysaccharide (Streeter et al, 1995) though these substances appear unlikely to persist for long periods of time in soil.…”

Effects of drought on nitrogen fixation in soybean root nodules

“…Six soybean seeds, cv Flint, were planted and each seed was irrigated with a fresh culture of B. japonicum. The wild-type strain was USDA 438 (NPS + ) and the NPS-strain was a transposon insertion mutant of 438 (Streeter et al . 1995).…”

“…1992). It is certain that the nodule‐specific polysaccharide (NPS) is a bacterial product, not a plant product, and that bacteroids are unable to hydrolyse NPS after they synthesize it (Streeter et al . 1995).…”

An experimental and modelling exploration of the host-sanction hypothesis in legume–rhizobia mutualism

“…The occurrence of low N 2 -fixing and ineffective rhizobia cheating strains in the same plant is common (Singleton and Tavares, 1986;Bronstein, 2001), and accumulation of resources by some non-fixing rhizobia in bacteroid stage has been proposed as cheating advantage at plant´s expenses (Denison, 2000). However, this accumulation is a general metabolic consequence of reduced carbon demand from the plant (Lodwig, 2003) and not necessarily implies rhizobia further survival advantages (Streeter et al, 1995).…”