Alfalfa Root Nodule Invasion Efficiency Is Dependent on Sinorhizobium meliloti Polysaccharides

Abstract

The soil bacterium Sinorhizobium meliloti is capable of entering into a nitrogen-fixing symbiosis with Medicago sativa (alfalfa). Particular low-molecular-weight forms of certain polysaccharides produced by S. meliloti are crucial for establishing this symbiosis. Alfalfa nodule invasion by S. meliloti can be mediated by any one of three symbiotically important polysaccharides: succinoglycan, EPS II, or K antigen (also referred to as KPS). Using green fluorescent protein-labeled S. meliloti cells, we have shown that there are significant differences in the details and efficiencies of nodule invasion mediated by these polysaccharides. Succinoglycan is highly efficient in mediating both infection thread initiation and extension. However, EPS II is significantly less efficient than succinoglycan at mediating both invasion steps, and K antigen is significantly less efficient than succinoglycan at mediating infection thread extension. In the case of EPS II-mediated symbioses, the reduction in invasion efficiency results in stunted host plant growth relative to plants inoculated with succinoglycan or K-antigen-producing strains. Additionally, EPS II-and K-antigen-mediated infection threads are 8 to 10 times more likely to have aberrant morphologies than those mediated by succinoglycan. These data have important implications for understanding how S. meliloti polysaccharides are functioning in the plant-bacterium interaction, and models are discussed.

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…Sinorhizobium meliloti strain 1021 requires the exopolysaccharide succinoglycan to establish a functional symbiosis with Medicago truncatula and Medicago sativa 36,143 . Bacterial mutants that are defective in succinoglycan production cannot initiate infection thread formation and yield nodules that are devoid of bacteria or bacteroids 36,143 .…”

“…(TABLE 1). Unlike the normal condition, aberrant infection threads are bulbous and winding, as if the bacteria have continued to proliferate even though the tip of the infection thread has halted its inward progression 36,39 . In contrast to exoY-mutant-containing CCRHs, 70% of exoH-mutant-containing CCRHs initiate infection threads, but they all abort before reaching the base of the root hair 39 .…”

“…meliloti produces the exopolysaccharides succinoglycan (also known as exopolysaccharide I, EPSI) and galactoglucan (EPSII), which facilitate infection thread formation 36,37 . Succinoglycan is a polymer of an octasaccharide repeating unit modified with acetyl, succinyl and pyruvyl substituents 38 …”

“…, is more efficient than galactoglucan in mediating infection thread formation on M. sativa and is the only exopolysaccharide produced by S. meliloti that can mediate the formation of infection threads on M. truncatula 36,37 . An S. meliloti exoY mutant, which cannot produce any succinoglycan, can form CCRHs but initiates almost no infection threads -these mutants remain trapped in a microcolony at the tip of the root hair 39 (FIGS 3,4; TABLE 1).…”

“…Bacterial mutants that are defective in succinoglycan production cannot initiate infection thread formation and yield nodules that are devoid of bacteria or bacteroids 36,143 . Succinoglycan is a polymer of an octasaccharide repeating unit composed of one galactose and seven glucose residues with acetyl, succinyl and pyruvyl modifications 38,144 (see figure).…”