Role of Exopolysaccharides of Rhizobium leguminosarum bv. viciae as Host Plant-Specific Molecules Required for Infection Thread Formation During Nodulation of Vicia sativa

Abstract

Mutants of Rhizobium leguminosarum bv. viciae bacteria that are affected in the biosynthesis of exopolysaccharides (EPS) are unable to effectively nodulate their host plants. By studying defined mutants, we show that R. legumi-nosarum bv. viciae strains require EPS for formation of infection threads in Vicia sativa (vetch) as well as for efficient induction of tight root hair curling. Results of coinoculation experiments with the EPS-deficient pssD111 mutant of R. leguminosarum bv. viciae in combination with heterologous EPS-producing strains indicated that vetch has certain structural requirements for rhizobial EPS to function in symbiosis. We hypothesize that EPS accelerates root hair curling and infection to such an extent that rhizobial root penetration precedes a plant defense response.

Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes

“…If the infection thread exits the epidermal cell, it does so by fusing with the distal cell wall, and bacteria enter the intercellular space between the epidermal cell and the underlying cell layer. Invagination and tip growth, similar to those seen at the beginning of infection thread growth, occur in the underlying cell, and a thread filled with bacteria is propagated further toward the root interior (170,173,175). Branching of the thread as it grows through the root and enters the nodule primordium increases the number of sites from which bacteria can exit the thread and enter nodule cells, ensuring that a sufficient number of nodule cells are colonized.…”

“…However, plants that form determinate nodules do not have such a requirement (16,31,54,64,67,68,81,88,94,95,121,134,175). The roles of rhizobial exopolysaccharides in symbiotic nodulation have been most thoroughly studied in S. meliloti.…”

“…Infection threads also cannot initiate normally in vetch root hairs inoculated with R. leguminosarum bv. viciae exo mutants (175). In the S. meliloti-alfalfa symbiosis and other symbioses, the Fix Ϫ phenotypes of exo mutants can be reversed by the addition to roots of small amounts of purified, lowmolecular-weight succinoglycan (6,44,65,167).…”

“…Pellock et al suggested that succinoglycan may be involved in organizing the root hair cytoskeleton in such a way that tip growth of the infection thread is rapid and highly polarized (121). Others have shown that exopolysaccharides are able to modulate (down regulate) defense responses in plants (57,114,115,174,175). Thus, rhizobial mutants lacking exopolysaccharides may cause abnormal infection thread growth because the plant mounts an inappropriate defense response to the invading rhizobia.…”

Exopolysaccharide Biosynthesis in Rhizobium leguminosarum: From Genes to Functions

“…It was found that Alg14 was needed for the correct positioning of Alg13 on the cytosolic face of the endoplasmic reticulum membrane mediating the formation of the active UDP-N-acetylglucosamine transferase complex [65,66]. Mutations in the pssE and pssD genes fully abolished EPS production and as a consequence resulted in defects of nodule infection [67][68][69].…”