S. meliloti, R. etli), or grouped in a large chromosomal region called a symbiotic
island (e.g., in M. loti and B. japonicum) [20, 71].
Nod factors initiate specific signaling cascades in root hairs and root cortex
resulting in expression of the early genes of symbiotic interaction. The products of
these genes cause deformation of the root hairs followed by root hair curling to trap
bacteria and induce formation and growth of infection threads to transport the
bacteria to the root cortex [29, 94]. Nod factors also promote nodule formation
a result of stimulation of cell proliferation in the cortex root due to changes
in phytohormone levels and induction-specific plant genes (nodulins) [63, 72, 75,
94]. Nod factors are perceived by plant Nod factor receptors (e.g., NFR1 and
NFR5 in Lotus japonicus), which are LysM-domain-containing receptor kinases
[12, 95]. Direct binding of Nod factors to the extracellular LysM domains of
the receptor complex leads to activation of the downstream nodulation signaling
pathways [96]. In particular, after Nod factor perception by plant receptor kinases,
Ca
2+ oscillation occurring in root hair cells initiates downstream signaling events
[20, 95]. Specificity in Nod factor binding is thought to be critical for recognition
between the prospective symbiotic partners.
2.4.2 Surface Polysaccharides
Many studies reported that rhizobial invasion of the host nodule via the infection
threads is strongly influenced by a complex variety of bacterial polysaccharides
in addition to Nod factors [28, 30, 97–99]. Rhizobia produce at least seven
different types of cell-surface polysaccharides: extracellular polysaccharides, lipopolysaccharides, capsular polysaccharides, gel-forming polysaccharide, K-antigen
polysaccharides, cyclic glucans, and high-molecular mass neutral polysaccharides
(glucomannans) [30, 31, 91, 97]. Polysaccharides contribute to various stages
of symbiotic development including root colonization, host-plant recognition,
infection thread formation, and nodule invasion. Bacterial polysaccharides are
also important for the evasion of plant immune responses and as protectants against
ROS [94, 97, 100].
Bacterial polysaccharides are recognized by specific plant receptors called
collectively lectins. Lectins are glycoproteins, which are abundant in legume seeds
and present on tips of growing root hairs. Lectins have no enzymatic activity;
however, binding carbohydrate residues, they facilitate the attachment of bacteria
to the host plant and modulate some processes of symbiosis [20, 28, 101].
Rhizobial polysaccharides lightly connected with the bacterial surface and
secreted in large amounts into the soil are named exopolysaccharides (EPSs).
EPSs are a major component of the cell surface and play a significant role in
secondary attachment. Rhizobial EPSs are chemically diverse species- or strainspecific heteropolymers and homopolymers that are composed of linear or branched
repeating units containing monosaccharides (D-glucose, D-mannose, D-galactose)
and D-galacturonic acid, substituted with noncarbohydrate moieties (e.g., acetyl,
pyruvyl, succinyl, etc.) [20, 28, 31]. These molecules demonstrate highly variable
compositions between strains and species [76, 91], but low-molecular mass fractions
were the most active in the infection process [102]. The role of EPSs was the best
300
U. Y. Stambulska and M. M. Bayliak
island (e.g., in M. loti and B. japonicum) [20, 71].
Nod factors initiate specific signaling cascades in root hairs and root cortex
resulting in expression of the early genes of symbiotic interaction. The products of
these genes cause deformation of the root hairs followed by root hair curling to trap
bacteria and induce formation and growth of infection threads to transport the
bacteria to the root cortex [29, 94]. Nod factors also promote nodule formation
a result of stimulation of cell proliferation in the cortex root due to changes
in phytohormone levels and induction-specific plant genes (nodulins) [63, 72, 75,
94]. Nod factors are perceived by plant Nod factor receptors (e.g., NFR1 and
NFR5 in Lotus japonicus), which are LysM-domain-containing receptor kinases
[12, 95]. Direct binding of Nod factors to the extracellular LysM domains of
the receptor complex leads to activation of the downstream nodulation signaling
pathways [96]. In particular, after Nod factor perception by plant receptor kinases,
Ca
2+ oscillation occurring in root hair cells initiates downstream signaling events
[20, 95]. Specificity in Nod factor binding is thought to be critical for recognition
between the prospective symbiotic partners.
2.4.2 Surface Polysaccharides
Many studies reported that rhizobial invasion of the host nodule via the infection
threads is strongly influenced by a complex variety of bacterial polysaccharides
in addition to Nod factors [28, 30, 97–99]. Rhizobia produce at least seven
different types of cell-surface polysaccharides: extracellular polysaccharides, lipopolysaccharides, capsular polysaccharides, gel-forming polysaccharide, K-antigen
polysaccharides, cyclic glucans, and high-molecular mass neutral polysaccharides
(glucomannans) [30, 31, 91, 97]. Polysaccharides contribute to various stages
of symbiotic development including root colonization, host-plant recognition,
infection thread formation, and nodule invasion. Bacterial polysaccharides are
also important for the evasion of plant immune responses and as protectants against
ROS [94, 97, 100].
Bacterial polysaccharides are recognized by specific plant receptors called
collectively lectins. Lectins are glycoproteins, which are abundant in legume seeds
and present on tips of growing root hairs. Lectins have no enzymatic activity;
however, binding carbohydrate residues, they facilitate the attachment of bacteria
to the host plant and modulate some processes of symbiosis [20, 28, 101].
Rhizobial polysaccharides lightly connected with the bacterial surface and
secreted in large amounts into the soil are named exopolysaccharides (EPSs).
EPSs are a major component of the cell surface and play a significant role in
secondary attachment. Rhizobial EPSs are chemically diverse species- or strainspecific heteropolymers and homopolymers that are composed of linear or branched
repeating units containing monosaccharides (D-glucose, D-mannose, D-galactose)
and D-galacturonic acid, substituted with noncarbohydrate moieties (e.g., acetyl,
pyruvyl, succinyl, etc.) [20, 28, 31]. These molecules demonstrate highly variable
compositions between strains and species [76, 91], but low-molecular mass fractions
were the most active in the infection process [102]. The role of EPSs was the best
300
U. Y. Stambulska and M. M. Bayliak