illustrated in Sinorhizobium-Medicago symbiosis. It was shown that succinoglycan,
a major surface EPS in bacteria S. meliloti, is required for the initiation and
elongation of infection threads, and increased succinoglycan production enhances
nodulation capacity [98, 99]. However, the symbiotic role of EPS is more complicated in the Mesorhizobium-Lotus interaction [103]. Several EPS mutants of M. loti
R7A formed uninfected nodule primordia on roots of L. japonicus and
L. corniculatus, whereas other mutants formed effective nodules [103]. It was
proposed that EPSs are able to modulate the host immunity and its ability depends
on the length of EPSs. Full-length EPSs and EPS minor mutants can suppress plant
innate immunity allowing infection, whereas significant modified EPSs trigger plant
defense responses resulting in block of infection [12, 103]. In addition, the expression of predicted defense-related genes significantly increased in M. truncatula
inoculated with a succinoglycan-deficient mutant compared with the control strain
producing succinoglycan [98].
Lipopolysaccharides (LPS) are typical components of the outer membrane in
the gram-negative bacteria. Rhizobial LPSs consist of three structural regions: an
O-chain polysaccharide that is attached to a core oligosaccharide, which is attached
to an acylated saccharide known as the lipid A. Lipid A is a hydrophobic component,
which anchored LPS into the phospholipid layer of the outer membrane [30,
31]. LPSs also play an essential role in legume-rhizobium symbiosis, but
in later steps of this interaction (i.e., differentiation of bacteria into bacteroids) [12,
30, 97]. LPSs from different strains of R. leguminosarum had the same chemical
structure regardless of the symbiotic properties of bacteria. However, the content
of LPSs may vary in different R. leguminosarum strains [104]. The structures of
lipid A in different rhizobial species have a variation in the glycosyl component
of its backbone and acylation pattern [12]. Depending on the structure,
lipid A can have no effects or either activate or inhibit the host innate
immune response [105]. As a result, lipid A can differently affect nodulation
process [12, 30, 97].
Rhizobia also can synthesize several types of basic cyclic glucans, which contain
15–30 glucose residues, depending on species of microorganisms and the type
of glucan (β-(1,2)-, β-(1,3)-, and β-(1,6)- glucans) [31]. Free-living rhizobia and
bacteroids can synthesize cyclic polysaccharides, which contain about 13 glucose
residues linked by β-(1,6)- and β-(1,3)-glycosidic bonds [106]. Cyclic neutral
β-(1,2)-glucans are located in the periplasmic space and play an important role
during hypoosmotic adaptation and plant infection [31]. Cyclic glucans are able to
increase the solubility of legume flavonoids and thus to make nodulation more
effective. These oligosaccharides can also serve as host-specific determinants of
rhizobia, since irrespective of the nodule type formed by the host plant, when
rhizobia lack cyclic glucans [12].
2.4.3 Secreted Proteins
In addition to the non-proteinaceous host specificity determinants described above
(Nod factors and surface polysaccharides), a third class of rhizobial signals that
affect symbiosis consists of secreted proteins [64]. Rhizobia have several secretion
13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
301
a major surface EPS in bacteria S. meliloti, is required for the initiation and
elongation of infection threads, and increased succinoglycan production enhances
nodulation capacity [98, 99]. However, the symbiotic role of EPS is more complicated in the Mesorhizobium-Lotus interaction [103]. Several EPS mutants of M. loti
R7A formed uninfected nodule primordia on roots of L. japonicus and
L. corniculatus, whereas other mutants formed effective nodules [103]. It was
proposed that EPSs are able to modulate the host immunity and its ability depends
on the length of EPSs. Full-length EPSs and EPS minor mutants can suppress plant
innate immunity allowing infection, whereas significant modified EPSs trigger plant
defense responses resulting in block of infection [12, 103]. In addition, the expression of predicted defense-related genes significantly increased in M. truncatula
inoculated with a succinoglycan-deficient mutant compared with the control strain
producing succinoglycan [98].
Lipopolysaccharides (LPS) are typical components of the outer membrane in
the gram-negative bacteria. Rhizobial LPSs consist of three structural regions: an
O-chain polysaccharide that is attached to a core oligosaccharide, which is attached
to an acylated saccharide known as the lipid A. Lipid A is a hydrophobic component,
which anchored LPS into the phospholipid layer of the outer membrane [30,
31]. LPSs also play an essential role in legume-rhizobium symbiosis, but
in later steps of this interaction (i.e., differentiation of bacteria into bacteroids) [12,
30, 97]. LPSs from different strains of R. leguminosarum had the same chemical
structure regardless of the symbiotic properties of bacteria. However, the content
of LPSs may vary in different R. leguminosarum strains [104]. The structures of
lipid A in different rhizobial species have a variation in the glycosyl component
of its backbone and acylation pattern [12]. Depending on the structure,
lipid A can have no effects or either activate or inhibit the host innate
immune response [105]. As a result, lipid A can differently affect nodulation
process [12, 30, 97].
Rhizobia also can synthesize several types of basic cyclic glucans, which contain
15–30 glucose residues, depending on species of microorganisms and the type
of glucan (β-(1,2)-, β-(1,3)-, and β-(1,6)- glucans) [31]. Free-living rhizobia and
bacteroids can synthesize cyclic polysaccharides, which contain about 13 glucose
residues linked by β-(1,6)- and β-(1,3)-glycosidic bonds [106]. Cyclic neutral
β-(1,2)-glucans are located in the periplasmic space and play an important role
during hypoosmotic adaptation and plant infection [31]. Cyclic glucans are able to
increase the solubility of legume flavonoids and thus to make nodulation more
effective. These oligosaccharides can also serve as host-specific determinants of
rhizobia, since irrespective of the nodule type formed by the host plant, when
rhizobia lack cyclic glucans [12].
2.4.3 Secreted Proteins
In addition to the non-proteinaceous host specificity determinants described above
(Nod factors and surface polysaccharides), a third class of rhizobial signals that
affect symbiosis consists of secreted proteins [64]. Rhizobia have several secretion
13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
301