xylem and phloem branch penetrate the nodules. Hence, nodules are newly plant
organs, which contain infection bacterial zone, where molecular nitrogen fixation
occurs; conducting fabrics, in which vegetative photoassimilates arrive to bacteria
and products of nitrogen fixation are transported to plants; and meristem, which
is responsible for the growth of nodules [72]. At the final stage of nodule formation,
the bacteria intensively multiply in the cytoplasm of the nodule cell. They are
surrounded by additional membranes and lose the flagella, gradually being pulled
out and gaining the appearance of girdle sticks. In this state, they continue to
multiply and turn into different forms of bacteroids [73]. Transformation of bacteria
into nitrogen-fixing bacteroids completes nodule formation [7, 70].
2.2
Chemotaxis of Rhizobia to Root Exudates as an Early Event
in Symbiotic Initiation
The host plant and rhizobia exhibit a strong mutual specificity. Since many microorganisms, including pathogenic and symbiotic ones, are present in the soil, the host
plant must first identify rhizobia as beneficial partners [1, 5, 8]. To initiate communication, plants secrete large amounts of different low-molecular mass compounds;
some of them are used by microorganisms as carbon and energy sources (carbohydrates, amino acids, organic acids), whereas others can be signal molecules for
attraction of the homologous (compatible) bacteria (flavonoids) [3, 18, 20]. Root
and seedling exudates induce directed movement (chemotaxis) of nodule bacteria
to the host plant that is an early stage in the interaction between micro- and
macrosymbionts [11, 19]. Reacting to root exudates, bacteria are concentrated in
the rhizosphere zone at a width of ~ 100 microns, and their number increases by
2–3 orders of magnitude [11, 74]. Bacteria can recognize the host plant via nonspecific and specific chemotaxis reaction. The result of the first one is the movement
of bacteria to simple molecules (carbohydrates, organic acids, and amino acids); the
result of the second is the movement to large molecules (hormones, lectins, and
enzymes) [14, 20]. In the laboratory, chemotaxis activity can be assessed by measuring growth zone (chemotaxis zone) of rhizobia after inoculation onto agar plates
containing different chemical compounds [15] (Fig. 3).
Rhizobia are positively chemotactic to various legume epidermal exudates,
including carbohydrates, amino acids, dicarboxylic and hydroxyaromatic acids,
and many phenolic compounds [18, 20, 75, 76]. The composition of root exudates
can be different between plant species and allows the selective requirement of
specific groups of microorganisms [77]. For example, pea plants select their symbiont Rhizobium leguminosarum by the excretion of homoserine into the rhizosphere
[78]. Root exudates also play an important role in plant defense through the secretion
of phytochemicals that can inhibit the growth of certain microbes. The ability to
tolerate these chemicals can play an important role in the ability to colonize the
plant [3].
Flavonoids are the most important compounds in legume root exudates [18, 79,
80]. They act as signal molecules and induce a specific symbiotic response in
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U. Y. Stambulska and M. M. Bayliak
organs, which contain infection bacterial zone, where molecular nitrogen fixation
occurs; conducting fabrics, in which vegetative photoassimilates arrive to bacteria
and products of nitrogen fixation are transported to plants; and meristem, which
is responsible for the growth of nodules [72]. At the final stage of nodule formation,
the bacteria intensively multiply in the cytoplasm of the nodule cell. They are
surrounded by additional membranes and lose the flagella, gradually being pulled
out and gaining the appearance of girdle sticks. In this state, they continue to
multiply and turn into different forms of bacteroids [73]. Transformation of bacteria
into nitrogen-fixing bacteroids completes nodule formation [7, 70].
2.2
Chemotaxis of Rhizobia to Root Exudates as an Early Event
in Symbiotic Initiation
The host plant and rhizobia exhibit a strong mutual specificity. Since many microorganisms, including pathogenic and symbiotic ones, are present in the soil, the host
plant must first identify rhizobia as beneficial partners [1, 5, 8]. To initiate communication, plants secrete large amounts of different low-molecular mass compounds;
some of them are used by microorganisms as carbon and energy sources (carbohydrates, amino acids, organic acids), whereas others can be signal molecules for
attraction of the homologous (compatible) bacteria (flavonoids) [3, 18, 20]. Root
and seedling exudates induce directed movement (chemotaxis) of nodule bacteria
to the host plant that is an early stage in the interaction between micro- and
macrosymbionts [11, 19]. Reacting to root exudates, bacteria are concentrated in
the rhizosphere zone at a width of ~ 100 microns, and their number increases by
2–3 orders of magnitude [11, 74]. Bacteria can recognize the host plant via nonspecific and specific chemotaxis reaction. The result of the first one is the movement
of bacteria to simple molecules (carbohydrates, organic acids, and amino acids); the
result of the second is the movement to large molecules (hormones, lectins, and
enzymes) [14, 20]. In the laboratory, chemotaxis activity can be assessed by measuring growth zone (chemotaxis zone) of rhizobia after inoculation onto agar plates
containing different chemical compounds [15] (Fig. 3).
Rhizobia are positively chemotactic to various legume epidermal exudates,
including carbohydrates, amino acids, dicarboxylic and hydroxyaromatic acids,
and many phenolic compounds [18, 20, 75, 76]. The composition of root exudates
can be different between plant species and allows the selective requirement of
specific groups of microorganisms [77]. For example, pea plants select their symbiont Rhizobium leguminosarum by the excretion of homoserine into the rhizosphere
[78]. Root exudates also play an important role in plant defense through the secretion
of phytochemicals that can inhibit the growth of certain microbes. The ability to
tolerate these chemicals can play an important role in the ability to colonize the
plant [3].
Flavonoids are the most important compounds in legume root exudates [18, 79,
80]. They act as signal molecules and induce a specific symbiotic response in
296
U. Y. Stambulska and M. M. Bayliak