rhizobial genes. Exactly which flavonoid in the rhizosphere a compatible bacterium
perceives can be difficult to determine since plants secrete a complex mixture of
flavonoids [1, 7, 71]. In the bacterial cell, flavonoids induce NodD-mediated expression of bacterial nodulation (nod) genes, which encode the enzymes required for
the synthesis of bacterial Nod factors, a family of lipochitooligosaccharides essential
for initiation and development of symbiotic interaction in most legumes [17, 65].
In the response to plant flavonoids, compatible rhizobia can elicit qualitative
and quantitative composition of these compounds in root exudates of the respective
host plants [18, 82]. In particular, root exudates of Ph. vulgaris inoculated with
R. leguminosarum bv. phaseoli contained higher amount of the flavonoid phytoalexin coumestrol and its isoflavonoid precursor daidzein than did exudates of sterile
plants [83]. Other study showed the increased quantities of daidzein, naringenin,
liquiritigenin, and isoliquiritigenin in root exudates of P. vulgaris after inoculation
with homologous rhizobia [84]. It was shown that rhizobia stimulate production
of flavonoids via increasing activities of phenylalanine ammonia lyase and chalcone
synthase involved in the plant phenylpropanoid biosynthesis pathway [85]. Most
flavonoids function as nod gene inducers in nanomolar and micromolar concentrations. Mixture of flavonoids seems to be more efficient in induction of nod gene
expression than a single type [86, 87]. At the same time, different flavonoids can
have distinct roles in nodulation process as was observed in Medicago truncatula
inoculated by Sinorhizobium meliloti [80]. Rhizobia are able to degrade plant
flavonoids with formation of a number of flavonoid derivatives and other phenolic
metabolites; some of them may act as nod gene inducers [11, 20]. Certain flavonoids
may act simultaneously as inducers or inhibitors of nod gene expression depending
on rhizobial species, as in the case of genistein and daidzein [86]. Both these
compounds activate nod genes in Bradyrhizobium japonicum and Rhizobium
sp. NGR234 and are repressors of nod gene expression in R. leguminosarum bvs.
trifolii and viciae [79].
Plant flavonoids penetrate the bacteria and activate bacterial NodD proteins,
which are members of the LysR family of transcriptional activators. NodD proteins
are encoded by nodD genes constitutively expressed in bacterial cells [7, 67,
88]. Flavonoid-activated NodD proteins bind to conserved DNA sequences (nodboxes) in the promoters of inducible nodulation genes (nod genes) with forming
a bend in DNA at the binding site [19, 89]. This binding triggers the expression of
responsive nod genes. NodD proteins from different rhizobial species respond
to different sets of flavonoids. For example, the daidzein and genistein, isoflavonoids
of soybean, induce nod gene expression in B. japonicum. At the same time, daidzein
prevents production of Nod factors in the noncompatible Sinorhizobium meliloti,
which responds positively to the flavone luteolin and does so in a NodD-dependent
manner [1, 90].
Rhizobia species may contain one to five homological NodD proteins, which
can be activated by flavonoids or several non-flavonoid compounds like jasmonates
[19]. The different NodD proteins determine, at least partially, the bacterial specificity to the host, and they are adapted to recognizing defined flavonoid compounds
produced by different legumes [91]. In R. meliloti, the different nodD genes (nodD1,
298
U. Y. Stambulska and M. M. Bayliak
perceives can be difficult to determine since plants secrete a complex mixture of
flavonoids [1, 7, 71]. In the bacterial cell, flavonoids induce NodD-mediated expression of bacterial nodulation (nod) genes, which encode the enzymes required for
the synthesis of bacterial Nod factors, a family of lipochitooligosaccharides essential
for initiation and development of symbiotic interaction in most legumes [17, 65].
In the response to plant flavonoids, compatible rhizobia can elicit qualitative
and quantitative composition of these compounds in root exudates of the respective
host plants [18, 82]. In particular, root exudates of Ph. vulgaris inoculated with
R. leguminosarum bv. phaseoli contained higher amount of the flavonoid phytoalexin coumestrol and its isoflavonoid precursor daidzein than did exudates of sterile
plants [83]. Other study showed the increased quantities of daidzein, naringenin,
liquiritigenin, and isoliquiritigenin in root exudates of P. vulgaris after inoculation
with homologous rhizobia [84]. It was shown that rhizobia stimulate production
of flavonoids via increasing activities of phenylalanine ammonia lyase and chalcone
synthase involved in the plant phenylpropanoid biosynthesis pathway [85]. Most
flavonoids function as nod gene inducers in nanomolar and micromolar concentrations. Mixture of flavonoids seems to be more efficient in induction of nod gene
expression than a single type [86, 87]. At the same time, different flavonoids can
have distinct roles in nodulation process as was observed in Medicago truncatula
inoculated by Sinorhizobium meliloti [80]. Rhizobia are able to degrade plant
flavonoids with formation of a number of flavonoid derivatives and other phenolic
metabolites; some of them may act as nod gene inducers [11, 20]. Certain flavonoids
may act simultaneously as inducers or inhibitors of nod gene expression depending
on rhizobial species, as in the case of genistein and daidzein [86]. Both these
compounds activate nod genes in Bradyrhizobium japonicum and Rhizobium
sp. NGR234 and are repressors of nod gene expression in R. leguminosarum bvs.
trifolii and viciae [79].
Plant flavonoids penetrate the bacteria and activate bacterial NodD proteins,
which are members of the LysR family of transcriptional activators. NodD proteins
are encoded by nodD genes constitutively expressed in bacterial cells [7, 67,
88]. Flavonoid-activated NodD proteins bind to conserved DNA sequences (nodboxes) in the promoters of inducible nodulation genes (nod genes) with forming
a bend in DNA at the binding site [19, 89]. This binding triggers the expression of
responsive nod genes. NodD proteins from different rhizobial species respond
to different sets of flavonoids. For example, the daidzein and genistein, isoflavonoids
of soybean, induce nod gene expression in B. japonicum. At the same time, daidzein
prevents production of Nod factors in the noncompatible Sinorhizobium meliloti,
which responds positively to the flavone luteolin and does so in a NodD-dependent
manner [1, 90].
Rhizobia species may contain one to five homological NodD proteins, which
can be activated by flavonoids or several non-flavonoid compounds like jasmonates
[19]. The different NodD proteins determine, at least partially, the bacterial specificity to the host, and they are adapted to recognizing defined flavonoid compounds
produced by different legumes [91]. In R. meliloti, the different nodD genes (nodD1,
298
U. Y. Stambulska and M. M. Bayliak