Flavonoids Induce Nodulation

Rhizobium and Legume Root Nodulation
Leguminous Plants
The Discovery of Symbiotic Nitrogen Fixation
Rhizobium Classification
Cultural Characteristics
Genetics
Flavonoids Induce Nodualation
Novel Approaches to Extend Root Nodulation
Ecology
Infection
Lectins
Structure of the Nodule
Function of the Nodule
Nitrogenase Hydrogenase Interrelationship
Stem Nodulating Legumes
Factors Affecting Nodulation
Temperature and Light
Hydrogen Ion Concentration
Mineral Nutrition

Flavonoids Induce Nodulation

Flavonoids Induce Nodulation
In the last decade, the following root secreted flavonoids from leguminous plants have been known to induce transcription of nodulation (nod) genes:

Plant source	Compounds
Alfalfa seed (Medicago sativa)	Luteolin
Clover seedling (Trifolum spp.)	Geraldone
Soybean seeding (Glycine max)	Genistein, Daidzein
Bean seed (Phaseolus vulgaris)	Delphinidin, Petunidin
Bean root (P. vulgaris)	Naringenin
Vetch root exudate(Vicia sativa sub sp. nigra)	Naringenin, Liquiritigenin

In some strains of R. meliloti, as many as 15 nodulation genes have been identified. Some of them are barely expressed in R. meliloti in a free-living state but are induced to express themselves manifold in the presence of root exudates. In alfalfa (lucerne, Medicago sativa), the induction factor in the root exudate is a flavone known as luteolin (3',4',5', 7'-tetrahydroxy flavanone). During the initiation of symbiosis, the two symbionts (the Rhizobium and the legume) interact at the molecular level of gene expression by signalling to each other. The product of nod D gene in Rhizobium interacts with the flavonoid compounds secreted by the roots and this follows the activation of other nod genes in a host dependent manner. Several flavonoid molecules produced by legumes have been found to induce or block the transcription of nod genes in Rhizobium and Bradyrhizobium.

A schematic diagram of events at the molecular level in the root region in Rhizobium-legume interaction. The protein product of nod D gene in Rhizobium is activated by contact with plant signal by way of flavonoid compounds. This then interacts on the promoter gene which controls nod ABC genes action to induce nodule formation. A schematic diagram of events at the molecular level in the root region in Rhizobium-legume interaction. The protein product of nod D gene in Rhizobium is activated by contact with plant signal by way of flavonoid compounds. This then interacts on the promoter gene which controls nod ABC genes action to induce nodule formation

The active inducers in root exudates of clover are 7,4-dihydroxyflavone, umbelliferone and formononetin, and soybean rootsrelease naringenin, genistein and diadzein (isoflavones). The positive action of nod genes in R. leguminosarum biovar trifolii is mediated by nod D gene and the interaction of nod D gene product with the plant secreted inducer and anti-inducer compounds. There are nodule inducer as well as anti-inducer regions on the clover root, the plant cells behind the root tip zone being the major site for the secretion of the inducer compounds.

This region is followed upwards with regions on the root alternatively producing stimulatory or inhibitory substances. Apparently, the primary determining factor for sites of nodule initiation in clover and other legumes could be the ratio of stimulator (inducer) : inhibitor (anti-inducer) in the vicinity of potential infection sites or the infection thread during infection. The anti-inducers are coumarins and isoflavones. Three components are necessary for induction of nod genes-1) the nod promoter, (2) the inducing substance produced by the plant which is of flavonoid nature, and 3) the nod D gene product.

Some reports indicate that adding flavonoids (10 mm of luteolin or naringenin) to the root region of certain varieties of alfalfa (Medicago sativa) increased nodulation and N2 fixation by R. melitoli under controlled defined experimental conditions. Experiments have also shown that certain flavonoids could be extracted from soils and are also present in roots of non-legumes such as wheat.

It is too early to say whether these flavonoids present in soil and wheat root could really have a role in root nodulation, but the findings assume significance when we are aware that nitrogen fixing nodules could be induced on wheat and rape seedlings by inoculation with specific rhizobia in the presence of hydrolytic enzymes.

The regulation of common nod genes in Bradyrhizobium japonicum appears to be similar to that of R. meliloti-alfalfasymbiosis. A positive activator nod D1 interacts with flavonoid compounds (daidzein and genestein) resulting in the induction of common nod genes.

In R. meliloti a 14 Kb nif/fix gene cluster with essentially three symbiotically transcription units, have been delineated. These include the structural genes for nitrogenase (nif HDK), a regulatory gene (nif A), and genes of unspecified function (fx ABC).

The development of nitrogen-fixing nodule involves the synthesis of a group of plant proteins, called nodulins. It has been demonstrated that there are nearly 100 nodulins involved in nodule development. Leghaemoglobin is a nodulin gene product. In soybean, the nodulin gene family in the host responsible for the synthesis of leghaemoglobin is activated 7-8 days after infection. Other enzymes in the nodules, the uricase and glutamine synthetase are also controlled by nodulins.

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