Lectins

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

Lectins

Lectins
Several investigators are trying to unravel the factors behind the specific recognition between a Rhizobium strain and its homologous host. This recognition phenomenon has been implicated to plant lectins (proteins) which specifically bind to carbohydrate receptors on the rhizobial cell. The current evidence points out that the receptors for the specific binding of clover and soybean lectins to rhizobia are polysaccharides of the bacterial capsule.

Most of the work on lectins have relied on seed lectin as source material but recent reports indicate that 'trifoliin', a specific plant protein has been isolated from the roots of clover and confirmed to exist on root surface by using immunofluorescence techniques. Some workers believe that the lectin hypothesis does not explain the enigmatic question of rhizobial invasion into leguminous roots, Although controversial, the lectin theory of Rhizobium-plant recognition did generate considerable investigation on specificity in Rhizobium-legume symbiosis.

A-An adult plant in its natural habitat, floating in a pond of water	B-Laser scanning confocal optisection of the open wound in the primary root where a lateral root has emerged	C-Higher magnification of B showing fluorescent bacteria deep within the open cavity of the root wound

D-Round nodule primordium developing at the base of the root cortex	E-Primary root showing nodules attached to the base of lateral roots	F-Transmission Electron Micrograph of intercellular infection and development of tubular intracellular infection threads which branches within the nodule having bacteria in a single row

The synthesis of trifoliin A in white clover has been demonstrated by means of studies on the incorporation of labelled amino acids and tracing the lectin in the root exudate. R. trifoliiin grown in defined media produced polysaccharide receptors for trifoliin A which changed with the age of culture.

These complementary protein-carbohydrate molecules (cross-reactive antigens) interact resulting in the recognition of the two symbionts with each other at the site of infection. In soybean, it has been demonstrated that a fluorescein isothiocyante (FITE) labelled soybean, it has been demonstrated that a Fluorescein isothiocyante (FITE) labelled soybean lectin preparation binds to cells of Bradyrhizobium japonicum which induced nodulation in plants Exceptions to this rule, notable in tropical legumes nodulated by promiscuous cowpea miscellany rhizobia have been reported.

For instance, concanvalin A from the tropical legume Canavalia ensiformis (Jack bean) can bind to many rhizobia unable to nodulate its roots.

There has been considerable amount of work on trifoliin. The surface of infective encapsulated R. trifolii contains an immunochemically unique polysaccharide that is antigenetically cross-reactive with a component on clover epidermal cells. Only those thizobial strains which infect clover have this cross-reactive antigen (CRA) and clover roots preferentially absorb infective R. trifolii and its corresponding CRA. These sites on root hairs known as receptor sites accumulate at root hair tips and decrease towards the base of the root hairs. On the contrary CRA is uniformly distributed along the root hair. Trifoliin specifically agglutinates R. trifolii and 2-deoxyglucose inhibits such agglutination.

Similarly, 2-dexoglucose inhibits the specific attachment mechanism of R. trifolii to clover root hairs, the nature of carbohydrate receptors for the lectin on Rhizobium cell has been the subject of several investigations. The structural differences in the lipopolysaccharide (LPS) and expolysaccharides (EPS) of Rhizobium are high and a search for relevant residues that function in recognition has become a major issue.

Within 4 hours, R. trifolii establishes a pattern of attachment on root hairs by clumping at the tip only by polar attachments along the site which can be inhibited by 2-deoxy-D-glucose. This initial reversible reaction becomes irreversible later with homologous Rhizobium and host combinations. These phases of attachment of rhizobia to root hairs appears to be controlled by nod genes.

Lectin Mediated binding of Rhizobium trifolii cells to the wall of root hair of clover seedlings

1. Enlarged diagram of a root hair of clover seedling	2. Root cortex
3. Root hair cell interior	4. Root hair cell wall
5. Interior of R. trifolii cell	6. Rhizobium cell wall
7. Cross reactive antigens, one of which is carbohydrate receptor on bacterial cell wall	8. Clover Lectin, Trifoliin

Many genes are involved in the synthesis of Rhizobium polysaccharide (gum) which surrounds a colony of cells. Mutants defective in polysaccharide synthesis are often characterized by poor infectivity and nodule formation. The precise role of various polysaccharides such as capsular polysaccharides (CPS), exopolysaccharides (EPS) and lipopolysaccharides (LPS) and cyclic B-1, 2-glucans produced by rhizobia are not very clear. It has been demonstrated that addition of EPS from the parent strain to plants together with some acidic exopolysaccharide lacking (EPS) mutants could restore the ability to induce nitrogen-fixing nodules which may point out the direct involvement of polysaccharides in the initiation of symbiosis.

Home | Site map | Submit Article | Directory | Search