MICROBIOLOGY

PRESENTATION

TOPIC RHIZOBIUM

BY RASHIKA SOOD

What is Rhizobium?

Rhizobium are symbiotic diazotrophs (prokaryotic organisms that carry out dinitrogenfixation) that form a endoymbioticassociation with legumes.

• Rhizobium is a genus of Gram negative Soil Bacteria.

• They are non sporing bacteria.

•Rod shaped cells.0.5-0.9 um x 1.2-3.0 um in size.

WHO DISCOVERED RHIZOBIUM?

Martinus Beijerinck (1851–1931) was one of the great general microbiologists whomade fundamental contributions to microbial ecology and manyother fields. He isolated the aerobic nitrogen-fixing bacteriumBacillus radicicola a root nodule bacterium also capable of fixing nitrogen (later named Rhizobium)

Rhizobium can be found in Root nodules of legumes.

Legumes are herbaceous woody plants that produce seeds in pods; examples of legumes include peas, beans, alfalfa, vetches, clovers, lupines, trefoils, locust, and mimosa. (fabaceae)

It is difficult to isolate rhizobiumfrom the soil..

Rhizobium are predominantly aerobic chemoorganotrophs and grow well in the presence of oxygen and utilize a wide range of relatively simple carbohydrates and amino compounds.

What is Rhizobium’s

Nutrition?

The rhizobia doesn’t grow wellwell on the peptone media used routinely.

Yeast mannitol (YM) is the most generally suitable for their growth.

Optimal growth of most strains occurs at a temperature range of 25-30° C and at a pH of 6.0-7.0

Colonies are white pigmented.Colonies are circular, convex, semi-translucent, raised and mucilaginous.

Despite their usual aerobic metabolism, many strains are able to grow well under microaerophillic conditions at oxygen tensions of less than 0.1 atm.

Mobile by a single polar flagellum or two to six peritrichousflagella.

MOBILITY

PROTECTION

OF

RHIZOBIUM

•Rhizobium is surrounded by a slimy capsule made of exopolysaccharide, which protects it from drying out. And also helps the bacterium stick to root hairs during various stages of its life cycle.

•Rhizobium contains nitrogenase, which is irreversibly damaged by exposure to oxygen.

But Rhizobium is aerobic and requires large amount of energy for Nitrogen fixation. It solves this “oxygen dilemma” by protecting the nitrogenase through Oxygen Scavenging Chemicals, specifically Leghaemoglobin a transport protein which effectively removes oxygen from module.

It also has exceptionally fast and efficient aerobic metabolism so very little Oxygen is stockpiled.

Host plant Bacterial symbiont

Alfalfa Rhizobium melilotiClover Rhizobium trifoliiSoybean Bradyrhizobium japonicumBeans Rhizobium phaseoliPea Rhizobium leguminosarumSesbania Azorhizobium caulinodansSome other Rhizobium sp. From Rhizobiaceae-R. Cellulosilyticum R. daejeonense R. etli R. galegae R. gallicum R. giardinii R. hainanenseR. huautlenseR. Indigoferae R. leguminosarum R. loessense R. lupini R. lusitanum R. mongolense R. miluonense R. sullae R. tropici R. undicola R. yanglingenseBoth plant and bacterial factors determine specificity

Rhizobium-legume symbioses

The Nodulation Process

1. Chemical recognition of roots and Rhizobium

2. Root hair curling3. Formation of infection thread4. Invasion of roots by Rhizobia5. cortical cell divisions and formation of

nodule tissue6. Bacteria fix nitrogen which is

transferred to plant cells in exchange for fixed carbon

RhizosphereFlavonoidsnod-geneinducers

Nod-factor

Chemical recognition of roots and rhizobium. Very early event.

Tip of a clover root hair.

The rhizobia shown here are clustered on the surface of the root. Soon they will start to invade the roots and begin a symbiotic partnership that will benefit both organisms.

2. Root hair curling.Root hair curls to trap the colony of rhizobium.

3. Formation of infection thread to penetrate root cortex and reach vascular tissue.

Pea plants inoculated with R. leguminosarum carrying the plasmid pHC60 that constitutively expresses green fluorescenstprotein (GFP).Curled root hairs (arrows) of pea plant.

Infection thread (green fluorescence) reaching the base of the root hair in plants.

3. Invasion of roots by Rhizobia

4. cortical cell divisions and formation of nodule tissue

Bacteria converts to bacteroids and begin to

form nitrogenase enzyme.

Rhizobium fixes nitrogen which is transferred to plant cells in exchange for fixed carbon

Hope youEnjoyed N’ learnt something!