Have you pondered on what happens to the mixture of gases found in the atmosphere? The air we breathe consists of nitrogen and oxygen as principal gases by volume and in combination, these gases make up to 99% dry atmosphere. All homo sapiens require oxygen for cellular and metabolic functions. Conventionally, through the process of respiration and photosynthesis, oxygen is exchanged between the atmosphere and life. Nitrogen which is the most bountiful element found is a limiting nutrient in many biomes and also a key element in the synthesis of proteins, nucleic acids, DNA and RNA which are pivotal for all living being as they are involved in the transfer of genetic information which is the most important of all biological molecules and crucial for all living things.
We inhale oxygen with nitrogen and hydrogen together as they co-exist in nature but unfortunately, N2 gas which dissolves in our bloodstream evaporates and exits our body during exhalation as the homeostatic environment of our body constantly maintains equilibrium. Proteins that contain oxygen, hydrogen and nitrogen are the most regular form of nitrogen as both humans and animals cannot consume nitrogen from air or soil thereby directing vegetation as a prime source of nitrogen
Why Plants Need to Absord Nitrogen?
- Plants have undergone abundant and extensive evolution than most other forms of lives on earth showing a wider range of adaptation for assimilating nitrogen for their growth.
- Atmospheric nitrogen in form of nitrogen gas and oxides of nitrogen but a majority of plants cannot assimilate the nitrogen directly or indirectly without the aid of the nitrogen fixation process.
- The atmospheric nitrogen deposited in Earth’s exterior is uptaken by peculiar nitrogen-fixing bacteria which provides the required nourishment for plant growth and is purely exclusive for prokaryotic plant species.
- In a nutshell, the intricate process of nitrogen fixation lies in the rudimentary symbiotic relationship between the plant and bacteria wherein the bacteria extract nitrogen from the atmosphere and fix it to a required form for plant growth.
- Many plants acquire nitrogen from either decaying organic matter or nitrifying bacteria present in the soil while other plants recruit nitrogen-fixation bacteria in their root nodules which converts atmospheric nitrogen into nitrate.
An extremely typical symbiotic relationship is pegged between the Rhizobiaceae bacteria which infect plant hosts in a particular taxonomic family, the Leguminosae.
For a prolonged time the Rhizobium bacteria were grouped in one genus and over the years they have been reclassified into three genera with Rhizobiacea
- Brady rhizobium
Rhizobia are Gram-negative and are mobile by a single polar flagellum or two to six peritrichous flagella.
How Does It Infect?
- The Rhizobium’s specific infection mechanism is extremely peculiar and interesting as it first attaches itself to the plant host’s epidermal root hair as the host secretes substances that serve as a chemoattractant for the species of Bradyrhizobium and Rhizobium and induces chemotaxis towards the defined root part.
- After attachment, the rhizobia secrets substances that leads to branch, curl and ultimately deforming young root hairs and also leading to the successful penetration of bacteria to form “infection thread” which are a specialized invasion structure.
- The root nodule is formed as a result of mitosis and cell growth in the plant foot cortex following which the bacteria actively infect the host cell and differentiates into ‘Bacteroides’ which ultimately fix nitrogen by reducing nitrogen to ammonia.
Genes Responsible For Nodulation
Symbiotic nitrogen-fixing genes can be broadly classified into three types of genes
- nod genes
- nif genes
- fix genes
- Nodulin or Nod genes are plant genes specific to nodules while nodulation genes or nod genes are the rhizobia genes that participate in nodule formation.
- When the leguminous root secretes signals as either phenolics and flavonoids which can easily diffuse through bacterial membranes they are recognized by positive transcriptional factor encoded by nod D.
- Nod genes can be of two types common node genes and host-specific nod genes, these nod genes serve as significant determinants of bacteria in the signal exchange between the symbiotic associates.
- The signals in the form of flavonoids will activate the nod D will result in the binding to the nod box and also in transcription in nod A, B, C and other gene expressions.
- Nod factors are lipo-chitooligosaccharides a chitin derivative signalling molecule secreted by Rhizobia during the highly regulated pathway which leads to the formation of root nodules and Bacteroides in the host cells.
- Structure specificity of Nod factors is resolved nod genes of rhizobium which encode for the protein that regulate nod factors by substituting chemical structure to the lipochito-oligosaccharide structure.
- The common nod genes are nod A, B and C which are commonly present in all rhizobia strains while nod P, Q, H are host specific nod genes.
- Nod A, B, C embraces a significant value as they encode necessary enzyme for nod factor synthesis.
What Happens If The Nod Genes Undergo Mutation?
Since these nod genes are quintessentially important for the nodulation process what would happen if they get inactivated by mutation? Easy, if there are no nod genes, there will be no modulation process ultimately leading to the lack of necessary nitrogen for growth.
- The cluster of the nod D, A, B, C genes are most common since they are structurally preserved and functionally replicable between species of Rhizobium, Azorhizobium and Brady rhizobium
- Mutations in the cluster of nod genes are reported to have a similar phenotype which is no root hair curling or cell division in plants which represent typical characteristics of such bacteria.
- Rhizobium genes which are recruited in the exopolysaccharides synthesis (Exo genes), lipopolysaccharide (lps gene), K-antigen when mutated, destruct the ability to infect by inducing the inability to form infection threads which will result in non-fixing empty nodules.
- Inactivation of nod genes will develop varieties of the phenotype including the absence of modulation, delayed yet effective nodulation, acts ad epistatic ally to host-range gene
- The studies by Liu, R & associates, Martinez, E., & associates have pegged that nevertheless of the host, mode of infection, nodule location and development inactivation of the nod A, B, C gene cluster will impair the ability to evoke any symbiotic response in the plant including the crucial steps such as root hair curling, Infection thread formation, Cortical cell division and nodule formation.
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Gettfert, M. (1992). Regulation and function of rhizobial nodulation genes. Microbiology Review.
Gresshoff, G. C.-A. (1991). PLANT GENETIC CONTROL OF NODULATION. Annual Review of Microbiology.
Long, S. R. (1989). RHIZOBIUM GENETICS. Annual Review of Genetics.
VANDERLEYDEN, P. V. (1995). The Rhizobium-Plant Symbiosis. MICROBIOLOGICAL REVIEWS, 124-142.