Genetics of Nodulation and N Fixation

  • Root nodule bacteria and symbiosis with legumes One of the most interesting and important plant bacterial interactions is that between leguminous plants and certain gram-negative nitrogen fixing bacteria.
  • Rhizobium, Brady rhizobium, Sino rhizobium, Mesorhizobium and Azorhizobium are gram negative motile rods.
  • Infection of the roots of a leguminous plant with the appropriate species of one of these genera leads to the formation of root nodules that are able to convert gaseous nitrogen to combined nitrogen, a process called nitrogen fixation.
  • Nitrogen fixation by legume Rhizobium, symbiosis is of considerable agricultural importance, as it leads to very significant increases in combined nitrogen in the soil.
  • Because nitrogen deficiencies often occur in unfertilized bare soil, modulated legumes are at a selective advantage under such conditions and can grow well in areas where other plants cannot.
  • CIG refers, the groups of leguminous plants that will develop effective nodules when inoculated with the rhizobia obtained from the nodules from any member of that legume group.
Stages in root nodule formation

The stages in the infection and development of root nodules are not fairly well understood. They include

  1. Recognition of the correct parameter on the part of both plant and bacterium and attachment of the bacterium to the root hairs.
  2. Excretion of nod factors by the bacterium.
  3. Invasion of the root hairs by the bacterial formation of an infection thread.
  4. Travel to main root via the infection thread.
  5. Formation of deformed bacterial cells, bacteroid, within the plant cells and development of the nitrogen fixing state.
  6. Continued plant and bacterial division and formation of the mature root nodule.
Nodulation events
  • Normal root hair
  • Exudation of organic substances
  • Accumulation of rhizobia in the rhizosphere
  • Orientation and binding of rhizobia
  • IAA production
  • Root hair curling and deformation
  • Formation of infection thread by rhizobia
  • Formation of sheperd’s crook cells and entry of infection thread
  • Thread containing bacteroid extending into root hair cells
  • Entry of infection thread into cortex and branching
  • Nodule development
Attachment and infection
  • The roots of leguminous plants secret a variety of organic compounds that stimulate the growth of a rhizosphere micro flora.
  • This stimulation is not restricted to the rhizobia but occurs with a variety of rhizosphere bacteria.
  • If there are rhizobia in the soil, they grow in the rhizosphere and build up to high population densities.
  • Attachment of bacterium to plant in the legume Rhizobium symbiosis is the first step in the formation of nodules.
  • A specific adhesion protein called rhicadhesinis present on the surface of all species of Rhzobium and Bradyrhizobium.
  • Rhicadhesin is a calcium-binding protein and may function by binding calcium complexes on the root hair surface.
  • Other substances, such as carbohydrate-containing protein called lectins, also play in plant bacterium attachment.
  • Initial penetration of Rhizobium cells into the root hair is via the root hair tip. following binding, the root hair curls as a result of the action of substances excreted by the bacterium called nod factor and the bacteria enter the root hair and induce formation by the plant of acellulosic tube, called infection thread, which spreads done the root hair.
  • Root cells adjacent to the root hairs subsequently become infected by rhizobia and nod factors stimulate plant cell division, eventually leading to formation of the nodule.

Bacterioids

  • Bacteriods are specifically referred to a swallon deformed Rhizobium cell found in the root nodule, capable of nitrogen fixation The Rhizobium bacteria multiply rapidly with in the plant cells and are transformed into swollen, misshapen and branched forms called bacteroid.
  • When the plant dies, the nodules can be deteriorates, releasing bacteria into the soil. The bacteroid forms are incapable of division, but there are always a small number of dormant rod-shaped cells present in the nodule.
  • These now proliferate; using some of the products of the deteriorating nodule as nutrients, and the bacteria can initiate the infection in other roots or maintain a free-living existence in the soil.

Lectins

  • Plant proteins which specifically bind to carbohydrate receptors (polysaccharides) in the rhizobia cell Genetics of nodule formation Genes directing specific steps in nodulation of a legume by a strain of Rhizobium are called nod genes.
  • Many nod genes from different Rhizobium species are highly conserved and are borne on large plasmids called sym plasmids.
  • In addition to nod genes which direct specific nodulation events, sym plasmids contain specificity gene, which restrict a strain Rhizobium to a particular host plant.
  • Indeed, cross inoculation group specificity can be transferred across species of rhizobia by simply transferring the respective sym plasmid.
  • In the sym plasmid of Rhizobium leguminosarum bio varviciae, nodgenes are located between two clusters of genes for nitrogen fixation the nifgenes.
  • Ten nod genes have been identified in this species. The nod ABC genes are involved in the production of oligosaccharides called nod factors, which induce root hair curling and trigger plant cell division, eventually leading to formation of the nodule.
  • In Rhizobium leguminosarum bio varviciae, the gene nodD encodes a regulatory protein; this controls transcription of other nod genes.
Nod D genes
  • Genes directing specific steps in nodulation of a legume by a strain of rhizobium is called Nod genes
  • Nod genes are born on large plasmids, called sym plasmids
  • Nod genes are located between two clusters of genes for N2 fixation called Nif genes
  • Nod gene consists of 8 genes
  • nod A,B,C,D,E,F,L,M
  • nod D controls the function of all nod genes
Nif Genes
  • Genes responsible for N fixation are called Nif genes
  • 22 genes are involved, arranged in 7 /8 clusters
  • Nif Q,B,A,L,F,M,Z,W,V,S,U,X,N,E,Y,T,K,D,H,J
  • KDH –control Nitrogenase enzyme complex
Factors affecting nodulation
  • Temperature and light
  • Combined Nitrogen
  • Hydrogen iron concentration
  • Mineral nutrition-Co,Mo,P,Ca
  • Genetic factors
  • Ecological factors
  • Salinity and alkalinity
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