Title Biological N fixing Microbes Symbiotic N-fixers-.pdf ✅

Soil 122 Biological N-Fixing Microbes Symbiotic N-fixers ➢ Discovery and classification ➢ Nodulation process Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. The process of biological nitrogen fixation occurs exclusively in selective prokaryotes. Broadly the BNF has been divided into two major groups.  Symbiotic N-fixation  Non-symbiotic N-fixation Symbiotic Nitrogen Fixation What is Symbiosis? “An intimate association between two different organisms for which the coexistence is deemed to be mutually beneficial” Symbiotic nitrogen fixation occurs in plants that harbor nitrogen-fixing bacteria within their tissues. Each of these (Plant and Bacteria) is able to survive independently, but only together can fix atmospheric nitrogen. The symbiotic diazotrophs belongs to three groups of prokaryotes. ➢ Legume-Rhizobium ➢ Cyanobacteria-plants/fungi ➢ Actinomycetes-trees Among all three groups of symbiotic association, Legume –Rhizobium symbiosis has been widely studied and wel developed. Legume –Rhizobium Symbiosis: Discovery and Classification Historical perspective of Legume –Rhizobium Symbiosis: ❖ In 1838 Boussinggault showed certain leguminous plants could fix atmospheric N and thus increase N-content in soil ❖ Hellriegel and Wilferth in 1886 demonstrated for the first time the ability of pea to convert dinitrogen from atmosphere and locate the nodules in roots. ❖ Bejeirink in 1888 could isolate microorganisms from the root nodule and designated as Bacillus radicola ❖ In 1889 frank renamed the organisms as Rhizobium means root living with the following characters.
❖ Aerobic and gram negative ❖ Non spore forming but rod shaped ❖ Motile by sub polar flagella ❖ Live as normal component in soil ,but cannot fix N ❖ It fix atmospheric N when reside inside nodule ❖ Utilize wide variety of carbohydrates Based on the ability of Rhizobium to infect diverse leguminous crop –in 1932, Fred proposed the concept of “Cross Inoculation Group” of Rhizobium. The cross inoculation group refers “a collection of leguminous species that develop nodules when exposed to rhizobium, isolated from the nodules of any member of that particular group” . Fred proposed seven such cross inoculation group Cross Inoculation Group and Rhizobium – Legume Association Cross Inoculation Group Rhizobium Species Legumes Included Alfalfa Group Rhizobium meloloti Alfalfa, Sweet Clover, Fenugreek Clover Group Rhizobium trifoli Clovers Pea Group Rhizobium leguminosarum Pea, Lentil Bean Group Rhizobium phaseoli Beans Lupine Group Rhizobium lupine Lupines Soybean Group Rhizobium japonicum Soybeans Cowpea Group Rhizobium sp. Cowpea, Peanut, Limabean etc. In 1981 Allen and Allen reported that the family leguminaceae comprise 748 genera and 19000 species of which 49% genera and 16% species were examined for nodulation that resulted 41% genera 14% species could produce nodule. Jordan in 1984 classified the Rhizobia into two groups based on ➢ the growth pattern and ➢ nif and nod genes location If growth is fast and nif & nod genes locate in plasmid ,it is termed as Rhizobium where as if slow growth and nif & nod genes locate in chromosomes ,termed as Bradyrhizobiumi In 1988 Dreyfus et al proposed another genus as Azorhizobium capable of producing stem as well as root nodule. Chen et al in 1988 proposed another genus as Sinorhizobium and 1997 the fifth genus by Jarvis et al as Mesorhizobium.
Character distinguishing different Rhizobial genera Genera Growth Rate Nif and nod genes location Fast Slow Chromosome Plasmid Rhizobium Bradyrhizobium Azorhizobium Sinorhizobium Mesorhizobium + - - + - - + + - + - + + + + + - + + - What characters are common in symbiosis? ➢ Host specificity ➢ Exist always in harmony Events Leading to Establishment of Legume Rhizobium Symbiosis ❑ Root system of leguminous plant secrets flavonoids. ❑ Flavonoides fuction as attractant ❑ Response from Rhizobia by producing certain protein called as NoD-D ❑ This NoD-D protein are activated by root exudates i.e.flavonoides. ❑ Flavonoides and NoD-D protein interact and induce bacterial nod-genes ❑ These nod-genes produce certain polysaccharides called Nod factors ❑ Once the nod-factors produce, it initiates nodulation in host plants in several ways. Functions of Nod-factors: ▪ Root hair curling and formation of infection thread ▪ Initiate cell division in cortex ▪ Transcription of nodulin genes in host genome Development of Root Nodule: ❖ Curling of root hair and Deformation: Bacteria swim nto the root surface, they don’t breach the root hair, but they pass into a infection thread through invagination (Root hair curling by many strains ,but enters into infection thread by a specific strain ) ❖ Invagination: Invasion from root hair or crack of epidermis cell ❖ Formation of Infection Thread: Infection threads formed by invagination of root hair cell envelope and consist of root hair cell wall externally surrounded by cell membrane. At the same time rhizobia enters through tunnel and continues cell division .Infection thread also continues its growth through epidermis and move till it reaches the cortex. ✓ Infection thread branches ✓ Cortex cell also branches.
✓ In the cortex cell the rhizobia are released through unwalled portion of infection thread into dividing cortex cell into a small droplet by a process called endocytosis. ✓ Each droplets containing one or more bacteria are enclosed by a plant cell membrane .At this stage rhizobia transformed from rod to irregular shape known as bacteroid (After rhizobia transformed into bacteriod ,they donot multiply further ,but initiate activation of certain genes) ✓ The membrane surrounding the bacteroid is called Peribacteroid membrane (PBM) ✓ Bacteroid activating certain genes and produce “hemin”protein and transported across the PBM membrane into cytoplasm ✓ In response to “hemin” protein the host produce another protein “globin” and ultimately produce “Laghaemoglobin” Functons of Laghaemoglobin:  It constitute 20-30% of total protein in cytoplasm  It present in cytoplasm of host cortical cell containing symbiosome ,but not within the symbiosome.  Regulate oxygen concentration into the bacteroid at lower rate. Functions of PBM: ❖ Bacteriod enclosed in a host derived membrane called PBM ❖ It regulates metabolic exchanges in between bacteroid and its surrounding environment i.e. in host cytoplasm ❖ Organic acids such as malate transported into bacteroid ❖ NH3 ,product of bacteroid transported to the host cel cytoplasm