LECTURE 2. RHIZOSPHERE SYSTEM

Rhizosphere systemRoot function/rhizosphereDiversity and Ecology of Associated MicrobesRhizodeposition or Plant-Derived compounds from plant rootsRoot ColonizationMicrobe-plant interactions in the rhizosphereEffect of biological activity in rhizosphere on plant

PLANT ROOTS

Root Anatomy

Root FunctionsAnchoringAbsorption of nutrientsstorage

Root renewalPerpetually changingIn young plantselongation and branching; primary root a few cm to 9 cm per daySecondary roots grow slower, Constantly renewedRadicles die a few days after formationIn cotton 50% of root tissue disappeared in 100 daysEquilibrium between formation of new roots and disappearance of old roots during last 2/3 of growth.Ripening stage, production of new roots stop, followed by dimunition of root mass.

Importance of rootsInteraction between roots and microbesDead root cells are precious nutritive resources for soil organisms

Rhizosphere systemZone or volume of soil under the influence of rootsExtend > 5mm from roots, area of increased microbial activitiesNot uniform; consist of several gradients that change as distance from root increasesApproximately defined as the soil that remains attached to roots when plant is carefully uprooted, then gently shaken.

Root anatomyEpidermisCortexEndodermisRoot hairs extension of epidermal cells, important for ion uptakeRoot cap- protect the meristem as root moves through soil; produce mucilage and substance that enhance plant cell growthRhizosphere boundaries: rhizosphere, rhizoplane, endophytes

Rhizosphere - the area of soil directly influenced by plant roots, variable. Soils that remains after shaking. 1 - 4 mm or more from root surfaceRhizoplane - surface of plant root, root hairs present large surface area (> 6 m2 for wheat plant). Only 4-10 % of rhizoplane is in direct contact with soil microbes. About 2 mm width.Difficult to distinguish rhizosphere & rhizoplane due to mucilageRoots primary, root hairs- increase surface area for absorption of ionsSoil-root interface has high microbial community

Energy Flux of rhizosphereIncreased microbial populations due to abundance energised substratesA portion of compounds photosynthesised in aerial parts is transported to roots through phloemThe carbon fixed; is transferred to roots; some stored as reserves or incorporated into new tissue; consumed for cell respiration (CO2); lost to external environment (Rhizodeposition/root exudates)

Rhizosphere EnvironmentPhysical and chemical properties of soil:-Soil texture, structure, moisture, temperature, pH, fertilization, etc.

Diversity and Ecology of Associated MicrobesMicrobial Populations of RhizosphereVast number of microbial speciesBacteria, fungi, algae, protozoa, nematode, virusesVariable nutrition patterns:- autotrophs, chemoautotrophs, heterotrophs, chemoheterotropsSaprophytes, parasites, associative, symbiontsOxygen requirements, temperature, physiology

Population decrease with increasing distanceRhizosphere/bulk soil (R/S) ratio estimate how strongly the rhizosphere affects organisms determine the ability of organisms to colonize rhizosphere (candidate as inoculant)R/S between 5 and 20, can also be >100

BacteriaMost numerous typically 106 - 109 g -1 rhizosphere soilLarge R/S ratioAbundance of non-sporulating rodsPseudomonads and Gram - negative bacteria are competitive and occupy large portion of rhizosphereGram-positive rods & cocci is lowerActinomycetes has smaller R/S ratio

FungiConsiderable fungal growth along root surfaces ; 12-14 mm hyphae mm -2 of root surface 105 to 106 g 1 soil

Composition of microflora of rhizosphere Vary depending on the family of plants; legumes have high R/S ration than crucifersVary with plant age; high effect at vegetative growth compared to reproductive and maturation stageReplacing plant chromosome will change the microbial populationDependent on root exudates

pH of RhizosphereDiffers from pH of adjacent soil; about 2 pH unitsDepend on soil buffering capacity and plant typesMore acidic than soil; pH gradient along rootsProton (H+) are excreted when cell elongateElectric current circulating towards inside of growing roots and towards outside of older zones

Absorption of mineral ions coupled to hydrolysis of ATP by membrane ATPase; transfer of proton from cytoplasm towards outside cell creates a pH and electric potential gradientConcentration of H+ is not constant depend on nature of ions taken up by roots; eg. Fertilizer NH4+ (pH decrease) or NO3- (pH increase)

Cont.

Rhizodeposition or Plant-Derived compounds from plant rootsExudates- Low molecular wt. compounds that leak from all cells into the intercellular spaces then to the soil. Not metabolically mediated Secretions- Low and high mol. wt. mucilages. Result of metabolic processMucilagesOriginate in root capHydrolysates of cell wall and sloughed root cap cellsSecreted by epidermal cellProduced by bacterial degradation of old, dead epidermal cells

c.Mucigel- Gelatinous material at root surface; Natural and modified plant mucilages, bacterial cells and metabolites, colloidal mineral and organic matter.Important in maintaining contact between root and soilFound on root/root hairs than root tipd.Lysates compounds released by lysis of epidermal cells.

Exudate composition (water soluble, insoluble, volatile)Sugars, amino acids, vitamins, tannin, Alkaloid, phosphatides, growth factors, flourescent substance, stimulants, inhibitors.Composition varies with plant genome and stage of development and environmental factors (light, temperature, water, redox potential, minerals, phytotoxicity, reduction in leaf surface (by insect or diseases)

Functions of root exudate:

Provide carbon , energy and nutrientsSugars- sources of carbon for microbial growthAmino acids- source of NOrganic acids- metal chelation, affects pH, absorption and translocation of nutrientsVitamins- influence bacterial population, species.

Exudation Rate and factors affectingGradients of release; greatest amount of exudate from cells involved in cell elongation and lateral root formation. Concentration declines exponentially with increase in distance from roots.Can be increased by extreme temperature, water stress, phosphorus deficiency, increase light intensity, increase microbial population, herbicides, pathogens, foliar treatments, symbiotic associations.Can be reduced by N deficiency, decrease light intensity, decrease microbial population

Root ColonizationFactors influencing rhizosphere colonizationMicrobial characteristics:- nutrient requirements, early growth rate, cellulase production, antibiotic tolerance and production, siderophore production, tolerance to fungicides or chemicals.Plant characteristics :- plant species, plant age, plant genetics, foliar treatmentsEnvironment:- soil type, texture, moisture, atmosphere, temperature, fertility, pesticides applied

Only 7 -15 % root surface occupied by microbesRhizoplane bacteria scattered in small colonies in holes, tears, crevices, junctions of epidermal cells in rootsRecognition signals

Colonization on rhizoplaneMostly by epiphytes and saprophytesNot distributed uniformly; only 10% of surface of young roots; ~ 37% in older rootsColonies extend very little compared to root elongationAbility of bacteria to disperse along root is limitedImplanted at the junction of epidermal cells and at base of absorbent hairs where exudation is most intense and protective mucilage is thickestLive on senescent cells of cortical cells, epidermal cells

EndophytesMicrobes can penetrate into central cylinder of rootsEntrance through rapture of endodermis during emergence of secondary rootsMinimal colonization by microbes except when attacked by pathogensBacteria can move from roots to stems and leaves

Microbe-plant interactions in the rhizosphereN2-fixationMycorrhizal associationsPlant growth promotionDecompositionnutrient cycling

Rhizospheric affinity/signalsMicrobes receive signals from plants that triggers chemotaxis process directed towards rootsSignals consist of mainly sugars and amino acids that diffuse in soil solutionRapid respond resulting in successful colonizationLectins in legumes root hair tips important in early stages of nodulation, it may bind the Rhizobium to the root tipsN-acyl homoserine lactones in root exudate regulate expression of bacterial genes for quarum sensing in plant-microbe interaction

RhizocompetenceThe ability of microbes to grow, function and maintain itself in rhizosphere; through production of antibioticResistant to foreign antibioticsOptimal utilization of available molecules

Microbiostasis and effect of exudatesMicrobiostasis or dormancy of microbesRoot exudate affect germination of clamydospores- stimulate parasites and symbionts- eg. Root exudate of Pinus sp. trigger germination of ectomycorrhizae spore of Suilus sp. but not other fungal species- hatching of parasitic nematode eggs

Microbial Movement towards hostAttraction of microbes to roots is associated with amino acids, fatty acids, aldehydes, alcoholsEg. Luteoline (alfalfa seeds) attract SinorhizobiumDaidzein, genistein (soybean roots) attract Phytopthora sojae

Attachment/fixation on hostSecretion of adhesive materials attract cells to plantMass bacterial cells affix themselves by forming extracellular fibrils of cellulose material

Effect of biological activity in rhizosphere on planti. Recycling of mineral elementsRoot exudate increase microbial population; increase predators; increase enzymes, decomposition and NH4+ release

ii. Decrease Redox PotentialRespiration of roots and microbes consume O2; reduce redox potential; increase solubility of certain cations (Fe and Mn)Increase toxic compounds

iii. Synthesis of growth factorsBiosynthesis of hormones; giberrelin, auxin, cytokinin, ethyleneiv. Chelating agentsSiderophores that has high affinity for Fe3+organic acids( oxalic or ketogluconic acid) involve in P solubilizationv. Production of toxic compoundsantibiotics; rhizobitoxin produce by certain Bradyrhizobium cause chlorosis in soybeanvi. Detoxification of toxic compounds

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