The Rhizosphere

Topic 14a:The RhizospherePlant-microbe interactions :•I) Region around a germinating seed = the spermosphere:•-Influence of microbes on plant at this stage can affect plant establishment •-Microbes involved can include:•a) Microbes present under seed coat (having gained access from air during flowering stage)•b) Microbes present on outer surface of seed coat(introduced during storage of seed, from air or surroundings)•c) Soil microbes stimulated bycompounds releasedby seed2 major types of root systems; fibrous &taprootII) Regions around, on or withindeveloped plant root•Terms vary, depending on author•1) Rhizosphere = volume of soil under immediate influence of plant root & root products, where microbial growth is usuallyenhanced (& most intense within micrometers of root)•-Rhizosphere usually assumed to extend from root surface (rhizoplane) out into soil for a few mm•2) Rhizoplane is ~ 1.0 um thick•3) Endorhizosphere = histosphere = cortosphere•=interior root cells colonized by microbes, especially epidermal cells & cortical cells •*See p. 72 of lecture packet; parts of plant rootSelected root zones/tissues:•1) Root cap:•-Outermost root cap cells areabraded/sloughed off as they contact soil particles, and will later be colonized by soil microbes•-Outermost root cap cells covered by root-derived mucigel that acts as lubricant•-Plant mucigelcan be found from root cap up to root hair zone•-Root cap cells produce most of root’s mucigel supplyRoot cap•Plant mucigel is largely composed of pectin & hemicellulose•-Possible plant mucigel roles:•a) Aid in nutrient absorption•b) Protect root tips from physical or chemical damage or desiccation•-Root cap often has no microbial colonists; maybe because root cap grows too quickly through soil to be readily colonized; or too much abrasion at root tip; &/or chemicalinhibitors of microbial growth may be present at root tipRoot hairs 2) Epidermis of root; is one celllayer thick•-Often covered by thin cuticle (waxy layer)•-In some plants, some epidermal root cellsgive rise to root hairs that greatly increaseabsorptive root areaRoot hairsWheat root hairs (left); soybean root hairs (right)3) Cortex (cortical tissue)•Cortical root cells often contain starchgrains & other storage compounds•-Often large air spaces exist betweencortical cells4) Endodermis •Is one cell layerthick•Encloses vascularcylinderAxenic root:•-= root having controlled, defined number & type of microbes associated with it•-Sterile root can be considered axenic •-Root with one desired bacterial associate is axenicMaterials released from root stimulate soilmicrobes nearby•**See p. 71 lecture packet; list of terms •-Root exudates include: carbohydrates; amino acids; other organic acids; vitamins; etc.•-**Non-sterile roots (with resident microbes) release more root exudates than roots grown in sterile solution—WHY?•Possible reasons include:-Æ•a) Due to physical damage to the root tissue caused by microbes•b) Microbes releasing chemical metabolites which cause increased root exudation•--Also; root exudation will be greater from a root growing in soil (where root is physically abraded by soil particles) than from root growing in aqueous nutrient solution •Many plant-microbe studies have been conducted in soil-free systems, to simplify system & reduce # of variables (so are hydroponic systems)•-More variability between plnat species in amounts & kinds of amino acids roots exude, than amounts & kinds of carbohydrates exuded•-Exuded organic acids from TCA cycle, etc. may alter rhizosphere pH & chelate metal ions (& be metabolized by microbes)Factors affecting exudation:•1) Plant species & developmental stage of plant•**Most exudate research has been conducted with germinating seeds or seedlings (since is easy to make & keep them axenic)•-Fewer studies have been conducted on mature plant root exudates•2) Temperature extremes (too hot or too cold) may cause exudation increases & decreases •3) Light (affecting photosynthesis) affects exudation •4) Soil moisture; low soil moisture can stress root, increasing exudation •5) Soil atmosphere; periods of anaerobiosis can alter exudates•6) Plant nutrition•7) Plant-injury factors (physical or chemical)8) Microbial effects on root exudation via:•a) microbes affecting root cell permeability (by altering membranes, etc.)•b) by altering root metabolism•c) by absorbing and/or altering exudates •d) by altering nutrient availability to rootMany root-associated microbes produce plant hormones = plant growth regulating substances (PGRs) which can affect plant metabolism. •These compounds are also produced naturallyby plants•-Includes:•a) Ethylene (C2H4); volatile; gas; formed bymicrobial activity in waterlogged (anaerobic)soil•-Some plants (like rice) respond to ethylene by developing aerenchyma= air spaces in cortex;allowing more rapid movement of air fromshoots to roots, helping alleviate anaerobicstressEthylene helps ripen fruit; promote leaf abscisionand senescence; aerenchyma development; etc.Root aerenchymab) Non-volatile plant growth regulators,including:•1) Gibberellins;•Work withauxins topromote stemtissueelongation &division;•Etc.2) Auxins(like indoleacetic acid = IAA)•-Promotes cellelongation &enlargement; promotelateral & adventitiousroot development;etc.3) Cytokinins promote cell division & differentiation;can form galls/tumors on plants (asin Agrobacterium)4) Abscisic acid often antagonizes action of otherplant hormones;inhibits growth•May be possible to manageactivity/occurrence of PGR producing bacteria around roots, to aid plant growth in stressful environments•-EX.: Enhanced dry weight accumulation noted in some field trials where plants are inoculated with free-living nitrogen fixing bacteria (Azospirillum spp.; Azotobacter spp.; etc.) may be due to enhanced development of roots & root hairs on plants, due to PGRs produced by bacteria •Increased root surface area can allow plant to better exploit nutrient resources of root zone soil; get greater production of plant material, etc.•-While root-associated, free-living N2fixer often keeps the nitrogen it fixes as part of its microbial biomass; may only make the fixed N available for plant uptake after bacterial cell dies & is mineralized in soilSoil microbes also mineralize OMin plant root zone•OM ---ÆCO2+ inorganic nutrients •Mineralization rate may be higher in rhizosphere soil than in non-rhizosphere soil (due to readily available root-derived carbon stimulating microbial activity & stimulating mineralization of humified soil OM, etc.); root can have “priming” effect accelerating decomposition of soil humic materials, pesticides, etc. in rhizosphere R/S values = (# microbes in rhizosphere) divided by (# microbes in non-rhizosphere soil)•1) R/S values for unicellular bacteria:•-Range of values ~ 2 to 20; but sometimesover 100•-Range due to different rhizosphere effects ofdifferent plant species & stage of plant growth •Unicellular bacterial numbers often respondmore quickly & extensively to rhizosphereinfluence than other microbial groups•By plate count estimates, unicellular bacteria are the most numerous soil microbes (range ~ 106to 108CFU/g soil; sometimes even higher)2) R/S values for actinomycetes:•-Usually less (often less than 10) than for unicellular bacteria•-Often the culturable microbial group with the lowest R/S value•-By plate count, usually second most numerous group of soil microbes (range from ~ 105to 108CFU/g soil)•-Question: Do these colonies arise mostly from dormant arthrospores or vegetative cells?3) R/S values for fungi:•-Usually the third most numerous group of soil microbes by plate count•Crop plant R/S values for fungi range from 3 to 20•-Question: Do these colonies arise mostly from dormant propagules or actively growing hyphae?Patterns of microbial growth on rootsurfaces:•Only 4.0-10.0% of grass root surface is colonized by microbes; so only a fraction of the root surface is occupied by microbes•-Often, highest #s of bacteria are found in cell expansion (elongation) zone, a major site of root exudation•-Older, senescing root zones & materials can have highest #s of associated microbes on & within the root•Root hairs are often colonized•-Bacterial microcolonies(~ 100 cells percolony) frequent on root surfaceRhizobacteria attached to plant root Bacteria on plant root surface Fungal hypha on plant root surfacePseudomonas fluorescens cells attachedto barley root P. fluorescens cells attached to barleyroot hairFluorescence micrograph of bacteria Bacteria attached to root hairsattached to root。