genic microorganisms affecting plant health are major and chronic threats to food production and ecosystem stabil-ity worldwide. Crop rotation, breeding for resistant plant varieties and the application of pesticides are insuffi cient to control root dias of important crop plants. An initia-tive, simple explanation of how the biological control of soil-borne pathogens could work? There is a large body of literature describing potential u of plant associated bacte-ria, the so called plant growth-promoting bacteria (PGPB) as agents stimulating plant growth and managing soil and plant health. The most widely studied group of PGPB is plant growth-promoting rhizobacteria (PGPR) that colonize the root surface and the cloly adhering soil interface, the rhizosphere. The widely recognized mechanism of biocon-trol mediated by PGPB is competition for an ecological niche/substrate, production of inhibitory allelochemicals, and induction of systemic resistance (ISR) in host plants to a broad spectrum of pathogens2. Induced resistance is a physiological “state of enhanced defensive capacity” elic-ited by specifi c environmental stimuli, whereby the plant’s innate defens are potentiated against subquent biotic challenges. This enhanced state of resistance is effective against a broad range of pathogens and parasites15. The two most clearly defi ned forms of induced resistance are systemic acquired resistance (SAR), and induced systemic resistance (ISR), which can be differentiated on the basis of the nature of the elicitor and the regulatory pathways involved (Fig. 1).
SAR can be triggered by exposing the plant to virulent, avirulent, and nonpathogenic microbes. Depending on the plant and elicitors, a t period of time is required for the establishment of SAR wherein accumulation of pathogenesis-related proteins (chitina and glucana), and salicylic acid takes place. ISR is potentiated by plant growth-promoting rhizobacteria (PGPR), of which best characterized are strains that belong to genus Pu-domonas that cau no visible damage to the plant’s root system3. Unlike SAR, ISR does not involve the accumula-tion of pathogenesis-related proteins or salicylic acid, but instead, relies on pathways regulated by jasmonate and ethylene5,52.
Plant roots relea substantial amounts of C- and N- containing compounds into the surrounding soil. Microor-ganisms are attracted to this nutritous environment and u the root exudates and lysates for growth and multiplication on the surface of root and in the adjacent rhizosphere soil. Becau of the rapid consumption of the nutrients, bacterial growth in the rhizosphere remains nutrient-limited where roots are ldom colonized for more than about 15% of their surface area. The rhizosphere micrfl ora plays an important role in plant development and acclimation to environmen-tal stress3. Since the rhizosphere microfl ora is extremely diver, a dynamic interplay between the members of the microbial community occurs which is mediated by syn-ergistic and antagonistic interactions within the limits of the nutrients available. In addition, signals are exchanged between fu
ngi and bacteria and plant roots which refl ect a highly dynamic belowground communication network. Plant growth-promoting rhizobacteria can suppress dis-eas through antagonism between bacteria and soil-borne pathogens, as well as by inducing a systemic resistance in the plant against both root and foliar pahogens. The induced resistance constitutes an increa in the level of basal re-sistance to veral pathogens simultaneously, which is of benefi t under natural conditions where multiple pathogens exist. Several specifi c Pudomonas strains have been reported to induce systemic resistance , carnation, cucumber, radish, tobacco, and Arabidopsis. In addition, veral other bacterial strains are reported to inducing resistance in different plant species, whereas others show specifi city, indicating specific recognition between bacte-ria and plants at the root surface. In carnation, radish and Arabidopsis, the O-antigenic side chain of the bacterial outer membrane lipopolysaccharide acts as an inducing determinant along with other bacterial traits. Pudobac-tin siderophores have been implicated in the induction of resistance in tobacco and Arabidopsis together with other siderophore, pdomonine. The siderophores may explain induction of resistance associated with salicylic
Fig. 1 The pathogen-induced SAR and the rhizobacteria-mediated ISR signal transduction pathways in Arabidopsis (From Pieter et al (2002) “Signaling in Rhizobacteria-Induced Systemic Resistance in Arabidopsis thaliana”. Reproduction with permission.
acid (SA) in radish. Although SA induces phenotypically similar systemic acquired resistance (SAR), it is not a necessary component of the systemic resistance induced by most rhizobacterial strains. In
stead, rhizobacteria-me-diated induced systemic resistance (ISR) is dependent on jasmonic acid (JA) and ethylene (ET) signaling in the plant. Upon challenge inoculation of induced plants with a patho-gen, leaves expressing SAR exhibit a primed expression of SA-responsive defen-related genes, whereas leaves expressing ISR are primed to express JA/ET-responsive genes. A combination of ISR and SAR can increa protec-tion against pathogens that are resisted through both path-ways, as well as extend protection to a broader spectrum of pathogens than ISR or SAR alone3,4. Here we focus on ISR with emphasis on extensively studied group of biocontrol PGPR consisting of certain fl uorescent pudomonads and other organisms that protect a range of crop plants from important, mostly fungal root pathogens.
Induced-resistance systems in plants
An induced-resistance system in plants is very complex which has been partially elucidated in veral model plant systems viz., Arabidopsis. There are three generally rec-ognized pathways of induced resistance in Arabidopsis wherein two of the are involved in the direct production of pathogenesis-related (PR) proteins; in one pathway, the production of PR proteins is generally the result of attack by pathogenic microorganisms whereas in the other, PR proteins are generally produced as a result of wounding, or necrosis-inducing plant pathogens; both pathways however ha
ve alternate mechanisms for induction. Typically, the pathogen-induced pathway relies on salicylic acid (SA) that is produced by the plant as a signaling molecule, whereas the wounding pathway relies on jasmonic acid (JA) as the signaling molecule. The compounds and their analogues induce similar respons when they are applied exogenous-ly and no doubt, there is considerable cross talk between the pathways5. The JA induced pathway has been designated as induced systemic resistance (ISR) and this term is also ud to refer to quite different process that are initiated by rhizobacteria.
The salicylate- and jasmonate-induced pathways are characterized by the production of a cascade of PR pro-teins which include antifungals (chitinas, glucanas and thaumatins), and oxidative enzymes (viz., peroxidas, polyphenol oxidas and lipoxygenas) respectively. Low-molecular weight compounds with antimicrobial properties (phytoalexins) can also accumulate. The third type of induced resistance is one which is provoked by non-pathogenic root-associated bacteria and is referred to as rhizobacteria induced systemic resistance (RISR) which led to development of systemic resistance to plant dias. However, it is functionally very different, as the PR proteins and phytoalexins are not induced by root colonization by the rhizobacteria in the abnce of attack by plant-patho-genic microorganisms. Once pathogen attack occurs, the magnitude of the plant resp
on to attack is incread and dia is reduced. Thus, RISR results in potentiation of plant defence respons in the abnce of cascade of pro-teins that is typical of the SA-induced system. Arabidopsis as a model to study
Rhizobacteria-Mediated ISR
To study rhizobacteria-mediated ISR, an Arabidopsis-bad model system was developed becau this plant species has been excellently studied for molecular genetic rearch on plant-microbe interaction wherein non-pathogenic rhizo-bacterial strain Pudom onas fluorescens WCS 417r has been ud as an inducing agent. Colonization of Arabidop-sis roots by ISR-inducing WCS 417r bacterium protects the plants against different type of pathogens, including the bacterial leaf pathogen Ps. syringae pv. tomato and Xanthom onas cam pestris pv. armoraciae, the fungal root pathogen Fusarium oxysporum, the fungal leaf pathogen Alternaria brassicicola and the oomycete leaf pathogen Peronospora parasitica6.
Role of ISR
It is envisaged that in suppressive soils plant roots are as-sociated with microbial communities that have an overall benefi cial effect on plant health. Indeed veral biocontrol PGPR elicit ISR in the ho
st plant which allows plants to withstand pathogen attack to the leaves/roots without offer-ing total protection7. Many effective biocontrol PGPR elicit ISR, irrespective of antibiotic production8. The effects of three different strains of Pudomonas spp. mediating ISR in Arabidopsis thaliana have been investigated through transcriptome (expresd level of proteins) analysis of plants with roots that were colonized by one of the strains (P. fluorescens WCS 417r, P. thivervalensis and P. fluo-rescens CHA0). In each instance, the transcript levels in the leaves were not markedly , they varied by less than a factor of three, compared with the uninoculated control, and systemic respons that are typically en after attack by necrotizing pathogens. Challenge inoculation of plants with a leaf , P. syringae pv. tomato, showed that ISR-positive plants were ‘primed’ i.e., they re-acted faster and more strongly to pathogen attack by induc-
ing defen mechanism9. Studies conducted with A. thali-ana mutants indicated that JA/ethylene inducible defensive pathway was important for ISR, whereas the SA-inducible pathway was meant for mediating systemic acquired resis-tance (SAR). In bean, ISR elicited by P. putida strain, was associated with elevated level of hexenal (volatile antifun-gal compound) and with enhanced expression of enzymes that are involved in hexenal synthesis8.
The foremost question that comes to mind is which bacte-rial signals elicit ISR? Phl- (2,4-diacetylphl
oroglucinol) mu-tants of P. fl uorescens CHA0 were less effective than the wild type in protecting Arabidopsis from the leaf pathogen Pero-nospora parasitica and application of phl to the roots trig-gered ISR to this pathogen10. Sharma et al34 (2007) have been described molecular characterization of rhamnolipid which is considered to be determinant of biocontrol activity wherein a detailed screening of bacterial isolates from the Central Hi-malayan region for plant growth promoting and antimycelial activity against Pythium and Phytophthora strains have been employed. They afforded ven isolates of which three were particularly effective against the incidence of damping-off in fi eld trials on chile and tomato. In this investigation an initial spectroscopic survey of the methanolic extracts of the ven bacterial isolates showed complex mixtures apart from tho from Pudomonas sp. GRP3, one of the most promising isolates bad on fi eld studies. Strain GRP3 was lected for structural characterization of its condary metabolites, par-ticularly glycolipids. The extracellular condary metabolites were enriched by Amberlite XAD-16 adsorber resin followed by paration and structural analysis using TLC, LC-MS, MS-MS and NMR spectroscopy. Acquired data show the prence of a number of mono- and di-rhamnolipids, that include Rhamno (Rha)-C8-C10, Rha-C10-C8, Rha-C10-
C10, Rha- C10-C12:1, Rha-C10-C12, Rha-Rha-C8-C10, Rha-Rha-C10-C10, Rha-Rha-C10-C10:1, Rha-Rha-C10-
C12, Rha-Rha-C10-C12:1, Rha-Rha-C12-C12:1, and Rha-Rha-C12-C12 in strain GRP3. Since rhamnolipids are involved in the lysis of the plasma membrane of zoospores of fungi, application of such rhamnolipid-producing rhizobacte-ria could facilitate control of damping-off plant pathogens.
SA-overproducing recombinant of P. fl uorescens strain P3 showed enhanced protection of tobacco against TMV compared with the wild type P3 which indicate that –SA might also stimulate defence. In another Pudomonas bio-control strain, a combination of siderophores pyocyanin and pyochelin em to be most effective for inducing resistance in tomato. The PGPR, P. fl uorescens GRP3 showed ISR in rice against sheath blight. The plant-growth stimulating volatile 2,3-butanediol that is found in Bacillus spp. can also initiate ISR. It is diffi cult to recover specifi c ISR elici-tors in veral ISR-competent strains of fl uorescent pudo-monads, therefore, it has been propod that a combination
of siderophore, O-antigen and fl agella might account for the
ISR effect11,12,13,14.
Rearchers have been described role of siderophores
which is one of the determinants of ISR in effecting plant
nutrition wherein they overcome problem of iron non-avail-
ability particularly in calcareous soils by incorporation of
siderophore producing strains of fl uorescent pudomonads
(F LPs). Sidrophore producing bacterium Pudomonas
strain GRP
3
was employed in a pot experiment to asss
the role of microbial siderophores in the iron nutrition of
mung bean employing Fe-citrate, Fe-EDTA, and Fe(OH)
3 in different concentration. The plant showed a reduction
如何开办幼儿园
of chlorotic symptoms and enhanced chlorophyll level in
bacterized plant. Bacterization with GRP
3
incread peroxi-
摔拼音
da activity and lowered catala activity in roots. There
was also a signifi cant increa in total and physiologically
available iron. Such siderophore producing system has the
potential of improving iron availability to plants and reduce
fertilizer usage33. Sharma et al35 (2007) reported effi cacy of
bacterial isolate to protect chile and tomato plants under
natural vegetable nurry and artifi cially created pathogen
厦门方特水上乐园infested (Pythium and Phytophthora spp.) nurry condi-
tions. Chile and tomato plants were harvested after 21 d of
蝴蝶习性sowing and analyd for peroxida and phenylalanine am-
monia lya (PAL) activities (ISR responsive proteins and
not SAR-responsive). They found that Pudomonas sp.
strains FQP PB-3, FQP-PB-3 and GRP
3
were most effective
in increasing shoot length together with incread activity最厉害的人
of peroxida and PAL., which are well known as indicators
of an active lignifi cation process.
The mechanism of rhizobacteria-induced systemic
resistance (RISR)
The generally non-specific character of IR constitutes an
increa in the level of basal resistance to veral pathogens
concomitantly, which is of benefi t under natural conditions
where multiple pathogens may be prevented15. To under-
stand the phenomenon of rhizobacteria-mediated ISR it is
important to gain insight into the bacterial plant mecha-
nisms involved and to unravel the requirements for ISR
induction, signaling, and expression.
Induction of ISR
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Beneficial rhizobacteria do not obviously damage their
host/cau localized necrosis, therefore, the eliciting factors
produced by ISR-triggering rhizobacteria must be different
from elicitors of pathogens. There is comparatively little
information on the bacterial determinants that trigger ISR. Mechanism of elicitation shows veral similarities to the generation of certain non-specifi c defen reactions in plant cells that occur in respon to general pathogen-associ-ated molecular patterns (PAMPs); common components are prent in microorganisms which appear to be recognized by eukaryotic cells 16. Cell surface components viz., LPS and fl agella can act as trigger of defence-associated reaction in suspension-cultured plant cells and leaves 17. Both the factors of the rhizobacterial strain WCS 358 have the ability to elicit ISR when applied as purifi ed components to root system of Arabidopsis plants upon challenge inoculation of treated plants with the causal agent of bacterial speck dis-ea. The pathogenic bacterium P . syringae pv. tomato (Pst) which results in chlorotic and necrotic symptoms on the plants was reduced to an extent comparable to that on plants grown in soil containing wild type strain WCS 35818.A non-specifi c induction of ISR by rhizobacteria is also incompatible with an obrved differential induction of sys-temic resistance in different plant species
and in ecotypes. Several rhizobacterial strains appear to be equally effective in ISR in different plant species whereas others show nar-row specifi city which is indicative of a plant species-specifi c recognition between bacteria and receptors on the root sur-face. Three WCS strains of P . fl uorescens mentioned earlier elicit ISR in different plant species (Table 1). For a limited number of ISR-eliciting rhizobacterial strains the inducing determinants (s) have been identifi ed through mutant analy-sis and application of isolated components (Table 2).
Signalling in pathogen-induced SAR
二级路由器Identifi cation of critical steps in the signal transduction pathway for SAR has been studied by employing mutant and transgenic plants. A phenolic compound structurally rembling SA was required for the establishment of SAR was borne out when SA was determined to be an endog-enous compound in plants which incread in amount upon elicitation. Recently, it has been hypothesized that local SA levels are incread upon induction which is associated
with the generation of a mobile signal that is transported
throughout the plant whereby initiating further local SA production in distant leaves. This level of SA is necessary and suffi cient to confer the systemically induced state 19.There is neither an understan
ding about the trig-ger which is responsible for incread SA production in the plant, nor has it been established how SA exerts its resistance-inducing action. The protein NPR1, an an-kyrin-repeat family protein which structurally rembles the inhibitor of IF -kB, necessary for SA action in plant, plays a role in animal innate immunity. A redox change caus oligomers of NPR1 in the cytoplasm to be reduced to monomers under the infl uence of SA. The monomers are transported into the nucleus where they interact with specifi c TGA transcription factors to allow the expressions of genes encoding pathogenesis-related proteins (PRs)20. The conclusions led to the hypothesis that the status of SAR relies on the prence of PRs.
Signalling in rhizobacteria-induced systemic resistance (RISR)
Signalling in ISR appears more complex than that in SAR. Several ISR-eliciting rhizobacterial strains have been de-scribed which are also capable of producing SA whereas others do not. Two criteria can be ud to explain this: (i) the ISR should be associated with the induction of PRs and, (ii) both ISR and the induction of PRs should be abolished in Nah G plants (SA defi cient). ISR against tobacco mosaic virus (TMV) and Botrytis cinerea is abolished in tobacco and tomato plants upon challenge inoculation with 7NSK2, and in Arabidopsis against P . syringae pv. maculicola after elicitation by B. pumilus SE3421,22 whereas it is maintained in all other combinations tested (Table
3). Strain WCS 358, which does not produce SA, elicit ISR in Arabidopsis whereas other rhizobacterial strain that can produce SA in vitro does not elicit , WCS 374 on Arabidopsis which otherwi elicits ISR in a SA-independent way viz., Serratia marcescens on tobacco or P . fl uorescens CHA0 on
Arabidopsis 10
; this data indicates that rhizobacterial produc-tion of SA is not generally required for induction of SAR.
Table 1 Differential induction of systemic resistance (SR) by Pudomonas spp. in different plant species Plant Species P . putida WCS 358
P . fl uorescens WCS 374
P . fl uorescens WCS 417
References
Arabidopsis + - + 36
Bean and Tomato
+
ND
+
18
Carnation - ND + 37Radish - + + 38
库尔勒海拔多少米Several ISR-eliciting strains have also been shown to activate the PR-1α promoter in a transgenic GUS reporter line of tobacco, including S. marcescens 90-166, that was subquently shown to induce resistance in tobacco in a SA-independent manner23,24. Downstream of SA in the SAR signaling pathway, the protein NPR1 plays an important role and this protein is necessary for ISR in Arabidopsis. Despite this SA is not necessary for ISR in this system. Mutant npr1 plants do not express ISR after treatment with WCS 417 and refl ect that NPR1 ems to play a central role in reaching the induced state whether triggered by avirulant pathogens or by non-pathogenic rhizobacteria. Recently, evidence was provided which demonstrated that NPR1 is translocated to the
nucleus upon induction of SAR, where it activates PR gene expression by physically interacting with a subclass of basic leucine zipper protein transcription factor that binds to promoter quences required for SA-inducible PR gene expression both in vitro and in vivo25,26,27. However, downstream of NPR1, the signaling pathways must diverge again becau SAR is associated with the accumulation of PRs whereas in ISR-induced plants such accumulation does not commonly occur (Fig. 1). Expression of ISR
Expression of ISR is similar to SAR upon challenge inocula-tion with pathogen wherein dia verity is reduced; the number of diad plants also diminishes. This reduction is associated with decread growth of the pathogen and reduced colonization of induced tissues which refl ects upon the ability of plant to resist the pathogen. The spectrum of dias against which ISR and SAR are effective overlaps only partly, becau of the differences in defen signaling. It has been demonstrated in Arabidopsis, that pathogens are resisted by either SA-dependent, or by JA- and/or ethylene dependent defens or both. SA is an important signaling molecule in both locally and systemically induced resistance respons; however, rearch on rhizobacteria mediated ISR signaling has demonstrated that JA and ethylene play the key roles28. Thus, expression of ISR is phenotypically quite similar to SAR, and relies not only on a different type of bio-logical induction but
occurs also through different defen-re-lated activities. Plant defen , phytoalexins can also contribute to plant resistance but available information shows that in mutants of Arabidopsis that are impaired in the
Table 2Bacterial determinants of induced systemic resistance in different plant species.
Bacterial strain Plant species Determinant References
B. amyloliquefaciens IN 937a Arabidopsis 2,3-butanediol 13
B. subtilis GB03 Arabidopsis 2,3-butanediol 13
P. aeruginosa 7 NSK2Bean SA
40
Tobacco SA 39
Tomato Pyochelin
&Pyocyanin 11 P. fl uorescens CHA0Arabidopsis 2,4 DAPG 10
Tobacco Siderophore 41
Tomato2,4 DAPG 42 P. fl uorescens Q2-87Arabidopsis 2,4 DAPG 43
P. fl uorescens WCS 374Radish LPS 44
Siderophore and Fe-regulated compounds45 P. fl uorescens WCS 417Arabidopsis LPS36
Carnation LPS 46
Radish LPS 44
Fe-regulated compounds45 P. putida WCS 358Arabidopsis LPS, Siderophore, Flagella18
Bean LPS, Siderophore18
Tomato LPS, Siderophore18 P. fl uorescens GRP3Rice Siderophore12
Rhizobium etli G12Potato LPS 47
S.marcescens 90-166Tobacco Fe-regulated compounds48
