This article is from the May 2009 issue of. published by The American Phytopathological Society

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1 This rticle is from the My 2009 issue of pulished y The Americn Phytopthologicl Society For more informtion on this nd other topics relted to plnt pthology, we invite you to visit APSnet t

2 Biologicl Control Seed Tretment with 2,4-Dicetylphloroglucinol-Producing Pseudomonds Improves Crop Helth in Low-pH Soils y Altering Ptterns of Nutrient Uptke Ros E. Rudles, Eric Stone, nd Brin B. McSpdden Grdener Deprtment of Plnt Pthology, The Ohio Stte University, Ohio Agriculturl Reserch nd Development Center, Wooster Accepted for puliction 5 Jnury ABSTRACT Rudles, R. E., Stone, E., nd McSpdden Grdener, B. B Seed tretment with 2,4-dicetylphloroglucinol-producing pseudomonds improves crop helth in low-ph soils y ltering ptterns of nutrient uptke. Phytopthology 99: Seed tretment with 2,4-dicetylphloroglucinol (DAPG)-producing Pseudomons strin meliorted iotic stress disorder in corn cused y growth in low-ph soil. In two consecutive growing sesons, Wood1Rtreted seed gve rise to plnts tht grew tller (P 0.05), hd fewer folir lesions (P 0.10), nd provided greter yields (P 0.1) thn the negtive controls when grown in soil with ph < 5.0. Under controlled conditions, seed tretment with Wood1R lso reduced folir lesion severity (P 0.05 in two of three experiments) ut filed to increse shoot or root growth in young seedlings grown in cidic soil. Significnt (P 0.05) ptterns of ltered minerl nutrient uptke (i.e., generlly incresing P nd Mg while reducing Al) were oserved to occur s result of Wood1R seed tretment under oth sets of growing conditions. In contrst, suppression of seedling dmping-off disese ws not indicted in this low-ph soil, ecuse no difference in crop stnd ws oserved for ny experiment. Additionlly, Wood1R-medited growth inhiition of seedling pthogens ws reduced in vitro t ph < 5.0, indicting tht secretion of ntifungl metolites my not occur in low-ph soils. This is the first report of n iotic stress meliortion of cid soil stress-relted symptoms y DAPG-producing pseudomond. Additionl keywords: phld+ Pseudomons fluorescens. Corresponding uthor: B. B. McSpdden Grdener; E-mil ddress: mg+@osu.edu doi: / PHYTO The Americn Phytopthologicl Society Diverse plnt-ssocited pseudomonds hve een shown to promote crop helth (25). Pseudomons fluorescens cteri tht produce the ntiiotic 2,4-dicetylphloroglucinol (DAPG) re well known for their cpcity to suppress diverse soilorne diseses (43), especilly tke-ll (7,30). DAPG producers re mrked y the presence of phld, n essentil gene involved in the iosynthetic pthwy of 2,4-DAPG (1), nd the mechnistic importnce of the DAPG production for iocontrol of seedling dmping-off nd root rots is well estlished (16,39). These cteri cn e redily recovered from most griculturl soils (17), especilly on cerels such s whet (26) nd corn (24). The strin Wood1R of phld+ pseudomonds which elongs to the genotype D ws identified s the dominnt genotype in corn fields in the stte of Ohio (24), genotype tht ws previously identified s superior rhizosphere colonist nd iocontrol strin (30,31). Recently, Rotenerg et l. (33) demonstrted tht the undnce of phld+ pseudomonds generlly ws positively correlted with corn stnds nd yields, though the precise mechnisms y which such responses occur re not entirely cler. In ddition, lthough widely dispersed, it is not cler how the ctivities of DAPG producers vry with soil conditions. Nturl or rtificil cidifiction of soils cn negtively impct crop growth nd yield, ecuse low soil ph cn significntly lter the vilility of severl minerl elements (22,38). For instnce, incresed soil cidity cn led to excessive uptke of luminum, mngnese, nd iron while simultneously reducing the vilility of essentil plnt nutrient ctions (clcium, mgnesium, nd potssium) nd nions (phosphtes nd sulfte). Acid soil conditions generlly led to poor root development tht susequently leds to nutrient nd wter deficiencies in the plnt (22). In ddition, imlnces of essentil minerl nutrients supplies ultimtely ffect yields (38). Although good griculturl mngement involves regulr liming of soils to mitigte cidifiction, millions of frmers still work lnd with excessively low soil ph (37). Little is known out the effect of low soil ph on DAPG producers nd their interction with plnts. Nsey et l. (29) performed n ssy testing the effects of strin F113 nd G22, which is DAPG mutnt of F113, under cid conditions (ph 4.4) on pe plnts. In tht study, no differences in plnt growth were oserved etween treted nd untreted plnts, even though the inoculted strins still ltered microil community structure to mesurle degree. Furthermore, Rotenerg et l (33) noted tht the undnce of ntive popultions of DAPG producers ws generlly negtively correlted with soil ph ut positively correlted with stnds nd yields under typicl growing conditions. In this study, we report the unexpected finding tht DAPG-producing pseudomonds hve the ility to control iotic nutrient stress disorder cused y low soil ph (i.e., ph < 5.0). MATERIALS AND METHODS Seed tretment. The strin Wood1R of P. fluorescens ws isolted from soils of Wood County, OH nd ws chrcterized to elong to the genotype D of phld+ pseudomonds (24). P. fluorescens strin Wood1R ws streked from glycerol stock into 1/3 King Medi B (KMB) gr nd llowed to grow for 48 h t room temperture. Then, the cteri were scrped from the gr plte nd were resuspended in 1/10 tryptic soy roth (TSB). The concentrtion of the cteri in the suspension ws quntified y mesuring the opticl density with n ELx800 Universl Microplte Reder (Biotek Instruments, Inc., Vermont). Then, cteril concentrtion of log 6 cells per grm of seed ws 506 PHYTOPATHOLOGY

3 pplied in 1% volume to mss rtio. An equivlent volume of 1/10 TSB ws dded to seed used s negtive control. Plnting ws done within 1 h fter treting the seed. Field iossy. The corn hyrid SC1091 (Seed Consultnts, Inc., Wshington Courthouse, OH) ws plnted t rte of 69,000 seeds/h during mid-my for two consecutive yers. The field ws locted on reserch frm sited t the Ohio Agriculturl Reserch nd Development Center (OARDC) in Wooster, OH. The tretments were plnted in locked fctoril design, with ech tretment eing plnted cross two nturlly occurring regions, delineted zone I nd zone II, with differing soil ph nd minerl concentrtions (Tle 1). There were two replictes per tretment consisting of rows tht were 76 cm prt nd 12 meters long. In 2006, the two soil zones were ssessed together ecuse the phenomenon ws not oserved until mid-july. Stnd counts were tken 29 dys post-plnting when the mjority of plnts hd reched V3 (n = 32) in oth zones together. Becuse the stress differentil ws not oserved until mid-july, plnt height nd percentges of lef re covered with lesions were ssessed when plnts in zone II nd the surrounding field were etween V6 to V8. Yields were ssessed y mechnicl hrvesting on 1 Novemer using the SPC 40 two-row reserch comine with the Hrvest Mster Plot Hrvest Dt System (ALMACO, Nevd, IA). In 2007, stnd counts nd height from oth soil zones were tken 28 dys post-plnting when most of the plnts hd reched V3 (n = 40). In zone II, eight plnts per tretment were collected 48 dys post-plnting to mesure plnt height, folir lesions, nd nutrient uptke. Susequently, the corn ers were hnd hrvested from ech tretment nd ech zone on 9 Octoer nd yields were clculted using the following formul: (kg er 1 ) (stnd m 2 ) (10,000 m 2 h 1 ) (ton 1,000 kg 1 ) (0.86). The 0.86 fctor is n estimtion of the mount of moisture sed on the numer used in the yer 2007 y the Ohio Corn Performnce Test progrm (28). The percentges of lef re covered with lesions were clculted sed on the Horsfll-Brrtt grding system (4). Experiments conducted under controlled conditions. Four experiments were performed with soils collected from the field descried ove. The first experiment (GH06) ws designed to determine if the nture of the crop stress oserved in the field ws sed on soil qulity differences. The experiment consisted of rndomized complete design in which untreted seed of the hyrid SC1091 were plnted in soils of zone I nd zone II (n = 8). The soils for this experiment were collected during the first week of August 2006 from untreted res of the field. Ech soil smple ws composite of three susmples collected 50 cm prt nd pproximtely 15 cm deep. The soils were ir dried for 5 to 7 dys nd then plced into smll plstic pots (10 cm in dimeter y 10 cm in height) for conducting the iossy. For the second greenhouse ssy (GH07), the remining soils from zone II from the August 2006 smpling were comined nd liquoted into similrly sized pots. For the third (GH07) nd fourth (GH07c) experiments, the soils from zone II were collected on 25 June nd 25 Octoer 2007, respectively, dried, nd mixed with the sme pproch descried ove. Experiment GH07c ws done in growth chmer. The 2007 experiments were estlished to determine the effect of Wood1R on corn helth when plnted into zone II soil. The corn ws plnted into the potted soils descried ove. Four seeds were plnted per pot. Three independent experiments were estlished in completely rndomized designs with n = 5, 4, nd 20 replictes for GH07, GH07, nd GH07c, respectively. Ech experiment consisted of two experimentl tretments (i.e., seed treted with or without P. fluorescens strin Wood1R). The plnts were collected 28 dys post-plnting nd were mesured for height, folir lesions, root weight, nd fresh shoot weight. Mesurements of ll the plnts in ech pot were tken nd then verged. Soil nutrient content nd plnt nutrient uptke quntifiction. Soil nd tissue smples were nlyzed for nutrient content y the Soil Testing nd Reserch Lortory (STAR l) t the OARDC immeditely fter smpling. Ech independent soil smple consisted of the entire content of soil from the whole pot or the entire volume of soil tht dhered to the root during smpling time for the controlled-conditions nd field experiments, respectively. The tissue smple consisted of composite smple of ll of the whole plnts in pot for the pot ssys nd the whole plnt collected for the field experiments. The numer of soil nd plnt smples consisted of n = 5, 4, nd 20 for GH07, GH07, nd GH07c, respectively, nd n = 8 for the field experiment. The soil nd plnt tissue smples were ir dried t 25 to 35 nd 60 C, respectively, nd susequently ground nd sieved through 2-mm rss mesh. The soil ph ws estimted y prepring 1:1 deionized wter nd soil slurry nd, fter 10 min, the ph ws mesured with n electronic ph meter (42). The ville phosphorus (P) ws determined with the Bry nd Kurtz P1 test (8). In ddition, exchngele K, C, nd Mg nd the totl ction exchnge cpcity were quntified with the mmonium cette extrction method (41). The extrctle P, K, C, Mg, S, Al, Cu, Fe, Mn, nd Zn were clculted with the Mehlich 3 Extrction nd Induced Coupled Plsm (ICP) Minerl Anlysis fter microwve digestion. The shoots were evluted for mjor elements (P, K, C, Mg, S, Al, Cu, Fe, Mn, nd Zn) with the ICP minerl nlysis fter microwve digestion (13,15). In vitro inhiition ssy. Pthogen growth inhiition medited y Wood1R ws tested t different ph levels. Sucultures of Fusrium grminerum, Rhizoctoni solni, nd Pythium grminicol were incuted in potto dextrose gr (PDA). Agr plugs of the cultures were trnsferred to PDA mended with 1 M KCl or 2 M NOH in order to otin desired ph of in intervls of 0.5 ph units. Fresh cultures of Pseudomons fluorescens strin Wood1R grown in 1/3 KMB were scrped nd then resuspended in 1/10 TSB. Then, 10-µl liquots contining pproximtely log 8 cells/ml were trnsferred onto the gr plte with the pthogen voiding direct contct. The pltes were scored for pthogen growth inhiition 6 dys fter preprtion on n ordinl scle of 0 (myceli overgrowing the cteri), 1 (mycelil growth to ner <1 mm from the edge of the cteri), or 2 ( cler inhiition zone of >1 mm etween the cteri nd the pthogens). Ech ssy included three independent replicte ssy pltes per ph tretment nd the entire experiment ws conducted twice. Sttisticl nlyses. Becuse the homogeneity of vrince ssumption ws violted in severl instnces, nonprmetric nlyses of vrince nd ssocition were pplied (36). All the sttisticl nlyses were performed using Minit softwre (relese 14.20; Minit Inc., Stte College, PA). TABLE 1. Chemicl properties of the field soils used in this study Soil ph P (ppm) K (ppm) c C (ppm) c Mg (ppm) c CEC (Meq 100 g 1 ) C (%) d Mg (%) d K (%) d Zone I Zone II 4.7*** * 122.5*** 18.0*** 13.5** 4.7*** 1.1*** 1.3 Medin vlues of the smples (n = 8) ssyed differ significntly mong soils ccording to the Kruskl Wllis test; *, **, nd *** indicte P 0.1, 0.05, nd 0.001, respectively. Similr ptterns of differences were oserved in ll susequent smplings of the sme two field loctions. Bry P-1 extrction method. c Exchngele C, Mg, nd K, one-molr (1 M) mmonium cette extrction method. d Extrctle C, Mg, nd K, Mehlich 3 extrction. Vol. 99, No. 5,

4 RESULTS TABLE 2. Chnges in medin plnt height, folir lesion severity, nd yields of field-grown corn resulting from seed tretment with Pseudomons fluorescens strin Wood1R when grown in low-ph soil Tretment Height (cm) Lef re with lesions (%) Yields (ton h 1 ) c 2006 Wood1R >1.5x y 10.5* Negtive control x >2y Wood1R 53.8** 1.5* 12.2 Negtive control Medin vlues of the treted smples ssyed within the sme yer differ significntly from the negtive control ccording to the Kruskl Wllis test; *, **, nd *** indicte P 0.10, 0.05, nd 0.001, respectively. Smple sizes were for n = 4 (2006) nd n = 8 (2007). Height (x) nd lef re covered with lesions (y) for 2006 were mesured when plnts were t V6 growth stge nd, lthough precise vlues were not otined t the time, field notes confirm tht ll plnts in the treted plot met the differentils noted. In 2007, dt represents plnts collected from the field t V4 growth stge. The lef re covered with lesions ws ssessed using the Horsfll nd Brrtt rting system. c In 2006, plots trversing zone I nd zone II were mechniclly hrvested. In 2007, only zone II sections were hrvested e hnd. Crop helth improvement under field conditions. The helth of field-grown corn plnts grown in low-ph soil ws mrkedly improved when the seed hd een treted with the DAPGproducing Pseudomons strin Wood1R (Tle 2). This response ws first noted in the 2006 field tril round the V7 stge. Although precise mesurements were not tken t tht time, field notes indicted tht crop height nd oveground iomss in suplots grown from Wood1R-treted seed were pproximtely doule tht for djoining sections of suplots where the seed hd not een pretreted with the cteri. Inversely, the percent lef re ffected y stress-relted folir discolortion ws noticely less in plots contining Wood1R-inoculted plnts. In 2007, the effects of seed tretment on crop helth were more precisely quntified, nd similr positive responses to the seed tretment were oserved (Tle 2). At V4, corn plnts treted with Wood1R were tller (P < 0.05) nd the percentge of lef re covered with lesions ws less (P < 0.10). Additionl plnt mesurements tken in 2007 (i.e., root weight nd fresh shoot weight mesured t V4) coincided with the differences noted in the plnt height mesurements. Medin root nd shoot fresh weights were greter (P < 0.05) for the plnts treted with P. fluorescens strin Wood1R (dt not shown). These two mesurements were positively correlted (Spermn s ρ 0.665, P 0.01) with height, confirming tht the plnt height mesurements were good proxies for plnt helth. Conversely, plnt height nd fresh shoot weight were oth negtively correlted with the percentge of lef re covered with lesions (ρ 0.503, P 0.05). In contrst, no significnt differences in crop stnd were oserved in zone II soils in either growing yer (P 0.3). Confirmtion of the phenomenon under controlled conditions. Under controlled conditions, the plnt stress cused y the zone II soils ws noticely lessened when seed were treted with P. fluorescens strin Wood1R (Tle 3). Results of GH07 nd GH07c experiments showed significnt reduction in the percent lef re covered with lesions on plnts rising Wood1Rtreted seed (P < 0.05 nd 0.001, respectively). In these experiments, no significnt differences in height (P 0.14) or root weight (P 0.25) were oserved in ny of the three experiments, indicting tht plnt growth promotion effects were not typiclly occurring during erly growth stges when the plnt were grown on cidic soil. As in the two field studies, no differences in stnd counts were oserved in these three studies (P 0.50 for ech), indicting tht dmping-off ws not significntly ffected y the Wood1R seed tretment under cidic soil conditions. Relting crop helth improvement to soil nutrient content nd plnt uptke. Significnt correltions (P 0.05 for ll) etween plnt height nd folir P, C, Mg, S, nd Cu (positive for ech) s well s B nd Mn (negtive for ech) were oserved (Fig. 1). On the other hnd, the percentge of lef re covered with lesions ws positively correlted to Mn only nd negtively correlted with P, C, Mg, nd Cu (P 0.05 for ll). These sttisticl ssocitions re consistent with the dignosis tht corn grown on zone II cid soils were suffering from n iotic stress disorder. Nutrient uptke in corn plnts grown in the low-ph soil otined from zone II ws ltered when P. fluorescens strin Wood1R ws pplied to seed prior to plnting (Tle 4). Most strikingly, plnts treted with P. fluorescens strin Wood1R were etter le to ccumulte folir P in ll four experiments, though the noted increses were significnt only in field07 nd GH07c (P < 0.10 for oth). Although not s striking, folir levels of K, Mg, nd S were generlly higher nd Al levels were generlly lower in Wood1R-treted plnts (i.e., in three of four experiments for ech). These dt indicte tht the Wood1R seed tretment led to significnt meliortion of the iotic stress disorder developing in response to the excessively cidic soil conditions. In vitro inhiition ssy. Becuse seedling dmping-off ws not suppressed y Wood1R in zone II soil conditions, we hypothesized tht low ph decresed the expression of ntifungl ctivities of the strin. To test this, pthogen growth inhiition iossys were performed in vitro t vrying ph levels. No inhiition of Pythium grminicol nd R. solni y Pseudomons fluorescens strin Wood1R occurred on PDA with ph < 5.0. However, Wood1R ws le to consistently inhiit pthogen growth ph 5.0 (P < 0.05). In contrst, inhiition of F. grminerum y Wood1R ws consistently oserved cross ll of the cidity levels tested (P < 0.05). These dt indicte tht soil cidity my negtively impct some of the iocontrol cpcities of DAPGproducing strins such s Wood1R, further counter-indicting the role of direct pthogen inhiition in the oserved growth promotion response seen in the low-ph soil of zone II. DISCUSSION In this study, we provide the first evidence tht DAPG producers cn lso ct to meliorte iotic stresses ssocited with excessively low soil ph. Specificlly, folir lesions induced on corn y growth on low-ph soil were reduced significntly on plnts tht were seed-treted with the phld+ P. fluorescens strin Wood 1R (Tles 2 nd 3). The oserved lesions ppered to TABLE 3. Pseudomons fluorescens Wood1R-medited reduction in the percentge of folir lesions on corn plnts grown in low ph soil under controlled conditions Seed tretment Lef re with lesions (%) GH07 P. fluorescens strin Wood1R 34.0** Negtive control 53.5 GH07 P. fluorescens strin Wood1R 16.8 Negtive control 7.1 GH07c P. fluorescens strin Wood1R 3.0*** Negtive control 13.8 Medin vlues of the treted smples rted 28 dys post plnting differed significntly from the negtive control ccording to the Kruskl Wllis test; *, **, nd *** indicte P 0.1, 0.05, nd 0.001, respectively. GH07, n = 5; GH07, n = 4; nd GH07c, n = 20. Percentge of lef re covered with lesions ws clculted with the Horsfll-Brrtt disese rting system nd includes white nd purple necrotic lesions mesured 28 dys post-plnting. 508 PHYTOPATHOLOGY

5 result from iotic nutrient stress ecuse they occurred only on plnts grown in zone II soils. Furthermore, the lesions themselves ppered similr to those generted y vrious iotic nutrient imlnces, including P deficiency nd Al toxicity (38,44). One could nticipte tht ph could hve significnt impct on rhizosphere microorgnisms physiologicl nd metolic chrcteristics (10,11,35). However, the fct tht such microil responses could led to such striking improvement in crop helth under conditions of cid soil stress ws surprising to us. The generlity of the noted stress meliortion phenotype mong the different strins of DAPG producers is not cler. It is well estlished tht high degree of genetic nd phenotypic diversity occurs mong DAPG producers (26), nd such vriility cn ffect their ility to compete in the rhizosphere of whet (20) nd other crop plnts (6). Here, we used only Wood1R, strin representing the dominnt phld genotype found on corn grown in Ohio (24) nd previously reported to e superior root colonizer (30,31). It will e interesting to see whether other DAPG producers hve similr cpcities to reduce plnt stress cused y low soil ph nd whether such phenotype is correlted with rhizosphere undnce of such cteri on different crops. Some other rhizocteri hve een shown to improve plnt helth y ltering nutrient uptke or nutrient vilility. It hs long een known tht Rhizoium nd Brdyrhizoium spp. lrgely determine nitrogen content in legumes through the process of symiotic nitrogen fixtion (21). In ddition, some Pseudomons strins cn increse minerl nutrient uptke sed on their cpcity to soluilize phosphorus (5) or desulfurize sulfonte (18). Some other rhizocteri cn indirectly medite nutrient uptke nd, consequently, improve plnt helth through interctions with mycorrhize (9). Still other cteril inoculnts hve een shown to reduce stress cused y hevy metl uptke (3). Generlly, one cn conclude tht eneficil effects of rhizocteri re gretest under conditions of limittion or stress to the plnt. This seems to e true whether the stress is iotic (e.g., imposed y soil with ph < 5.0) or iotic (e.g., imposed y the root pthogen complex). Here, the exct mechnisms y which Wood1R meliortes the physiologicl stress imposed on corn y growth in low-ph soil remin uncler. Further work should revel the generlity of the oserved phenomenon nd its reltive importnce compred with the disese-suppressing ctivities expressed y Wood1R nd relted strins in different soils with vrying chemicl properties. A chnge in root growth rte or morphology my prtilly explin the Wood1R-induced stress meliortion. It is well known tht plnt s cpcity to otin wter nd minerl nutrients from the soil is ssocited with its ility to extend its root system (34). However, under dry or nutrient-limiting conditions, root growth rtes cn slow down. Moreover, chnge in root rchitecture hs een identified s n importnt voidnce strtegy y which plnts dpt to iotic stresses (22). Whether such ctivities occur in situ in field-grown crops such s corn remins to e investigted. Studies hve shown tht phosphorus uptke in corn nd severl other crop species is highly influenced y the density of root nd root hirs (14,27). Recently, Brzelton et l. (2) demon- Fig. 1. Rnk correltions etween different mesures of plnt helth nd nutrient composition of corn plnts grown under field conditions in the low-ph soil from zone II. Significnt Spermn s coefficients re noted; *, **, nd *** indicte P 0.1, 0.05, nd 0.001, respectively (n = 8). TABLE 4. Chnges in folir nutrient composition of corn developing from seed treted with Pseudomons fluorescens strin Wood1R when grown in low-ph soil Extrctle minerls (ppm) c Experiment Tretment P K C Mg S Al B Cu Fe Mn Zn Field ,167* 20,737 3,896* 1,796* 1, * ND 630 1, ,649 23,032 3,145 1,402 1, ND 598 1, GH07 + 1,765 19,803 5,149 2,227* 2,112 55** 15 ND 95 5, ,654 19,379 4,822 1,860 2, ND 98 6, GH07 + 2,281 39,372 3,488 1,279 2, , ,205 37,288 3,591 1,358 2, , GH07c + 3,779* 22,523 2,338* 2,366 2,365* ** 3** 745 3,965 81** 3,083 22,179 2,417 2,183 2, , All corn (hyrid SC1091) ws grown in low-ph soils otined from zone II. Presence (+) or sence ( ) of Wood1R seed tretment. c Medin vlues of the treted smples ssyed within the experiment differ significntly from the negtive control ccording to the Kruskl Wllis test; *, **, nd *** indicte P 0.1, 0.05, nd 0.001, respectively; ND = not detected. Vol. 99, No. 5,

6 strted tht ppliction of DAPG-producing pseudomonds or exogenously pplied DAPG itself could increse root hir density nd secondry root growth in tomto seedlings. Preliminry in vitro ssys conducted in soil extrct gr medi showed tht root length of seed treted with P. fluorescens strin Wood1R ws similrly decresed; however, detiled evlution of secondry root growth nd root hir formtion ws not conducted (dt not shown). Nonetheless, our field dt showed tht the root fresh weight of plnts treted with Wood1R ws positively correlted with P, Mg, nd Cu nd negtively correlted with Al nd Fe (Fig. 1), suggesting tht root development might hve een improved y the enhnced uptke of such nutrients (Tle 4). Becuse the reltive undnce of DAPG producers hs een correlted with improved stnds nd yields of corn grown under vrious field conditions (33), we hypothesized tht multiple mechnisms expressed y such strins cn contriute to improved crop helth. It is well estlished tht DAPG producers cn medite specific pthogen suppression nd provide for iologicl control of severl different root diseses (43). Although ntiiosis of root pthogens is one estlished mechnism of ction (17,39), DAPG producers, nd DAPG itself, cn lso induce plnt host defenses pthwys nd lter crop root morphology under certin conditions (2,12,23,40). Such different host responses indicte tht single strin (or popultion) of DAPG-producing rhizocteri cn ct in multiple wys to promote crop helth (32). This ide is not new, ecuse iocontrol nd plnt-growth promotion ttriuted to vrious rhizocteri hve een previously referred to s two sides of the sme coin (19). However, it now seems cler tht the full spectrum of rhizocteril ctivities tht led to improvements in crop helth must certinly extend eyond the commonly mesured responses of disese suppression nd erly-seson growth promotion. It will e essentil to identify nd chrcterize the expression of these different mechnisms if we re to fully explin the yield enhncements generted through seed inocultion with DAPG-producing pseudomonds nd other iocontrol microes. ACKNOWLEDGMENTS We thnk D. Snchez, B. Brdll, nd the Snyder Frm field crew for their ssistnce in plnting nd hrvesting of the field plots; nd R. E. Rudles thnks M.-S. Benitez for her support throughout the project. LITERATURE CITED 1. Bnger, M. G., nd Thomshow, L. S Identifiction nd chrcteriztion of gene cluster for synthesis of the polyketide ntiiotic 2, 4-dicetylphloroglucinol from Pseudomons fluorescens Q2-87. J. Bcteriol. 181: Brzelton, J. N., McSpdden Grdener, B. B., nd Coenen ,4- Dicetylphloroglucinol lters plnt root development. Mol. Plnt-Microe Interct. 21: Burd, G. I., Dixon, G., nd Glick, B. R A plnt growth-promoting cterium tht decreses nickel toxicity in seedlings. Appl. Environ. Microiol. 64: Cmpell, C. L., nd Mdden, L. 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