microbiology ecology vol 1 issue 6

8/12/2019 microbiology ecology vol 1 issue 6

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he Role of Pigmentation, Ultraviolet Radiation Tolerance, andeaf Colonization Strategies in the Epiphytic Survival ofhyllosphere Bacteria

L. Jacobs1,2, T.L. Carroll1 and G.W. Sundin1,2,3

Department of Plant Pathology and Microbiology, Texas A & M University, College Station, TX 77843, USADepartment of Plant Pathology, Michigan State University, East Lansing, MI 48824, USACenter for Microbial Ecology, Michigan State University, East Lansing, MI 48824, USA

ceived: 4 September 2003 / Accepted: 19 November 2003 / Online publication: 23 September 2004

bstract

henotypic mechanisms that enhance bacterial UVRrvival typically include pigmentation and DNA repairechanisms which provide protection from UVA andVB wavelengths, respectively. In this study, we exam-ed the contribution of pigmentation to field survival inavibacter michiganensis and evaluated differences in

opulation dynamics and leaf colonization strategies.wo C. michiganensis pigment-deficient mutants were

gnificantly reduced in UVA radiation survival in vitro;ne of these mutants also exhibited reduced field popu-ions on peanut when compared to the wild-type strain

ver the course of replicate 25-day experiments. TheVR-tolerant C. michiganensis strains G7.1 and G11.1aintained larger epiphytic field populations on peanutmpared to the UVR-sensitive C. michiganensis T5.1.

piphytic field populations ofC. michiganensis utilizede strategy of solar UVR avoidance during leaf coloni-tion resulting in increased strain survival on leaves afterVC irradiation. These results further demonstrate the

mportance of UVR tolerance in the ability of bacterialrains to maintain population size in the phyllosphere.owever, an examination of several bacterial speciesom the peanut phyllosphere and a collection of envi-nmental Pseudomonas spp. revealed that sensitivity toVA and UVC radiation was correlated in some but not

of these bacteria. These results underscore a need torther understand the biological effects of different solaravelength groups on microbial ecology.

Introduction

The phyllosphere (plant leaf surface) presents a harshenvironment for the growth and survival of microor-ganisms; nutrients are limited, and stress conditions,including fluctuating water availability, osmotic stress,and exposure to solar UV radiation (UVR), are prevalent.Solar radiation is categorized by photobiologists intothree wavelength classes (UVA, 320 to 400 nm; UVB, 290to 320 nm; UVC,


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atmosphere by oxygen and ozone, high-energy UVCwavelengths have been substituted for UVB in manylaboratory assessments of UVR survival involving DNArepair. For example, we have found that UVC-sensitivityassays are more effective at discriminating survival dif-ferences among closely related strains [30]; also, UVCirradiation provides sensitivity data for bacteria and fungi

that are highly tolerant or insensitive to elevated doses ofUVB [1, 25, 29].

Phyllosphere bacterial communities are diverse andare usually composed of organisms displaying a range ofsensitivity to DNA-damaging UVR wavelengths and ahigh percentage of pigmented organisms [2, 9, 12, 29,33]. Mechanisms to combat UVA-induced oxidativestress in bacteria are complex and interactive and includethe production of carotenoid pigmentation and/or theexpression of enzymes such as catalase and superoxidedismutase (SOD). Bacterial pigmentation also affords

protection from UVA wavelengths or from visiblewavelengths that can activate photosensitizing chemicalsthat are inhibitory to cells. Carotenoid pigments pro-duced by Erwinia herbicola (Pantoea agglomerans) andClavibacter michiganensis play an important role in UVAsurvival [3, 29]. The yellow pigment xanthomonadinproduced byXanthomonas campestris increased the sur-vival of cells in the presence of visible light and thephotosensitizing agent toluidine blue, and also protectedlipids from peroxidation [24]. Similar to the phyllo-sphere, carotenoid pigmentation is prevalent in organ-

isms inhabiting other solar UVR-exposed habitats such asfreshwater phytoplankton assemblages [18], and inatmospheric bacterial isolates [34]. However, the con-tribution of pigmentation to environmental survival,especially in the phyllosphere, has not been adequatelyaddressed.

We are interested in the role of solar UVR in mod-ulating phyllosphere bacterial communities and in thespecific UVR survival strategies that contibute to in-creasedfitness of bacterial phyllosphere inhabitants. Ourprevious studies have focused on C. michiganensis as a

model peanut epiphyte; most isolates ofC. michiganensisrecovered from peanut are pigmented and highly tolerantto all UVR wavelength classes, and this organism makesup the majority of the culturable bacterial communityfrom peanut late in the growing season in Texas [12, 29].In this study, we examined the contribution of pigmen-tation to field survival and examined differences in thepopulation dynamics of C. michiganensis strains usingtwo UVR-tolerant and one UVR-sensitive strain. We alsoevaluated the ecological strategy of UVR avoidance infield populations of C. michiganensis. Finally, we report

the results of comparative examinations of the in vitrosurvival of phyllosphere bacteria and Pseudomonas spp.following irradiation with UVA or DNA-damaging UVCwavelengths.

Methods

Bacterial Strains and Growth Conditions. The bacterialstrains used in this study are listed in Table 1. The peanutepiphytes C. michiganensis G7.1, G11.1, and T5.1 werechosen for comparative study because these strains dif-fered in sensitivity to UVC radiation (G7.1, highly UVC

tolerant; G11.1, medium UVC tolerant; T5.1, UVC sen-sitive) and produced different colored pigments in cul-ture (Table 1). All bacterial strains were grown in Luria-Bertani (LB) medium (Difco) or Kings medium B (KB)[16] at 28C. Pigment-deficient mutants of C. michi-

ganensis G7.1 and T5.1 were generated using ethylmethane sulfonate (EMS) as previously described [29].The antibiotic rifampicin (75 lg mL)1) was added tomedia where necessary. For enumeration of inoculatedstrains in field experiments, KB medium was amendedwith rifampicin (75 lg mL)1) for bacterial selection and

nystatin (300 U mL)1

) for fungal growth inhibition(KBrn).

In Vitro UVA and UVC Sensitivity Analyses. UVAsensitivity was determined in vitro using an XX-15 lamp(UVP Products; San Gabriel, CA). Preparation of bac-terial suspensions prior to irradiation was done aspreviously described [12, 30]. The cell suspensions wereplaced on a rocking shaker (five revolutions per minute)under the UVA lamp source or agitated by hand underthe UVC lamp source to ensure mixing to minimize cell

survival due to shading. We examined cell survival(compared to a nonirradiated strain) following five toseven incremental doses per strain; higher doses wereachieved by increasing the time of exposure. The energyoutput of the UVA (4548 J m)2 s)1) and UVC (1.5 Jm)2 s)1) lamps was monitored with a UVX radiometer(UVP Products) fitted with the appropriate wavelengthdetector.

Field ExperimentsBacterial Population Dynam-

ics. Field experiments were conducted at a site adja-

cent to the Texas A & M campus in College Station,Texas, using replicated microplots of peanut (Arachishypogeae cv. Florunner). In one set of experiments, wecompared the population dynamics ofC. michiganensisG7.1 and G7.1pig), and T5.1 and T5.1pig) inoculatedseparately onto plants. In a second set of experiments, wecompared the dynamics ofC. michiganensis G7.1, G11.1,and T5.1 inoculated separately or as a 1:1:1 mixture. Thecircular microplots (diameter = 0.7 m) contained fivepeanut plants which were hand-sown at a depth of 2 to3 cm. Each field experiment consisted of three replicate

plots per treatment arranged in a randomized blockpattern, and the experiments were conducted twice.Temperature, relative humidity, and precipitation weremonitored at a site 500 m from the field plot. Data

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adings were taken every 15 s and logged using a CR-10talogger (Campbell Scientific, Logan, UT). Solar UVBadiance during the hours 1100 to 1500 was monitored

wice per week throughout the experiments using a UVBtector (SED240/UVB-1/W) attached to an IL-1700

search radiometer (International Light, Newburyport,A). The detector was placed at 0.3 m above canopyight. UVB irradiance was measured every second, ande readings were integrated over the 4-h period yieldingquantitative output in J m)2.

Cells ofC. michiganensisstrains were grown on KBrnr 48 h prior to use as inocula for plant experiments.

he cells were resuspended in 0.1 M potassium phos-hate buffer (pH 7.0), and suspensions were adjusted

rbidimetrically to 1 108

cfu mL)1

. Bacterial inoc-um was applied to the peanut leaves (at approximately00) with a hand-held sprayer until the leaf surfaces

ere uniformly wet. Samples consisted offive individualaves taken from each replicate plot for a total of 15aves per strain per sampling date. Leaves of the sameze and age were chosen randomly from the top of theant canopy. Plant leaf samples were taken at 1200. Eachaf was placed in a sterile plastic bag and transported toe laboratory on ice for immediate processing. Leavesere weighed and placed in 10 mL prechilled buffer

.1 M potassium phosphate, pH 7.0, 0.1% peptone),llowing which bacterial cells were removed by a 7-minnication treatment in an ultrasonic bath (Model 250T,

WR Scientific; Houston, TX). Samples (0.1 mL) from

appropriate dilutions of the sonicate were plated on KBrnmedium. Bacterial colonies were counted following 72 to96 h incubation at 25C. In experiments examining the

dynamics of mixed C. michiganensis populations, strainswere simultaneously enumerated on the same plates ascolonies ofC. michiganensisG7.1 (orange), G11.1 (pink),and T5.1 (creamy light yellow) could be easily differen-tiated by color.

Enhanced UVR Survival of C. michiganensis During

Leaf Colonization. In these experiments, we evaluatedthe survival ofC. michiganensis G7.1, G11.1, and T5.1 onUVC-irradiated peanut leaves by comparing populationson UVC-irradiated leaves with populations recovered

from nonirradiated leaves. Inoculum was prepared asdescribed above. Strains (G7.1, G11.1, T5.1) were eitherinoculated individually or as a 1:1:1 mixture. Immedi-ately and at 1, 2, 3, 4, 6, 8, and 10 days after inoculation,four leaves per treatment were randomly excised, placedin sterile plastic trays, and brought to the laboratory onice. The leaves were then irradiated on the abaxial andadaxial leaf surfaces with UVC radiation by using the XX-15 lamp as described above. A UVC dose of 225 J m)2

was used for strains G7.1 and G11.1, and a dose of 100 Jm)2 was used for strain T5.1; the UVC doses were chosen

because they resulted in 10% cell survival in vitro. Anadditional four leaves were excised per treatment, andthese leaves were not irradiated as a control. After irra-diation, leaves were processed by sonication (to effec-

ble 1. Bacterial strains used in the study and their relevant characteristics

ain Relevant characteristics UV MIDc

a Source or reference

evibacteriumsp.9 Produces an orange pigment on KB 50 [29]rkholderia cepacia

DB01 Soil isolate NT C.F. Gonzalezavibacter michiganensis

G7.1 Rif R

, produces an orange pigment on KB 250 [12]G7.1pig) White, pigment-deficient mutant of G7.1 250 This study G11.1 Rif R, produces a pink pigment on KB 100 [12]T5.1 Rif R, produces a cream-colored pigment on KB 50 [12]T5.1pig) White, pigment-deficient mutant of T5.1 50 This study

rtobacteriumsp.G28 Produces an orange pigment on KB 200 [29]ntoeasp.

T18 Produces a yellow pigment on KB 100 [29]eudomonas cichorii02959 Plant isolate NT G.W. Sundin

eudomonas fluorescensf5 Soil isolate NT C.L. Bender

eudomonas syringae pv. syringaeB728a Bean pathogen NT G.A. BeattieB86-17 Bean pathogen, encodes rulAB operon NT [32]eudomonas syringae pv. tomato

DC3000 Tomato pathogen NT J. Murillo

he UV-MIDc is defined as the minimal dose of UVC radiation (J m)2) required to inhibit growth [29].

T: not tested.

6 J.L. JACOBS ET AL.: ULTRAVIOLETRADIATION ANDPHYLLOSPHERE BACTERIA


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tively quantify cells located in external leaf sites) andbacterial populations were enumerated by dilution plat-ing as described above. Percent survival values weredetermined by comparing counts of cells recovered fromUVC-irradiated and nonirradiated leaves. A ratio wasthen derived by dividing the in planta percent survivalvalues by the corresponding percent survival value

determined in vitro. Independent experiments wereconducted for each strain and for the mixture in both2000 and 2001.

Results

Selection and Characterization of Pigment-Deficient Mu-

tants of C. michiganensis G7.1 and T5.1. The role ofpigmentation in UVA radiation sensitivity and epiphyticsurvival on peanut leaves in thefield was studied usingC.

michiganensis strains G7.1 and T5.1. A chemical muta-genesis procedure involving EMS addition to stationary-phase cultures ofC. michiganensisG7.1 and T5.1 [29] wasutilized to generate pigment-deficient mutants of thesestrains. White, nonpigmented colonies were selected onKBr plates, and their similarity to the parental strain wasdetermined by comparison of profiles generated usingfatty acid methyl ester analysis (data not shown). StrainG7.1 produces an orange pigment in culture, and non-pigmented colonies were easily distinguishable on KBr.Nonpigmented colonies of strain T5.1 (originally creamy

light yellow) were also selected. The in vitro sensitivity toUVA and UVC radiation was compared for both parentaland pigment-deficient mutants of G7.1 and T5.1. WhileUVC radiation sensitivity for the pigment-deficient mu-tants was unchanged compared to the appropriateparental strain (data not shown), both mutants exhibitedmarked reductions in UVA survival following doses of100 kJ m)2 or higher (Fig. 1). These experiments alsorevealed that strain T5.1 was reduced in UVA survivalwhen compared to G7.1 (Fig. 1).

We examined the population dynamics ofC. michi-

ganensisG7.1, G7.1pig)

, T5.1, and T5.1pig)

in replicatedfield plots of peanut. Prior to the initiation offield studies,the results of growth chamber experiments (no UVRpressure) indicated that all strains consistently maintainedequivalent populations on peanut leaves over 7- to 10-dayperiods (data not shown). Each strain was inoculated andevaluated individually, and the experiments were con-ducted simultaneously. Two separate experiments wereconducted with each strain and corresponding pigment-deficient mutant. Maximum daily temperatures averaged36C during the course of these experiments with no

measurable rainfall occurring during any experiment.Four-hour (1100 to 1500) solar UVB irradiance averaged12.4 kJ m)2 during these experiments. Although we didnot monitor solar UVA irradiance during these experi-

ments, the results of several other studies indicate thatdose levels of solar UVA radiation are typically 2285times greater than solar UVB levels [4, 7, 20]. If the solarUVA irradiance followed this range under the conditionsof our studies, we would expect the average 4-h UVAdoses to range from 272 to 1054 kJ m)2, levels that wouldbe expected to significantly affect the survival of the C.

michiganensisstrains that we examined.Following inoculation, populations of G7.1pig)

exhibited small but consistent reductions of up to sev-enfold when compared to G7.1 over the course of the 25-day experiment (Fig. 2A). Differences were statisticallysignificant (P < 0.05) on five occasions (indicated byasterisks in Fig. 2A). In contrast, populations of T5.1pig)

and T5.1 on peanut leaves were nearly identicalthroughout the course of the experiment (Fig. 2B).Similar results were observed when the experiment wasrepeated (data not shown).

In Vitro UVC Survival and Population Dynamics on

Field-Grown Peanut of C. michiganensis G7.1, G11.1, and

T5.1. The objective of these experiments was to

Figure 1. Survival ofC. michiganensis G7.1 (h), G7.1pig) (m),

T5.1 (r

), and T5.1pig

)

()) after UVA irradiation. Each datumpoint represents the mean ( the standard error of the mean) from

three replicate experiments.



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mpare the population dynamics of three strains fromthin a bacterial species that differed in UVR sensitivity.though we would have preferred to use UVB radiationr the strain sensitivity assays, C. michiganensis strains7.1 and G11.1 were insensitive to UVB doses as high as kJ m)2 (data not shown); therefore, we used UVC

diation in these experiments. In vitro analyses of UVCnsitivity indicated that strain G7.1 was significantlyore tolerant than T5.1 with survival differences of000-fold following irradiation with a 250 J m)2 dose

ig. 3). Strain G11.1 was only slightly more sensitive toVC radiation than G7.1 (Fig. 3).

Separate field experiments were conducted toamine the population dynamics of C. michiganensis

rains G7.1, G11.1, and T5.1 on peanut; strains wereoculated either individually or as a 1:1:1 mixture of allree strains. Comparisons of strains inoculated indi-

dually indicated that strain G7.1 consistently colonizedanut leaves at levels that were 10- to 100-fold higheran T5.1 (Fig. 4A). These results were observed in eachplicated experiment performed (data not shown).

opulations of strain G11.1 were similar to that of G7.1roughout the 10-day experiments (Fig. 4A). Theperiments involving mixed inoculum again showedat G7.1 and G11.1 maintained similar population levels

n peanut while populations of T5.1 were reduced up to-fold compared to G7.1 in the three strain mixtureig. 4B).

Enhanced UVR Survival of C. michiganensis During

eaf Colonization in Field Experiments. The effect ofaf colonization strategy on increasing strain survival on

leaves following UVC irradiation was evaluated in fieldexperiments. Populations of G7.1, G11.1, and T5.1 wereestablished separately or as a 1:1:1 mixture on peanutleaves in replicated field plots. We then enumeratedpopulations from replicate sets of irradiated and nonir-

radiated leaves taken at eight time points over a 10-dayperiod following inoculation. Mean populations weredetermined from four individual leaves, with an addi-tional set of four leaves irradiated with UVC. An in plantaUVC survival percentage was generated by dividing themean values from the irradiated/nonirradiated popula-tions, yielding a value which was then divided by the invitro percent survival of the strain at the correspondingUVC dose to yield an in planta/in vitro survival (IPIVS)ratio. Ratios of >1 indicated that UVC survival was ele-vated for the in planta populations.

The IPIVS ratio of strain G7.1 inoculated individu-ally following a 225 J m)2 UVC dose was between twoand three for thefirst 3 days of the experiment, and thenincreased to greater than 10 on days 4, 6, and 8 (Fig. 5A).

gure 2. Population dynamics of (A) C. michiganensisG7.1 (d),

.1pig) (s), and (B) T5.1 (m), and T5.1pig) (n) recovered from

e phyllosphere offield-grown peanut from replicatedfield plots.

ch datum point represents the mean ( the standard error of the

ean) from samples consisting of 15 individual leaves. Significant

fferences in populations (P< 0.05) are indicated by asterisks (*).

Figure 3. Survival ofC. michiganensis G7.1 (h), G11.1 (s), and

T5.1 (m) after UVC irradiation. Each datum point represents the

mean ( the standard error of the mean) from three replicate

experiments.



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A decrease to seven was observed on day 10; however,this value indicated that the in planta survival remainedsevenfold elevated compared to the in vitrosurvival value(Fig. 5A). The IPIVS ratios for G11.1 remained less than4 until day 10 (Fig. 5B) and fluctuated considerably forT5.1 from a high of about 10 on day 2 to a value of 1 onday 4 (Fig. 5C). Although fluctuations in the IPIVS ratio

were observed in these experiments, we did not observelarge fluctuations in population numbers from nonirra-diated leaves (data not shown).

Experiments performed with the strains inoculatedas a mixture revealed that the IPIVS ratios for G7.1 andG11.1 were similar to those observed when the strainswere inoculated individually (Fig. 5A, B). However, theIPIVS ratio for T5.1 decreased considerably from days 6to 10 in mixed inoculations compared to T5.1 inoculatedalone (Fig. 5C).

Comparative UVA and UVC Sensitivity of Phyllo-sphere Isolates and Pseudomonas Strains. Little isknown about the interrelationship of UVB/UVC andUVA survival strategies within bacteria. We initiatedstudies to address this by first determining the sensi-tivity of a group of peanut phyllosphere strains andPseudomonas strains following UVC and UVA irradia-tion. We chose the peanut phyllosphere isolates Brevi-bacterium sp. S9, C. michiganensis G7.1, G11.1, andT5.1,Curtobacterium sp. G28, and Pantoea sp. T18; eachof these isolates was previously characterized as UVC

tolerant except for C. michiganensis T5.1 and Pantoeasp. T18, which were UVC sensitive [12]. We also in-cluded plant pathogen strains P. cichorii 302959, P. sy-ringae pvs. syringae B728a and B86-17 and P. syringaepv. tomato DC3000, and soil isolates P. fluorescens Pf5and Burkholderia cepacia DB01. UVC and UVA irradi-ation experiments were done as described in theMethods section and were repeated at least three timesfor each strain. Each of the UVC-tolerant peanutphyllosphere strains produced a pink or orange pigmentwhen cultured on KB medium, and C. michiganensis

T5.1 andPantoea sp. T18 produced a yellow pigment onKB. UVA irradiation experiments revealed that Brevi-bacterium sp. S9 and Pantoea sp. T18 were relativelyinsensitive exhibiting killing of


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lerant to UVC radiation and was more sensitive toVA radiation; in contrast,B. cepaciaDB01 was sensitive

UVC radiation, but showed increased survival com-

pared to theP. syringaestrains following UVA irradiation(Fig. 7A, B). P. fluorescens Pf5 was the most sensitiveorganism examined (Fig. 7A, B).

Figure 6. Survival ofBrevibacterium sp. S9 (n), Pantoea sp. T18

(h), C. michiganensis G7.1 (d), G11.1 (s), T5.1 (m), and Cur-

tobacteriumsp. G28 (n) after irradiation with UVA. Each datum

point represents the mean from three replicate experiments.

Figure 7.Survival ofB. cepacia DB01 (m),P. cichorii302959 (h),

P.fluorescensPf5 (s),P. syringaepv. syringaeB728a (r) and B86-17 (n), P. syringae pv. tomato DC3000 (n) after irradiation with

(A) UVC or (B) UVA. Each datum point represents the mean from

three replicate experiments.

gure 5.In planta/in vitrosurvival ratio ofC. michiganensisG7.1), G11.1 (B), and T5.1 (C) on peanut leaves irradiated with

VC. Open symbols indicate strains inoculated alone; filled sym-

ls indicate strains (G7.1, G11.1, and T5.1) inoculated in a 1:1:1

io. Thein planta/in vitrosurvival ratio is determined by dividing

e percent survival of strains from irradiated peanut leaves by the

rcent survival of strains irradiated in saline solution. The dashed

es denote a ratio of 1; a value >1 indicates enhanced survival in

anta. UVC doses used were 225 J m)2 for G7.1 and G11.1 and

0 J m)2 for T5.1.



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Discussion

The modulation of phyllosphere microbial populationsby solar UVB radiation has been demonstrated in severalrecent studies [10, 12, 13, 28, 29], but the role of UVAradiation, bacterial pigmentation, and the effect of dif-ferential strain sensitivity to DNA-damaging UVR

wavelengths are less understood. In this study, the fieldsurvival on peanut leaves ofC. michiganensis G7.1pig)

(EMS-selected mutant that does not produce its orangepigment in culture) was reduced compared to the wild-type strain on all 11 sampling dates and significantlyreduced on five of 11 sampling dates over a 25-dayperiod. The size reductions were never greater than 10-fold, but still implicate the importance of pigmentationas a UVA-survival mechanism in the environment andsuggest that ambient solar UVA radiation can reducephyllosphere bacterial populations.

No difference was observed between populations ofC. michiganensis T5.1 and T5.1pig) in field experiments;however, the creamy yellow-colored pigment producedby T5.1 is not the color typical of most carotenoids [6].Thus, although strain T501pig) was greatly reduced inUVA survival in vitro, it is possible that other UVA-survival mechanisms compensated for the loss of pig-mentation in this strain, enabling survival under theambient UVA conditions existing in our experiments. Analternative explanation is that the UVR sensitivity ofstrain T5.1 is such that any additional mutations lower-

ing in vitro UVR sensitivity do not affect (i.e., furtherreduce) epiphytic survival in nature.

Since we used EMS mutagenesis to generate theG7.1pig) and T5.1pig) strains, it is possible that addi-tonal mutations were also present in the mutants thatalso affected their epiphytic survival. However, because ofthe lack of tools available in Clavibactergenetics, we feltthat the EMS method afforded the best opportunity toexamine the effect of pigmentation on epiphytic survivalusing field studies. The results of our studies, accompa-nied by recent increases in the genetic understanding of

Clavibacter/Corynebacterium species [17], provides thejustification for future examinations of additional phe-notypes that affect UVA sensitivity and efforts to con-struct and examine the survival of defined geneticmutants in the field.

Assessment of the role of UVA radiation in phyllo-sphere ecology is difficult because of the lack of aneffective screening filter that would eliminate UVAwavelengths above plant canopies while permitting UVBand visible wavelengths to pass. Recent experiments uti-lizing short-term sunlight exposures of the fungus

Metarhizium anisopliaeand the bacterium P. syringaepv.syringae comparing survival following nonscreened,UVB-screened, or UVB + UVAscreened exposures havehighlighted the importance of both UVA and UVB

wavelengths in mediating damage to cellular componentsand cell death [5, 19].

Pigmentation has long been recognized as a potentialmechanism for increased microbial survival in solarradiationimpacted ecosystems. In eukaryotic microbes,pigments such as melanin are effective sunscreens forsolar UVB radiation and significantly enhance survival

[11, 36]. Bacterial cells are too small to effectively utilizeUVR self-shading mechanisms [8], and this observationis borne out with the importance of active mechanismssuch as DNA repair and photoreactivation in UVBradiation survival [14, 27, 28, 32]. The pigments pro-duced by many phyllosphere bacteria resemble carote-noids, pigments whose function is to quench activeoxygen species that are generated by exposure to UVAradiation [35]. Indeed, laboratory-derived nonpigmentedmutants ofC. michiganensis were more sensitive to UVAwavelengths, but their sensitivity to UVC wavelengths

was unchanged [29]. Thus, although pigmented phyllo-sphere isolates are also typically more tolerant of UVCradiation than nonpigmented isolates [12], the pigmen-tation is most likely providing UVA protection only.

In this study, we found that the UVC-tolerant C.michiganensis G7.1 and medium UVC-tolerant G11.1maintained epiphytic populations on peanut that were10- to 100-fold larger than the more sensitive strainT5.1 when these strains were either inoculated individ-ually or inoculated as a mixture. The difference in pop-ulation size may also be related to other aspects of

epiphytic fitness (e.g., desiccation tolerance, alteredcompetitive ability) as strain T5.1 consistently exhibitedreduced population size on peanut leaves. However, theincreased tolerance of strains G7.1 and G11.1 to UVCand UVA radiation, relative to strain T5.1, could alsoindicate that UVR tolerance is an important phenotypefor the maintenance of population size in the phyllo-sphere.

Avoidance of solar UVR may be an important eco-logical strategy of a range of bacterial phyllosphere resi-dents. We have previously demonstrated in growth

chamber experiments that phyllosphere populations ofC.michiganensis G7.1 and T5.1 on peanut survived UVCirradiation at increased levels compared to cells irradiatedin vitro [10]. In this study, we observed that field pop-ulations of C. michiganensis G7.1, G11.1, and T5.1exhibited increased UVC survival when inoculated sep-arately on peanut leaves. The reduction in IPIVS ratio ofstrain T5.1 on later sampling dates when inoculated aspart of a mixture with G7.1 and G11.1 suggests that in-terstrain competitive interactions may also play a role ina strains ability to colonize leaves and/or access UVR-

shaded sites on leaves. It is unlikely that these non-pathogenic organisms are colonizing internalprotectedsites as pathogenic P. syringae strains inhabit on theircognate host plant [26, 37]. However, external physically



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aded leaf sites, such as those associated with leaf ve-ation, or shading of cells within aggregates represent

ossibilities that might result in the survival increasesserved in our studies.

Our initial attempt to determine if correlationsisted in bacterial survival following irradiation withNA-damaging UVB/UVC or with UVA wavelengths

owed that a relationship was not clear. For example,agglomerans exhibited the highest UVA survival of

e peanut epiphytes tested, but also exhibited thewest UVC survival. C. flaccumfaciens, an organismhose UVC survival level is similar to that of C.ichiganensis G7.1, was increased in UVA sensitivityhen compared to G7.1 at higher dose levels. UVA andVC sensitivities were relatively similar for the P. flu-escens and P. syringae strains; however, B. cepaciaB01 and P. cichorii 302959 differed in comparativensitivity between the UVA and UVC wavelength

oups. These results illustrate that a larger knowledgese on the effects of UVA and UVB/UVC wavelengths

n microbial ecology is required before a generalizedtermination of UVA and UVB/UVC sensitivity ofdividual bacterial strains will provide revealing bio-gical information.

Bacterial survival in solar UVR-impacted habitats isainly affected by UVR irradiance (UVA + UVB) and

uration of exposure. UVR exposure affects ecologicalness through necessitating a shift in energy resources

om cellular growth to DNA repair or other processes to

meliorate UVR-mediated damage. Intuitively, higher-ergy UVB wavelengths would seem to be of utmost

mportance because of their ability to damage DNA.hromosomal DNA damage blocks replication andanscription and must be repaired to ensure cell survival.VA wavelengths may have additional ecological effects;gher-energy, i.e., lower, UVA wavelengths are capablekilling cells, whereas UVA wavelengths >375 nm canused as ecological cues for fungal sporulation [17]. An

nderstanding of how bacterial cells simultaneouslyrceive and respond to the solar UVA and UVB radia-

on present within their environment, the coordination,any, of cellular resources into DNA repair and UVA-rvival processes, and subsequent effects on fitnesspresents an important unsolved problem in microbialology.

cknowledgments

his work was supported by the Agricultural Experimentations of Texas and Michigan. We thank the researchersted in Table 1 for bacterial strains, S. Sabaratnam and

Gunasekera for critical reviews of the manuscript, andree anonymous reviewers whose comments strength-ed the manuscript.

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