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APPLIED AND ENVIRONMENTAL MICROBIOLOGY,
0099-2240/00/$04.00⫹0Aug. 2000, p. 3393–3398 Vol. 66, No. 8
Copyright © 2000, American Society for Microbiology. All Rights Reserved.
Rhizosphere Soil Aggregation and Plant Growth Promotion of
Sunflowers by an Exopolysaccharide-Producing Rhizobium sp.
Strain Isolated from Sunflower Roots
YOUNES ALAMI, WAFA ACHOUAK, CHRISTINE MAROL, AND THIERRY HEULIN*
CEA/Cadarache, DSV-DEVM, Laboratoire d’Ecologie Microbienne de la Rhizosphe`re (LEMiR),
UMR 163 CNRS-CEA, 13108 Saint-Paul-lez-Durance Cedex, France
Received 3 March 2000/Accepted 7 June 2000
Root-adhering soil (RAS) forms the immediate environment where plants take up water and nutrients for
their growth. We report the effect of an exopolysaccharide (EPS)-producing rhizobacterium (strain YAS34) on
the physical properties of sunflower (Helianthus annuus L.) RAS, associated with plant growth promotion,
under both water stress and normal water supply conditions. Strain YAS34 was isolated as a major EPS-
producing bacterium from the rhizoplane of sunflowers grown in a French dystric cambisol. Strain YAS34 was
assigned to the Rhizobium genus by 16S ribosomal DNA gene sequencing. Inoculation of sunflower seeds and
soil with strain YAS34 caused a significant increase in RAS per root dry mass (dm) (up to 100%) and a
significant increase in soil macropore volume (12 to 60 m in diameter). The effect of inoculation on sunflower
shoot dm (up to ⴙ50%) and root dm (up to ⴙ70%) was significant under both normal and water stress
conditions. Inoculation with strain YAS34 modified soil structure around the root system, counteracting the
negative effect of water deficit on growth. Using [
15
N]nitrate, we showed that inoculation made the use of
fertilizer more effective by increasing nitrogen uptake by sunflower plantlets.
Soil structure has a strong impact on a range of processes
influencing crop yield. The basic units of soil structure, named
aggregates, comprise solid material and pores. These aggre-
gates determine the mechanical and physical properties of soil
such as retention and movement of water, aeration, and tem-
perature (16). Aggregate formation is an important factor con-
trolling germination and root growth (17).
Several studies have shown that formation of stable aggre-
gates strongly depends on both the nature and the content of
organic matter (10, 12, 14, 18, 29). Unstable aggregates gen-
erally have a lower content of organic matter than do stable
ones (24). Plant roots contribute to soil organic material, and
thereby to soil aggregate stability, directly through the root
material itself (36) and indirectly through stimulation of mi-
crobial activity in the rhizosphere (4). It is generally believed
that microbial action on soil aggregation is due to the produc-
tion of exopolysaccharides (EPS) (25). This is supported by
experimental observations demonstrating that the amendment
of soil with microbial EPS results in an increased soil aggre-
gation (14, 26).
The influence of microbes on aggregate stability has largely
been studied in bulk soil (15, 25, 34). Relatively little attention
has been paid to the influence of microorganisms, particularly
EPS-producing rhizobacteria, on the aggregation of root-ad-
hering soil (RAS) (3, 36). Understanding the effects of micro-
organisms on RAS aggregation is important because RAS
forms the immediate environment where plants take up water
and nutrients for their growth. Factors liable to change the
physical properties of RAS can be expected to modify absorp-
tion of water and minerals by plants. In previous work, we
found that inoculation of wheat with Paenibacillus polymyxa
(selected for its nitrogen fixation activity in wheat rhizosphere)
in a silty topsoil increased the RAS mass-to-root tissue (RT)
mass ratio (RAS/RT ratio) by 57% (20). We recently demon-
strated that the EPS (levan) produced by P. polymyxa is impli-
cated in the aggregation of RAS on wheat (7). The same effect
on wheat was observed after inoculation with Pantoea agglom-
erans, with additional evidence of the importance of bacterial
activity in the regulation of water content of the rhizosphere by
improved soil aggregation (3).
The purpose of this study was to determine the influence of
inoculation with a rhizobacterium selected for its EPS produc-
tion on soil aggregation of sunflower RAS, and its conse-
quences for water and nitrogen uptake of sunflower plants,
under both water stress and normal water supply conditions.
MATERIALS AND METHODS
Soil. A silty sand clay soil (24.7% sand, 53.1% silt, and 18.3% clay) (dystric
cambisol), with a field capacity of 18.2%, was collected from the upper layer (0
to 30 cm) at the CETIOM (Centre Technique Interprofessionnel des Ole´agineux
Me´tropolitains) experimental station (Saint-Florent-sur-Cher, France). The sta-
tion had been under sunflower-wheat crop rotation for 10 years. Soil was sieved
(⬍2-mm pore size) and stored at 4°C until used. Total carbon and nitrogen
concentrations were 1.0 and 0.11%, respectively, total cation exchange capacity
(0.5 M ammonium chloride) was 14.9 meq per 100 g of dry soil, and pH was 6.8
(1:1, soil/water ratio).
Bacterial strain. Bacterial strain YAS34 was selected from the indigenous
sunflower taproot microflora for its ability to produce large amounts of gel-
forming EPS (2) on RCV-glucose medium (3). The sunflower plantlets were
grown in the soil of Saint-Florent-sur-Cher for 4 weeks. Strain YAS34 was
phenotypically identified using the Biolog GN MicroPlate system (Biolog Inc.,
Hayward, Calif.) based on bacterial ability to metabolize 95 carbon sources. The
Biolog GN microplates were inoculated with strain YAS34 according to the
recommendations of the manufacturer. The microplates were incubated for 4
and 24 h at 30°C and then read with a 590-nm filter (microplate reader, Dynatech
MR 5000). The results were analyzed with Biolog GN database version 3.0 to
assign strain YAS34.
Genotypic characterization of strain YAS34 was also carried out by sequencing
the entire small-subunit (16S) ribosomal DNA (rDNA) gene as previously de-
scribed (1). Strain YAS34 was grown in modified Luria-Bertani broth (1 g of
tryptone, 0.5 g of yeast extract, 0.5 g of NaCl, and4gofglucose, all per liter). The
16S rDNA gene was amplified by PCR, with the primer pairs fD1 and rD1 (37),
and sequenced (R. D. Anderson, D. T. Minnick, M. Veigl, and W. D. Sedwick,
1992, United States Biochemical Corp. publication).
* Corresponding author. Mailing address: CEA/Cadarache, DSV-
DEVM, Laboratoire d’Ecologie Microbienne de la Rhizosphe`re, UMR
163 CNRS-CEA, 13108 Saint-Paul-lez-Durance Cedex, France. Phone:
33 4 42 25 48 27. Fax: 33 4 42 25 66 48. E-mail: thierry.heulin@cea.fr.
3393
Bacterial inoculation and growth conditions. Strain YAS34 was grown over-
night in 300 ml of modified Luria-Bertani broth at 30°C. The culture was har-
vested in late log phase (10
7
to 10
8
CFU ml
⫺1
), washed twice, and resuspended
in 300 ml of sterile distilled water (SDW). Sunflower seeds (Helianthus annuus
L., cv. Albena, Rustica Prograin Ge´ne´tique, Mondonville, France) (75 to 85 mg
per seed) were decorticated and surface sterilized after incubation (1 h) under
vacuum in 2% (vol/vol) calcium hypochlorite solution. Seeds were rinsed three
times with SDW, immersed in 3% (vol/vol) hydrogen peroxide solution for 30
min, and washed again three times with SDW. The sterilized seeds were inocu-
lated with strain YAS34 by incubating them with 10 ml of bacterial suspension
(10
7
CFU per seed) in a sterile plastic tube (15 ml). Noninoculated control seeds
were incubated with 10 ml of SDW. The tubes were gently shaken on an orbital
shaker for 2 h before the seeds were planted (one seed per pot) at a depth of 2
cm. In addition, 5 kg of nonsterile air-dried soil was inoculated by spraying it with
290 ml of the bacterial suspension (10
7
to 10
8
CFU ml
⫺1
) and then placed in
plastic pots (6 cm in diameter by 19 cm in height). Each pot was filled with 250 g
of soil from Saint-Florent-sur-Cher. A noninoculated control soil (5 kg) was
sprayed with 290 ml of SDW. Twenty pots containing inoculated soil and inoc-
ulated seeds and 20 pots with uninoculated soil and uninoculated seeds were
placed in a controlled-environment chamber under a 26°C/18°C day/night cycle
and with a 16-h/8-h light/dark cycle (350-mol m
⫺2
s
⫺1
light intensity).
Soil moisture was adjusted to 70% of water-holding capacity (WHC) (soil
moisture, 13.3% of dry weight; matric potential, ⫺0.20 MPa) and maintained
constant during the experiment by daily sprinkling with SDW. Ten days after
sowing, 10 of the 20 replicates were submitted to water stress by discontinuing
watering. Fourteen days after planting (4 days of water stress), plantlets were
harvested (experiment 1). To evaluate the consequences of inoculation for ni-
trogen uptake by the plant, an independent experiment (experiment 2) was
performed using exactly the same protocol, except that each pot received 6.7 mg
of N at sowing. Nitrogen was applied to soil as Ca (
15
NO
3
)
2
(26.9 atom%
15
N
excess). After 4 days of water stress, soil moisture in the pots was about 9.5% of
dry weight (50% of WHC; matric potential, ⫺0.60 MPa) in experiment 1 and
7.5% of dry weight (40% of WHC; matric potential, ⫺1.0 MPa) in experiment 2.
Harvesting and determination of RAS. Ten plantlets per treatment were
sampled. Plant watering was stopped 24 to 36 h before harvesting to facilitate the
separation of RAS from bulk soil. Roots with adhering soil were carefully
separated from bulk soil by gentle mechanical agitation (Agitest; Stuart Scien-
tific) for 1 min. RAS was removed from RT by washing them in SDW. RAS dry
mass (dm) and root dm were measured after 24 h at 105°C, and the RAS/RT
ratio was calculated.
Estimation of strain YAS34 population in RAS and on roots. Three replicates
per treatment were randomly selected for the detection and enumeration of
strain YAS34 in RAS and RT fractions. Serial dilutions (in 0.85% KCl) of root
macerates and soil suspensions were performed, and aliquots of each dilution
were plated on RCV-glucose medium. Bacteria were incubated at 30°C for 72 h,
and mucoid colonies (similar to strain YAS34) were counted as CFU. To make
sure that mucoid colonies originated from the strain YAS34 inoculum, about
10% of the isolates counted as similar to YAS34 at the last dilution were
randomly selected and genotyped by a DNA fingerprinting method based on
rep-PCR with enterobacterial repetitive intergenic consensus (ERIC) sequences
as primers (35). The procedure of in vitro amplification of the DNA by PCR and
conditions for electrophoresis are described in detail elsewhere (19). The gels
containing ethidium bromide were photographed under UV illumination with
Polaroid 665 film (instant camera system, Polaroid MP4
⫹
).
Pore size distribution. Mercury intrusion porosimetry was performed to mea-
sure pore size distributions in RAS aggregates (5). RASs of three root systems
per treatment were pooled and placed in an intrusion porosimeter (Carlo Erba,
series 200), associated with the Macropore Unit Series 120 connected to an IBM
PC computer, according to the previously described mercury intrusion porosim-
etry method (3). The mercury intrusion porosimetry method is based on the
principle that mercury requires an overpressure (from 1.25 ⫻10
⫺3
to 200 MPa)
to enter a pore of air-dried millimetric soil aggregates, previously outgassed at
room temperature for 2 h. The pore radii intruded range from 4 to 10
6
nm. A
value for the surface tension of mercury of 0.48 N m
⫺1
and a contact angle of
141° were used with the Washburn equation (5), assuming cylindrical pores in the
calculation.
Nitrogen analyses. Four pots per treatment were randomly selected after 14
days of plant growth. From each pot, four compartments were sampled: nonad-
hering soil, RAS, RT, and shoots. The samples were finely ground, and nitrogen
analyses were carried out using the Kjeldahl-Olsen method. Isotopic analyses
were performed by mass spectrometry (Micromass VG 622), using the Ross and
Martin method as described by Guiraud (21).
Statistical analysis. Analyses of variance were performed with Statgraphics
software (version 5.0; STSC Software Products). The least significant difference
(LSD) and Newman-Keuls tests were used for multiple-range analyses.
Nucleotide sequence accession number. The strain YAS34 was deposited at
the Collection Nationale de Culture Microbienne, Institut Pasteur, Paris, France,
under no. I-1809. The accession number of the complete 16S rDNA sequence of
strain YAS34 in the GenBank database is AF239242.
RESULTS
Isolation and identification of strain YAS34. The EPS-pro-
ducing strain YAS34 was isolated from the rhizoplane of sun-
flowers (about 5 ⫻10
6
CFU g of root dm
⫺1
) growing in a
dystric cambisol (Saint-Florent-sur-Cher). This strain is gram
negative, catalase positive, and oxidase negative. Using the
Biolog GN MicroPlate system, strain YAS34 was identified as
Pantoea agglomerans (gamma-Proteobacteria subdivision). How-
ever, molecular identification based on sequencing of the total
16S rDNA gene of strain YAS34 (accession no. AF239242)
revealed that strain YAS34 belongs to the genus Rhizobium
(alpha-Proteobacteria subdivision). Comparison of the com-
plete 16S rDNA sequence with data bank sequences (Gen-
Bank and EMBL) showed that strain YAS34 had identity lev-
els of 99.5% with Rhizobium sp. strain USDA 1920 (GenBank
accession no. U89823), 98.9% with Rhizobium sp. strain OK50
(GenBank accession no. D14515), 98.0% with Rhizobium gal-
licum R602
T
(GenBank accession no. U86343), and 97.8%
with Rhizobium mongolense USDA 1844
T
(GenBank accession
no. U89817). On the basis of 16S rDNA analysis, it was con-
cluded that strain YAS34 is unlike any of the previously de-
scribed species of Rhizobium and so will be named in this work
Rhizobium sp. strain YAS34 rejecting unambiguously the phe-
notypic identification provided by the Biolog system.
Rhizosphere and root colonization with the Rhizobium sp.
strain YAS34. Neither water limitation (data not shown) nor
inoculation with strain YAS34 modified the total number of
culturable bacteria (enumerated on nutrient agar or RCV-
glucose medium) on RT and RAS fractions (Table 1). The
number of EPS-producing bacteria (per gram of root dm) on
roots was 10 times higher than that in RAS. Colonies of Rhi-
zobium sp. strain YAS34 were identified using ERIC-PCR
genomic fingerprinting of randomly selected mucoid colonies
(phenotypically similar to Rhizobium sp. strain YAS34). The
results showed that the ERIC-PCR patterns of 95% of mucoid
YAS34-like colonies were identical to that of strain YAS34.
Consequently, we conclude that 95% of mucoid colonies in
both experiments (experiments 1 and 2) originated from the
inoculated strain YAS34. Although the inoculation did not
modify the total number of culturable bacteria, the percentage
of Rhizobium sp. population (consisting of isolates with the
same ERIC genotype as strain YAS34) in the inoculated treat-
ments was four to eight times higher than that in uninoculated
controls, making up to 54% of root-associated bacteria enu-
merated on RCV-glucose medium (Table 1).
TABLE 1. Colonization of RAS and RT of sunflower by
Rhizobium sp. strain YAS34 in two independent experiments
Part
colonized Expt
no.
c
Total culturable bacteria
a
(n⫽6) Rhizobium sp. (%)
b
(n⫽3)
Control Inoculated Control Inoculated
RAS 1 8.1 ⫾0.3 7.9 ⫾0.5 3 ⫾123⫾5
2 7.8 ⫾0.3 7.6 ⫾0.3 5 ⫾138⫾3
Roots 1 9.7 ⫾0.3 9.5 ⫾0.5 8 ⫾431⫾7
2 8.3 ⫾0.6 8.3 ⫾0.5 8 ⫾154⫾5
a
Counted on nutrient agar medium (in log CFU per gram dm); 6 out of 20
plantlets were analyzed (n⫽6).
b
Isolates with the same ERIC-PCR profile as Rhizobium sp. strain YAS34 (in
percentage of total culturable bacteria counted on RCV-glucose medium); 3 out
of the 20 plantlets were analyzed (n⫽3).
c
1⫽16-day-old plantlets, without nitrogen supply; 2 ⫽15-day-old plantlets,
with nitrogen supply.
3394 ALAMI ET AL. APPL.ENVIRON.MICROBIOL.
Effect of inoculation and water limitation on RAS mass. The
quantitative effect of these two factors on RAS was estimated
by the RAS/RT ratio for each sunflower plantlet, where RAS
was the dm of RAS and RT was the root dm. In both experi-
ments, with (experiment 2) and without (experiment 1) nitro-
gen supply, the average RAS/RT ratio decreased significantly
in water stress conditions, up to 50% in uninoculated samples
and up to 40% in inoculated samples (Table 2). Under both
water supply conditions, the mass of RAS of inoculated plant-
lets was significantly higher than that of the uninoculated con-
trol treatment (Table 2). In the experiment without additional
nitrogen supply (experiment 1), inoculation of sunflowers with
strain YAS34 increased the RAS/RT ratio by 70 and 104%
under normal and water stress conditions, respectively (Table
2). In the experiment with nitrogen supply (experiment 2), the
increase in the RAS/RT ratio due to inoculation was lower but
still significant: ⫹22% under normal water conditions and
⫹52% under water limitation (Table 2).
RAS porosity. The curves fitted in Fig. 1 describe the influ-
ence of water stress and inoculation with strain YAS34 on the
distribution of pore volume of sunflower RAS in experiment 1.
The four cumulative curves were trimodal. Phase I corre-
sponded to the mercury entrance in main pore volumes with a
neck throat radius ranging from 6 to 40 m (V, macropores).
The second (v
1
) and third (v
2
) phases represented mercury
entrance in pores with a radius ranging from 0.6 to 6 m(v
1
,
mesopores) and a radius of less than 0.6 m(v
2
, micropores)
(Fig. 1). The effects of water stress and of bacterial inoculation
were mainly observed on volumes of macropores ranging from
6to30m in pore radius (V). For example, in the absence of
inoculation, the water stress increased the macropore volume
from about 30 mm
3
g
⫺1
in the control to 40 mm
3
g
⫺1
in water
stress conditions (curve A versus curve C, Fig. 1). This increase
in macropore volumes was responsible for the increase (110
versus 120 mm
3
g
⫺1
) of the cumulative pore volume (V ⫹v
1
⫹
v
2
). The main effect of inoculation with strain YAS34 on RAS
porosity was observed in normal water conditions. This effect
was an increase in cumulative pore volume (from 110 to 130
mm
3
g
⫺1
), resulting from an increase in macropore volume
(from 30 to 50 mm
3
g
⫺1
) (Fig. 1). The mercury porosimetry
curves of the inoculated treatment were identical in normal
and water stress conditions (curves B and D).
Plant growth. In experiment 1 (no nitrogen supply), there
was an overall negative effect of water stress (for 4 days) on
FIG. 1. Effect of inoculation with Rhizobium sp. strain YAS34 and soil hydric
treatment on the cumulative mercury pore volume of RAS aggregates. (A)
Noninoculated control treatment in drained soil; (B) inoculated treatment in
drained soil; (C) noninoculated control treatment in dry soil; (D) inoculated
treatment in dry soil.
TABLE 2. Effects of soil water content and inoculation of sunflowers with Rhizobium sp. strain YAS34 on plant growth parameters
and the production of RAS in two independent experiments
a
Compartment Expt
no.
b
Value for treatment group
c
:LSD n
Significance of effect
d
:
NI/NWS Inoc/NWS NI/WS Inoc/WS Inoculation Water stress Interaction
Shoots 1 41.6 a 48.2 b 40.2 a 43.7 a 4.0 9 *** * NS
2 75.1 a 114.5 c 81.1 a 91.8 b 8.2 12 *** ** ***
RT 1 14.8 ab 17.9 b 11.5 a 14.8 ab 3.8 9 * * NS
2 51.0 a 89.2 c 59.2 ab 65.2 b 8.3 12 *** ** ***
RAS 1 287 b 565 c 109 a 281 b 92 9 *** *** NS
2 714 b 1,555 c 488 a 878 b 189 7 *** *** **
RAS/RT ratio 1 19.4 b 32.8 c 9.5 a 19.4 b 5.0 9 *** *** NS
2 14.0 b 17.1 c 9.1 a 13.8 b 2.2 7 *** *** NS
a
Abbreviations: NI, noninoculated; NWS, non-water stressed; Inoc, inoculated; WS, water stressed; NS, nonsignificant.
b
Experiment 1, 16-day-old plantlets without nitrogen supply; experiment 2, 15-day-old plantlets with nitrogen supply.
c
For shoots, RT, and RAS, values are shown as milligrams per plant; for RAS/RT ratio, values are milligrams to milligrams. Values followed by the same letter(s)
indicate homogeneous groups (P⬍0.05 [LSD]) after line-by-line variance analysis.
d
*, P⬍0.05; **, P⬍0.01; ***, P⬍0.001.
VOL. 66, 2000 EPS-PRODUCING RHIZOBIUM IN THE SUNFLOWER RHIZOSPHERE 3395
sunflower growth. However, inoculated plantlets, whatever the
water conditions, showed a significantly greater shoot dm
(⫹20%, P⬍0.001) than did the uninoculated controls (Table
2). There was also a significant effect of inoculation on root dm
(P⬍0.05). In experiment 2 (in the presence of nitrogen sup-
ply), water stress caused a significant decrease in shoot and
root dm’s only in the inoculated treatments (Table 2). Inocu-
lated-stressed plantlets had 20 and 27% lower root and shoot
dm’s, respectively, compared with inoculated nonstressed
plantlets. However, as in the previous experiment (experiment
1), after 14 days of growth, inoculated sunflower plantlets
showed a significant increase in shoot dm compared with uni-
noculated controls: ⫹10 and ⫹50% increase under water stress
and normal water supply conditions, respectively (P⬍0.001).
This significant stimulatory effect was also detected for root dm
(⫹70%) under normal water supply conditions (Table 2). The
greater dry matter of inoculated plantlets might explain their
sensitivity to water stress, compared with uninoculated plant-
lets.
Nitrogen uptake. In noninoculated control plantlets, water
stress had no significant effect on the total nitrogen content
(QN) or the percentage of fertilizer N recovery (FNR) (Table
3). Four days of water stress significantly decreased the QN of
inoculated sunflower plantlets (⫺14% in shoots). In spite of
this reduction, the QN of shoots from inoculated plantlets
subjected to water stress conditions was significantly higher
than that of uninoculated controls grown under normal and
water-stressed conditions (⫹12%) (Table 3). No significant
difference was detected for the nitrogen content of roots in dry
conditions between inoculated and uninoculated treatments
(Table 3). However, in normal water supply conditions, the
roots of inoculated plantlets showed a significant increase in
nitrogen content (approximately ⫹57%, P⬍0.05). The same
trend was also reflected in
15
N uptake: 6.5% of the
15
N-labeled
nitrogen was recovered in sunflower plantlets of uninoculated
controls compared with 9.0 and 15.4% in inoculated stressed
plantlets and inoculated nonstressed plantlets, respectively
(Table 3). This enhancement of
15
N uptake by sunflower plant-
lets due to inoculation with Rhizobium sp. strain YAS34 was
concomitant with a significant increase in
15
N content in RAS
from the inoculated nonstressed treatment (three times high-
er), compared with noninoculated controls (Table 3). No sig-
nificant effect of inoculation on
15
N content in RAS was de-
tected under water stress conditions. In bulk soil, neither
inoculation nor water stress had any effect on nitrogen content
(data not shown).
DISCUSSION
Colonization of sunflower rhizosphere by Rhizobium sp.
strain YAS34. Rhizobium sp. strain YAS34 was found naturally
associated with sunflower roots at high frequency (5 ⫻10
6
CFU g of root dm
⫺1
), and we showed that inoculation could
improve this association. Rhizobium sp. strain YAS34 was able
to colonize the rhizosphere of sunflowers and to persist in it
for at least 2 weeks at a high level (up to 30% of total cultur-
able bacteria counted on RCV-glucose) (Table 1). Recent ex-
periments have shown that rhizobia are also good colonizers
of nonlegume roots such as rice (Y. G. Yanni, R. Y. Rizk, V.
Corich, A. Squartini, and F. B. Dazzo, Proc. 15th Symbiotic
Nitrogen Fixation Conf., p. 17, 1995 [abstract]) and canola,
maize, and lettuce (9, 27). Interestingly, the Rhizobium sp.
strain YAS34 population was still high after 4 days of water
stress, in both rhizoplane and RAS fractions. This survival
ability under water-limited conditions may be due, at least in
part, to EPS production. The EPS layer may maintain a hy-
drated microenvironment around microorganisms during des-
iccation (13). Hartel and Alexandre (22) and later Roberson
and Firestone (30) demonstrated that desiccation survival of
Rhizobium sp. and Pseudomonas sp., respectively, required an
increased EPS production.
Effect of inoculation on aggregation and porosity of RAS. As
expected, the increase in the EPS-producing strain YAS34
population in the sunflower rhizosphere after inoculation sig-
nificantly increased the RAS/RT ratio, whatever the water
conditions. Similar results were obtained for wheat plantlets
inoculated with either P. polymyxa (20) or Pantoea agglomerans
(3). This significant increase in RAS mass around the roots
of sunflower plantlets inoculated with Rhizobium sp. strain
YAS34 could be the result of either an increase in soil adhe-
sion to roots or a higher soil aggregate stability around roots,
or both. This aggregation effect of strain YAS34 may be due to
EPS production. Purified xanthan and alginate (produced by
Xanthomonas sp. and Azotobacter vinelandii, respectively) can
improve aggregate formation (11). The polysaccharides are
apparently adsorbed on soil particle surfaces and cement par-
ticles together (6, 13). On the other hand, it was shown previ-
ously that microbial biomass and polysaccharide production
are increased in association with the stimulation of microbial
populations in the rhizosphere of various plants (23).
The factors that favor root cap polysaccharide production
may be expected to improve soil adhesion to roots and/or RAS
aggregation. Hence, the effect of Rhizobium sp. strain YAS34
TABLE 3. Effects of soil water content and inoculation of sunflowers with Rhizobium sp. strain YAS34 on
nitrogen uptake by the plant in experiment 2
a
Compartment Value
type
Value for treatment group
d
:LSD n
Significance of effect
e
:
NI/NWS Inoc/NWS NI/WS Inoc/WS Inoculation Water stress Interaction
Shoots QN
b
3.30 a 4.43 c 3.35 a 3.80 b 0.27 4 *** ** **
% FNR
c
6.68 a 15.36 c 6.46 a 9.04 b 2.15 4 *** *** ***
Roots QN 0.88 a 1.38 b 1.03 a 1.10 a 0.27 4 ** NS *
% FNR 2.09 a 5.04 b 2.38 a 2.88 a 1.17 4 *** * **
RAS QN 1.05 ab 1.98 c 0.73 a 1.20 b 0.34 4 *** *** NS
% FNR 0.31 a 0.94 b 0.15 a 0.33 a 0.20 4 *** *** **
a
Abbreviations: NI, noninoculated; NWS, non-water stressed; Inoc, inoculated; WS, water stressed; NS, nonsignificant.
b
QN is in milligrams of N per plantlet.
c
FNR ⫽(QN
sample
⫻E
sample
)/(QN
fertilizer
⫻E
fertilizer
) where QN is the total nitrogen in a sample or applied fertilizer and Eis the isotopic excess of
15
N (percent).
d
Values followed by the same letter(s) indicate homogeneous groups (P⬍0.05 [LSD]) after line-by-line variance analysis.
e
*, P⬍0.05; **, P⬍0.01; ***, P⬍0.001.
3396 ALAMI ET AL. APPL.ENVIRON.MICROBIOL.
in sunflower rhizosphere on soil aggregation may also be partly
indirect, through a stimulation of root exudation.
On plantlets subjected to water stress, whatever the inocu-
lation treatment, the RAS/RT ratio values were lower than
those of plantlets growing under normal water supply condi-
tions. Of particular interest in this study is the finding that this
RAS/RT ratio of stressed and inoculated plantlets is as great as
that of noninoculated nonstressed plantlets (Table 2). This
suggests that inoculation of sunflowers with strain YAS34 may
limit the negative effect of dry conditions on RAS aggregation.
This effect may also be related to the production of EPS by
strain YAS34. Microbial EPS may both increase WHC of soil
(31) and reduce water loss during desiccation (30).
Concomitantly with this bacterial effect on the RAS/RT ra-
tio, a significant increase in soil macropore volume (corre-
sponding to 12 to 60 m in pore diameter) associated with an
increase in total pore volume was observed at harvesting (day
14), whatever the water supply conditions (Fig. 1). Exactly the
same increase in soil macropore volume due to the inoculation
was detected at an intermediate stage (day 12 [data not shown]).
According to Wu et al. (38), pores between aggregates of 120
to 600 m (diameter) should be in the range of 12 to 60 m
(diameter). According to the model of Oades and Waters (28),
roots and fungal hyphae contribute to the formation of macro-
aggregates (diameter ⬎250 m), whereas formation of meso-
and microaggregates (diameter ⬍250 m) involves plant and
microbial debris and bacteria. Our results suggest that bacte-
ria, probably via their EPS production, also contribute to mac-
roaggregate formation. Theoretically, new aggregates can be
obtained from either breakdown of larger aggregates or accre-
tion of mesoaggregates (33). Concerning the increase in the
RAS/RT ratio, the second process seems to be a better expla-
nation for the increased RAS macroporosity observed after
inoculation with strain YAS34. By colonizing mesopores, the
inoculated bacteria may bind these aggregates into larger ones
through their EPS production, generating a new macroporosity
(Fig. 1).
Plant growth and N assimilation. Rhizobia are well known
for their beneficial effect on plant growth resulting from sym-
biotic N
2
fixation with legumes. There is also some evidence in
the literature that some rhizobia are able to colonize nonle-
gume roots and promote their growth (32; Yanni et al., Proc.
15th Symbiotic Nitrogen Fixation Conf.). Chabot et al. (8)
attributed promotion of maize and lettuce growth observed in
the field after inoculation with Rhizobium leguminosarum bv.
phaseoli to phosphate solubilization, whereas Noel et al. (27)
suspected indoleacetic acid production for promotion of let-
tuce growth by R. leguminosarum, in gnotobiotic conditions.
The stimulation of sunflower N uptake after inoculation with
Rhizobium sp. strain YAS34 may be explained by the increase
in RAS macroporosity (12 to 60 m) (Fig. 1) and increased
plant growth due to the production of plant growth hormones.
According to Oades and Waters, in these pores the water trans-
fer depends on plant root suction (28). A better water supply
may also enhance plant growth, especially in water-limiting
conditions, and so enhance the nitrogen uptake. Another hy-
pothesis to be tested is that strain YAS34 can enhance the
diffusion rate of nitrate through its EPS toward plant roots, as
has been demonstrated previously for glucose in EPS-amended
clay, compared with pure clay (13). Among the main results is
the finding that stressed sunflower plantlets inoculated with
strain YAS34 had as much shoot dry matter as the uninocu-
lated nonstressed control. Strain YAS34 seems to modify soil
structure around the root system in such a way that the water
supply, plant growth, and N uptake of sunflowers are increased,
counteracting the effect of water deficit on growth. Further
work under “real” conditions is needed to check for the rhi-
zosphere and root colonization of strain YAS34 and its effects
on plant growth.
Conclusions. We demonstrate that Rhizobium sp. strain
YAS34, specifically selected for EPS production, acts as a plant
growth-promoting rhizobacterium with a nonlegume, even in
dry conditions. The main effects of sunflower inoculation with
Rhizobium sp. strain YAS34 were the increase of RAS mass,
macropore volume of RAS, and N uptake by the plant and
finally plant growth. Strain YAS34 was also able to relieve the
effect of water stress on sunflower growth, which is particularly
important in Mediterranean areas, where crops are often sub-
jected to lengthy dry periods. Most of these inoculation effects
may be related to EPS production. To confirm the involvement
of EPS in this process, further experiments will be performed
with a knockout mutant of strain YAS34 deficient in EPS
production.
ACKNOWLEDGMENTS
This work was partly supported by CETIOM (Y.A.), for which we
thank A. Merrien.
We thank the CETIOM team of Saint-Florent-sur-Cher for soil
sampling. We also thank G. Guiraud for supervision of
15
N experi-
ments and M. Ryder (CSIRO, Adelaide, Australia) for critically read-
ing the manuscript.
REFERENCES
1. Achouak, W., R. Christen, M. Barakat, M.-H. Martel, and T. Heulin. 1999.
Burkholderia caribensis sp. nov., an exopolysaccharide-producing bacterium
isolated from vertisol microaggregates in Martinique. Int. J. Syst. Bacteriol.
49:787–794.
2. Alami, Y., T. Heulin, M. Milas, R. de Baynast, A. Heyraud, and A. Villain.
August 1998. Polysaccharide, microorganism and method for obtaining
same, composition containing it and application. European patent 97-1624
970212.
3. Amellal, N., G. Burtin, F. Bartoli, and T. Heulin. 1998. Colonization of
wheat roots by EPS-producing Pantoea agglomerans and its effect on rhizo-
sphere soil aggregation. Appl. Environ. Microbiol. 64:3740–3747.
4. Angers, D. A., and G. R. Mehuys. 1989. Effects of cropping on carbohydrate
content and water stable aggregation of a clay soil. Can. J. Soil Sci. 69:
373–380.
5. Bartoli, F., R. Philippy, and G. Burtin. 1991. Aggregation in soil with small
amounts of swelling clay. I. Aggregate stability. J. Soil Sci. 39:593–616.
6. Ben-Hur, M., and J. Letey. 1989. Effect of exopolysaccharides, clay disper-
sion, and impact energy on water infiltration. Soil Sci. Soc. Am. J. 53:
233–238.
7. Bezzate, S., S. Aymerich, R. Chambert, S. Czarnes, O. Berge, and T. Heulin.
2000. Disruption of the Paenibacillus polymyxa levansucrase gene impairs
ability to aggregate soil in the wheat rhizosphere. Environ. Microbiol. 2(3):
333–342.
8. Chabot, R., H. Antoun, and M. P. Cescas. 1996. Growth promotion of maize
and lettuce by phosphate-solubilizing Rhizobium leguminosarum biovar
phaseoli. Plant Soil 184:311–321.
9. Chabot, R., H. Antoun, J. W. Kloepper, and C. J. Beauchamp. 1996. Root
colonization of maize and lettuce by bioluminescent Rhizobium leguminosa-
rum biovar phaseoli. Appl. Environ. Microbiol. 62:2767–2772.
10. Chaney, K., and R. S. Swift. 1984. The influence of organic matter on
aggregate stability of some British soils. J. Soil Sci. 35:223–230.
11. Chaney, K., and R. S. Swift. 1986. Studies on aggregate stability. I. Refor-
mation of soil aggregates. J. Soil Sci. 37:329–335.
12. Chenu, C. 1993. Clay or sand polysaccharide associations as models for the
interface between micro-organisms and soil: water-related properties and
microstructure. Geoderma 56:143–156.
13. Chenu, C., and E. B. Roberson. 1996. Diffusion of glucose in microbial
extracellular polysaccharide as affected by water potential. Soil Biol. Bio-
chem. 28:877–884.
14. Cheshire, K. V. 1979. Nature and origin of carbohydrates in soils. Academic
Press, Inc., New York, N.Y.
15. Degens, B. P., G. P. Sparling, and L. K. Abbott. 1994. The contribution from
hyphae, roots and organic carbon constituent to aggregation of a sandy loam
under long-term clover-based and grass pastures. Eur. J. Soil Sci. 45:459–
468.
16. Dickson, E. L., V. Rasiah, and P. H. Groenevelt. 1990. Comparison of four
prewetting techniques in wet aggregate stability determination. Can. J. Soil
Sci. 71:67–72.
17. Dinel, H., P. E. M. Levesque, P. Jambu, and D. Righi. 1992. Microbial
VOL. 66, 2000 EPS-PRODUCING RHIZOBIUM IN THE SUNFLOWER RHIZOSPHERE 3397
activity and long-chain aliphatics in the formation of stable soil aggregates.
Soil Sci. Soc. Am. J. 56:1455–1463.
18. Elustondo, J., D. A. Angers, M. R. Laverdie`re, and A. N’Dayegamiye. 1990.
Etude comparative de l’agre´gation et de la matie`re organique associe´e aux
fractions granulome´triques de sept sols sous culture de maı¨s en prairie. Can.
J. Soil Sci. 70:395–402.
19. Frey, P., P. Frey-Klett, J. Garbaye, O. Berge, and T. Heulin. 1997. Metabolic
and genotypic fingerprinting of fluorescent Pseudomonas associated with the
Douglas fir-Laccaria bicolor mycorhizosphere. Appl. Environ. Microbiol. 63:
1852–1860.
20. Gouzou, L., G. Burtin, R. Philippy, F. Bartoli, and T. Heulin. 1993. Effect of
inoculation with Bacillus polymyxa on soil aggregation in the wheat rhizo-
sphere: preliminary examination. Geoderma 56:479–490.
21. Guiraud, G. 1984. Contribution du marquage isotopique a` l’e´valuation des
transfert d’azote entre les compartiments organiques et mine´raux dans les
syste`mes sol-plante. Ph.D. thesis. Universite´ Pierre et Marie Curie, Paris,
France.
22. Hartel, P. G., and M. Alexandre. 1986. Role of extracellular polysaccharide
production and clays in the desiccation tolerance of cowpea Bradyrhizobia.
Soil Sci. Soc. Am. J. 50:1193–1198.
23. Haynes, R. J., and G. S. Francis. 1993. Changes in microbial biomass C, soil
carbohydrate composition and aggregates stability induced by growth of
selected crop and forage species under field conditions. J. Soil Sci. 44:
665–675.
24. Haynes, R. J., and R. S. Swift. 1990. Stability of soil aggregates in relation to
organic constituents and soil water content. J. Soil Sci. 41:73–83.
25. Lynch, J. M., and E. Bragg. 1985. Microorganisms and soil aggregate sta-
bility. Adv. Soil Sci. 2:133–171.
26. Martens, D. A., and W. R. T. Frankenberger, Jr. 1993. Soil saccharide
extraction and detection. Plant Soil 149:145–147.
27. Noel, T. C., C. Sheng, C. K. Yost, R. P. Pharis, and M. F. Hynes. 1996.
Rhizobium leguminosarum as plant growth-promoting rhizobacterium: direct
growth promotion of canola and lettuce. Can. J. Microbiol. 42:279–283.
28. Oades, J. M., and A. G. Waters. 1991. Aggregate hierarchy in soils. Aust. J.
Soil Res. 29:815–828.
29. Reid, J. B., and M. J. Goss. 1981. Effect of living roots of different plant
species on the aggregate stability of two arable soils. J. Soil Sci. 32:521–541.
30. Roberson, E. B., and M. Firestone. 1992. Relationship between desiccation
and exopolysaccharide production in a soil Pseudomonas sp. Appl. Environ.
Microbiol. 58:1284–1291.
31. Shanmunagathan, R. T., and J. M. Oades. 1982. Effect of dispersible clay on
the physical properties of the B horizon of red-brown earth. Aust. J. Soil Res.
20:315–324.
32. Terouchi, N., and K. Syono. 1990. Rhizobium attachment and curling in
asparagus, rice and oat plants. Plant Cell Physiol. 31:119–127.
33. Tespra, R. 1990. Formation of new aggregates and weed seed behaviour in
a coarse and in a fine-textured loam soil: a laboratory experiment. Soil
Tillage Res. 15:285–296.
34. Tisdall, J. M., and J. M. Oades. 1980. The effects of crop rotation on
aggregation in a red-brown earth. Aust. J. Soil Res. 18:423–434.
35. Versalovic, J., T. Koeuth, and J. R. Lupski. 1991. Distribution of repetitive
DNA sequences in eubacteria and application to fingerprinting of bacterial
genomes. Nucleic Acids Res. 19:6823–6831.
36. Watt, M., M. E. McCully, and C. E. Jeffree. 1993. Plant and bacterial
mucilages of the maize rhizosphere: comparison of their soil binding prop-
erties and histochemistry in a model system. Plant Soil 151:151–165.
37. Weisburg, W. G., S. M. Barns, D. A. Pelletier, and D. J. Lane. 1991. 16S
ribosomal DNA amplification for phylogenetic study. J. Bacteriol. 173:697–
703.
38. Wu, L., J. A. Vomocil, and S. W. Childs. 1990. Pore size, particle size,
aggregate size, and water retention. Soil Sci. Soc. Am. J. 54:952–956.
3398 ALAMI ET AL. APPL.ENVIRON.MICROBIOL.
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