Growth promotion effects of the endophyte Acinetobacter
johnsonii strain 3-1 on sugar beet
Yingwu Shi & Kai Lou & Chun Li
Received: 24 August 2011 /Accepted: 5 November 2011 /Published online: 18 November 2011
# Springer Science+Business Media B.V. 2011
Abstract An endophytic bacteriumn identified as Acineto-
bacter johnsonii strain 3–1 was isolated from surface-
sterilized roots of Beta vulgaris. Its effect on sugar beet
seedling growth was studied using pot assays and field
experiments. This strain promoted beet seedling growth
following seed inoculation by seed dipping. Plant height
and dry weight of beet increased by 19% and 69%,
respectively, compared with controls. Strain 3–1 exhibited
the ability to increase absorption of N, P, K, and Mg
elements from soil and increase the content of vitamins B
and C, and protein within beet. In addition, the strain also
produced a phytohormone-auxin, produced nearly twice as
much IAA as that produced by strain 2–2, and was able to
solubilize phosphates. The concentration of dissolved P in
the medium was 180.5 mg L−1 after 4 days of incubation.
In field experiments, strain 3–1 significantly increased the
content of sucrose, fructose, and the yield of the beet. The
growth-promoting properties of Acinetobacter johnsonii
strain 3–1 indicates that this promising isolate merits
further investigation into its symbiosis with beet plants
and its potential application in agriculture.
Keywords Sugar beet . Endophytic bacteria . Acinetobacter
johnsonii . Growth promotion . Colonization
1 Introduction
Sugar beet is the most important sugar crop in the
developing world. Fertilizers and pesticides are the most
important inputs for beet production but using growth-
promoting bacteria can contribute to making beet cultivation
sustainable and less dependent on fertilizers. Among such
bacteria are rhizobacteria, which colonize plant roots and
consume root exudates and lysates (Pieterse et al., 2002;
Antoun and Prevost, 2006). Some strains, the growth-
promoting rhizobacteria (PGPR), can promote plant growth.
They improve the cycling of nutrients and minerals such as
nitrogen, phosphate and can be used as biofertilizers
(Kennedy et al., 2004). The growth-promoting effects of
PGPR are affected by environmental conditions and there-
fore are not stable. Endophytes including a variety of
bacteria, fungi, and actinomycetes and can be isolated from
cultivated crops (Liu and Tang 1996) as well as from wild
plants (Brooks et al. 1994) both monocotyledons (Fisher
et al. 1992) and dicotyledons (El-Shanshoury et al. 1996).
Healthy roots of sugar beet have been shown to harbour
endophytic bacteria and healthy leaves to harbour endo-
phytic fungi (Jacobs et al. 1985; Larran et al. 2004), little is
known about the behaviour of these microorganisms during
the growing season. Endophytes occupy microniches within
plant tissues and some have been found to be growth-
promoting endophytes (PGPE). These endophytes are
unaffected by competition from other microorganisms or
conditions in the rhizosphere. As a result, a new approach
has been developed in recent years to reduce fertilizer use
which consists of treating seeds and seedlings with PGPE.
Y. Shi :K. Lou (*)
Institute of Microbiology,
Xinjiang Academy of Agriculture Science,
Urumqi, Xinjiang, China
e-mail: loukai02@mail.tsinghua.edu.cn
Y. Shi
Key Laboratory of Green Chemical Processes of Xinjiang
Production and Construction Corps, College of Agronomy,
Shihezi University,
Shihezi, Xinjiang, China
C. Li
Department of Biotechnology, School of Life Science
and Technology, Beijing Institute of Technology,
Beijing, China
Symbiosis (2011) 54:159–166
DOI 10.1007/s13199-011-0139-x
Thus, inoculation of pepper with endophytic Pseudomonas
fluorescens significantly increased fresh weight, height, and
stem diameter (Lucas-Garca et al. 2003). In an previous
study, we isolated 221 endophytic bacterial isolates from
tissues sterilized sugar beets from a widely planted beet
cultivar (Beta vulgaris L. var. saccharifera) variety
KWS2409 in a semi-arid region in east Urumuqi in
Xinjiang,China. In the present study, we assess the effect
of Acinetobacter johnsonii strain 3–1 on the growth of beet
seedlings and on the ability of their root system to absorb
mineral nutrients.
2 Materials and methods
2.1 Sugar beet variety, bacterial strain, and soil
The soil for the pot experiments was collected from a fertile
field in northern Xinjiang. Chemical and physical proper-
ties of soil were analysed (Table 1). Total carbon, C, was
determined by muffle furnace heating the soil to 500°C;
total nitrogen, N, by the Kjeldahl method; and total
phosphorus, P, by molybdenum blue colorimetry (Riehm
1985). Calcium, extractable magnesium, potassium, and
sodium were measured using an atomic absorption spec-
trophotometer (Schachtschnabel and Heinemann 1974); pH
was determined with a pH meter; and total salt content was
assessed by measuring the electrical conductivity of soil.
Sugar beet (Beta vulgaris L. var. saccharifera) variety
KWS2409 and Acinetobacter johnsonii strain 3–1 (GenBank
accession number EU594557) were used, with Chryseobac-
terium indologenes strain 2–2 (GenBank accession number
EU594563) as control for 3–1, in the study assessing the
effect of bacteria on plant growth. Sugar beet seeds (Beta
vulgaris L. var. saccharifera) variety KWS2409 were
obtained from Toutunhe farm in a semi-arid region in east
Urumuqi in Xinjiang,China.
2.2 Plant growth and inoculation in pots
Both strains 3–1 and 2–2 were grown in a liquid medium
(3 g beef extract, 5 g peptone, 5 g NaCl, and 1 g glucose, in
1 L water, pH 7.0–7.2) shaken at 200 rpm at 30°C for 48 h,
after which the bacterial cells were precipitated by centrifu-
gation at 5,000×g for 5 min. The pellets were suspended in
sterile distilled water and the concentration of the suspension
adjusted to 108 cfu ml−1 with a spectrophotometer (Mayak
et al. 2004). Sugar beet seeds were surface-sterilized with
70% alcohol for 8 min followed by 5% hydrogen peroxide
solution for 2 h and then rinsed in sterile water (Caitriona et al.
2004). The disinfected seeds were immersed in the bacterial
suspension for 6 h and then sown in pots, 10 seeds per pot.
After germination, the stand was thinned to keep six plants in
each pot. Ten replicates were maintained, with beet seeds
immersed in distilled water instead of the bacterial suspen-
sion serving as control (CK). The pots were kept in a growth
chamber with the day/night temperatures set at 30°C/25°C
and 200 μmol photons m−2 s−1 of light supplied throughout
the 12 hday. Soil moisture was maintained at 60% of the
soil’s moisture-holding capacity. Germination percentage
was recorded 7 days later and data on plant height, fresh
weight and dry weight, and the contents of N, P, K, and
Mg and of Vitamin B,Vitamin C and protein recorded
4 weeks after germination.
2.3 Re-isolation of endophytic bacteria
Endophytic bacteria from the seedling plant tissues were re-
isolated 4 weeks after germination. To do this, fresh
samples of sugar beet plants were washed thoroughly with
tap water to remove adhering soil and debris, rinsed in 95%
ethanol, and then flamed. This minimised carry-over of
external bacteria when cross-sectioning at various locations
using a sterile knife. Samples were aseptically removed from
the core, periphery, and secondary root emergence zone
(crease) areas. Each sample of root tissue was aseptically
weighed, then added to 100 mL of sterile saline (0.85%), and
blended for 2 min in a Waring blender. The blended samples
were initially diluted to standardise all preparations. This was
followed by additional serial decimal dilution in sterile saline.
Endophytic bacteria were isolated using an aerobic spread-
plate method. Dilution volumes of 0.1 mL were plated in
triplicate on nutrient agar (NA) supplemented with 2.0 g/L
sucrose. The spread plates were incubated at room tempera-
ture for 7 d and colony counts recorded by standard methods
(Kodaka et al. 2005). Pure cultures were subsequently
isolated and bacterial isolates were stored on agar slants.
2.4 Identification of endophytic bacteria
Pure cultures of the bacterial isolates were prepared for
identification purposes. The bacterial re-isolates 3–1 and
Table 1 Chemical and physical properties of soila in the top (0–25 cm) layer
Parameter Ct g/kg Nt P K Mg Na Ca Total salts pH
Value 24.1 1.28 1.1 0.15 0.04 0.14 0.12 1.32 7.85
aThe soil was yellow brown
160 Y. Shi et al.
2–2 were identified using the Vitek AutoMicrobic System
(Vitek AMS; Vitek Systems, Inc., Hazelwood, MO.). The
Vitek test was repeated twice. Isolates were characterized
by Vitek AMS, colony morphology, catalase production,
oxidase test, and gram stain.
2.5 Phylogenetic analysis of the isolated bacteral strains
The 16S rRNA gene of genomic DNA isolated was
extracted from pure bacterial colonies following standard
protocols. The primers used for PCR were pF (5 -AGA
GTT TGATCC TGG CTC AG-3 ) and pR (5 -AAG GAG
GTG ATC CAG CCG CA-3).The PCR products with the
expected size (about 1,500 bp) were purified using a DNA
Gel Extraction Kit and cloned into pMD18-T vector
followed by sequencing. Sequence analysis was performed
using the BLAST algorithm (http://www.ncbi.nlm.nih.
gov). Bacterial identifications were based on 16S rRNA
gene sequence similarity. For further characterization of
isolates 3–1 and 2–2, a neighbor-joining phylogenetic tree
was constructed with the MEGA 4.0 program.
2.6 Measuring the ability of strain 3–1 to solubilize
phosphates
Strains 3–1 and 2–2 were grown in PKO (Ca3(PO4)2)
medium Pikovaskaia’s medium, Yao 2004, per liter:
Ca3(PO4)2 3.0~5.0 g, sucrose 10.0 g, (NH4)2SO40.5 g,
NaCl 0.2 g, MgSO4·7H2O 0.1 g, KCl 0.2 g, yeast extract
0.5 g, MnSO41 mL (0.004 g/L), FeSO4 (Fe-EDTA) 0.1 mL
(0.002 g/L), agar 15 g, pH value 7.0±0.2 () and incubated
at 30°C for 12 h with vigorous shaking. The concentration
of the bacterial suspension was adjusted to 108 cfu ml−1 and
the suspension used as the primary inoculum. Two
millilitres of the primary inoculum were added to 200 ml
PKO medium whereas 2 ml distilled water added to 200 ml
PKO medium served as control. Each treatment was
repeated three times. The phosphates-solubilizing capability
of Strains 3–1 and 2–2 were measured every 24 h for 7 days.
Each time, a sample of the culture medium was centrifuged
at 10,000 rpm for 20 min, and pH of the supernatant
determined with a pH meter. Dissolved P (H2PO4
−) was
determined using molybdenum blue colorimetry (Zhao
et al. 2003; Zhang 2008). The pellet was suspended in
0.1 mol·L−1 HCl, rinsed three times in sterilized water,
and optical density (OD660nm) of the suspension was
measured.
2.7 Measuring the ability of strain 3–1 to secrete IAA
The IAA assay was performed using the method described
by Patten and Glick (2002) with some modifications. Both
the strains, namely strains 3–1 and 2–2, were multiplied
overnight in 5 ml of DF salts minimal medium (Dworkin
and Foster 1958) which had the following composition (for
1 L): 4.0 g KH2PO4, 6.0 g Na2HPO4, 0.2 g MgSO4·7H2O,
Fig. 1 Effect of Strain 3–1 on growth of sugar beet. From left CK-W,
seeds soaked in water; 3-1-N, seeds soaked in a suspension of Strain
3–1 in nutrient solution; CK-N, seeds soaked in nutrient solution
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
D
ry
W
ei
gh
t/g
0
5
10
15
20
25
30
CK-W CK-N 3-1-W 3-1-N 2-2-W 2-2-N CK-W CK-N 3-1-W 3-1-N 2-2-W 2-2-N
In
di
vid
ua
l p
la
nt
h
ei
gh
t/c
m
Fig. 2 Effect of seed inoculation on growth of beet seedlings. CK-W,
seeds soaked in water; CK-N, seeds soaked in Hoagland solution;
3-1-W, seeds soaked in a suspension of Strain 3–1 cells in water;
3-1-N, seeds soaked in a suspension of Strain 3–1 cells in
Hoagland solution; 2-2-W, seeds soaked in a suspension of Strain
2–2 cells in water; 2-2-N, seeds soaked in a suspension of Strain
2–2 cells in Hoagland solution. The results are representative of 48
plants. Error bars show the standard deviation
Growth promotion effects of the endophyte 161
2.0 g glucose, 2.0 g gluconic acid, and 2.0 g citric acid;
trace elements: 1 mg FeSO4·7H2O, 10 mg H3BO3, 11.1 mg
MnSO4·H2O, 124.6 mg ZnSO4·7H2O, 78.22 mg
CuSO4·5H2O, and 10 mg MoO3; pH 7.2; and 2.0 g
(NH4)2SO4 as the nitrogen source. The next day, 20 μL of
the primary inoculum was transferred into 5 ml of the DF
medium supplemented with 100 μg ml−1 L-tryptophan
(Sigma Chemicals) taken from a filter-sterilized 2 mg/ml
stock prepared in warm water. After incubation for 48, 72,
96, and 120 h, the density of each culture was measured
spectrophotometrically at 600 nm. The bacterial cells were
then removed from the culture medium by centrifuging at
6,000×g for 15 min; a 1 ml aliquot of the supernatant was
mixed vigorously with 4 ml of Salkowski’s reagent (Gordon
and Weber 1951) consisting of 150 ml of concentrated
H2SO4, 250 ml of distilled H2O, and 7.5 ml of 0.5 M
FeCl3·6H2O, and allowed to stand at room temperature for
20 min before the absorbance at 535 nm was measured.
The concentration of IAA in each culture medium was
determined by comparing with a standard curve.
2.8 Estimation of internal root colonization
Pot trials were designed to assess the internal colonization
of sugar beet roots after seedling inoculation through the
seed soaking method described above. Rifampicin-resistant
mutants of isolates 3–1 and 2–2 were prepared as described
by He et al. (2004). The obtained mutants were compared
with their wild types in relation to their ability to produce
auxins. None of these mutants differed morphologically
from their parental strains, and all mutants had identical
growth rates and auxin-production capability with their
parental strains. Sugar beet seedlings were prepared as
described above and inoculated with isolates 3–1 and 2–2.
Every week after planting (1–8), roots were sampled from
the soil, washed thoroughly in tap water, surface disinfested
as described above and the population densities of isolates
3–1 and 2–2 (log10 CFU g−1 fresh root weight) were
determined using NA amended with rifampicin (300 μgml−1 ,
Sigma). Each treatment was replicated five times with two
plants in each replicate for each sampling.
2.9 Field experiments to evaluate performance of strain 3–1
Field experiments were conducted at the Toutunhe Farm in
Urumuqi, Xinjiang province, China, from 2009 to 2010.
Strain 3–1 inoculation was made the same way as pot
experiment, inoculated by using the root drench method.
Beets were harvested shortly after the 7th week of testing,
at which time the plants were about 15 cm at the crown.
Table 2 Effect of inoculation with Strain 3–1 on the content (mg/kg fresh weight) of N, P, K, and Mg and of Vb2,Vc, and protein in beet
Treatment Nt P K Mg VB2(10
−3) VC Protein
CK-W 1789.23±13.56 14.44±0.59 3629.99±61.41 659.61±17.59 1.62±0.05 0.82±0.01 964.01±0.88
CK-N 1828.06±35.09 15.39±0.51 3657.90±9.06 669.64±8.80 1.79±0.07 0.86±0.01 1002.43±1.90
3-1-W 2005.11±5.72* 19.63±0.44* 3823.65±88.53* 857.05±3.30* 1.94±0.07* 0.93±0.02* 1497.25±1.46*
3-1-N 2118.56±14.35* 21.78±0.63* 3949.72±50.98* 877.41±8.65* 2.06±0.05* 0.96±0.01* 1546.67±3.49*
2-2-W 1824.20±28.58 16.39±0.61 3653.14±10.16 664.85±7.75 1.86±0.07 0.85±0.02 1354.39±3.70
2-2-N 1852.63±18.27 17.63±0.62 3701.36±29.83 687.13±10.03 1.95±0.05 0.9±0.02 1405.92±2.19
Average ± standard error from three separate experiments. Values with the symbol * indicate significant differences at P<0.05 from
corresponding control
A. johnsonii 3P09AC EU977715.1
A. johnsonii 2P08MC EU977619.1
A. johnsonii 2P01AC EU977659.1
A. johnsonii CCA0908 GU320671.1
A. johnsonii CAI-6 DQ257426.1
A. johnsonii CAI-27 DQ257430.1
A. johnsonii UFV-E05 EF114343.1
A. johnsonii 3P02MC EU977623.1
A. johnsonii 3-1 EU594557.1
A. johnsonii 3P08MA EU977634.14
51
30
1
9
76
1
Fig. 3 Phylogenetic analysis of
partial 16S rDNA sequence of
isolates Acinetobacter johnsonii
strain 3–1 obtained from sugar
beet. Numbers in parentheses
represent the sequencesaccession
number in GenBank. The
number at each branch points is
the percentage supported by
bootstrap
162 Y. Shi et al.
Beets were fertilized, irrigated, and weeds controlled
following typical production practices in northwest, China.
The sucrose and fructose assay was performed using a
spectrophotometric method (Taylor, 1995). Yield compar-
isons among treatments were made at harvest date, and
sugar content was determined by refractive method.
2.10 Statistical analysis
All experiments were arranged in completely randomised
block designs. Population data were transformed into log10
CFU g−1 fresh weight. Data were subjected to analysis of
variance, and treatment means were compared using Fisher’s
Protected LSD Test at P<0.05.
3 Results
3.1 Effect of strain 3–1 on plant growth
Results of seed inoculation are shown in Figs. 1 and 2.
Germination percentage was significantly higher in seeds
inoculated with Acinetobacter johnsonii strain 3–1 (3–1-W)
and so were the two other growth parameters, namely dry
weight and plant height. Germination rate of inoculated
seeds was 92%, 28.6% (in the text just give percentages to
the nearest whole percentage i.e. 92% and 29%) greater
than that of seeds that had been soaked in water (CK-W).
Dry weight of seedlings from the inoculated seeds was
about 3-fold greater, and the seedlings were 18.3% taller,
than those from seeds that had been soaked only in water
before sowing. Plants from seeds inoculated with strain
3–1 grew significantly better than those growing from
seeds that had been soaked in water (W) or in Hoagland
solution (N). Plants from seeds inoculated with strain 3–1
were 19.4% taller and 69.8% heavier than those from
uninoculated seeds. In comparison, the 2–2 strain did
increase the dry weight of beets, and it had little effect
on plant height, however.
3.2 Effect of strain 3–1 on plant nutrient uptake
The content of N, P, K, and Mg and of vitamin B, vitamin
C, and protein was determined 4 weeks after germination
(Table 2). Inoculation with Strain 3–1 significantly increased
the uptake of all the elements that were analysed: total N and
P following inoculation with strain 3–1 were greater by
10.8% and 26.4% respectively than their levels in CK-W,
and the levels of K and Mg were also higher. It was also
found that salt, that is soaking the seeds in a nutrient
solution, affected the uptake of nutrients from soil: levels of
N and P in CK-W were lower by 2.1% and 6.2% (In the test
just give the % to the nearest whole percentage, i.e. 2% and
6%) respectively than the levels in CK-N. Inoculation with
strain 3–1, however, counteracted this effect, levels of K+
and Mg2+ in strain 3–1-N being 7.4% and 23.6% higher
respectively than those in CK-N and levels of total Vitamin
B,Vitamin C, and protein being 13.1%, 10.4%, and 35.2%
Fig. 4 Relationship between growth of Strain 3–1 and level of
dissolved phosphorus. The results are representative of three
experiments, with distilled water added to PKO medium as control.
Error bars indicate standard deviation
Fig. 5 Relationship between dissolved phosphorus (in the culture
medium inoculated with Strain 3–1) and pH of the culture medium.
The results are representative of three experiments. Error bars indicate
standard deviation
Table 3 Production of IAA by Strain 3–1 in a medium supplemented
with tryptophan
Time (h) IAA production (μg/ml/OD600 unit)
Strain 3–1 Strain 2–2
24 21.53+2.17 10.12+1.01
48 42.19+2.87 15.97+1.60
72 95.84+3.00 20.49+2.05
96 126.97+3.23 25.64+2.56
120 119.45+3.40 33.23+3.32
144 51.07+3.86 33.20+3.32
168 49.56+3.36 29.10+2.91
A average + standard error from three separate experiments
Growth promotion effects of the endophyte 163
higher respectively in strain 3–1-N than those in CK-N.
Strain 2–2 also influenced the ability of sugar beet to
accumulate ions, although not to the same extent as strain
3–1: levels of N, P, and K+ in 2–2-W
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