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Exploration, isolation and characterization of indigenous rhizobacteria
from patchouli rhizosphere as PGPR candidates in producing IAA and
solubilizing phosphate
To cite this article: H Halimursyadah et al 2022 IOP Conf. Ser.: Earth Environ. Sci. 951 012055
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3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
IOP Conf. Series: Earth and Environmental Science 951 (2022) 012055
IOP Publishing
doi:10.1088/1755-1315/951/1/012055
1
Exploration, isolation and characterization of indigenous
rhizobacteria from patchouli rhizosphere as PGPR candidates
in producing IAA and solubilizing phosphate
H Halimursyadah*, Syamsuddin, Nurhayati, DN Rizva
Agrotechnology Department, Faculty of Agriculture, Universitas Syiah Kuala, Jalan
Tgk. Hasan Krueng Kalee 3, Darussalam-Banda Aceh 23111, Indonesia
*Email: halimursyadah@unsyiah.ac.id
Abstract. Microorganisms that are active and aggressive colonizing the rhizosphere are known
as rhizobacteria. They are able to act as biofertilizers, bioprotectants, biostimulants and
bioremediation. This study aims to identify and characterize groups of rhizobacteria present in
the patchouli rhizosphere that can produce IAA compounds and have the ability to solubilize
phosphate in the soil. Soil samples were taken from the patchouli rhizosphere at Purwosari
Village, Nagan Raya, Aceh Province, Indonesia. This study used quantitative and qualitative
descriptive analysis through serial dilutions to obtain rhizobacterial strains. Parameters observed
were macroscopic and microscopic characteristics, gram test, IAA production and phosphate
solubilization. The study obtained 37 isolates of rhizobacteria from Purwosari (PS), comprising
25 isolates of gram positive and 12 isolates of gram negative. The rhizobacteria PS 5/1 produced
the lowest IAA at 21.66 ppm, whereas isolate 5/6 C produced the highest IAA at 83.38 ppm.
Twenty-five isolates of rhizobacteria could solubilize phosphate while the remaining 12 isolates
did not have this ability. The rhizobacteria PS 7/1 resulted in the highest PSI at 2.55 and isolates
PS 8/7 produced the lowest PSI at 1.33. The rhizobacteria isolates that can produce IAA and
phosphate solubilizing have the potential to be used as PGPR candidates.
1. Introduction
Patchouli (Pogostemon cablin Benth.) is one of the important export commodities from Aceh which
includes patchouli oil, as the main fixative constituent in modern perfumery, and patchouli leaves. In
2018, the Agricultural Quarantine Agency recorded that nationally patchouli leaves were exported as
many as 297.7 tons with a value of Rp 470 million, and in 2019 they increased to 1,227 tons with a value
of Rp 1.3 billion. In 2020, however, there was a decrease in volume, reaching only 591.3 tons, and yet
due to a surge of price, the economic value could reach Rp 6.8 billion [1]. As of 2021, the Aceh Jaya
Department of Industry and Trade states that the average production of patchouli oil for farming
communities in Aceh Jaya District ranges from 5-6 tons per day [2].
Due to its benefits, the optimization of patchouli cultivation is necessary to ensure that its production
remains sustainable. One of the efforts to achieve this is by utilizing microorganisms in situ to support
plant growth, such as rhizobacteria. Rhizobacteria known as Plant Growth Promoting Rhizobacteria
(PGPR) are a group of microorganisms living in the rhizosphere that exert benefits for plants. PGPR
activities provide advantages for plant growth through direct or indirect mechanisms. The direct effects
3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
IOP Conf. Series: Earth and Environmental Science 951 (2022) 012055
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doi:10.1088/1755-1315/951/1/012055
2
of PGPR in promoting plant growth occur through various mechanisms, including fixation of free
nitrogen that is transferred into plants, production of siderophores that chelate iron (Fe) and make Fe
unavailable to pathogens, and solubilization of minerals such as phosphorus and synthesis of
phytohormones. Indirect effects in increasing plant growth, on the other hand, occur through suppression
of phytopathogens [3].
The roles of rhizobacteria in the rhizospheric area consist of providing nutrients in the soil,
decomposing organic matter, mineralizing organic matter, stimulating plant growth, and being a
biological agent (controlling pests and plant diseases). Soil microbes have many important roles in the
cycle of organic elements for life, such as producing the Indole Acetic Acid (IAA) hormone. IAA
hormone is an endogenous auxin that plays a role in root development, inhibits the growth of side shoots,
stimulates abscission, and plays a role in the formation of xylem and phloem tissues.
The ability to fix nitrogen, phosphate solubilizing, and produce siderophores, hydrogen cyanide
(HCN), chitinases, proteases, and cellulases are characteristics of suitable rhizobacteria. Phosphate
solubilizing bacteria can synthesize phytase and phosphatase enzymes that play a role in the hydrolysis
of organic P to dissolve inorganic P and produce organic acids that help release metal-fixed P [4]. In
addition, according to [5], the increase in plant growth by rhizobacteria can occur through one or more
mechanisms related to the physiological characteristics of bacteria and conditions in the rhizosphere.
Some rhizobacterial strains are able to synthesize IAA from precursors (basic materials) contained in
root exudates as well as from organic matter (plant and animal residues), whereas in general plants they
are unable to produce IAA in sufficient quantities for growth and development. Rhizobacteria that can
produce IAA and phosphate solubilizing came from the genera Pseudomonas, Bacillus, and Serratia.
The purpose of the research was to find out the rhizobacteria that produce IAA and phosphate
solubilization.
2. Materials and methods
This study was conducted at the Laboratory of Seed Science and Technology, Faculty of Agriculture,
Syiah Kuala University on October 2020 - April 2021. The initial stage was exploration aimed at taking
soil samples from the rhizosphere of a healthy patchouli plant growing in Purwosari, Nagan Raya, Aceh
Province, Indonesia.
2.1. Isolation and characterization
Isolation of rhizobacteria was carried out using the serial dilution method up to 10-9. The incubation
process was conducted at room temperature for 3-7 days. Soil samples were taken for a total of 10 kg
from 10 different points in the area. Then, the soil was dried at room temperature (28 - 300C). After
2x24 hours of drying, the soil was first sieved with a 9-mesh sieve shaker. Next, the soil was weighed
at 1 gram and placed it in a test tube, and later added with 9 mL of distilled water, and then shaken until
evenly distributed, hereinafter referred to as the first dilution of 10-1. The 1 mL pipette from 10-1 was
afterward put into a test tube containing 9 mL of diluent solution, hereinafter referred to as the second
dilution 10-2, up to the final dilution 10-9. Then, 1 mL at 10-9, 10-8 and 10-7 dilutions was taken and poured
into a petri dish by adding Sucrose Peptone Agar medium, which was still a liquid (±450C), and then
made a duplicate. Next, it was shaken homogeneously and waited until the medium solidified, after
which the petri dish could be inverted and then incubated for 2 days (pour method). Afterward,
observation and calculation for the number of colonies were done. Colonies can be distinguished by
surface appearance (shiny or gloomy), by colour (white, yellow, red, brown, orange, blue, green, and
purple), and by density (soft like slime, soft like butter, and some are hard and dry). Furthermore,
identification of rhizobacteria was carried out to obtain pure cultures from local indigenous
rhizobacteria. Parameters observed were the ability to produce IAA, phosphate solubilizing bacteria,
and gram test.
3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
IOP Conf. Series: Earth and Environmental Science 951 (2022) 012055
IOP Publishing
doi:10.1088/1755-1315/951/1/012055
3
2.2. IAA production test
IAA production test was conducted qualitatively and quantitatively. The isolates were tested
qualitatively using Salkowski’s reagent. PGPR isolates were inoculated on flat Nutrient Agar media
supplemented with tryptophan with a concentration of 100 ppm using the T streak technique [6]. Then,
they were incubated at room temperature for 48 hours. Salkowski’s reagent was dripped onto PGPR
isolates that had been grown in the Nutrient Agar media until evenly distributed. Furthermore, isolates
that had been dripped with Salkowski’s reagent were stored in a dark room for 30 minutes. Positive
results were indicated by a change in the colour of the isolate colony to light yellow.
2.3. Phosphate solubilization test
Rhizobacteria isolates were taken using a sterile needle and grown on Pikovskaya media aseptically.
The medium was incubated for 7 x 24 hours at 370C. Observation of colony growth capable of forming
a clear zone was indicated as isolates capable of solubilizing phosphate. Colony diameter and clear zone
were measured at 7 x 24 hours). The formula for measuring Phosphate Solubilization Index (PSI) was
the ratio of total diameter (halo) and colony diameter. PSI category: 1.59 Low; 1.6 – 2.12 Medium; 2.15
– 2.59 High; 2.6 – 3 Very High [7].
3. Results and discussion
3.1. Isolation and identification of rhizobacteria from patchouli rhizosphere
The geographical position of indigenous rhizobacteria taken from the patchouli rhizosphere at Purwosari
Village, Nagan Raya, Aceh Province, Indonesia. The exploration of rhizobacteria as PGPR candidates
resulted in 37 isolates coded in PS. The performance of rhizobacteria isolates is shown in Figure 1.
Figure 1. Performance of several rhizobacteria isolates macroscopically (top) and microscopically
(bottom).
Further, candidate isolates as PGPR were identified macroscopically to identify the shape, colour,
edge, and elevation. The process of identifying rhizobacteria isolates was based on the key to the
determination in the book “Bergey’s Manual of Systematic Bacteriology” [8] through the biochemical
tests. The results of the identification of rhizobacteria isolates were presented in Table 1.
PS 4/2
PS 6/3A
PS 8/1
PS 8/4K
PS 8/4U
3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
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Table 1. Macroscopic characteristics and gram test of indigenous rhizobacteria isolates from patchouli
rhizosphere.
Table 1 shows that the results of macroscopic identification reflect the diversity of rhizobacteria
isolates obtained based on shape, colouration, surface or edge conditions, and elevation. The
rhizobacterial isolate candidates that had been obtained were tested further to examine their ability to
produce several compounds, namely growth stimulants (biostimulants) and nutrient providers
(biofertilizers). Rhizobacteria are bacteria that colonize plant roots which produce phytohormones to
support plant growth and development as well as induce plant resistance. In addition, its role is also a
biocatalyst to support the availability of important organic acids for plants. The presence of rhizobacteria
as an environmental conservation agent to maintain the biodiversity of root microbes helps support
environmentally friendly agriculture that can increase agricultural yields.
Isolate
Code
Macroscopic
Gram staining
Shape
Colour
Edge
Elevation
PS 5/1
Irregular
Greenish Brown
Undulate
Drop-like
Negative
PS 5/2 C
Irregular
Brown
Irregular
Flat
Positive
PS 5/2 H
Spindle
Brown
Undulate
Flat
Positive
PS 5/3
Circular
Greenish Dark Brown
Undulate
Flat
Negative
PS 5/4
Circular
Brown
Irregular
Flat
Negative
PS 5/5
Spindle
Brown
Undulate
Flat
Positive
PS 5/6 C
Circular
Brown
Entire
Flat
Positive
PS 5/6 PK
Circular
Brown
Entire
Flat
Positive
PS 5/7
Circular
Yellowish Brown
Entire
Flat
Positive
PS 6/1
Circular
Dark Brown
Undulate
Flat
Positive
PS 6/2
Circular
Yellowish White
Lobate
Raised
Positive
PS 6/3 A
Irregular
Light Brown
Undulate
Raised
Negative
PS 6/3 C
Circular
Yellowish White
Entire
Flat
Negative
PS 6/4
Circular
Dark Brown
Entire
Flat
Positive
PS 6/5
Circular
Light Brown
Undulate
Flat
Positive
PS 4/1
Circular
Brown
Undulate
Flat
Negative
PS 4/2
Irregular
Brown + Yellow
Undulate
Convex
Negative
PS 4/3
Irregular
Greyish Brown
Lobate
Flat
Positive
PS 4/4
Circular
Light Brown
Entire
Flat
Negative
PS 4/5
Circular
Brown
Entire
Drop-like
Positive
PS 4/6
Spindle
Brown
Entire
Flat
Positive
PS 4/7
Spindle
Greyish Green
Entire
Flat
Negative
PS 7/1
Spindle
Brownish Yellow
Undulate
Flat
Positive
PS 8/1
Spindle
Purple
Entire
Raised
Negative
PS 8/2
Spindle
Clear Yellow
Entire
Raised
Positive
PS 8/3
Circular
Brown
Entire
Flat
Positive
PS 8/4 K
Irregular
Yellow + Orange
Entire
Convex
Positive
PS 8/4 P
Spindle
White
Entire
Convex
Positive
PS 8/4 U
Spindle
Whitish Purple
Entire
Convex
Positive
PS 8/5
Spindle
Clear Yellow
Entire
Convex
Positive
PS 8/6
Circular
Clear Yellow
Undulate
Raised
Positive
PS 8/7
Rhizoid
Clear
Entire
Flat
Positive
PS 8/8 C
Irregular
Brown
Entire
Raised
Negative
PS 8/8 K
Spindle
Brown
Entire
Raised
Positive
PS 8/8 PK
Irregular
Brown
Entire
Flat
Positive
PS 8/9
Rhizoid
Yellow Orange
Entire
Convex
Negative
PS 8/12
Circular
Clear Brown
Entire
Flat
Positive
3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
IOP Conf. Series: Earth and Environmental Science 951 (2022) 012055
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PS 4/2
PS 6/3A
PS 8/1
PS 8/4K
PS 8/4U
(Positive Gram)
(Positive Gram)
(Negative Gram)
(Positive Gram)
(Positive Gram)
Figure 2. Gram staining of several rhizobacteria isolates from patchouli rhizosphere.
The results of macroscopic tests on 37 isolates of rhizobacteria based on shape, colour, surface, edge
and elevation can be presumed to belong to several groups of soil bacterial genera such as Bacillus,
Pseudomonas, Chromobacterium, Erwinia, Flavobacterium, Micrococcus, and Serratia sp. The results
of the gram staining test of rhizobacteria produced 25 isolates of gram-positive reaction rhizobacteria
and 12 isolates of gram-negative reaction rhizobacteria (Table 1, Figure 2). The response of bacteria to
gram stain reactions is an important property to help determine the group or type of bacteria. For
example, the Bacillus group as one of the rhizosphere-dwelling bacteria can benefit plants with gram-
positive reactions which form anaerobic and aerobic endospores [9]. Pseudomonas spp is a gram-
negative straight or curved rod-shaped bacterium, measuring about 0.6x2 m, found singly, in pairs, and
sometimes forming short chains, does not have spores, does not have a sheath, and has a flagellum.
Certain Pseudomonas bacteria have two or three flagella, and so they are always active. The Serratia
group of bacteria are gram-negative bacteria from the family Enterobacteriaceae. The research has
indicated that the biological pigments produced by these bacteria have antifungal, immunosuppressive,
and antiproliferative activities [10].
PS 5/1
PS 6/1
PS 6/3A
PS 8/6
PS 8/8K
Undulate
Undulate
Undulate
Undulate
Entire
Figure 3. Microscopic observation of the edges of several rhizobacterial isolates.
The analysis carried out on the identification of rhizobacteria isolates as PGPR included the ability
of rhizobacteria to produce IAA compounds and the ability to solubilize phosphate qualitatively and
quantitatively. The test results of the ability of rhizobacteria isolates to produce IAA and the ability to
solubilize phosphate were presented in Table 2.
The results showed that 37 isolates of rhizobacteria were able to produce IAA as a whole. The PGPR
candidate isolates of rhizobacteria was capable of producing the IAA hormone compound to stimulate
plant growth. The rhizobacteria isolated from the exploration was able to produce IAA in the range of
values of 21.66 – 83.38 ppm. The rhizobacteria isolate PS 5/1 produced the lowest concentration of IAA
at 21.66 ppm with an absorbance value of 0.38 ml-1 filtrate, while isolate PS 5/6C produced the highest
concentration of IAA at 83.38 ppm with an absorbance value of 1.5 ml-1 filtrate. IAA compounds
released by rhizobacteria as exudates can be absorbed by plant roots which will be used to stimulate
meristematic cell division and initiation in plant physiological processes. Rhizobacteria are soil bacteria
that aggressively colonize plant rhizosphere. The dynamic rhizosphere rich in energy sources from
organic compounds released by plant roots (root exudates) is a habitat for various types of microbes to
develop, and at the same time a place for microbial hub and competition [11]. Each plant secretes root
3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
IOP Conf. Series: Earth and Environmental Science 951 (2022) 012055
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6
exudates with different compositions so that they also act as microbial selectors; its effects can increase
the development of certain microbes and inhibit the development of other microbes [12][3]. The more
root exudation is, the greater the number and diversity of microbes are. This condition will increase
competition in the rhizosphere colonization process. Rhizobacteria are the most efficient competitor
microbes that can shift from the position of native microbes in the rhizospheric zone to the middle age
of the plant [13].
Table 2. Isolates of rhizobacteria that produce IAA and phosphate solubilizing.
Isolate Code
IAA
Phosphate Solubilizing
Absorbance
Value (μ/ml
filtrate)
IAA Concentration
(ppm)
Halo
Diameter
(cm)
Colony
Diameter
(cm)
Phosphate
Solubility
Index
PS 4/1
0.51
28.38
1.47
0.73
2.01
PS 4/2
0.77
43.16
1.60
0.70
2.28
PS 4/3
0.69
38.83
1.00
0.70
1.42
PS 4/4
0.91
50.61
1.23
0.63
1.95
PS 4/5
0.93
52.00
-
-
-
PS 4/6
0.58
32.61
1.67
0.80
2.08
PS 4/7
0.82
45.61
-
-
-
PS 5/1
0.38
21.66
-
-
-
PS 5/2C
0.50
27.83
-
-
-
PS 5/2H
0.53
29.83
-
-
-
PS 5/3
0.50
28.16
1.20
0.70
1.71
PS 5/4
0.84
47.00
1.50
0.77
1.94
PS 5/5
1.01
56.16
1.20
0.70
1.71
PS 5/6C
1.50
83.38
1.47
0.80
1.83
PS 5/6 PK
0.62
34.94
1.37
0.80
1.71
PS 5/7
0.72
40.27
-
-
-
PS 6/1
0.61
34.05
1.57
0.70
2.24
PS 6/2
0.60
33.38
1.40
0.63
2.22
PS 6/3A
0.65
36.50
1.67
0.70
2.38
PS 6/3C
0.81
45.33
1,00
0.73
1.36
PS 6/4
0.62
34.50
-
-
-
PS 6/5
0.67
37.38
-
-
-
PS 7/1
0.64
36.00
1.97
0.77
2.55
PS 8/1
0.53
29.61
-
-
-
PS 8/2
0.65
36.16
1.60
0.77
2.07
PS 8/3
0.54
30.16
1.67
0.80
2.08
PS 8/4K
0.57
32.05
1.90
0.80
2.37
PS 8/4P
0.48
27.05
1.10
0.80
1.37
PS 8/4U
0.46
25.88
1.93
0.83
2.32
PS 8/5
0.60
33.50
1.50
0.70
2.14
PS 8/6
0.69
38.72
-
-
-
PS 8/7
0.46
25.94
1.03
0.77
1.33
PS 8/8C
0.60
33.61
1.63
0.80
2.03
PS 8/8K
0.41
23.27
1.43
0.80
1.78
PS 8/8 PK
0.69
38.61
1.97
0.80
2.46
PS 8/9
0.64
35.83
-
-
-
PS 8/12
0.42
23.83
1.53
0.80
1.91
3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
IOP Conf. Series: Earth and Environmental Science 951 (2022) 012055
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Figure 4. PS 8/4P (a) rhizobacteria isolates and PS 5/1 (b) rhizobacteria isolates producing IAA.
IAA hormone is a growth regulator that plays a role in the process of plant growth. IAA produced
by microbes in the rhizosphere can increase the number of root hairs and size and the number of
adventitious roots of plants [14]. The IAA hormone is synthesized as a secondary metabolite produced
under conditions of suboptimal bacterial growth or when the amino acid precursor tryptophan is
available. It has been reported that IAA production by bacteria can vary among different species and
strains, and it is also influenced by culture condition, growth stage, and substrate availability. Moreover,
isolates from the rhizosphere are more efficient auxin producers than isolates from the bulk soil [15]
[16].
In addition, the test results of 37 isolates of rhizobacteria differed in their ability to solubilize
phosphate. There were 10 isolates that did not have the ability to solubilize phosphate (Table 2). The
ability of rhizobacteria to solubilize phosphate is an indication that rhizobacteria can act as PGPR that
can stimulate plant growth and release bounded-phosphate in the soil to become available to plants. The
rhizobacteria isolates PS 7/1 produced the highest PSI of 2.55 and the isolates PS 8/7 produced the
lowest PSI of 1.33.
PS 4/2
PS 6/3A
PS 8/1
PS 8/4K
PS 8/4U
+
+
-
+
+
Figure 5. Rhizobacteria isolates (+) able to solubilize phosphate indicated by the formation of a clear
zone; while rhizobacteria isolates (-) had no ability to solubilize phosphate.
Rhizobacteria isolates capable of phosphate solubilizing were characterized by the formation of a
clear zone around the bacterial colonies when grown on solid Pikovskaya medium (Figure 5). The test
for the ability of rhizobacteria to solubilize phosphate was marked by the formation of a clear zone
around the bacterial colonies tested on Pikovskaya medium due to the breakdown of Ca3(PO4)2
contained in the medium [17]. Rhizobacteria have different abilities in dissolving phosphate. Phosphate
solubilizing rhizobacteria improve plant phosphorus nutrition by mobilizing inorganic and organic
phosphates. Several mechanisms of phosphate solubilization have been reported. Phosphate solubilizing
microbe produces different types and amounts of organic acids. Some groups of organic acids produced
by rhizobacteria are citric acid, succinic acid, fumaric acid, gluconic acid, oxalic acid, malic acid, and
gluconic acid [18]. Organic acids produced by microorganisms vary in quality and quantity in liberating
phosphate. Different mechanisms of inorganic phosphate solubilization are acidification due to the
production of organic acids and inorganic acids, as well as H+ excretion, exopolysaccharide production,
and siderophore production [19]. Phosphate-solubilizing bacteria (PSB) have the ability to solubilize
insoluble phosphorus (P) and release soluble P. Phosphate solubilizing rhizobacteria convert unavailable
P to available P to meet the needs of plants through reshuffle and absorption [20].
3rd International Conference on Agriculture and Bio-industry (ICAGRI 2021)
IOP Conf. Series: Earth and Environmental Science 951 (2022) 012055
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8
Colony size does not always affect the formation of a clear zone around the colony because a large
colony diameter does not always result in a large clear zone diameter. In principle, the clear zone area
can show the ability of rhizobacteria to solubilize phosphate, but it cannot show the amount of phosphate
concentration dissolved in the medium. Isolates of rhizobacteria that form clear zones faster and have a
broad Phosphate Solubilization Index (PSI) value are phosphate solubilizing bacteria that have the
potential as biofertilizers by solubilizing phosphate elements bound to other elements such as Fe, Al,
Ca, and Mg, and thus, phosphate elements become available [21]. A biofertilizer consisting of a
consortium of bacteria that has the ability to dissolve phosphate, dissolve potassium, and produce auxin
can have a high potential for increasing growth and yields [22]. Isolated rhizobacteria with codes PS
4/1, PS 4/2, PS 4/3, PS 4/4, PS 4/6, PS 5/3, PS 5/4, PS 5/5, PS 5/6C, PS 5/6K, PS 6/1, PS 6/2, PS 6/3A,
PS 6/3C, PS 7/1, PS 8/2, PS 8/3, PS 8/4K, PS 8/4P, PS 8/4U, PS 8/5, PS 8/7, PS 8/8C, PS 8/8K, PS
8/8PK, and PS 8/12 were bacterial isolates that have the potential as biological fertilizers because of
their ability to form clear zones compared to other isolates.
4. Conclusion
The results of exploration, identification, and characterization produced 37 isolates of indigenous
rhizobacteria from patchouli rhizosphere located in Purwosari, Nagan Raya, Aceh Province, Indonesia.
Thirty-seven rhizobacteria isolates had the ability to produce IAA. The rhizobacteria isolate PS 5/1
gained the lowest IAA at a concentration of 21.66 ppm, while isolate 5/6 C produced IAA at 83.38 ppm.
Twenty-five rhizobacteria isolates could dissolve phosphate and the remaining 12 isolates did not have
this ability. The rhizobacteria isolate PS 7/1 produced the highest PSI at 2.55 and the isolates PS 8/7
gained the lowest PSI at 1.33. Rhizobacteria isolates that produce IAA and able to solubilize phosphate
have the potential to be used as PGPR candidates.
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