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The study entitled “Isolation and characterization of beneficial rhizosphere
microorganisms from ragi grown in Attappady hill tract of Kerala” was conducted
during the year 2019-2021 in the Department of Agricultural Microbiology, College
of Agriculture, Vellayani, Thiruvananthapuram with the objective to isolate
nitrogen fixing and phosphate solubilizing bacteria from soil and root samples
collected from Attapady and select efficient isolates through in vitro screening
process and assess the plant growth promotion activity by roll towel assay.
Thirty six nitrogen fixing bacteria and eight phosphate solubilizing bacteria
were isolated from root and soil samples from ragi grown in Attappady region. The
isolates were designated as ATY1-ATY36 for NFB isolates and PSB1-PSB8 for
PSB isolates. Twenty isolates were selected after eliminating isolates with
similarities in colony morphological characters, in order to avoid repetition among
the isolates obtained from the same sample. Both NFB and PSB isolates were
characterized by morphology and biochemical tests.
All selected isolates were subjected to plant growth promotion activity
studies. The Indole Acetic Acid (IAA) production showed a wide range of variation
from 101.22 µg mL-1
to 3.26 µg mL-1
. Gibberellic Acid (GA) production of all the
twenty isolates were done and the results ranged between 10.07 µg mL-1 and 2.18
µg mL-1
. Maximum IAA and GA production was recorded in isolate ATY10.
Extracellular ammonia production of the bacterial isolates ranged between
176.53±5.07 μmol mL−1 and 75.59 μmol mL−1. The nitrogen content of NFB
isolates ranged between 30.75 µg mL-1 and 9 µg mL-1
. The isolate ATY33 had
maximum nitrogen content. The selected PSB isolates were subjected to plate assay
and broth assay using Pikovskaya’s medium. The isolate PSB1 recorded maximum
zone of solubilization (18.23 mm), phosphate solubilizing index (2.82) and
quantification of phosphate solubilization (53.41 mg L-1
).
The bacterial isolates were screened based on weighted average ranking. In
NFB, ATY10, ATY34 and ATY35 were selected for further studies and in PSB,
the isolates PSB1, PSB3 and PSB4 were selected. The selected isolates were
164
identified as Pantoea agglomerans ATY10, Rhizobium sp. ATY34, Ensifer
adhaerens ATY35, Burkholderia territorii PSB1, Burkholderia cepacia PSB3 and
Aeromonas hydrophila PSB4 by 16s rRNA sequencing. Effect of these isolates
were assessed for plant growth promotion in vitro in ragi seeds using roll towel
assay. Maximum germination percentage was recorded by Aeromonas hydrophila
PSB4 (81.24 per cent) and the minimum number of days taken for germinationwas
recorded by Pantoea agglomerans ATY10 treated seeds. Also, the treatment
Pantoea agglomerans ATY10 recorded maximum shoot length (3.31 cm), root
length (7.76 cm), seedling length (11.08 cm), seedling vigour index (876.62), root
dry weight (0.443 mg) and root shoot ratio (0.50). The treatment Burkholderia
territorii PSB1 recorded maximum root fresh weight (3.762 mg) and shoot dry
weight (0.877 mg).
The six isolates were subjected to assess the multiple traits such as
phosphorus, potassium and silicate solubilizing capacity, siderophore production
and antifungal activity. Burkholderia territorii PSB1 showed maximum zone of
inhibition against Rhizoctonia solani (3.16 mm) and Sclerotium rolfsii (0.58 mm).
The maximum antagonistic activity against Fusarium sp. and Helminthosporium
sp. was recorded by Burkholderia cepacia PSB3 with 4.13 mm and 5.66 mm of
zone of inhibition respectively. Burkholderia territorii PSB1 and Burkholderia
cepacia PSB3 solubilized potassium alumino silicate in agar also, with clearance
zone of 11.00 mm and 12.66 mm (in diameter), respectively. None of the isolates
solubilized magnesium trisilicate in Bunt and Rovira medium. The siderophore
production was observed in Aeromonas hydrophila PSB4 isolate in Chrome Azurol
S (CAS) agar medium plates.
Based on the results of the present study, it can be concluded that Pantoea
agglomerans ATY10 is the superior isolate among NFB isolates for plant growth
promotion. Also, the isolate Burkholderia territorii PSB1 is best among PSB
isolates for plant growth promotion and biocontrol activity.