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Rice (Oryza sativa L). is one of the world's most significant cereal crops. It is widely
cultivated food crop in the world, providing staple food to 2.7 billion people. Rice cultivation
is facing many challenges due to abiotic stresses like drought, acidity, flood and high
temperature. Among the major yield-limiting biotic factors, disease management is one of the
most important problems in paddy cultivation. Among the several diseases in rice, sheath
blight (Rhizoctonia solani) is a fungal disease that has a high impact on the rice crop from
tillering to the heading stage. Rhizospheric microbes, particularly actinomycetes have gained
attention due to their ability to promote plant growth, control diseases and reduce harm to the
environment. The use of actinomycetes as bio-control agents is sustainable and
complementary in improving crop yield and protection that is affordable to farmers.
Actinomycetes have been used to mitigate drought and acidic stress. Hence, the present study
was carried out on “Evaluation of drought and acid-tolerant actinomycetes for rice (Oryza
sativa L).” with the objective of plant growth-promotion and Rhizoctonia solani management
under pot culture studies.
Ten rhizospheric soil samples were collected from drought and acidic rice-growing
areas of the Thrissur and Palakkad districts. All the actinomycetes isolates were subjected to
morphological, cultural, and biochemical characterization. The actinomycetes isolates showed
gram-positive reaction and varied in spore chain morphology viz., filamentous, spiral and
closed spiral.
In vitro screening of actinomycetes isolates for plant growth-promoting traits showed
that all the actinomycetes isolates were positive for indole acetic acid production. Out of ten
isolates, seven isolates were positive for the production of IAA and two isolates viz., MNT7 and CHT-1 were high indole acetic acid producers. IAA production by actinomycetes
isolates ranged from 4.41 (μg/ml) to 5.99 (μg/ml). The MNT-7 (Streptomyces
spinoverrucosus) isolate showed the highest IAA production (5.99 μg/ml) followed by CHT1 (5.43 μg/ml) and PTK-4 (Streptomyces spp.) (5.23 μg/ml). All the isolates fixed atmospheric
nitrogen which ranged from 2.80 μg/ml to 10.50 μg/ml. The MNT-7 (Streptomyces
spinoverrucosus (10.50 μg/ml), VLK-2 (9.10 μg/ml) and VER-4 (Streptomyces cellulosae)
(7.70 μg/ml) isolates were the most efficient nitrogen-fixing isolates. The amount of phosphate
solubilization varied between 3.28 μg/ml to 57.56 μg/ml. nine isolates showed potassium
solubilization in the range of 1.86 μg/ml to 5.07 μg/ml.
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In vitro screening of actinomycetes for antagonistic properties did not show
ammonia and siderophore production. Among the ten actinomycetes isolates, MNT-7
(Streptomyces spinoverrucosus) and PTK-4 (Streptomyces spp.) isolates produced
hydrogen cyanide. Only MNT-7 (Streptomyces spinoverrucosus) (71.23%) and PTK-4
(Streptomyces spp) (68.82%) isolates inhibited the growth of Rhizoctonia solani indicating
their biocontrol potential.
In vitro screening of actinomycetes for acidic pH and water stress tolerance showed
that MNT-7 (Streptomyces spinoverrucosus) isolate was tolerant to acidic pH upto pH 3.5
and MNT-7 (Streptomyces spinoverrucosus) and PTK-4 (Streptomyces spp.)isolates were
water stress tolerant upto 20 % PEG.
The three most promising (MNT-7, PTK-4 and VER-4) actinomycetes were
identified as Streptomyces spinoverrucosus, Streptomyces spp., Streptomyces cellulosae
respectively.
On the basis of the plant growth-promoting activities, acidic pH and water stress
tolerance, native actinomycetes isolates of Streptomyces spinoverrucosus (MNT-7),
Streptomyces spp. (PTK-4) and Streptomyces cellulosae (VER-4) evaluated under pot
culture. MNT-7 (Streptomyces spinoverrucosus) significantly increased all the growth
parameters and yield also recorded the minimum sheath blight percent disease incidence of
6.79 under pot culture studies.
The present study revealed that Streptomyces spinoverrucosus (MNT-7) was the
most promising PGPR for plant growth promotion and sheath blight disease management.
To, the best of knowledge, Streptomyces spinoverrucosus is the first report of PGPR
tolerant to drought and acidic pH. However, further evaluation field is needed to confirm
the findings.