Evaluation of drought and acid tolerant actinomycetes for rice (Oryza sativa L.)

Abstract

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. 134 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.