Isolation and in vitro screening of silicate solubilizing bacteria from paddy rhizosphere

Abstract

The study entitled “Isolation and in vitro screening of silicate solubilizing bacteria from paddy rhizosphere”, was conducted during 2018-2020, in the Department of Agricultural Microbiology, College of Agriculture, Vellayani, Thiruvananthapuram, with the objective of isolation and in vitro screening of bacteria which are capable of solubilizing insoluble form of silicate. Bacteria capable of solubilizing silicates were isolated from the rhizosphere soils collected from different upland and low land paddy fields by serial dilution and plate count method using Bunt and Rovira medium supplemented with 0.25 per cent magnesium trisilicate. Based on the clear halo zone formed around the bacterial colonies on solid media, they were identified as Silicate Solubilizing Bacteria (SSB). Twenty seven isolates of bacteria capable of solubilizing insoluble form of silicate (magnesium trisilicate) were obtained from different locations and were allotted code numbers from SSB 1 to SSB 27. These isolates were subjected to plate and broth assay in Bunt and Rovira medium supplemented with 0.25 per cent magnesium trisilicate. After three days of incubation of test plates at room temperature, all the twenty seven isolates solubilized magnesium trisilicate and produced clearing zone around the bacterial colonies on solid media. The size of clearance zone ranged from 3 mm to 13 mm in plates. The maximum clearance zone of 13 mm was recorded with the isolate SSB 14 which was significantly superior to all other isolates. In broth culture, SSB 20 showed the highest silicate solubilization of 94.65 mg L-1. Based on plate as well as broth assay of all the twenty seven isolates obtained, five isolates viz., SSB 3, SSB 14, SSB 18, SSB 20 and SSB 22 which showed the maximum clearance zone in plate and silicate solubilization in broth were selected as superior isolates. All the isolates obtained were subjected to plate and broth assay for phosphate solubilization in Pikovskaya’s medium and potassium solubilization in Aleksandrov medium. Among them, fourteen isolates showed phosphate solubilization in plates and the clearance zone ranged from 0.87 mm to 5.50 mm. The maximum clearance zone of 5.50 mm was recorded with the isolate SSB 22 which was on par with SSB 23 with 5 mm clearance zone in plate and highest solubilization of 41.92 mg L-1 in broth was shown by SSB 22 which was significantly superior to all other isolates. Twelve isolates showed potassium solubilization in plates and the clearance zone ranged from 2.25 mm to 5.50 mm. Maximum clearance zone of 5.50 mm was recorded with the isolate SSB 8 which was significantly superior to all other isolates. The highest potassium solubilization of 37.50 mg L-1 in broth was observed with isolates SSB 1, SSB 2, SSB 7, SSB 8, SSB 13, SSB 18, SSB 21 and SSB 22 which were found to be statistically on par. Acid production by the five superior SSB isolates, SSB 3, SSB 14, SSB 18, SSB 20 and SSB 22 was detected as a yellow halo around the bacterial colonies in bromophenol blue amended Bunt and Rovira medium. All the five superior isolates tested showed positive results for acid production. The antagonistic activity of the five superior SSB isolates were assessed against major pathogens of paddy viz., Rhizoctonia solani, Magnaporthe grisea, Helminthosporium oryzae and Xanthomonas oryzae pv. oryzae following dual culture method. Out of the five isolates tested, three isolates (SSB 18, SSB 20 and SSB 22) inhibited Rhizoctonia solani. Three isolates (SSB 3, SSB 18 and SSB 22) showed antagonism against Magnaporthe grisea and four isolates (SSB 3, SSB 18, SSB 20 and SSB 22) inhibited Helminthosporium oryzae. The bacterial pathogen, Xanthomonas oryzae pv. oryzae was inhibited by three isolates (SSB 18, SSB 20 and SSB 22). Among all the five isolates tested against different phytopathogens, SSB 18 was found superior with the maximum zone of inhibition of 9.65 mm, 14.45 mm, 10.80 mm and 11.50 mm against Rhizoctonia solani, Magnaporthe grisea, Helminthosporium oryzae and Xanthomonas oryzae pv. oryzae respectively. The five superior isolates were characterized based on morphological and biochemical characters. The results revealed that all the isolates were rod shaped, Gram positive endospore formers. Based on the results of present study, it can be concluded that SSB 3, SSB 14, SSB 18, SSB 20 and SSB 22 are the superior silicate solubilizing bacterial isolates. Among them, SSB 18 showed the highest antagonistic activity against major pathogens of paddy.