Journal of Advances in Biology & Biotechnology

Aims: This study aimed to recover Bacillus spp. and fluorescent Pseudomonas spp. from the rice rhizosphere, determine their antagonistic effects on Xanthomonas oryzae pv. oryzae (Xoo), and assess their capacity for biological control and plant growth promotion under glasshouse conditions.

Study Design: Bacillus spp. and fluorescent Pseudomonas spp. were isolated from rhizosphere soil obtained from rice-growing fields. Antagonism towards Xanthomonas oryzae pv. oryzae was examined in vitro through preliminary inhibition tests and a double-layer assay. Isolates showing promise were subsequently assessed for disease suppression and plant growth-promoting properties in glasshouse experiments.

Place and Duration of Study: Department of Plant Pathology, ICAR-IIRR, Rajendranagar, 2018.

Methodology: Following collection, rice-field rhizosphere samples were processed for bacterial isolation; samples intended for recovery of spore-forming bacteria underwent heat treatment. Bacillus and Pseudomonas cultures were recovered and purified on PDY and PAM media, respectively. Antagonism against Xanthomonas oryzae pv. oryzae (Xoo) was first examined on MWA plates and then assessed quantitatively by the chloroform-vapour double-layer procedure. To examine plant growth promotion (PGP), TN-1 rice seeds were treated with bacterial suspensions formulated in talc and gum acacia, sown in sterilised soil, and evaluated for germination, root length and shoot height.

Results: A total of 60 bacterial isolates were obtained from rice rhizosphere soil, consisting of 45 Bacillus spp. and 15 fluorescent Pseudomonas isolates. In vitro antagonism towards Xoo was observed in 26 of the Bacillus isolates and in all 15 Pseudomonas isolates. Two isolates within each bacterial group produced the greatest percentage inhibition; inhibitory responses differed among the other isolates. The 15 Pseudomonas isolates were also examined for hydrogen cyanide (HCN), siderophore and volatile inhibitory compound production, together with phosphate solubilisation. Every isolate gave a positive response in three of these four assessments. Only P-3, P-6, P-10 and P-11 reacted weakly positively for HCN, with the other isolates testing negative. Bacillus isolates showed positive responses in tests including starch hydrolysis but failed to grow anaerobically, in keeping with their strictly aerobic character. Five Pseudomonas isolates (P-4, P-5, P-6, P-7 and P-8) and five Bacillus isolates (ARI 1-2, ARI 2-4, ARI 1-3, D1-2-2 and D2-1-1) were then examined for antagonism towards Xoo in vivo under glasshouse conditions. Treatment with P-7 yielded a lesion length of 18.5 cm, compared with 20.37 cm for the control. These selected isolates were additionally assessed for their influence on rice growth. P-4, P-6 and P-7 showed favourable plant growth-promoting effects in assessments of seed germination and root and shoot development. Germination exceeded the control in most treatments, with the highest germination following pretreatment with P-8; P-7, P-5 and D2-1-1 also improved germination. Seedling height was additionally evaluated relative to untreated plants. P-6 gave the greatest shoot length (18.33 cm), while the longest roots (9.17 cm) occurred following P-4 treatment. Taken together, the observations indicate growth-promoting activity among the tested Bacillus and Pseudomonas isolates, notably P-4, P-6 and P-7.

Conclusion: Rice-rhizosphere Bacillus and fluorescent Pseudomonas isolates displayed both antagonism towards Xanthomonas oryzae pv. oryzae (Xoo) and traits associated with plant growth promotion. Among the promising Pseudomonas strains, P-4, P-6, P-7 and P-8 supported germination and seedling development, whereas P-7 also produced shorter bacterial blight lesions in the glasshouse. These findings indicate their potential as bio-inoculants for improving rice growth and contributing to bacterial blight management.