Novel Strains of Xanthomonas oryzae pv. oryzae UV Mutated Induce Systemic Resistance in Rice against Bacterial Leaf Blight Disease

Abstract

Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is the most devastating bacterial disease of rice production worldwide. Studies were conducted to develop management strategies using attenuated Xoo strains that were nearly or equally effective with previous ISR-P^® uses (Pseudomonas fluorescens SP007s product). Bacterial suspension of the virulent wildtype strain Xoo-19 was placed on nutrient agar and exposed to ultraviolet (UV) radiation (65 W) for 3 min. Two UV-induced biotype strains (M-407 and M-690) that survived the irradiation and reduced the growth rate and pathogenicity were selected and characterized. They differed from the wildtype Xoo-19 in colony morphology, production of bacteriocin and exopolysaccharide, enzyme activity (cellulase and protease), and reduced motility on the surface of agar. They were investigated further for their ability to control BLB and activate defense-related enzymes, β-1,3-glucanase and peroxidases (POX) under greenhouse conditions, compared with a naturally-occurring avirulent strain of Xoo obtained in this study (Xoo-7) and the strain SP007s. Twenty one-day old rice plants of two cultivars (susceptible cv. Khao Dauk Mali105 or KDML105 and resistant cv. Suphanburi 1) separately pretreated with 1×10⁸ cfu/mL of each of four antagonist strains as foliar sprays, were inoculated with the wildtype strain Xoo-19 at 3 d after pretreatment. All four strains, namely M 690, M-407, Xoo-7 and SP007s, provided significant (P = 0.05) disease reduction by 53.7, 49.5, 45.2, and 58.1% with KDML105 and 62.1, 51.4, 47.9, and 62.1% with Suphanburi 1, respectively, when assessed 14 d post inoculation compared to the nontreated control. Strain M-690 was equal to or nearly as effective in disease reduction as SP007s with no significant (P = 0.05) difference between both cultivars tested. The M-690 and SP007s strains also showed the highest accumulation of β-1,3-glucanase and POX activity in rice plants that correlated with their disease reduction ability. Control efficacy of all four antagonist strains showed a difference in the two-rice cultivars, with Suphanburi 1 being higher than KDML105 in both disease reduction and defense-related enzyme accumulation.