Evaluation of Paenibacillus PGPR Strains for Growth Promotion and Biocontrol of Rice Sheath Blight

Abstract

A study was conducted to evaluate PGPR strains belonging to the genus Paenibacillus for their potential as biological control agents of rice sheath blight disease caused by Rhizoctonia solani. Paenibacillus strains are known to produce a wide variety of antimicrobial compounds with activity against both fungi and bacteria, which led us to hypothesize that this genus of bacteria may harbor potent biological control agents of Rhizoctonia solani. First, a preliminary screening of 335 Paenibacillus strains belonging to 56 species was conducted to identify strains with antagonistic capability to R. solani. In total, 21 strains belonging to the species P. peoriae, P. jamilae, and P. polymyxa exhibited antagonism. These strains were then evaluated in several in vitro experiments to further characterize their antagonism to R. solani, including an advanced antibiosis assay, a sclerotia germination assay, and a detached leaf assay. All but one of the Paenibacillus strains completely inhibited the germination of R. solani sclerotia. In addition, nine strains in total significantly reduced the average disease rating in a detached leaf assay as compared to the disease control. These nine strains were then evaluated in greenhouse disease assays for their capacity to antagonize R. solani in planta. Six of the evaluated nine strains had significantly lower lesion length percentages, and four of these strains (JJ-195, JJ-1580, JJ-1710, JJ-1824) also had significantly lower disease ratings compared to the disease control. Paenibacillus strains showing antagonistic activity were also evaluated for their ability to promote the early growth of rice; however, no significant differences were observed. In addition, the colonization dynamics of three selected strains were evaluated in the rice phyllosphere utilizing rifampicin-resistant generated mutants. Populations of all three strains decreased rapidly over the selected time points: 0, 1 week, and 2 weeks after inoculation suggesting that these strains are not well-suited to the phyllosphere, and that a large population size was not necessary for some strains to have effective biological control of R. solani.