Fostering resistance in common bean: Synergistic defense activation by Bacillus subtilis HE18 and Pseudomonas fluorescens HE22 against Pythium root rot

Abstract

Root rot of common bean caused by Pythium aphanidermatum is a serious disease that causes significant losses to the production and yield of this crop. This study aimed to biocontrol the root rot disease of common bean using Bacillus subtilis HE18 and Pseudomonas fluorescens HE22 as effective bioagents and promote the growth of this plant. Using the in vitro dual culture assay, B. subtilis HE18 and P. fluorescens HE22 inhibited the radial growth of the pathogenic P. aphanidermatum by 67.95 and 48.39%, respectively. Chemical analysis of the cell-free filtrates of both bioagents using GC/MS demonstrated the production of twenty and thirteen volatile compounds by B. subtilis HE18 and P. fluorescens HE22, respectively, which may account for their significant antifungal efficacy. In the greenhouse, the application of a combination of both bio-inoculants displayed lower severity of pre- and post-emergence damping off and high plant survival, recording 10.1, 6.7, and 83.3%, respectively. The expression of the defense-related genes JERF3, GLU, and PR1, was examined in the treated plants using qPCR, where the highest upregulation levels were recorded for the infected plants that were treated with the combined biocontrol agents. Treating of the infected common bean with a combination of B. subtilis HE18 and P. fluorescens HE22 increased the activity levels of POD and PPO antioxidant enzymes and enhanced the phenol content. Combined treating of the infected plants with both bioagents increased the TSS to 14.15 °Brix, compared with the infected, untreated plants (7.05 °Brix). The dual application of both bioagents expressed a significant increment in Chl. a and b pigments, enhanced the levels of carotenoids, and increased the total pigments. Treating of the infected and control plants with both bio-inoculants caused an appreciable increment in the shoot height, root length, and shoot and dry weights, respectively. Moreover, the same treatment increased the pod weights, pod length and width, and the number of seeds pod−1, in addition to the yield plant−1. Finally, we concluded that both bioagents acted synergistically against the fungal pathogen, as they targeted multiple sites in the fungal cell and promoted the growth parameters and yields of the treated common bean plants.