Effects of Bacillus cereus strain Jdm1 on Meloidogyne incognita and the bacterial community in tomato rhizosphere soil.

Bacillus cereus strain Jdm1 was isolated and tested for activity as a biocontrol agent to suppress Meloidogyne incognita. Petri dish test results indicated that Jdm1 culture supernatant significantly inhibited the second-stage juvenile (J2) activity and egg hatching, and also decreased the number of galls on tomato roots in the pot test. Control efficiency reached 43%, with improved growth compared to control plants. In field tests, control efficacies were greater than 50% 30 day post-inoculation, before decreasing. Furthermore, when avermectins were included to manage M. incognita, the yield of tomatoes was increased significantly. The effect of Jdm1 on the bacterial community in the tomato rhizosphere soil was monitored using PCR-denaturing gradient gel electrophoresis (PCR-DGGE) on field plants. DGGE band patterns and principal component analysis showed that application of Jdm1 did not permanently imperil the bacterial community, which recovered soon after inoculation, despite being impacted initially. The plant growth stage had a much greater influence on the bacterial community in tomato rhizosphere soil.

Taxonomic structure and functional association of foxtail millet root microbiome.

The root microbes play pivotal roles in plant productivity, nutrient uptakes, and disease resistance. The root microbial community structure has been extensively investigated by 16S/18S/ITS amplicons and metagenomic sequencing in crops and model plants. However, the functional associations between root microbes and host plant growth are poorly understood. This work investigates the root bacterial community of foxtail millet (Setaria italica) and its potential effects on host plant productivity. We determined the bacterial composition of 2882 samples from foxtail millet rhizoplane, rhizosphere and corresponding bulk soils from 2 well-separated geographic locations by 16S rRNA gene amplicon sequencing. We identified 16 109 operational taxonomic units (OTUs), and defined 187 OTUs as shared rhizoplane core OTUs. The ß-diversity analysis revealed that microhabitat was the major factor shaping foxtail millet root bacterial community, followed by geographic locations. Large-scale association analysis identified the potential beneficial bacteria correlated with plant high productivity. Besides, the functional prediction revealed specific pathways enriched in foxtail millet rhizoplane bacterial community. We systematically described the root bacterial community structure of foxtail millet and found its core rhizoplane bacterial members. Our results demonstrated that host plants enrich specific bacteria and functions in the rhizoplane. The potentially beneficial bacteria may serve as a valuable knowledge foundation for bio-fertilizer development in agriculture.

Isolation and characterization of a rhizobacterial antagonist of root-knot nematodes.

The rhizobacterial strain Jdm2 was isolated from the rhizosphere of the traditional Chinese medicinal herb Trichosanthes kirilowii in Jiangsu province, China, and was identified as Bacillus subtilis. Exposure of cell-free filtrate of the strain to the root-knot nematode Meloidogyne incognita under in vitro conditions caused substantial mortality of the second stage juvenile (J2) and significantly reduced egg hatchability. A greenhouse trial demonstrated that 56 days after treatment with Jdm2, the number of galls associated with M. incognita infection in the tomato (Solanum lycopersicum) roots was significantly reduced compared to controls, and the disease severity of infected plants was lower in treated plants (36%) compared to water control (75%). Consistently, in the field trial, the biocontrol efficacy of Jdm2 reached 69%, 51% and 48% after 30, 60 and 90 days post-transplantation, respectively. As indicated by PCR-DGGE analysis, inoculation with Jdm2 strain had an effect on the bacterial community of the tomato rhizosphere at the first stage, but was not able to imperil the bacterial community stability for long time. The novel bacterial strain Jdm2 enhances plant growth and inhibits nematode activity, and has the potential to be a safe and effective microbial pesticide.