Brassica rapa subsp. Chinensis juice enhances Bacillus subtilis selectively in leafy green production

Bacillus subtilis (BS) is a well-known beneficial microorganism for plants but is not competitive in the plant rhizosphere microbiome. We report the selective support of Brassica rapa subsp. Chinensis (Xiao Bai Cai) juice (XBCJ) on BS both in hydroponic nutrient solution and the plant rhizosphere of lettuce. After 2 weeks of being inoculated in the lettuce rhizosphere, the Bacillus population was enumerated at 3.30 ± 0.07 log CFU/unit in the BS group and at 5.20 ± 0.39 log CFU/unit in the BS + XBCJ group (p < 0.05). Accordingly, lettuce crops from the BS + XBCJ group were significantly higher than the control group for all of the tested biomass-related parameters (p < 0.05). The treatment did not significantly affect the texture, colour, moisture contents, total phenolic contents, or antioxidant activities of the lettuce crops (p > 0.05). Non-target ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) suggested that phenolic compounds could be the key class of phytochemicals being responsible for the selectivity. High-throughput RNA-based 16S rRNA gene sequencing and analysis were performed to depict the influence of BS and XBCJ over the global microbiome compositions of plant rhizosphere.