Gravitational water flow enhances the colonization of spinach roots in soil by plant growth-promoting Pseudomonas

Root colonization by a plant growth-promoting Pseudomonas (H3R strain) was examined to determine whether gravitational water flow affects the bacterial distribution on spinach roots. Firstly, the effect of the amount of irrigation water or soil moisture content on the distribution of the H3R strain resistant to three antibiotics was examined using a rhizobox. When a large amount of water was irrigated from the top of the rhizobox, the Pseudomonas strain inoculated onto the spinach seed was detected in the lower part of the rhizobox. However, when water was supplied from the side of the rhizobox with a sprayer, the Pseudomonas strain could not be detected in the lower part of the rhizobox. Secondly, to visualize root colonization by the inoculated bacteria, Pseudomonas strains were marked with the lux gene. Spinach seeds were inoculated with the lux-marked Pseudomonas strains and grown in a pot. A plastic tube with a hole at the upper, middle, or lower position was buried in a pot. Water was supplied from a hole of the tube. The bacterial distribution on roots was observed based on bioluminescence. Only when water was supplied from the upper hole, did the bacteria colonize the whole root. When water was supplied from the middle or lower hole, the bacteria remained around the seeds. These results indicated that the effect of the gravitational water flow by irrigation is important for the spread of colonization of spinach roots by Pseudomonas strains.