Changes in bacterial community structure induced by mycorrhizal colonisation in split-root maize

Maize plants were grown in an autoclaved quartz sand-soil mix to which the bacterial communities of the soil and the mycorrhizal inocula were reintroduced. The root systems of the plants were divided with the two halves growing in separate pots. There were five different treatments: plants with both root halves non-mycorrhizal either at high or low P availability (nm-nm HP and nm-nm LP) or mycorrhizal plants grown at low P availability with one side of the root system non-mycorrhizal while the other side was inoculated with Glomus intraradices (GI-nm) or G. mosseae (GM-nm) or plants with both sides mycorrhizal but each side inoculated with a different fungus (GI-GM). The plants were harvested after 3 or 6 weeks. Shoot dry weight and shoot P concentration of the nm-nm HP plants were always higher than the plants grown at low P supply. Acid phosphatase activity in the rhizosphere was similar in all treatments and did not change over time. However, after 6 weeks alkaline phosphatase activity was higher in the rhizosphere of both root halves in the mycorrhizal plants as compared to the non-mycorrhizal plants. Mycorrhizal colonisation increased from 15-34% after 3 weeks to 78-87% after 6 weeks with no significant difference between GI and GM. The bacterial community structure, assessed by denaturing gradient gel electrophoresis (DGGE), changed over time and was specific for each of the three compartments, non-rhizosphere soil, rhizosphere soil and root surface. While the two P levels in the non-mycorrhizal treatments had no significant effect on the bacterial communities, mycorrhizal colonisation changed the bacterial community structure on the root surface and in the non-rhizosphere soil. The bacterial communities of the GI-GM plants differed more from the non-mycorrhizal plants than those of the plants with only one half of the root system mycorrhizal. The bacterial communities of both root halves of the GI-nm plants did not differ from each other and were very similar to those of the non-mycorrhizal plants. After 3 weeks, the bacterial communities of the two sides of the GM-nm plants differed: the mycorrhizal side of the resembled that of the GI-GM plants, while the non-mycorrhizal side of the root system was similar to that of the nm-nm plants. However after 6 weeks, the bacterial community structures of the two sides of the root system of the GM-nm plants were similar and differed from those of the nm-nm plants. It is concluded that the effect of mycorrhizal colonisation on the bacterial community structure in the rhizosphere may, at least in part, be plant-mediated.