Effects of transgenic poplars expressing increased levels of cellular cytokinin on rhizosphere microbial communities

• Microbial biomass and community structure were compared at three locations.
• Biomasses differed between the transgenics and the control at one location.
• At the other locations, the biomasses were similar but community structures differed.
• These variations among sites were temporal and inconsistent.

Considerable effort has been made in biotechnology to increase plant biomass. Altering cellular levels of plant hormones, including cytokinin, by genetic modification, has been one way to achieve the goal as it is involved in a variety of processes related to plant growth and development. However, the alteration inevitably may change physiological and biochemical characteristics of plants, and thus could affect the relationship between plants and other organisms interacting with the plants such as microorganisms inhabiting in the rhizosphere. To determine if these indirect effects on rhizosphere microorganisms, mediated by hormonal changes in plants, do occur, we investigated the microbial biomass and community structure associated with transgenic Populus trees with altered cellular cytokinin levels, using phospholipid fatty acid (PLFA) analysis. Three transgenic lines expressing increased levels of cellular cytokinin (T1403, T1410, and T1413) and their non-transgenic isoline (BH) were planted at three locations (Suwon, Cheongwon, and Jinju) in 2011. Soil samples were collected near the base of each tree monthly, from May to September. Indicator PLFAs were utilized to calculate the microbial (bacterial and fungal) biomass, and PLFA profiles were developed to characterize the structure of those communities. Over the growing season, soils from Cheongwon and Jinju had similar microbial biomasses (PLFAs indicating functional groups) whereas, at Suwon, the biomass associated with the rhizosphere of Line T1413 was significantly different from that of the other transgenics and the control. At Cheongwon and Jinju, the structure of the rhizosphere microbial communities differed significantly between Lines T1403 or T1410 and BH, but only in May and June. By contrast, those structures were similar in all sampling months for each line at Suwon. Our results indicate that the influence resulting from genetic modification of the poplar trees on the rhizosphere microbial community is only temporary and inconsistent depending upon location and genetic line.