Development of a simple root model to study the effects of single exudates on the development of bacterial community structure.

The plant root interface is a hot spot for microbial activities. Root exudates are the key compounds that drive microbial performance. However quality and amount of root exudates are highly dynamic in time and space, thus a direct influence of a single compound on a microbial community composition is fairly impossible to study in nature. Therefore it was the aim of this project to develop an artificial root model (ARM), and investigate the influence of three compounds which have often been described as root exudates acting as model compounds for carbohydrates, organic acids and amino acids (glucose, malic acid and serine) on the development of bacterial communities and time on the ARM based on 16S rRNA derived TRFLP pattern. The ARM consisted of a slide covered with low melting agarose, where 8 different compounds which have been described as typical root exudates were embedded. The ARMs were incubated in soil for 2, 5, 9 and 20 days, before the analysis of the developed bacterial community structure was done. The bacterial community composition was in good agreement after 9 days of incubation of the ARM in soil with the root associated microflora of Arabidopsis thaliana shortly before flowering. The single compounds of the exudates mix had different effects on the development of ARM derived bacterial communities. Whereas the experiments where glucose was omitted gave no significant differences in the development of bacterial communities over time compared to the ARM where the standard mixture of exudates had been applied, there was a pronounced effect visible mainly after two days of incubation of the ARM in the experiments where no malic acid was added to the exudate mixture. At later time points ARMs with standard exudates' mixture and those where malic acid had been omitted, the bacterial community composition did not differ. The experiments where serine was omitted mainly induced shifts in the bacterial community composition compared to the ARM with standard exudates' mixture at the latest sampling time point (20 days of incubation).