Simultaneous down-regulation of enzymes in the phenylpropanoid pathway of plants has aggregated effects on rhizosphere microbial communities

We have investigated the effects of genetic modifications to lignin biosynthesis in tobacco on the structural and functional characteristics of the microbial community in the rhizospheres of plants with down-regulations to cinnamyl alcohol dehydrogenase (CAD), caffeic acid O-methyltransferase (COMT) and cinnamoyl-CoA reductase (CCR) singly and in all the double and triple combinations, and corresponding unmodified plants. The mass of root produced was reduced by the down-regulation of enzymes in the phenylpropanoid pathway, but there were no differences in the microbial biomass in the rhizosphere per mass of roots. The composition of the rhizosphere microbial communities differed between modified and unmodified plants, with the modified plants having smaller proportions of fungal biomass as indicated by phospholipid fatty acid. Subtle changes to both root chemistry and rhizosphere microbial community function associated with down-regulation of CAD have the potential to modify carbon cycling processes.