Effect of crop rotation and soil cover on alteration of the soil microflora generated by the culture of transgenic plants producing opines

The culture of transgenic Lotus corniculatus plants producing opines, which are bacterial growth substrates, leads to the selection of rhizospheric ba cteria able to utilize these substrates. We have investigated the fate of t he opine-utilizing community over time under different experimental conditi ons following elimination of selective pressure exerted by the transgenic p lants. These plants were removed from the soil, which was either left unpla nted or replanted with wild-type L. corniculatus or wheat plants. The densi ty of opine-utilizing bacteria in the fallow soils remained essentially unc hanged throughout the experiment, regardless of the soil of origin (soil pl anted with wild-type or transgenic plants). When wild-type Lotus plants wer e used to replace their transgenic counterparts, only the bacterial populat ions able to utilize the opines were affected. Long-term changes affecting the opine-utilizing bacterial community on Lotus roots was dependent upon t he opine studied. The concentration of nopaline utilizers decreased, upon r eplacement of the transgenic plants, to a level similar to that of normal p lants, while the concentration of mannopine utilizers decreased to levels i ntermediate between transgenic and normal plants. These data indicate that: (i) the opine-utilizing bacterial populations can be controlled in the rhi zosphere via plant-exudate engineering; (ii) the interaction between the en gineered plants and their root-associated micro-organisms is transgene spec ific; and (iii) alterations induced by the cultivation of transgenic plants may sometimes be persistent. Furthermore, opine-utilizing bacterial popula tions can be controlled by crop rotation. Therefore, favouring the growth o f a rhizobacterium of agronomic interest via an opine-based strategy appear s feasible.