1. The runoff from nitrogen-rich fertiliser applied to farmland creates major ecological problems. Plants such as legumes have symbiotic nitrogen-fixing bacteria which can provide nitrogen naturally, without the need for adding fertiliser. However, these symbionts are not associated with crops such as corn.

Scientists have developed a strain of nitrogen-fixing bacteria that can associate with corn roots. Initially, this bacterium did not supply the corn with nitrogen when the crop needed it, but the scientists managed to alter the strain so that nitrogen was produced at the right times. 

Which of the following provides a possible explanation for how might they have achieved this?