The chemotaxis module is responsible for ensuring that our bacteria move towards roots. For this, the bacteria need to be able to sense a common root exudate. We have chosen E. coli chemotaxis to be rewired towards malic acid (also referred to as malate), a compound found in the TCA cycle. It is released from the roots at low concentrations. Since our chassis, Escherichia coli does not normally exhibit chemotaxis towards malate, we needed to engineer a malate-responsive sensor into the microbes that will enable them to perform chemotaxis towards roots.

Following chemotaxis towards the roots, our bacteria should be taken up into the roots. We want the bacteria to get taken up into the plant roots to ensure that the concentration of indole-3-acetic acid (IAA) in the plant is increased. If the bacteria remained outside the roots, this goal may also be reached but it may be harder to increase internal IAA concentration. In addition, uptake of bacteria into the roots followed by secretion of chemicals presents a novel platform for modifying plants without genetically modifying the plant genomes. In a paper published last year, Paungfoo-Lonhienne et al. showed that Arabidopsis and tomato plants are able to actively break down their cell wall to take up GFP-tagged E. coli and S. cerevisiae and use them as a source of nutrients.