This question has no definitive answer. In previous experience the percentage of cross-communicating strains has varied from 1-10% depending on the plant, etc.

2. What would happen if you picked strain 30-84I from the orange halo sector of the plate and re-streaked it onto LB agar? Strain 30-84 from the white halo sector?

Strain 30-84I is defective in the ability to synthesize its cognate AHL signal. Without this signal it can not express the phenazine biosynthetic genes and is white. The orange halo that is visible is due to the production of AHL signal by the test isolate that is recognized by strain 30-84I. Thus, re-streaking strain 30-84I away from the spotted isolate will remove the source of AHL signal and the indicator will once again appear white due to a lack of AHL signal for phenazine gene activation.

Strain 30-84 is the wildtype strain and is orange due to the production of phenazines via the AHL-mediated regulatory system. The white halo is due to the test isolate interfering with phenazine gene activation. The simplest explanation is that the test isolate is producing a compound that blocks either the action of the AHL signal or the function of PhzR. Thus, re-streaking strain 30-84 away from the spotted isolate should restore normal phenazine expression and the colonies will be orange.

3. Why would different bacteria cross-communicate? That is, list several possible ecological function(s) of cross-communication.

This is an interesting question in that there is no single answer. The simplest hypothesis is that bacteria must function as members of a mixed community to survive and that cross-communication allows them to organize these communities in some manner.

4. Discuss two experiments that you might perform to study the phenomenon of cross-communication further.

This question could have multiple answers. Two obvious experiments are: 1. Isolate and identify the signaling compounds themselves and see if the isolated compound has a similar effect, and 2. Determine whether cross-communication affects long term survival of strain 30-84.

5. Does the fact that a test isolate produces an AHL signal in vitro prove that the same isolate produces an AHL signal in planta?

No, it only demonstrates that some signal is produced under the in vitro conditions used. To demonstrate signal production in situ requires measuring cross-communication directly on the root.

6. Some of the test isolates might produce a clear zone in the lawn of P. aureofaciens. What might this clearing indicate about the interactions between the two strains?

The zone of growth inhibition probably indicates that the test isolate is producing a compound that is inhibiting the growth of P. aureofaciens. It could be an antibiotic, a bacteriocin, etc. Check the plate to see if strain 30-84 does grow in the region over time.