As we have outlined in this paper, much is known about
signaling in the iPGPR–legume symbiosis and a certain
amount of ‘reasoning by analogy’ will help understanding
signal exchange between ePGPR and plants. Within the
various ePGPR associations there is a much greater
diversity between proximity to the host plant, mode of
infection and, most importantly, mode of growth effects
than in the iPGPR. Signaling molecules from ePGPR can be,
as recently shown, volatile compounds. At this point,
production of volatile compounds from iPGPR is unknown.
A clear distinction between the two types of associations has
been made and with this understanding one can begin to fit
pieces of this puzzle together to enhance the plant growth
promoting effects. Research is being conducted on ePGPR
to determine the identities of signaling compounds produced
and of course, the detailed signaling response
mechanisms that follow. Additional research exploiting