Thirty-five years of research following the initial discovery by Ryan and coworkers
established plant resistance against insect herbivores as a highly dynamic process. In addition to the proteinase inhibitors, many more inducible factors have
been identified which contribute to direct defense and which have the potential to
enhance host plant fitness after herbivore attack. These are aspects that will be introduced
in this chapter to provide the background for a more detailed discussion of
the defensive role of individual proteins in the subsequent, more focused chapters
of this volume. Another aspect of induced resistance that has fascinated researchers
since the seminal Green-and-Ryan-paper is the systemic nature of the response:
defense proteins accumulate not only at the site of wounding but also systemically
in unwounded tissues of the infested plant. Obviously, a signal must be generated
locally as a consequence of insect feeding which is then propagated throughout the
plant, and able to induce the expression of defense proteins at distant sites (Green
and Ryan 1972; Ryan and Moura 2002). Our current understanding of systemic
wound signaling for direct defense will also be summarized here.