Three‐way interactions between plants, microbes and insects

Plants are important mediators of interactions between their associated microbe and insect communities (Van der Putten et al. 2001; Ohgushi 2005). Changes in plants induced by one species have cascading effects on interactions with other species, shaping their abundances and community structure (Ohgushi 2008). While the consequences of such indirect interactions for community structure have predominantly been examined within the plant-associated insect community (e.g. Van Zandt & Agrawal 2004; Poelman et al. 2008; Utsumi 2011), there is growing evidence that there are similar community-wide impacts of plant-mediated interactions between microbes and insects (e.g. Kluth, Kruess & Tscharntke 2001; Omacini et al. 2001; Katayama, Zhang & Ohgushi 2011; Tack, Gripenberg & Roslin 2012). This highlights the ecological importance of three-way interactions between plants, microbes and insects. The study of such ‘plant–microbe– insect’ (PMI) interactions (Fig. 1) is a research area that has been rapidly expanding in the past two decades. Molecular studies of the mechanisms underlying these three-way interactions, as well as ecological and evolutionary studies of the consequences of PMI interactions in natural communities, have recently given a large impetus to this young field. In this special feature, we have brought together eight papers reviewing different aspects of these recent advances in the field of PMI interactions. Research on PMI interactions has gradually bridged the traditionally separated subdisciplines of plant pathology, insect pathology and entomology. Plant pathologists early on realized that insects were not only important vectors of plant disease, but also one of the factors determining what was then called ‘host predisposition’ (Yarwood 1959; Schoeneweiss 1975). This term was used to describe any environmental alteration of the susceptibility of host plants to their pathogens, prior to their interaction. Similarly, in the 1980s, a series of reviews from entomologists appeared on the effects of plantand insect-associated microbes on plant resource exploitation by insects (e.g. Jones 1984; Hammond & Hardy 1988), culminating in the seminal book on microbial mediation of plant–insect interactions by Barbosa, Krischick & Jones 1991, which provided the first detailed and fascinating overview of the widespread, diverse and strong roles played by plantand insect-associated microbes in shaping plant–insect interactions. PMI interactions represent a broad research field, both in terms of the disciplines involved (from molecular