A look into the toolbox of multi‐talents: insect pathogenicity determinants of plant‐beneficial pseudomonads

The Pseudomonas fluorescens group of bacteria is very diverse and comprises members that make part of the beneficial rhizosphere microbiota that cooperates with the plant (Mendes et al., 2013; Venturi and Keel, 2016). Plant-beneficial activities of rhizosphere pseudomonads have been investigated since several decades. They include stimulation of plant growth and defense, mobilization of soil nutrients and suppression of phytopathogenic fungi, protists and bacteria via antimicrobial compounds (Haas and D efago, 2005). Recent research identified a phylogenetically distinct P. fluorescens subgroup, specified by strains of Pseudomonas protegens and Pseudomonas chlororaphis, which exhibits potent insecticidal activities as an extra (P echy-Tarr et al., 2008; Olcott et al., 2010; Ruffner et al., 2015; Flury et al., 2016). According to current knowledge, the insecticidal strains are capable of colonizing and killing representatives of three major insect orders, i.e. Lepidoptera, Diptera and Hemiptera, including several agricultural pests, following oral uptake (Olcott et al., 2010; Kupferschmied et al., 2013; Ruffner et al., 2013; Flury et al., 2016). In a typical course of infection, ingested entomopathogenic pseudomonads colonize the gut, breach the intestinal epithelial barrier, invade the hemocoel, proliferate, and eventually kill the insect (Kupferschmied et al., 2013). While the molecular mechanisms involved in plant-beneficial activities of these pseudomonads have been dissected in some detail (Haas and D efago, 2005; Gross and Loper, 2009; Kupferschmied et al., 2013), the virulence factors and mechanisms contributing to their capacity to invade and kill insects currently are largely unknown. A major insect virulence factor identified so far is the Fit toxin, which typically is produced by strains of P. protegens and P. chlororaphis and is significant for their pathogenicity towards Lepidopteran larvae (P echy-Tarr et al., 2008; Ruffner et al., 2013; Flury et al., 2016). Production of the insecticidal protein is specifically switched on in the insects, but not on plant roots (P echy-Tarr et al., 2013; Kupferschmied et al., 2014). Mutants lacking the Fit toxin retain substantial toxicity, stressing that insect pathogenicity of these bacteria is multifactorial. In this issue of Environmental Microbiology, Loper et al. (2016) report on the identification of additional insect pathogenicity factors of a representative insecticidal pseudomonad, i.e. P. protegens Pf-5, in an oral infection model. Their approach was inspired by the observation that P. protegens mutants defective for the global regulator GacA are severely impaired in oral toxicity to Dipteran and Lepidopteran insects (Olcott et al., 2010; Ruffner et al., 2013; Flury et al., 2016). GacA is known to positively control various traits contributing to beneficial and pathogenic activities of pseudomonads (Haas and D efago, 2005). In their study, Loper et al. (2016) focused on GacA-controlled toxic and lytic exoproducts of P. protegens and investigated their contribution to oral toxicity towards Drosophila melanogaster using directed mutational analysis. Several of their findings significantly expand our knowledge about the insect pathogenicity determinants contained in the rich toolbox of these multi-talented bacteria (Fig. 1). Loper et al. (2016) identified rhizoxin as a major factor in oral toxicity of P. protegens Pf-5 towards Drosophila. The macrolide molecule interferes with mitosis in eukaryotic cells, which enables broad antifungal, cytotoxic, and phytotoxic activities, and it has demonstrated function in plant pathogen suppression (Loper et al., 2008). Insect toxicity now emerges as an additional feature of rhizoxin, illustrating that P. protegens can deploy certain toxic secondary metabolites for plant-beneficial as well as for insect-pathogenic activities. Remarkably, the rhizoxin gene cluster is only present in a subset of P. *For correspondence. E-mail christoph.keel@unil.ch; Tel. 141 21 692 56 36; Fax 141 21 692 56 05.