A Novel Interaction between Plant-Beneficial Rhizobacteria and Roots: Colonization Induces Corn Resistance against the Root Herbivore Diabrotica speciosa

Abstract

A number of soil-borne microorganisms, such as mycorrhizal fungi and rhizobacteria, establish mutualistic interactions with plants, which can indirectly affect other organisms. Knowledge of the plant-mediated effects of mutualistic microorganisms is limited to aboveground insects, whereas there is little understanding of what role beneficial soil bacteria may play in plant defense against root herbivory. Here, we establish that colonization by the beneficial rhizobacterium Azospirillum brasilense affects the host selection and performance of the insect Diabrotica speciosa. Root larvae preferentially orient toward the roots of non-inoculated plants versus inoculated roots and gain less weight when feeding on inoculated plants. As inoculation by A. brasilense induces higher emissions of (E)-β-caryophyllene compared with non-inoculated plants, it is plausible that the non-preference of D. speciosa for inoculated plants is related to this sesquiterpene, which is well known to mediate belowground insect-plant interactions. To the best of our knowledge, this is the first study showing that a beneficial rhizobacterium inoculant indirectly alters belowground plant-insect interactions. The role of A. brasilense as part of an integrative pest management (IPM) program for the protection of corn against the South American corn rootworm, D. speciosa, is considered.

Neonate larvae of the specialist herbivore Diabrotica virgifera virgifera do not exploit the defensive volatile (E)-β-caryophyllene in locating maize roots

“…Presumably, transformed maize line 201-L1 and its isogenic line HiII vary solely on the emission of EbC and each produced similar amounts of other secondary metabolites and CO 2 (Degenhardt et al 2009). Second instars WCR were previously shown to select host plants with a suitable density of conspecific larvae using the induced EbC in a dosedependent manner (Robert et al 2012b), whereas D. speciosa Santos larvae did not respond to this plant volatile (Santos et al 2014). Second instar larvae apparently not only used EbC but also ethylene as a signal to locate suitable host plants (Robert et al 2012a).…”

Eliciting maize defense pathways aboveground attracts belowground biocontrol agents

“…This observation could be a boon for strategies using induced plant defenses to augment biological control in conjunction with other strategies to repel D. speciosa larvae27. Because larvae do not preferentially respond to elicitor treated plants, induced plant defense will likely not attract belowground larval pest populations in the field while augmenting control through recruitment of entomopathogenic nematode natural enemies.…”

Get Tough, Get Toxic, or Get a Bodyguard: Identifying Candidate Traits Conferring Belowground Resistance to Herbivores in Grasses

“…There has been some speculation that root hairs may offer some protection by preventing very small herbivores (e.g., neonates) from reaching and penetrating the root epidermis or may possibly provide refugia for natural enemies of herbivores such as entomopathogenic nematodes (Johnson et al, 2016a). Although not strictly a physical defense, root hairs may also increase the root surface available for colonization by beneficial soil microbes, which in turn can sometimes confer resistance to insect and nematode herbivores of grasses (Piskiewicz et al, 2009; Santos et al, 2014). Notably, root hairs can be induced, for example by plant-parasitic nematodes in barley (Haase et al, 2007).…”

“…The plant parasitic nematode Tylenchorhynchus ventralis is strongly controlled by soil microbes in a coastal foredune grassland (Piskiewicz et al, 2009) and colonization of maize by the rhizobacterium Azospirillum brasilense can deter, and reduce the performance of, western corn rootworm (Santos et al, 2014). However, another study of three plant species, including the grass Holcus lanatus , found no effect of the soil microbial community on defense against nematodes (Wurst et al, 2009).…”