Herbivore-Associated Bacteria as Potential Mediators and Modifiers of Induced Plant Defense Against Spider Mites and Thrips

Abstract

Induced plant defense, comprising contact with exogenous stimuli, production of endogenous signals alerting the plant, associated biochemical cascades, and local and/or systemic expression of the defense mechanisms, critically depends on the nature of the inducing agents. At large, bio-trophic pathogenic microorganisms and viruses usually trigger the salicylate (SA)-mediated pathway, whereas necro-trophic pathogens and herbivores usually trigger the jasmonate (JA)-mediated pathway in plants. The SA- and JA-mediated pathways do not operate independently but commonly interfere with each other. Several recent studies revealed abnormal plant responses upon herbivore attack in diverse plant-herbivore systems. Observed abnormalities range from suppression of the common JA-pathway, induction of the SA-pathway to no response, yet the underlying proximate causes and ultimate consequences of these variations are elusive. Strikingly, some studies provide compelling evidence that anti-herbivore plant responses may decisively depend on bacteria associated with the herbivore attacking the plant (HAB for herbivore-associated bacteria). HAB may influence herbivore recognition by the plant and alter the biochemical cascades inside plants. Here, I report cases in point of HAB manipulating induced anti-herbivore plant responses, suggest spatial and temporal categorization of HAB, and point at proximate and ultimate aspects of plant defense manipulation by HAB. Following, I overview the diversity of HAB of spider mites and herbivorous thrips, argue that, considering recently reported phenomena of abnormal plant responses upon spider mite attack, some of these HAB could represent important, but hitherto largely neglected, mediators/modifiers of induced plant defense against spider mites and thrips, and conclude with suggestions for future research.

Plant Defenses Against Tetranychus urticae: Mind the Gaps

“…These features support the mite polyphagous character and demonstrate its capability to counteract the effects of a variety of active components either synthesized by the plant as defenses or as components of the acaricides used for pest control. In addition, the presence of endosymbionts, mainly Wolbachia spp, in spider mites may alter their interaction with the host, manipulate the plant defenses and modify mite performance [19,20]. Biological control of spider mites through natural predators is a useful practice in closed greenhouses but not in open fields.…”

Interactions between white mealybugs and red spider mites sequentially colonizing coffee plants

“…The authors showed that red spider mite‐infested coffee plants were more resistant to conspecifics and the coffee leaf miner Leucoptera coffeella Guérin‐Méneville (Lepidoptera: Lyonetiidae), although these results contrasted with observations in the field, where multiple infestations by the two herbivores were frequently found. Inconsistencies in mite‐induced plant responses have been commonly reported in the literature as the ability of mites to manipulate plant defences seems to vary among populations and might be related to their associated bacteria (Schausberger, ). Therefore, the questions of whether red spider mites suppress coffee plant defences, and whether this ability varies among populations, deserve further investigation.…”

Herbivore Associated Microorganisms Modulating Induced Plant Defenses

“…More studies are needed to determine the mechanisms by which Herbivore-associated bacteria regulate JAinducible plant defenses (Frago et al, 2012). Further studies to achieve selective elimination of single bacterial taxa, to pinpoint which bacteria are responsible for manipulating plant defense (Schausberger, 2018). Physiological and genetic studies are needed to directly demonstrate the role of microbes in assisting detoxification in insect gut (Sikano et al, 2016).…”

“…Microbes have persistent associations with their hosts and that do not cause overt pathogenic symptoms. Insect symbionts can be divided into three general categories based on their location relative to the insect body: environmental (or external), digestive system (internal extracellular) and endosymbionts (internal intracellular) (Schausberger, 2018). Few symbionts are intracellular or housed within the body cavity and are passed between generations through eggs, resulting in maternal inheritance.…”